Articles de revues : « Seawater signature »

1

Kurzweil, Florian, Corey Archer, Martin Wille, Ronny Schoenberg, Carsten Münker et Olaf Dellwig. « Redox control on the tungsten isotope composition of seawater ». Proceedings of the National Academy of Sciences 118, n^o 18 (27 avril 2021) : e2023544118. http://dx.doi.org/10.1073/pnas.2023544118.

Texte intégral

Résumé :

Free oxygen represents an essential basis for the evolution of complex life forms on a habitable Earth. The isotope composition of redox-sensitive trace elements such as tungsten (W) can possibly trace the earliest rise of oceanic oxygen in Earth’s history. However, the impact of redox changes on the W isotope composition of seawater is still unknown. Here, we report highly variable W isotope compositions in the water column of a redox-stratified basin (δ186/184W between +0.347 and +0.810 ‰) that contrast with the homogenous W isotope composition of the open ocean (refined δ186/184W of +0.543 ± 0.046 ‰). Consistent with experimental studies, the preferential scavenging of isotopically light W by Mn-oxides increases the δ186/184W of surrounding seawater, whereas the redissolution of Mn-oxides causes decreasing seawater δ186/184W. Overall, the distinctly heavy stable W isotopic signature of open ocean seawater mirrors predominantly fully oxic conditions in modern oceans. We expect, however, that the redox evolution from anoxic to hypoxic and finally oxic marine conditions in early Earth’s history would have continuously increased the seawater δ186/184W. Stable W isotope compositions of chemical sediments that potentially preserve changing seawater W isotope signatures might therefore reflect global changes in marine redox conditions.

Styles APA, Harvard, Vancouver, ISO, etc.

2

Emmanuel, S., J. A. Schuessler, J. Vinther, A. Matthews et F. von Blanckenburg. « Iron isotope fractionation in marine invertebrates in near shore environments ». Biogeosciences Discussions 11, n^o 4 (11 avril 2014) : 5533–55. http://dx.doi.org/10.5194/bgd-11-5533-2014.

Texte intégral

Résumé :

Abstract. Chitons (Mollusca) are marine invertebrates that produce radula (teeth or rasping tongue) containing high concentrations of biomineralized magnetite and other iron bearing minerals. As Fe isotope signatures are influenced by redox processes and biological fractionation, Fe isotopes in chiton radula might be expected to provide an effective tracer of ambient oceanic conditions and biogeochemical cycling. Here, in a pilot study to measure Fe isotopes in marine invertebrates, we examine Fe isotopes in modern marine chiton radula collected from different locations in the Atlantic and Pacific oceans to assess the range of isotopic values, and to test whether or not the isotopic signatures reflect seawater values. Furthermore, by comparing two species that have very different feeding habits but collected from the same location, we infer a possible link between diet and Fe isotopic signatures. Values of δ56Fe (relative to IRMM-014) in chiton teeth range from −1.90 to 0.00‰ (±0.05‰ (2σ) uncertainty in δ56Fe), probably reflecting a combination of geographical control and biological fractionation processes. Comparison with published local surface seawater Fe isotope data shows a consistent negative offset of chiton teeth Fe isotope compositions relative to seawater. Strikingly, two different species from the same locality in the North Pacific (Puget Sound, Washington, USA) have distinct isotopic signatures. Tonicella lineata, which feeds on red algae, has a mean δ56Fe of −0.65 ± 0.26‰ (2σ, 3 specimens), while Mopalia muscosa, which feeds primarily on green algae, shows lighter isotopic values with a mean δ56Fe of −1.47 ± 0.98‰ (2σ, 5 specimens). Although chitons are not simple recorders of the ambient seawater Fe isotopic signature, these preliminary results suggest that Fe isotopes provide information concerning Fe biogeochemical cycling in near shore environments, and might be used to probe sources of Fe in the diets of different organisms.

Styles APA, Harvard, Vancouver, ISO, etc.

3

Emmanuel, S., J. A. Schuessler, J. Vinther, A. Matthews et F. von Blanckenburg. « A preliminary study of iron isotope fractionation in marine invertebrates (chitons, Mollusca) in near-shore environments ». Biogeosciences 11, n^o 19 (8 octobre 2014) : 5493–502. http://dx.doi.org/10.5194/bg-11-5493-2014.

Texte intégral

Résumé :

Abstract. Chitons (Mollusca) are marine invertebrates that produce radulae (teeth or rasping tongues) containing high concentrations of biomineralized magnetite and other iron-bearing minerals. As Fe isotope signatures are influenced by redox processes and biological fractionation, Fe isotopes in chiton radulae might be expected to provide an effective tracer of ambient oceanic conditions and biogeochemical cycling. Here, in a pilot study to measure Fe isotopes in marine invertebrates, we examine Fe isotopes in modern marine chiton radulae collected from different locations in the Atlantic and Pacific oceans to assess the range of isotopic values, and to test whether or not the isotopic signatures reflect seawater values. Values of δ56Fe (relative to IRMM-014) in chiton teeth range from −1.90 to 0.00 ‰ (±0.05‰ (2σ) uncertainty in δ56Fe), probably reflecting a combination of geographical control and biological fractionation processes. Comparison with published local surface seawater Fe isotope data shows a consistent negative offset of chiton teeth Fe isotope compositions relative to seawater. Strikingly, two different species from the same locality in the North Pacific (Puget Sound, Washington, USA) have distinct isotopic signatures. Tonicella lineata, which feeds on red algae in the sublittoral zone, has a mean δ56Fe of −0.65 ± 0.26‰ (2σ, 3 specimens), while Mopalia muscosa, which feeds on both green and red algae in the eulittoral zone, shows lighter isotopic values with a mean δ56Fe of −1.47 ± 0.98‰ (2σ, 5 specimens). Three possible pathways are proposed to account for the different isotopic signatures: (i) physiologically controlled processes within the chitons that lead to species-dependent fractionation; (ii) diet-controlled variability due to different Fe isotope fractionation in the red and green algal food sources; and (iii) environmentally controlled fractionation that causes variation in the isotopic signatures of bioavailable Fe in the different tidal regions. Our preliminary results suggest that while chitons are not simple recorders of the ambient seawater Fe isotopic signature, Fe isotopes provide valuable information concerning Fe biogeochemical cycling in near-shore environments, and may potentially be used to probe sources of Fe recorded in different organisms.

Styles APA, Harvard, Vancouver, ISO, etc.

4

Renson, Virginie, et Michael D. Glascock. « Lead Isotopes to Identify Underwater Ceramic Contamination : The Example of the Kyrenia Shipwreck (Cyprus) ». Minerals 11, n^o 6 (12 juin 2021) : 625. http://dx.doi.org/10.3390/min11060625.

Texte intégral

Résumé :

We present the lead isotopic composition of ceramic fragments from the Kyrenia shipwreck (Cyprus), selected from three chemical groups related to the Rhodes and Alimos regions (Greece). Fragments of the lead sheathing covering the ship’s hull and biogenic material formed on some of the ceramic sherds, were analyzed along with sherds from the Rhodes and Alimos source areas for comparison. The objective of this paper was to evaluate the impact of the seawater environment on the isotopic signature of the ceramics, and on our ability to use lead isotopes to source ceramics recovered from seawater. The results showed that the lead isotopic composition of the shipwreck ceramics was modified by its prolonged proximity to the lead hull sheathing. The lead signature of filtering marine organisms encrusted on the ceramics provided support for this hypothesis.

Styles APA, Harvard, Vancouver, ISO, etc.

5

Resing, Joseph A., et Francis J. Sansone. « The chemistry of lava-seawater interactions II : the elemental signature ». Geochimica et Cosmochimica Acta 66, n^o 11 (juin 2002) : 1925–41. http://dx.doi.org/10.1016/s0016-7037(01)00897-3.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

6

Paulukat, Cora, Geoffrey J. Gilleaudeau, Pavel Chernyavskiy et Robert Frei. « The Cr-isotope signature of surface seawater — A global perspective ». Chemical Geology 444 (décembre 2016) : 101–9. http://dx.doi.org/10.1016/j.chemgeo.2016.10.004.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

7

Ripperger, S., M. Rehkämper, D. Porcelli et A. N. Halliday. « Cadmium isotope fractionation in seawater — A signature of biological activity ». Earth and Planetary Science Letters 261, n^o 3-4 (septembre 2007) : 670–84. http://dx.doi.org/10.1016/j.epsl.2007.07.034.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

8

Bouchard, Laurianne, Ján Veizer, Laura Kennell-Morrison, Mark Jensen, Ken G. Raven et Ian D. Clark. « Origin and 87Rb–87Sr age of porewaters in low permeability Ordovician sediments on the eastern flank of the Michigan Basin, Tiverton, Ontario, Canada ». Canadian Journal of Earth Sciences 56, n^o 3 (mars 2019) : 201–8. http://dx.doi.org/10.1139/cjes-2018-0061.

Texte intégral

Résumé :

Porewater extractions and acid leachates of rock core from a 250 m thick sequence of low-permeability Ordovician-age shales and limestones, on the eastern flank of the Michigan Basin, were analysed for strontium isotope ratios in an attempt to infer porewater ages from observed 87Sr/86Sr enrichments. The porewaters originated as Ordovician seawater, which subsequently mixed with evaporated Silurian seawater infiltrating from above, and, to some extent, with a deep brine—with an enriched 87Sr/86Sr signature—from the underlying crystalline shield or deep basin. The porewater 87Sr/86Sr ratios are more radiogenic than contemporaneous seawater but show no obvious correlation to those leached from the solid rock phases. Accepting that the initial 87Sr/86Sr signatures in porewaters were dominated by Late Silurian brine, potentially with an additional deep brine component, the excess of radiogenic 87Sr appears to represent ingrowth from 87Rb decay over a time span of some 420 million years, approaching the depositional age of the rocks. Similarly, Rb/Sr errochron ages of acid leachates of solid phases, and the calculated initial 87Sr/86Sr isotopic ratios, are consistent with a proposition that the calcites inherited their Sr from Ordovician seawater and were dolomitized shortly afterwards by infiltrating Mg-enriched evaporative brine, indicating long-term conservative behaviour for the enclosing carbonate rocks. The errochron for leachates from (alumino)silicates yields a high initial 87Sr/86Sr, but with an errochron age of about 340 ± 48 Ma, likely owing to variable admixtures of diagenetic illite in the shales. Overall, the data provide evidence for a stable hydrologic regime since Paleozoic time.

Styles APA, Harvard, Vancouver, ISO, etc.

9

Tignat-Perrier, Romie, Aurélien Dommergue, Alban Thollot, Olivier Magand, Timothy M. Vogel et Catherine Larose. « Microbial functional signature in the atmospheric boundary layer ». Biogeosciences 17, n^o 23 (4 décembre 2020) : 6081–95. http://dx.doi.org/10.5194/bg-17-6081-2020.

Texte intégral

Résumé :

Abstract. Microorganisms are ubiquitous in the atmosphere, and some airborne microbial cells were shown to be particularly resistant to atmospheric physical and chemical conditions (e.g., ultraviolet – UV – radiation, desiccation and the presence of radicals). In addition to surviving, some cultivable microorganisms of airborne origin were shown to be able to grow on atmospheric chemicals in laboratory experiments. Metagenomic investigations have been used to identify specific signatures of microbial functional potential in different ecosystems. We conducted a comparative metagenomic study on the overall microbial functional potential and specific metabolic and stress-related microbial functions of atmospheric microorganisms in order to determine whether airborne microbial communities possess an atmosphere-specific functional potential signature as compared to other ecosystems (i.e., soil, sediment, snow, feces, surface seawater etc.). In the absence of a specific atmospheric signature, the atmospheric samples collected at nine sites around the world were similar to their underlying ecosystems. In addition, atmospheric samples were characterized by a relatively high proportion of fungi. The higher proportion of sequences annotated as genes involved in stress-related functions (i.e., functions related to the response to desiccation, UV radiation, oxidative stress etc.) resulted in part from the high concentrations of fungi that might resist and survive atmospheric physical stress better than bacteria.

Styles APA, Harvard, Vancouver, ISO, etc.

10

Liang, Cheng Hao, Jing Di Yao, Nai Bao Huang et Jian Hua Wu. « Research on the Shaft-Frequency Electric Field Character of Ship’s Physical Scale Model ». Advanced Materials Research 1035 (octobre 2014) : 62–66. http://dx.doi.org/10.4028/www.scientific.net/amr.1035.62.

Texte intégral

Résumé :

By signature gathering system, the shaft-frequency electric field of ship’s physical scale model is measured. At the same time, the mechanism of the corrosion related to shaft-frequency electric field is also described. Results show that the shaft-frequency electric field signature arises around a vessel and transmits in the surrounding seawater due to the current flow from impressed current cathodic protection (ICCP) system and the current flow resulting from the corrosion of dissimilar metals during propeller rotation, which contains both shaft fundamental and harmonics. The signature character of electric field is more and more marked with the shaft rotation speed increasing. The amplitude of electric shaft-frequency electric field by modulating the current of ICCP system is 10 times greater than that of corrosion current. It is proved that the using a 1:100 physical scale model of a vessel to study the shaft-frequency electric field character is an effective method and provided an new way.

Styles APA, Harvard, Vancouver, ISO, etc.

11

Mehta, Kurang, Jon L. Sheiman, Roel Snieder et Rodney Calvert. « Strengthening the virtual-source method for time-lapse monitoring ». GEOPHYSICS 73, n^o 3 (mai 2008) : S73—S80. http://dx.doi.org/10.1190/1.2894468.

Texte intégral

Résumé :

Time-lapse monitoring is a powerful tool for tracking subsurface changes resulting from fluid migration. Conventional time-lapse monitoring can be done by observing differences between two seismic surveys over the surveillance period. Along with the changes in the subsurface, differences in the two seismic surveys are also caused by variations in the near-surface overburden and acquisition discrepancies. The virtual-source method monitors below the time-varying near-surface by redatuming the data down to the subsurface receiver locations. It crosscorrelates the signal that results from surface shooting recorded by subsurface receivers placed below the near-surface. For the Mars field data, redatuming the recorded response down to the permanently placed ocean-bottom cable (OBC) receivers using the virtual-source method allows one to reconstruct a survey as if virtualsources were buried at the OBC receiver locations and the medium above them were a homogeneous half-space. Separating the recorded wavefields into upgoing and downgoing (up-down) waves before crosscorrelation makes the resultant virtual-source data independent of the time-varying near-surface (seawater). For time-lapse monitoring, varying source signature for the two surveys and for each shot is also undesirable. Deconvolving the prestack crosscorrelated data (correlation gather) by the power spectrum of the source-time function results in virtual-source data independent of the source signature. Incorporating up-down wavefield separation and deconvolution of the correlation gather by the source power spectrum into the virtual-source method suppresses the causes of nonrepeatability in the seawater along with acquisition and source signature discrepancies. This processing combination strengthens the virtual-source method for time-lapse monitoring.

Styles APA, Harvard, Vancouver, ISO, etc.

12

Weiss, Chester J. « The fallacy of the “shallow-water problem” in marine CSEM exploration ». GEOPHYSICS 72, n^o 6 (novembre 2007) : A93—A97. http://dx.doi.org/10.1190/1.2786868.

Texte intégral

Résumé :

The recent explosion of activity in offshore controlled-source electromagnetic (CSEM) exploration has shown, both theoretically and in practice for the time-harmonic case, that shallow-water environments (depths less than, say, [Formula: see text]) can pose a significant challenge for detection and characterization of thin resistive targets in the subsurface because the weakly attenuated atmospheric response overprints the weaker target response. As an alternative, the transient CSEM experiment is considered to explore the nature of the “airwave” signature when viewed from the perspective of CSEM time series. The signature of a thin, isolated, resistive horizon is computed by using the quasi-analytic solution for a 1D earth in response to excitation by a horizontal electric dipole antenna. That signature, clearly seen in the transient data, generally lies in the time interval between the arrivalof the airwave and the late-time seawater response.The fact that either decreasing the water depth or increasing the source-receiver offset broadens this time interval suggests a target response that is generally easier to identify in shallow water than when viewed from a frequency-domain perspective. The airwave arrival is nearly synchronous across an inline receiver array, whereas the response time of the conducting elements of the model — the sea, the sediments, and the target — increases with increasing source-receiver offset. At short offsets (less than a few kilometers), and in intermediate water depths (approximately [Formula: see text]), these signatures can overlap and be difficult to decouple. However, modeling results also show that moveout rates of the post-airwave arrival may be used to infer the presence and depth of thin resistive targets within the geologic section to depths of [Formula: see text] below [Formula: see text] of water.

Styles APA, Harvard, Vancouver, ISO, etc.

13

Narożny, Michał, Paweł Polański, Rafał Namiotko et Marta Czarnowska. « Relationship Between Basic Physiochemical Properties of Seawater and Magnitude of Underwater Electric Field ». Zeszyty Naukowe Akademii Marynarki Wojennej 215, n^o 4 (1 décembre 2018) : 63–75. http://dx.doi.org/10.2478/sjpna-2018-0027.

Texte intégral

Résumé :

Abstract Passive defense systems which minimize chance of vessel detection have to be utilized due to application of naval mines. Passive defense and signature minimization can be regarded as not only magnitude reduction but also as its shaping. Electric field magnitude at a given depth is a function of an electric field source but also it depends on physicochemical properties of seawater — temperature and salinity. In this paper results of underwater electric field simulations are presented. Cases of various depths, temperatures and salinities are shown. Computational results are compared to underwater electric field measurements performed with portable sensor.

Styles APA, Harvard, Vancouver, ISO, etc.

14

Hu, Yan, Fang-Zhen Teng, Terry Plank et Catherine Chauvel. « Potassium isotopic heterogeneity in subducting oceanic plates ». Science Advances 6, n^o 49 (décembre 2020) : eabb2472. http://dx.doi.org/10.1126/sciadv.abb2472.

Texte intégral

Résumé :

Oceanic crust and sediments are the primary K sinks for seawater, and they deliver considerable amounts of K to the mantle via subduction. Historically, these crustal components were not studied for K isotopes because of the lack of analytical precision to differentiate terrestrial variations. Here, we report a high-precision dataset that reveals substantial variability in oceanic plates and provides further insights into the oceanic K cycle. Sixty-nine sediments worldwide yield a broad δ41K range from −1.3 to −0.02‰. The unusually low values are indicative of release of heavy K during continental weathering and uptake of light K during submarine diagenetic alteration. Twenty samples of altered western Pacific crust from ODP Site 801 display δ41K from −0.60 to −0.05‰, averaging at −0.32‰. Our results indicate that submarine alteration of oceanic plates is essential for generating the high-δ41K signature of seawater. These regionally varying subducting components are heterogeneous K inputs to the mantle.

Styles APA, Harvard, Vancouver, ISO, etc.

15

Mincer, Tracy J., Paul R. Jensen, Christopher A. Kauffman et William Fenical. « Widespread and Persistent Populations of a Major New Marine Actinomycete Taxon in Ocean Sediments ». Applied and Environmental Microbiology 68, n^o 10 (octobre 2002) : 5005–11. http://dx.doi.org/10.1128/aem.68.10.5005-5011.2002.

Texte intégral

Résumé :

ABSTRACT A major taxon of obligate marine bacteria within the order Actinomycetales has been discovered from ocean sediments. Populations of these bacteria (designated MAR 1) are persistent and widespread, spanning at least three distinct ocean systems. In this study, 212 actinomycete isolates possessing MAR 1 morphologies were examined and all but two displayed an obligate requirement of seawater for growth. Forty-five of these isolates, representing all observed seawater-requiring morphotypes, were partially sequenced and found to share characteristic small-subunit rRNA signature nucleotides between positions 207 and 468 (Escherichia coli numbering). Phylogenetic characterization of seven representative isolates based on almost complete sequences of genes encoding 16S rRNA (16S ribosomal DNA) yielded a monophyletic clade within the family Micromonosporaceae and suggests novelty at the genus level. This is the first evidence for the existence of widespread populations of obligate marine actinomycetes. Organic extracts from cultured members of this new group exhibit remarkable biological activity, suggesting that they represent a prolific resource for biotechnological applications.

Styles APA, Harvard, Vancouver, ISO, etc.

16

D’ozouville, Noémi, Sophie Violette, Nathalie Gassama, Aline Dia et Nathalie Jendrzejewski. « Origin and modelling of water salinization in a coastal aquifer of the Bay of Bengal : The Kaluvelly watershed, Tamil Nadu, India ». Bulletin de la Société Géologique de France 177, n^o 6 (1 novembre 2006) : 333–45. http://dx.doi.org/10.2113/gssgfbull.177.6.333.

Texte intégral

Résumé :

Abstract Over-pumping of the Vanur sandstone aquifer has led to a lowering of the piezometric surface and degradation of water quality through increased salinization. This aquifer salinization is a good example of many similar problems in India and other parts of the world. The Vanur Formation is the main aquifer of a multilayered system bordered by the sea on the eastern side and partly overlaid by the brackish waters of the Kaluvelly swamp in the north. The origin of the salinity, which apparently is not simply seawater intrusion, and its dynamics are examined, using chemical and isotopic tools together with 1D hydrological modelling of the movement of the seawater/fresh water interface. The content of major elements and some trace elements as well as isotopic ratios (18O/16O, D/H and 87Sr/86Sr) were measured in groundwater, surface and rainwater during five sampling surveys (January 1999 to October 2001). Available data on rainfall, piezometric and hydrogeologic records were used. We identified human contamination by F, Li in parts of the aquifer, which invalidated their use as tracers. The chemical composition of water from the Vanur aquifer shows a classical chemical evolution from the recharge area to the deeper confined area, consisting in increased water-rock interaction and a subsequent increase of solute species. However, the range of major compound concentration ratios for some wells does not follow this general trend. The non-consistent points are located in the most depressed area of the aquifer (−20 m amsl in June 2000), except in the north where the brackish water of the Kaluvelly swamps seems to enter the aquifer. In the depressed area, the sulphate signature corresponds to a mixing with a mineralized and sulphate-rich water body, likely to be Ramanathapuram sandstone water. It is due to the upward leakage from this underlying formation. A seasonal evaporation signature recorded by stable isotopes (δ18O, δD) suggests the addition of return irrigation flow. The 1D hydrodynamical model of the sea-water/fresh water movement was built with the available geological and hydraulic data. Hydrodynamic calculations show that seawater intrusion can be expected to occur within 3 to 20 years after the year 2000, depending on the value of unknown parameters (porosity) or boundary conditions (recharge, pumped volume). But we cannot rule out that a lithologic or tectonic barrier prevent any seawater intrusion inland; future geological investigation has to be done to confirm or infirm this hypothesis.

Styles APA, Harvard, Vancouver, ISO, etc.

17

Emmanuel, Laurent, Maurice Renard, René Cubaynes, Marc de Rafelis, Michael Hermoso, Laurence Lecallonnec, Antoine le Solleuz et Jacques Rey. « The “Schistes carton” of Quercy (Tarn, France) : a lithological signature of a methane hydrate dissociation event in the Early Toarcian. Implications for correlations between Boreal and Tethyan realms ». Bulletin de la Société Géologique de France 177, n^o 5 (1 septembre 2006) : 237–47. http://dx.doi.org/10.2113/gssgfbull.177.5.237.

Texte intégral

Résumé :

Abstract The geochemical events (negative shifts in δ13C and δ18O, and Mn peak) associated with the “Schistes carton” of Quercy (SW France) do not appear to be due only to lithological variations and a differential diagenesis. They correspond to the Lower Toarcian global event and seem to be connected to a destabilization phase of gas hydrates. During its oxidation by seawater, the release of methane gas with a very low carbon isotopic ratio (− 60%) led to the production of CO2 and carbonates with a negative δ13C. The consumption of oxygen in the seawater resulted in conditions that were first dysoxic and then anoxic. The reduction of burrowing and bioturbation associated with this oxidation resulted in the laminated appearance of the “Schistes carton”. The decrease in the oxygen content led to a decrease in MnO2 microparticles present in the water column and sediments. This created a complementary source of Mn2+ that was incorporated into the carbonates produced during this event. Furthermore, an increase in the average seawater temperature (δ18O) appears associated with this event. Finally, the apparent diachronism of geochemical events in the Boreal and the Tethyan realms is discussed.

Styles APA, Harvard, Vancouver, ISO, etc.

18

Hendry, K. R., G. E. A. Swann, M. J. Leng, H. J. Sloane, C. Goodwin, J. Berman et M. Maldonado. « Technical Note : Silica stable isotopes and silicification in a carnivorous sponge <i>Asbestopluma</i> ; sp. » Biogeosciences 12, n^o 11 (5 juin 2015) : 3489–98. http://dx.doi.org/10.5194/bg-12-3489-2015.

Texte intégral

Résumé :

Abstract. The stable isotope composition of benthic sponge spicule silica is a potential source of palaeoceanographic information about past deep seawater chemistry. The silicon isotope composition of spicules has been shown to relate to the silicic acid concentration of ambient water, although existing calibrations do exhibit a degree of scatter in the relationship. Less is known about how the oxygen isotope composition of sponge spicule silica relates to environmental conditions during growth. Here, we investigate the vital effects on silica, silicon and oxygen isotope composition in a carnivorous sponge, Asbestopluma sp., from the Southern Ocean. We find significant variations in silicon and oxygen isotopic composition within the specimen that are related to unusual spicule silicification. The largest variation in both isotope systems was associated with the differential distribution of an unconventional, hypersilicified spicule type (desma) along the sponge body. The absence an internal canal in the desmas suggests an unconventional silicification pattern leading to an unusually heavy isotope signature. Additional internal variability derives from a systematic offset between the peripheral skeleton of the body having systematically a higher isotopic composition than the internal skeleton. A simplified silicon isotope fractionation model, in which desmas were excluded, suggests that the lack of a system for seawater pumping in carnivorous sponges favours a low replenishment of dissolved silicon within the internal tissues, causing kinetic fractionation during silicification that impacts the isotope signature of the internal skeleton. Analysis of multiple spicules should be carried out to "average out" any artefacts in order to produce more robust downcore measurements.

Styles APA, Harvard, Vancouver, ISO, etc.

19

Hendry, K. R., G. E. A. Swann, M. J. Leng, H. J. Sloane, C. Goodwin, J. Berman et M. Maldonado. « Technical Note : Silica stable isotopes and silicification in a carnivorous sponge \\textit{Asbestopluma} sp. » Biogeosciences Discussions 11, n^o 12 (2 décembre 2014) : 16573–97. http://dx.doi.org/10.5194/bgd-11-16573-2014.

Texte intégral

Résumé :

Abstract. The stable isotope composition of benthic sponge spicule silica is a potential source of palaeoceanographic information about past deep seawater chemistry. The silicon isotopic composition of spicules has been shown to relate to the silicic acid concentration of ambient water, although existing calibrations do exhibit a degree of scatter in the relationship. Less is known about how the oxygen isotope composition of sponge spicule silica relates to environmental conditions during growth. Here, we investigate the biological vital effects on silica silicon and oxygen isotope composition in a carnivorous sponge, Asbestopluma sp., from the Southern Ocean. We find significant variations in silicon and oxygen isotopic composition within the specimen that appear related to unusual spicule silicification. The largest variation in both isotope systems was associated to the differential distribution of an unconventional, hypersilicified spicule type (desma) along the sponge body. The absence of an internal canal in the desmas suggests an unconventional silicification pattern leading to an unusually heavy isotopic signature. Additional internal variability derives from a systematic offset between the peripheral skeleton of the body having systematically a higher isotopic composition than the internal skeleton. A simplified silicon isotope fractionation model, in which desmas were excluded, suggests that the lack of a system for seawater pumping in carnivorous sponges favours a low replenishment of dissolved silicon within the internal tissues, causing kinetic fractionation during silicification that impacts the isotopic signature of the internal skeleton. Analysis of multiple spicules should be carried out to "average out" any artefacts in order to produce more robust downcore measurements.

Styles APA, Harvard, Vancouver, ISO, etc.

20

Mellon, Craig, Rachel Wood et Laetitia Pichevin. « Assessing the Reliability of Early Marine Cements in Recording Changes in Seawater Redox Conditions Across the Late-Devonian Mass Extinction ». Edinburgh Student Journal of Science 1, n^o 1 (4 juillet 2024) : 14–18. http://dx.doi.org/10.2218/esjs.9681.

Texte intégral

Résumé :

The Late-Devonian Mass Extinction (LDME) extinguished up to 40% of all marine species, with evidence suggesting marine anoxia was the primary cause. This study performs rare earth element analysis of early marine cements from reefal limestones from Australia and Scotland to show that these cements capture a marine anoxic signature across the LDME for the first time, directly implicating marine anoxia and by extension, the rise of land plants, as the cause of the LDME. This work also demonstrates that early marine cements serve as reliable proxies for recording changes in seawater redox conditions over whole-rock analysis.

Styles APA, Harvard, Vancouver, ISO, etc.

21

Merlivat, Liliane, Jacqueline Boutin, David Antoine, Laurence Beaumont, Melek Golbol et Vincenzo Vellucci. « Increase of dissolved inorganic carbon and decrease in pH in near-surface waters in the Mediterranean Sea during the past two decades ». Biogeosciences 15, n^o 18 (21 septembre 2018) : 5653–62. http://dx.doi.org/10.5194/bg-15-5653-2018.

Texte intégral

Résumé :

Abstract. Two 3-year time series of hourly measurements of the fugacity of CO2 (fCO2) in the upper 10 m of the surface layer of the northwestern Mediterranean Sea have been recorded by CARIOCA sensors almost two decades apart, in 1995–1997 and 2013–2015. By combining them with the alkalinity derived from measured temperature and salinity, we calculate changes in pH and dissolved inorganic carbon (DIC). DIC increased in surface seawater by ∼25 µmol kg−1 and fCO2 by 40 µatm, whereas seawater pH decreased by ∼0.04 (0.0022 yr−1). The DIC increase is about 15 % larger than expected from the equilibrium with atmospheric CO2. This could result from natural variability, e.g. the increase between the two periods in the frequency and intensity of winter convection events. Likewise, it could be the signature of the contribution of the Atlantic Ocean as a source of anthropogenic carbon to the Mediterranean Sea through the Strait of Gibraltar. We then estimate that the part of DIC accumulated over the last 18 years represents ∼30 % of the total inventory of anthropogenic carbon in the Mediterranean Sea.

Styles APA, Harvard, Vancouver, ISO, etc.

22

Hernández-Morales, Pablo, Jobst Wurl, Carlos Green-Ruiz et Diego Morata. « Hydrogeochemical Characterization as a Tool to Recognize “Masked Geothermal Waters” in Bahía Concepción, Mexico ». Resources 10, n^o 3 (4 mars 2021) : 23. http://dx.doi.org/10.3390/resources10030023.

Texte intégral

Résumé :

Geo-thermalism has been widely recognized on the Baja California Peninsula, especially during the last decade. The current research, carried out on Bahia Concepcion, evidences the existence of geothermal springs, which get recharged mainly by groundwater and seawater. The groundwater can be characterized as Na+-Cl− and Na+-HCO3− type, with a pH value close to neutrality. The slightly more acidic thermal sites presented temperatures between 32 °C and 59 °C at the surface. Based on the relationships of the Cl− and Br−, as well as the B/Cl−, and Br−/Cl− ratios, seawater was recognized as the main source of salinity. The spatial distribution is explained directly through marine intrusion, or via sprays and aerosols within the rainwater. Seawater ratios in thermal springs varied from 62% to 83%, corresponding mainly to shallow inflow, but seawater inputs into the deep thermal reservoir were also recognized. Temperatures in the geothermal deep reservoir were inferred from 114 to 209 °C, calculated through the SiO2 and Na+-K+ geothermometers. In addition to previously reported thermal sites at Bahía Concepción, and based on their elevated temperatures, two new sites were identified. Another five springs do not fulfill the commonly used definition, based on differential temperature, but show the typical hydrogeochemical signature of thermal water. A new approach to identify this low-temperature geothermal-influenced spring water by its hydrogeochemical composition is presented, for which the term “Masked Geothermal Waters” (MGW) is introduced. Our findings increase the area of the geothermal anomaly and, therefore, the potential of geothermal resources. The approach proposed in this research will also be useful to identify more MGW in other coastal areas.

Styles APA, Harvard, Vancouver, ISO, etc.

23

D'Andres, Joëlle, Mark A. Kendrick, Vickie C. Bennett et Allen P. Nutman. « Halogens in serpentinites from the Isua supracrustal belt, Greenland : An Eoarchean seawater signature and biomass proxy ? » Geochimica et Cosmochimica Acta 262 (octobre 2019) : 31–59. http://dx.doi.org/10.1016/j.gca.2019.07.017.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

24

Hao, Ruilin, Liyin Pan, Nana Mu, Xi Li, Xiaodong Fu, Shaoyun Xiong, Siqi Liu, Jianfeng Zheng, Min She et Axel Munnecke. « Multi-Phase Dolomitization in the Jurassic Paleo-Oil Reservoir Zone, Qiangtang Basin (SW China) : Implications for Reservoir Development ». Minerals 14, n^o 9 (5 septembre 2024) : 908. http://dx.doi.org/10.3390/min14090908.

Texte intégral

Résumé :

The age and dolomitization processes in the Paleo-oil reservoir zone, which is composed of massive dolostones found in the Qiangtang Basin (SW China), are still debated. In this research, the Long’eni-Geluguanna Area was selected. Macroscopic information, thin sections, and geochemical methods were used to investigate the dolomitization characteristics and the processes that controlled dolomitization. Five types of replacive dolomites and two types of dolomite cement were observed. Some of the dolomites displayed ghosts of primary sedimentary structures. Saddle dolomites were prevalent, occurring in the interparticle and moldic pores of the limestone which should have been filled at an early diagenetic stage. Ten microfacies types were identified. The foraminifera assemblage provides evidence that the studied interval is of Early Jurassic age. The δ13C values are similar to the contemporaneous seawater signature. The REE+Y patterns of limestones and dolostones exhibit similarities to that of seawater. The mean Na and Sr values are comparable to those of other near-normal seawater dolomites. The δ18O values of all lithologies are markedly depleted. The dolomitization started penecontemporaneously, with deposition. A general sand shoal setting with patch reefs developed. The dolomitizing fluids, near-normal seawater, was probably formed by slight evaporation on top of the shoal. Saddle dolomites in the interparticle and moldic pores might indicate hydrothermal activity, which also caused the recrystallization of some pre-existing dolomites. The recrystallization might have slightly increased the crystal size, demolished the ghost structures, formed saddle dolomites, and altered the REE+Y patterns. The recrystallization extent diminished with increasing distance from the fluids-providing fracture. Furthermore, the existence of protected areas within the sand shoal settings could enhance the vertical and horizontal heterogeneity of dolostone reservoirs.

Styles APA, Harvard, Vancouver, ISO, etc.

25

Tortola, Marco, Ihsan S. Al-Aasm et Richard Crowe. « Diagenetic Pore Fluid Evolution and Dolomitization of the Silurian and Devonian Carbonates, Huron Domain of Southwestern Ontario : Petrographic, Geochemical and Fluid Inclusion Evidence ». Minerals 10, n^o 2 (7 février 2020) : 140. http://dx.doi.org/10.3390/min10020140.

Texte intégral

Résumé :

Core samples from two deep boreholes were analyzed for petrographic, stable and Sr isotopes, fluid inclusion microthermometry and major, minor, trace and rare-earth elements (REE) of different types of dolomite in the Silurian and Devonian carbonates of the eastern side of the Michigan Basin provided useful insights into the nature of dolomitization, and the evolution of diagenetic pore fluids in this part of the basin. Petrographic features show that both age groups are characterized by the presence of a pervasive replacive fine-crystalline (<50 µm) dolomite matrix (RD1) and pervasive and selective replacive medium crystalline (>50–100 µm) dolomite matrix (RD2 and RD3, respectively). In addition to these types, a coarse crystalline (>500 µm) saddle dolomite cement (SD) filling fractures and vugs is observed only in the Silurian rocks. Results from geochemical and fluid inclusion analyses indicate that the diagenesis of Silurian and Devonian formations show variations in terms of the evolution of the diagenetic fluid composition. These fluid systems are: (1) a diagenetic fluid system that affected Silurian carbonates and was altered by salt dissolution post-Silurian time. These carbonates show a negative shift in δ18O values (dolomite δ18O average: −6.72‰ VPDB), Sr isotopic composition slightly more radiogenic than coeval seawater (0.7078–0.7087), high temperatures (RD2 and SD dolomite Th average: 110 °C) and hypersaline signature (RD2 and SD dolomite average salinity: 26.8 wt.% NaCl eq.); and (2) a diagenetic fluid system that affected Devonian carbonates, possibly occurred during the Alleghenian orogeny in Carboniferous time and characterized by a less pronounced negative shift in δ18O values (dolomite δ18O average: −5.74‰ VPDB), Sr isotopic composition in range with the postulated values for coeval seawater (0.7078–0.7080), lower temperatures (RD2 dolomite Th average: 83 °C) and less saline signature (RD2 dolomite average salinity: 20.8 wt.% NaCl eq.).

Styles APA, Harvard, Vancouver, ISO, etc.

26

Barkley, Hannah C., Anne L. Cohen, Yimnang Golbuu, Victoria R. Starczak, Thomas M. DeCarlo et Kathryn E. F. Shamberger. « Changes in coral reef communities across a natural gradient in seawater pH ». Science Advances 1, n^o 5 (juin 2015) : e1500328. http://dx.doi.org/10.1126/sciadv.1500328.

Texte intégral

Résumé :

Ocean acidification threatens the survival of coral reef ecosystems worldwide. The negative effects of ocean acidification observed in many laboratory experiments have been seen in studies of naturally low-pH reefs, with little evidence to date for adaptation. Recently, we reported initial data suggesting that low-pH coral communities of the Palau Rock Islands appear healthy despite the extreme conditions in which they live. Here, we build on that observation with a comprehensive statistical analysis of benthic communities across Palau’s natural acidification gradient. Our analysis revealed a shift in coral community composition but no impact of acidification on coral richness, coralline algae abundance, macroalgae cover, coral calcification, or skeletal density. However, coral bioerosion increased 11-fold as pH decreased from the barrier reefs to the Rock Island bays. Indeed, a comparison of the naturally low-pH coral reef systems studied so far revealed increased bioerosion to be the only consistent feature among them, as responses varied across other indices of ecosystem health. Our results imply that whereas community responses may vary, escalation of coral reef bioerosion and acceleration of a shift from net accreting to net eroding reef structures will likely be a global signature of ocean acidification.

Styles APA, Harvard, Vancouver, ISO, etc.

27

Moiroud, Mathieu, Emmanuelle Pucéat, Yannick Donnadieu, Germain Bayon, Michel Guiraud, Silke Voigt, Jean-François Deconinck et Fabrice Monna. « Evolution of neodymium isotopic signature of seawater during the Late Cretaceous : Implications for intermediate and deep circulation ». Gondwana Research 36 (août 2016) : 503–22. http://dx.doi.org/10.1016/j.gr.2015.08.005.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

28

Mazariegos, Junior G., Jennifer C. Walker, Xiaomei Xu et Claudia I. Czimczik. « Tracing Artificially Recharged Groundwater using Water and Carbon Isotopes ». Radiocarbon 59, n^o 2 (11 août 2016) : 407–21. http://dx.doi.org/10.1017/rdc.2016.51.

Texte intégral

Résumé :

AbstractWe conducted an isotopic analysis of groundwater in Orange County, California, USA, around the Talbert Seawater Injection Barrier to determine if recycled water, used to artificially recharge local aquifers, carries a unique isotopic signature that can be used as a tracer. From September 2014 to April 2015, we collected groundwater from six privately owned wells within the coastal groundwater basin, along with various surface waters. All water samples were analyzed for their stable isotopic composition (δ18O, δD), the δ13C and 14C signature of the dissolved inorganic carbon (DIC) pool, DIC concentration, pH, and salinity. The DIC of groundwater mixing with recycled water is enriched in 14C above natural background levels, with varying signal strength through time, depleted in δ13C, and low in DIC concentration. Water isotopes further suggest that recycled water is a mixture of Colorado River water and regional groundwater. In contrast, groundwater found further away from the injection barrier has carbon and water isotope composition consistent with regional groundwater and Santa Ana River water. Our findings imply that recycled water injected through the Talbert Barrier is isotopically unique, and that 14C enrichment may be used as an intrinsic tracer of artificial recharge within the basin.

Styles APA, Harvard, Vancouver, ISO, etc.

29

Samaoui, Samir, Ayoub Aabi, Abdellah Boushaba, Belkasmi Mohammed, Abdellah Nait Bba, Abderrahim Essaifi, Lahssen Baidder et Othmane Lamrani. « Metallogeny and Genesis of Fault-Filling Barite-Sulfide Veins (Ougnat, Morocco) : Petrography, Fluid Inclusion, and Sr-S Isotopic Constraints ». Geosciences 14, n^o 3 (18 mars 2024) : 83. http://dx.doi.org/10.3390/geosciences14030083.

Texte intégral

Résumé :

The Ougnat Massif of the eastern Anti-Atlas (Morocco) hosts barite and sulfide vein-type deposits of vital economic importance. With over 150 mineralized structures reported in the Ougnat Massif, the ore-bearing ones are predominantly composed of barite, quartz, calcite, and minor portions of sulfides. The mineralized veins are driven by NW-SE and NE-SW to E-W oblique-slip opening faults that cross both the Precambrian basement and its Paleozoic cover. The mineralized structures occur as lenses and sigmoidal veins that follow stepped tension fracture sets oblique to the fault planes. These geometries and kinematic indicators of these structures point to a predominantly normal-sinistral opening in a brittle-ductile tectonic setting. The S isotopic compositions of barite from the Ougnat Massif (+10.8 to +19.5‰) fall mostly within the range of δ34S values of Late Triassic to Jurassic seawater, thus suggesting that some of the SO2− in barite comes from seawater sulfate. This range of δ34S values also corresponds approximately to the hydrothermal barite context. The 87Sr/86Sr ratios of barite, which range from 0.710772 to 0.710816, lie between the radiogenic strontium isotopic compositions of deposition by hydrothermal solutions, and also coincide with the non-radiogenic isotopic signature of Triassic to Jurassic seawater. Based on a fluid inclusions study, the ore-forming fluids were a mixture of two or more fluids. A deep hot fluid with an average temperature of 368 °C leached the granodiorites and volcanic-sedimentary complex of the Ouarzazate Group. This fluid provided the hydrothermal system with most of the Ba, radiogenic Sr, and some of the dissolved S. A second, shallow fluid with an average temperature of 242 °C was derived from Late Triassic to Jurassic seawater. The barite mineralization of the Ougnat Massif constitutes a typical example of vein-type mineralization that occurred along the northern margin of the West African Craton and regionally tied to the central Atlantic opening.

Styles APA, Harvard, Vancouver, ISO, etc.

30

Belcher, Anna, Sophie Fielding, Andrew Gray, Lauren Biermann, Gabriele Stowasser, Peter Fretwell, Louise Ireland et Geraint A. Tarling. « Experimental determination of reflectance spectra of Antarctic krill (Euphausia superba) in the Scotia Sea ». Antarctic Science 33, n^o 4 (12 juillet 2021) : 402–14. http://dx.doi.org/10.1017/s0954102021000262.

Texte intégral

Résumé :

AbstractAntarctic krill are the dominant metazoan in the Southern Ocean in terms of biomass; however, their wide and patchy distribution means that estimates of their biomass are still uncertain. Most currently employed methods do not sample the upper surface layers, yet historical records indicate that large surface swarms can change the water colour. Ocean colour satellites are able to measure the surface ocean synoptically and should theoretically provide a means for detecting and measuring surface krill swarms. Before we can assess the feasibility of remote detection, more must be known about the reflectance spectra of krill. Here, we measure the reflectance spectral signature of Antarctic krill collected in situ from the Scotia Sea and compare it to that of in situ water. Using a spectroradiometer, we measure a strong absorption feature between 500 and 550 nm, which corresponds to the pigment astaxanthin, and high reflectance in the 600–700 nm range due to the krill's red colouration. We find that the spectra of seawater containing krill is significantly different from seawater only. We conclude that it is tractable to detect high-density swarms of krill remotely using platforms such as optical satellites and unmanned aerial vehicles, and further steps to carry out ground-truthing campaigns are now warranted.

Styles APA, Harvard, Vancouver, ISO, etc.

31

Burkholz, Celina, Neus Garcias-Bonet et Carlos M. Duarte. « Warming enhances carbon dioxide and methane fluxes from Red Sea seagrass (<i>Halophila stipulacea</i>) sediments ». Biogeosciences 17, n^o 7 (3 avril 2020) : 1717–30. http://dx.doi.org/10.5194/bg-17-1717-2020.

Texte intégral

Résumé :

Abstract. Seagrass meadows are autotrophic ecosystems acting as carbon sinks, but they have also been shown to be sources of carbon dioxide (CO2) and methane (CH4). Seagrasses can be negatively affected by increasing seawater temperatures, but the effects of warming on CO2 and CH4 fluxes in seagrass meadows have not yet been reported. Here, we examine the effect of two disturbances on air–seawater fluxes of CO2 and CH4 in Red Sea Halophila stipulacea communities compared to adjacent unvegetated sediments using cavity ring-down spectroscopy. We first characterized CO2 and CH4 fluxes in vegetated and adjacent unvegetated sediments, and then experimentally examined their response, along with that of the carbon (C) isotopic signature of CO2 and CH4, to gradual warming from 25 ∘C (winter seawater temperature) to 37 ∘C, 2 ∘C above current maximum temperature. In addition, we assessed the response to prolonged darkness, thereby providing insights into the possible role of suppressing plant photosynthesis in supporting CO2 and CH4 fluxes. We detected 6-fold-higher CO2 fluxes in vegetated compared to bare sediments, as well as 10- to 100-fold-higher CH4 fluxes. Warming led to an increase in net CO2 and CH4 fluxes, reaching average fluxes of 10 422.18 ± 2570.12 µmol CO2 m−2 d−1 and 88.11±15.19 µmol CH4 m−2 d−1, while CO2 and CH4 fluxes decreased over time in sediments maintained at 25 ∘C. Prolonged darkness led to an increase in CO2 fluxes but a decrease in CH4 fluxes in vegetated sediments. These results add to previous research identifying Red Sea seagrass meadows as a significant source of CH4, while also indicating that sublethal warming may lead to increased emissions of greenhouse gases from seagrass meadows, providing a feedback mechanism that may contribute to further enhancing global warming.

Styles APA, Harvard, Vancouver, ISO, etc.

32

Arslan, Niyazi, Meysam Majidi Nezhad, Azim Heydari, Davide Astiaso Garcia et Georgios Sylaios. « A Principal Component Analysis Methodology of Oil Spill Detection and Monitoring Using Satellite Remote Sensing Sensors ». Remote Sensing 15, n^o 5 (5 mars 2023) : 1460. http://dx.doi.org/10.3390/rs15051460.

Texte intégral

Résumé :

Monitoring, assessing, and measuring oil spills is essential in protecting the marine environment and in efforts to clean oil spills. One of the most recent oil spills happened near Port Fourchon, Louisiana, caused by Hurricane Ida (Category 4), that had a wind speed of 240 km/h. In this regard, Earth Observation (EO) Satellite Remote Sensing (SRS) images can effectively highlight oil spills in marine areas as a “fast and no-cost” technique. However, clouds and the sea surface spectral signature complicate the interpretation of oil spill areas in the optical images. In this study, Principal Component Analysis (PCA) has been applied of Landsat-8 and Sentinel-2 SRS images to improve information from the optical sensor bands. The PCA produces an output unrelated to the main bands, making it easier to distinguish oil spills from clouds and seawater due to the spectral diversity between oil, clouds, and the seawater surface. Then, an additional step has been applied to highlight the oil spill area using PCAs with different band combinations. Furthermore, Sentinel-1 (SAR), Sentinel-2 (optical), and Landsat-8 (optical) SRS images have been analyzed with cross-sections to suppress the “look-alike” effect of marine oil spill areas. Finally, mean and high-pass filters were used for Land Surface Temperature (LST) SRS images estimated from the Landsat thermal band. The results show that the seawater value is about −17.5 db and the oil spill area shows a value between −22.5 db and −25 db; the Landsat 8 satellites thermal band 10, depicting contrast at some areas for oil spill, can be determined by the 3 × 3 and 5 × 5 Kernel High pass and the 3 × 3 Mean filter. The results demonstrate that the SRS images should be used together to improve oil spill detection studies results.

Styles APA, Harvard, Vancouver, ISO, etc.

33

Hall, A. J., A. J. Boyce et A. E. Fallick. « A Sulphur Isotope Study of Iron Sulphides in the Late Precambrian Dalradian Easdale Slate Formation, Argyll, Scotland ». Mineralogical Magazine 52, n^o 367 (septembre 1988) : 483–90. http://dx.doi.org/10.1180/minmag.1988.052.367.06.

Texte intégral

Résumé :

AbstractPyritic slates from the late Precambrian, Middle Dalradian Argyll Group Easdale Slate Formation, contain mainly quartz, muscovite and chlorite with variable amounts of dolomite, albite and paragonite. Slates from Easdale Island and Cuan Ferry contain pyrite porphyroblasts with δ34S = + 12 to + 16‰. The pyrite grew during a post-tectonic retrogressive event at the expense ofpyrrhotine which formed during the main regional metamorphism of the Grampian orogeny by reduction of diagenetic pyrite. Slate from Oban contains abundant diagenetic framboidal pyrite and small syn-tectonic pyrite porphyroblasts with δ34S = +22‰. This pyrite was not all reduced to pyrrhotine on metamorphism so there was little retrogressive growth of pyrite. Metamorphism appears to have homogenized local (cm scale at least) isotopic inhomogeneities and preserved an average seawater-sulphate-sulphide isotopic fractionation value. Middle Dalradian seawater-sulphate had a δ34S value of about + 35‰, so the small fractionations are appropriate for bacteriogenic reduction in bituminous sediments, the heavier sulphide in the case of the Oban slate indicating more rapid reduction of sulphate. Lower Dalradian Appin Group, Ballachulish slate contains pyrite with δ34S = +15±2‰ and is best interpreted as forming in the same manner as the Easdale slates of Easdale Island and Cuan Ferry; the sharp increase in late Precambrian ocean-sulphate sulphur isotope signature from +15 to > +30‰ therefore occurred by Lower Dalradian Appin Group times.

Styles APA, Harvard, Vancouver, ISO, etc.

34

Ekka, Shail Vijeta, Yu-Hsuan Liang, Kuo-Fang Huang et Der-Chuen Lee. « Molybdenum Isotopic Fingerprints in Taiwan Rivers : Natural versus Anthropogenic Sources ». Water 15, n^o 10 (15 mai 2023) : 1873. http://dx.doi.org/10.3390/w15101873.

Texte intégral

Résumé :

Two rivers from Taiwan with different source inputs, the Danshuei (polluted) and Liwu (non-polluted), were selected to study the behavior of molybdenum (Mo) isotopes during weathering and riverine transport. In the Danshuei River, δ98/95Mo ranges from 0.83‰ to 1.50‰ (wet season) and 0.54‰ to 1.25‰ (dry season). With a few exceptions, δ98/95Mo in the Danshuei River is lighter during the wet season, while, in contrast, heavier in the dry season. In the Liwu River, δ98/95Mo varies from 0.54‰ to 1.30‰ and gets heavier along the mainstream. Using the MixSIAR model, three Mo sources are identified in the Danshuei River: seawater intrusion, rock–water interaction, and anthropogenic inputs. Seawater intrusion can explain the heavy δ98/95Mo downstream signal during the wet season contributing 14–39% from the MixSIAR model. However, the lighter δ98/95Mo signal during the dry season is most likely due to anthropogenic inputs in the middle and lower reaches of the Danshuei River contributing 75–98%. In the Liwu River, dissolved Mo isotopes correlate with SO42−/Na and (Sr/Na) ×1000 ratio, suggesting that pyrite oxidation coupled with carbonate weathering governs the heavy δ98/95Mo signature, with sequestration of light δ98/95Mo into secondary mineral phases in bedload sediments. Furthermore, these results have important implications for riverine Mo sources to the ocean, controlled by anthropogenic activity and weathering processes.

Styles APA, Harvard, Vancouver, ISO, etc.

35

Chan, P. T. W., J. Halfar, W. H. Adey, P. A. Lebednik, R. Steneck, C. J. D. Norley et D. W. Holdsworth. « Recent density decline in wild-collected subarctic crustose coralline algae reveals climate change signature ». Geology 48, n^o 3 (17 décembre 2019) : 226–30. http://dx.doi.org/10.1130/g46804.1.

Texte intégral

Résumé :

Abstract Warming surface ocean temperatures combined with the continued diffusion of atmospheric CO2 into seawater have been shown to have detrimental impacts on calcareous marine organisms in tropical and temperate localities. However, greater oceanic CO2 uptake in higher latitudes may present a higher oceanic acidification risk to carbonate organisms residing in Arctic and subarctic habitats. This is especially true for crustose coralline algae that build their skeletons using high-Mg calcite, which is among the least stable and most soluble of the carbonate minerals. Here we present a century-long annually resolved growth, density, and calcification rate record from the crustose coralline alga Clathromorphum nereostratum, a dominant calcifier in Pacific Arctic and subarctic benthic communities. Specimens were collected from the Aleutian Islands, Alaska (USA), a region that has undergone a long-term decline of 0.08 ± 0.01 pH units since the late 19th century. Growth and calcification rates remain relatively stable throughout the record, but skeletal densities have declined substantially since A.D. 1983. Strong correlations to warming sea-surface temperatures indicate that temperature stress may play a significant role in influencing the ability of corallines to calcify. Decreasing algal skeletal density may offset the benefits of continued growth and calcification due to a weakening in structural integrity, which could have detrimental consequences for the diverse reef-like communities associated with algal structures in mid-to-high latitudes.

Styles APA, Harvard, Vancouver, ISO, etc.

36

Hofmann, Laurie C., et Svenja Heesch. « Latitudinal trends in stable isotope signatures and carbon-concentrating mechanisms of northeast Atlantic rhodoliths ». Biogeosciences 15, n^o 20 (18 octobre 2018) : 6139–49. http://dx.doi.org/10.5194/bg-15-6139-2018.

Texte intégral

Résumé :

Abstract. Rhodoliths are free-living calcifying red algae that form extensive beds in shallow marine benthic environments (<250 m), which provide important habitats and nurseries for marine organisms and contribute to carbonate sediment accumulation. There is growing concern that these organisms are sensitive to global climate change, yet little is known about their physiology. Considering their broad distribution along most continental coastlines, their potential sensitivity to global change could have important consequences for the productivity and diversity of benthic coastal environments. The goal of this study was to determine the plasticity of carbon-concentrating mechanisms (CCMs) of rhodoliths along a latitudinal gradient in the northeast Atlantic using natural stable isotope signatures. The δ13C signature of macroalgae can be used to provide an indication of the preferred inorganic carbon source (CO2 vs. HCO3-). Here we present the total (δ13CT) and organic (δ13Corg) δ13C signatures of northeast Atlantic rhodoliths with respect to changing environmental conditions along a latitudinal gradient from the Canary Islands to Spitsbergen. The δ13CT signatures (−11.9 to −0.89) of rhodoliths analyzed in this study were generally higher than the δ13Corg signatures, which ranged from −25.7 to −2.8. We observed a decreasing trend in δ13CT signatures with increasing latitude and temperature, while δ13Corg signatures were only significantly correlated to dissolved inorganic carbon. These data suggest that high-latitude rhodoliths rely more on CO2 as an inorganic carbon source, while low-latitude rhodoliths likely take up HCO3- directly, but none of our specimens had ∂13Corg signatures less than −30, suggesting that none of them relied solely on diffusive CO2 uptake. However, depth also has a significant effect on both skeletal and organic δ13C signatures, suggesting that both local and latitudinal trends influence the plasticity of rhodolith inorganic carbon acquisition and assimilation. Our results show that many species, particularly those at lower latitudes, have CCMs that facilitate HCO3- use for photosynthesis. This is an important adaptation for marine macroalgae, because HCO3- is available at higher concentrations than CO2 in seawater, and this becomes even more extreme with increasing temperature. The flexibility of CCMs in northeast Atlantic rhodoliths observed in our study may provide a key physiological mechanism for potential adaptation of rhodoliths to future global climate change.

Styles APA, Harvard, Vancouver, ISO, etc.

37

Kusakabe, Minoru, Keisuke Nagao, Takeshi Ohba, Jung Hun Seo, Sung-Hyun Park, Jong Ik Lee et Byong-Kwon Park. « Noble gas and stable isotope geochemistry of thermal fluids from Deception Island, Antarctica ». Antarctic Science 21, n^o 3 (11 février 2009) : 255–67. http://dx.doi.org/10.1017/s0954102009001783.

Texte intégral

Résumé :

AbstractNew stable isotope and noble gas data obtained from fumarolic and bubbling gases and hot spring waters sampled from Deception Island, Antarctica, were analysed to constrain the geochemical features of the island's active hydrothermal system and magmatism in the Bransfield back-arc basin. The 3He/4He ratios of the gases (< 9.8 × 10-6), which are slightly lower than typical MORB values, suggest that the Deception Island magma was generated in the mantle wedge of a MORB-type source but the signature was influenced by the addition of radiogenic 4He derived from subducted components in the former Phoenix Plate. The N2/He ratios of fumarolic gas are higher than those of typical mantle-derived gases suggesting that N2 was added during decomposition of sediments in the subducting slab. The δ13C values of -5 to -6‰ for CO2 also indicate degassing from a MORB-type mantle source. The H2/Ar- and SiO2 geothermometers indicate that the temperatures in the hydrothermal system below Deception Island range from ~150°C to ~300°C. The δD and δ18O values measured from fumarolic gas and hot spring waters do not indicate any contribution of magmatic water to the samples. The major ionic components and δD-δ18O-δ34S values indicate that hot spring waters are a mixture of local meteoric water and seawater. Mn and SiO2 in spring waters were enriched relative to seawater reflecting water-rock interaction at depth.

Styles APA, Harvard, Vancouver, ISO, etc.

38

Vahrenkamp, Volker C. « Carbon-isotope signatures of Albian to Cenomanian (Cretaceous) shelf carbonates of the Natih Formation, Sultanate of Oman ». GeoArabia 18, n^o 3 (1 juillet 2013) : 65–82. http://dx.doi.org/10.2113/geoarabia180365.

Texte intégral

Résumé :

ABSTRACT The δ13C values of Albian to Cenomanian shallow-marine carbonate sequences of the Natih Formation have been collected from subsurface cores of a key location in Oman. The 450-m-thick stack of shelf carbonates is without significant gaps in deposition. The δ13C data range between 1‰ and 6‰, more-or-less tracking the evolution over time of δ13C in seawater established elsewhere in time-equivalent pelagic carbonate sequences. Anchored by biostratigraphy the isotope profile suggests several additional time correlations. It thus provides significantly enhanced stratigraphic resolution and a key section for regional correlations. In particular, the onset of Natih deposition (Natih F and G members) coincides with the Albian/Cenomanian boundary event, thus placing the base Natih into the Albian. The Natih C and D members were deposited mainly during the Mid-Cenomanian oceanic anoxic event, while the carbon-isotopes signature of the Natih A Member, which is at this locality incomplete due to erosion, documents the onset of the Cenomanian/Turonian boundary event (OAE2). This indicates a latest Cenomanian, possibly Early Turonian age for the top Natih at this subsurface location and suggests an Early Turonian age for the more complete Natih section exposed in the nearby Oman Mountains sections. Both organic-rich intervals of the Natih Formation (Natih E4b and B2) do not correlate with global Oceanic Anoxic Events indicating a rather local setting for source-rock deposition. This is further supported by an isotopic anomaly associated with the organic-rich Natih B. The anomaly is likely related to near-seabed diagenesis or a temporary limited water exchange of the intra-shelf basin with the open ocean and the incorporation of recycled carbon from oxidized organic matter into the water column and the inorganic carbon pool. The subsurface carbon-isotope profile correlates well with those from nearby outcrop and other subsurface sections adding further confidence that primary signatures are preserved and can be used for correlations. As in other Early Cretaceous shelf sequences of the Arabian Plate oxygen isotopes are lighter than expected for calcite deposited in equilibrium with Cretaceous seawater indicating most likely whole-scale recrystallization and stabilization during shallow burial at slightly elevated temperatures.

Styles APA, Harvard, Vancouver, ISO, etc.

39

Kolo, K., et Ph Claeys. « In vitro formation of Ca-oxalates and the mineral glushinskite by fungal interaction with carbonate substrates and seawater ». Biogeosciences 2, n^o 3 (26 octobre 2005) : 277–93. http://dx.doi.org/10.5194/bg-2-277-2005.

Texte intégral

Résumé :

Abstract. This study investigates the in vitro formation of Ca-oxalates and glushinskite through fungal interaction with carbonate substrates and seawater as a process of biologically induced metal recycling and neo-mineral formation. The study also emphasizes the role of the substrates as metal donors. In the first experiment, thin sections prepared from dolomitic rock samples of Terwagne Formation (Carboniferous, Viséan, northern France) served as substrates. The thin sections placed in Petri dishes were exposed to fungi grown from naturally existing airborne spores. In the second experiment, fungal growth and mineral formation was monitored using only standard seawater (SSW) as a substrate. Fungal growth media consisted of a high protein/carbohydrates and sugar diet with demineralized water for irrigation. Fungal growth process reached completion under uncontrolled laboratory conditions. The newly formed minerals and textural changes caused by fungal attack on the carbonate substrates were investigated using light and scanning electron microscopy (SEM-EDX), x-ray diffraction (XRD) and Raman spectroscopy. The fungal interaction and attack on the dolomitic and seawater substrates resulted in the formation of Ca-oxalates (weddellite CaC2O4·2(H2O), whewellite (CaC2O4·(H2O)) and glushinskite MgC2O4·2(H2O) associated with the destruction of the original hard substrates and their replacement by the new minerals. Both of Ca and Mg were mobilized from the experimental substrates by fungi. This metal mobilization involved a recycling of substrate metals into newly formed minerals. The biochemical and diagenetic results of the interaction strongly marked the attacked substrates with a biological fingerprint. Such fingerprints are biomarkers of primitive life. The formation of glushinskite is of specific importance that is related, besides its importance as a biomineral bearing a recycled Mg, to the possibility of its transformation through diagenetic pathway into an Mg carbonate. This work is the first report on the in vitro formation of the mineral glushinskite through fungal interaction with carbonate and seawater substrates. Besides recording the detailed Raman signature of various crystal habits of Mg- and Ca-oxalates, the Raman spectroscopy proved two new crystal habits for glushinskite. The results of this work document the role of microorganisms as metal recyclers in biomineralization, neo-mineral formation, sediment diagenesis, bioweathering and in the production of mineral and diagenetic biomarkers. They also reveal the capacity of living fungi to interact with liquid substrates and precipitate new minerals.

Styles APA, Harvard, Vancouver, ISO, etc.

40

Özyurt, Merve, M. Ziya Kırmacı, Ihsan Al-Aasm, Cathy Hollis, Kemal Taslı et Raif Kandemir. « REE Characteristics of Lower Cretaceous Limestone Succession in Gümüşhane, NE Turkey : Implications for Ocean Paleoredox Conditions and Diagenetic Alteration ». Minerals 10, n^o 8 (30 juillet 2020) : 683. http://dx.doi.org/10.3390/min10080683.

Texte intégral

Résumé :

Trace and rare earth elements (REEs) are considered to be reliable indicators of chemical processes for the evolution of carbonate systems. One of the best examples of ancient carbonate successions (Berdiga Formation) is widely exposed in NE Turkey. The Lower Cretaceous limestone succession of Berdiga Formation may provide a case study that reveals the effect of ocean paleoredox conditions on diagenetic alteration. Measurement of major, trace and REEs was carried out on the Lower Cretaceous limestones of the Berdiga Formation, to reveal proxies for paleoredox conditions and early diagenetic controls on their geochemistry. Studied micritic limestone microfacies (MF-1 to MF-3) indicate deposition in the inner platform to a deep shelf or continental slope paleoenvironment during the Hauterivian-Albian. The studied limestone samples mainly exhibit low Mg-calcite characteristics with the general chemical formula of Ca98.35–99.34Mg0.66–1.65(CO3). They are mostly represented by a diagnostic REE seawater signature including (1) slight LREE depletion relative to the HREEs (ave. 0.72 of Nd/YbN and ave. 0.73 of Pr/YbN), (2) negative Ce anomalies (Ce/Ce* = 0.38–0.81; ave. 0.57), (3) positive La anomaly (La/La* = 0.21–3.02; ave. 1.75) and (4) superchondritic Y/Ho (ave. 46.26). Studied micritic limestones have predominantly low Hf (bdl to 0.5 µg/g), Sc (bdl to 2 µg/g) Th (bdl to 0.9 µg/g) contents suggesting negligible to minor shale contamination. These findings imply that micritic limestones faithfully record chemical signals of their parental and diagenetic fluids. The succession also exhibits high ratios of Eu/Eu* (1.01–1.65; ave. 1.29 corresponding to the positive Eu anomalies), Sm/Yb (1.26–2.74; ave. 1.68) and La/Yb ratios (0.68–1.35; ave. 0.9) compared to modern seawater and wide range of Y/Ho ratios (29.33–70.00; ave. 46.26) which are between seawater and hydrogenetic Fe-Mn crusts. Several lines of geochemical evidence suggest water-rock interaction between parental seawater and basaltic rocks at elevated temperatures triggered by hydrothermal activity associated with Early Cretaceous basaltic magma generation. The range of Ce/Ce* values is suggestive of mostly oxic to dysoxic paleoceanographic conditions, with a sudden change to dysoxic conditions (Ce/Ce* = 0.71–0.81), in the uppermost part of the MF-1. This is followed by an abrupt deepening paleoenvironment with a relative increase in the oxic state of the seawater and deposition of deeper water sediments (MF-2 and MF-3) above a sharp transition. The differences in microfacies characteristics and foraminifera assemblage between MF-1 and overlying facies (MF-2 and MF-3) may also confirm the change in paleoceanographic conditions. Therefore, REEs data obtained from studied limestones have the potential to contribute important information as to regional paleoceanographic conditions of Tethys during an important period in Earth history.

Styles APA, Harvard, Vancouver, ISO, etc.

41

Harris, Paul (Mitch), Mara R. Diaz et Gregor P. Eberli. « The Formation and Distribution of Modern Ooids on Great Bahama Bank ». Annual Review of Marine Science 11, n^o 1 (3 janvier 2019) : 491–516. http://dx.doi.org/10.1146/annurev-marine-010318-095251.

Texte intégral

Résumé :

Great Bahama Bank (GBB) is the principal location of the formation and accumulation of ooids (concentrically coated, sand-size carbonate grains) in the world today, and as such has been the focus of studies on all aspects of ooids for more than half a century. Our view from a close look at this vast body of literature coupled with our continuing interests stresses that biological mechanisms (microbially mediated organomineralization) are very important in the formation of ooids, whereas the controlling factor for the distribution and size of ooid sand bodies is the physical energy. Mapping and coring studies of the modern ooid sand bodies on GBB provide insight into the rock record from different perspectives. An important consequence of the dual influence of ooid formation and distribution is that the geochemical signature of ooids is not in equilibrium with the seawater in which ooids form; therefore, extracting the paleophysical energy record from oolitic deposits is potentially more accurate than doing so for the paleochemical record.

Styles APA, Harvard, Vancouver, ISO, etc.

42

Kellock, Celeste, Maria Cristina Castillo Alvarez, Adrian Finch, Kirsty Penkman, Roland Kröger, Matthieu Clog et Nicola Allison. « Optimising a method for aragonite precipitation in simulated biogenic calcification media ». PLOS ONE 17, n^o 12 (2 décembre 2022) : e0278627. http://dx.doi.org/10.1371/journal.pone.0278627.

Texte intégral

Résumé :

Resolving how factors such as temperature, pH, biomolecules and mineral growth rate influence the geochemistry and structure of biogenic CaCO3, is essential to the effective development of palaeoproxies. Here we optimise a method to precipitate the CaCO3 polymorph aragonite from seawater, under tightly controlled conditions that simulate the saturation state (Ω) of coral calcification fluids. We then use the method to explore the influence of aspartic acid (one of the most abundant amino acids in coral skeletons) on aragonite structure and morphology. Using ≥200 mg of aragonite seed (surface area 0.84 m2), to provide a surface for mineral growth, in a 330 mL seawater volume, generates reproducible estimates of precipitation rate over Ωaragonite = 6.9–19.2. However, unseeded precipitations are highly variable in duration and do not provide consistent estimates of precipitation rate. Low concentrations of aspartic acid (1–10 μM) promote aragonite formation, but high concentrations (≥ 1 mM) inhibit precipitation. The Raman spectra of aragonite precipitated in vitro can be separated from the signature of the starting seed by ensuring that at least 60% of the analysed aragonite is precipitated in vitro (equivalent to using a seed of 200 mg and precipitating 300 mg aragonite in vitro). Aspartic acid concentrations ≥ 1mM caused a significant increase in the full width half maxima of the Raman aragonite v1 peak, reflective of increased rotational disorder in the aragonite structure. Changes in the organic content of coral skeletons can drive variations in the FWHM of the Raman aragonite ν1 peak, and if not accounted for, may confuse the interpretation of calcification fluid saturation state from this parameter.

Styles APA, Harvard, Vancouver, ISO, etc.

43

Blanchet, Cécile L. « A database of marine and terrestrial radiogenic Nd and Sr isotopes for tracing earth-surface processes ». Earth System Science Data 11, n^o 2 (24 mai 2019) : 741–59. http://dx.doi.org/10.5194/essd-11-741-2019.

Texte intégral

Résumé :

Abstract. The database presented here contains radiogenic neodymium and strontium isotope ratios measured on both terrestrial and marine sediments. The main purpose of this dataset is to help assess sediment provenance and transport processes for various time intervals. This can be achieved by either mapping sediment isotopic signature and/or fingerprinting source areas using statistical tools. The database has been built by incorporating data from the literature and the SedDB database and harmonizing the metadata, especially units and geographical coordinates. The original data were processed in three steps. Firstly, specific attention has been devoted to providing geographical coordinates to each sample in order to be able to map the data. When available, the original geographical coordinates from the reference (generally DMS coordinates) were transferred into the decimal degrees system. When coordinates were not provided, an approximate location was derived from available information in the original publication. Secondly, all samples were assigned a set of standardized criteria that help split the dataset into specific categories. For instance, samples were distinguished according to their location (“Region”, “Sub-region” and “Location” that relate to locations at continental to city or river scale) or the sample type (terrestrial samples – “aerosols”, “soil sediments”, “river sediments”, “rocks” – or marine samples – “marine sediment” or “trap sample”). Finally, samples were distinguished according to their deposition age, which allowed us to compute average values for specific time intervals. Graphical examples illustrating the functionality of the database are presented and the validity of the process was tested by comparing the results with published data. The dataset will be updated bi-annually in order to add more data points to increase the sampling density or provide new types of samples (e.g. seawater signature) and/or integrate additional information regarding the samples. It is publicly available (under CC4.0-BY Licence) from the GFZ data management service at https://doi.org/10.5880/GFZ.4.3.2019.001.

Styles APA, Harvard, Vancouver, ISO, etc.

44

Faÿ-Gomord, O., C. Allanic, M. Verbiest, R. Honlet, F. Champenois, M. Bonifacie, C. Chaduteau et al. « Understanding Fluid Flow during Tectonic Reactivation : An Example from the Flamborough Head Chalk Outcrop (UK) ». Geofluids 2018 (2018) : 1–17. http://dx.doi.org/10.1155/2018/9352143.

Texte intégral

Résumé :

Flamborough Head chalks are located at the extremities of E-W and N-S trending fault systems along the Yorkshire coast (UK). Rock deformation is expressed in Selwicks Bay where a normal fault is exposed along with a high density of calcite veins. The fault mineralization is tested using geochemistry. Crosscutting relationships are used to differentiate between three vein generations: a network of parallel veins that are oriented perpendicular to stratigraphy (Group I), hydraulic breccia with typical jigsaw puzzle structure (Group II), and a third generation of calcite veins crosscutting the two previous generations (Group III). Geochemical analyses revealed that all three generations possess the same chemical signature and must reflect successive pulses from the same mineralizing fluid source. Strontium isotope analyses showed that the veins have elevated 87Sr/86Sr ratios, that is, up to 7.110, while ratios of the chalk matrix equal 7.707. The latter value is in agreement with the signature of Late Cretaceous seawater. Consequently, the source of the fluid is external, reflecting an open system. The radiogenic Sr-isotope ratios, combined with low iron concentration, suggest that fluids migrated through sandy deposits. Fluid inclusion salinities range from 0 to 12 eq. wt% NaCl equiv. with a dominance of very low salinity inclusions, reflecting a meteoric signal. This leads to a model where meteoric fluids stored in an underlying confined sandstone aquifer were remobilized. The wide range of salinities could result from mixing of the meteoric fluid with some more saline fluids present in the rock sequence or from the dissolution of salts in the subsurface. In addition to the understanding of the local paragenetic evolution of the veining in Flamborough Head chalks, this study offers an insight into the way how fluid flows and mineralizes along fault zones.

Styles APA, Harvard, Vancouver, ISO, etc.

45

Potter, Eric G., Colter J. Kelly, William J. Davis, Guoxiang Chi, Shao-Yong Jiang, Morteza Rabiei et Brian J. McEwan. « Fluid sources in basement-hosted unconformity–uranium ore systems : tourmaline chemistry and boron isotopes from the Patterson Lake corridor deposits, Canada ». Geochemistry : Exploration, Environment, Analysis 22, n^o 1 (1 décembre 2021) : geochem2021–037. http://dx.doi.org/10.1144/geochem2021-037.

Texte intégral

Résumé :

The Patterson Lake corridor is a new uranium district located on the southwestern margin of the Athabasca Basin. Known resources extend almost 1 km below the unconformity in graphite- and sulfide-bearing shear zones within highly altered metamorphic rocks. Despite different host rocks and greater depths below the unconformity, alteration assemblages (chlorite, illite, kaolinite, tourmaline and hematite), ore grades and textures are typical of unconformity-related deposits. This alteration includes at least three generations of Mg-rich tourmaline (magnesio-foitite). The boron isotopic composition of magnesio-foitite varies with generation: the earliest generation, which is only observed in shallow samples from the Triple R deposit (Tur 1), contain the heaviest isotopic signature (δ11B ≈ 19–26‰), whereas subsequent generations (Tur 2 and Tur 3) yield lighter and more homogeneous isotopic signatures (δ11B ≈ 17.5–19.9‰). These results are consistent with precipitation from low-temperature, NaCl- and CaCl2-rich brine(s) derived from an isotopically heavy boron source (e.g. evaporated seawater) that interacted with tourmaline and silicates in the basement rocks and/or fluids derived from depth (with low δ11B values). The lower δ11B values in paragenetically later magnesio-foitite reflect greater contributions of basement-derived boron over time, whereas minor compositional variations reflect local metal sources (e.g. Cr, V, Ti) and evolving fluid chemistry (decreasing Na and Ca, increasing Mg) over time. The δ11B and chemical variation in magnesio-foitite over time reinforce the strong interactions with basement rocks in these systems while supporting incursion of basinal brines well below the unconformity contact.Supplementary material: Complete analytical dataset including reference materials are available at https://doi.org/10.6084/m9.figshare.c.5727555Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Styles APA, Harvard, Vancouver, ISO, etc.

46

Vesnaver, Aldo L., Flavio Accaino, Gualtiero Bohm, Gianni Madrussani, Jan Pajchel, Giuliana Rossi et Giancarlo Dal Moro. « Time‐lapse tomography ». GEOPHYSICS 68, n^o 3 (mai 2003) : 815–23. http://dx.doi.org/10.1190/1.1581034.

Texte intégral

Résumé :

In time‐lapse analysis, we have to distinguish the seismic response changes due to oil and gas production at a reservoir over the years from several other causes, such as the recording signature and random noise. In this paper, we focus our attention on the velocity macromodel provided by seismic tomography, which is a basic tool for the data regularization, its depth or time migration, and a possible final subtraction among different vintages. We show first that we cannot use just a single velocity model for all data sets, because of seasonal variations of the overburden velocity (which is mainly due to seawater temperature in marine cases and to the water table depth in land cases). However, we can exploit the basic assumption of time‐lapse analysis for constraining reflection/refraction tomography, i.e., by imposing the constraint that the layer structure and the local velocities do not change outside the reservoir (and in the shallowest part) over time. We thus get coupled models that are physically consistent, with a better spatial coverage and higher information redundancy. The new method is illustrated by a marine case history from the North Sea.

Styles APA, Harvard, Vancouver, ISO, etc.

47

Al-Bassam, Khaldoun S. « STABLE CARBON AND OXYGEN ISOTOPES OF SOME CARBONATE-FLUORAPATITES FROM CENOMANIAN AND TURONIAN SEQUENCES, BOHEMIAN CRETACEOUS BASIN, CZECH REPUBLIC ». Iraqi Geological Journal 51, n^o 1 (30 juin 2018) : 1–16. http://dx.doi.org/10.46717/igj.51.1.1ms-2018-06-23.

Texte intégral

Résumé :

Stable carbon and oxygen isotopes were analysed in the structural CO3 of carbonate-fluorapatite in phosphate nodules and coprolites present in distinctive horizons at the Cenomanian – Turonian boundary and in late Turonian sequence at the Bohemian Cretaceous Basin of the Czech Republic. The results indicate that the structural CO3 in the analysed carbonate-fluorapatites is depleted in δ13C relative to CO3 in carbonates of comparable age and all samples show negative δ18O values close to those recorded in mid-Cretaceous carbonate-CO3. The obtained δ13C values, however, are higher than those reported in apatite-CO3 of the late Cretaceous Tythyan upwelling-type phosphates. The δ13C results are interpreted to indicate that phosphogenesis have taken place in an environment rich in the light isotope of carbon, similar to that usually prevails in the organic-rich pore water below sedimentwater interface. The higher δ13C values in the studied apatites relative to their Tethyan analogues may reflect the signature of the well-documented δ13C excursion of the Oceanic Anoxic Event 2 at the Cenomanian-Turonian boundary. The δ18O values suggest paleotemperatures expressive of the mid-Cretaceous climatic warming and a lower paleosalinity than that of normal well-circulated seawater.

Styles APA, Harvard, Vancouver, ISO, etc.

48

Brengman, Latisha A., et Christopher M. Fedo. « Development of a mixed seawater-hydrothermal fluid geochemical signature during alteration of volcanic rocks in the Archean (∼2.7 Ga) Abitibi Greenstone Belt, Canada ». Geochimica et Cosmochimica Acta 227 (avril 2018) : 227–45. http://dx.doi.org/10.1016/j.gca.2018.02.019.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

49

Gancheva, Irina, Elisaveta Peneva et Violeta Slabakova. « Detecting the Surface Signature of Riverine and Effluent Plumes along the Bulgarian Black Sea Coast Using Satellite Data ». Remote Sensing 13, n^o 20 (13 octobre 2021) : 4094. http://dx.doi.org/10.3390/rs13204094.

Texte intégral

Résumé :

The clear and reliable detection of effluent plumes using satellite data is especially challenging. The surface signature of such events is of a small scale; it shows a complex interaction with the local environment and depends greatly on the effluent and marine water constitution. In the context of remote sensing techniques for detecting treated wastewater discharges, we study the surface signature of small river plumes, as they share specific characteristics, such as higher turbidity levels and increased nutrient concentration, and are fresh compared to the salty marine water. The Bulgarian Black Sea zone proves to be a challenging study area, with its optically complex waters and positive freshwater balance. Additionally, the Bulgarian Black Sea coast is a known tourist destination with an increased seasonal load; thus, the problem of the identification of wastewater discharges is a topical issue. In this study, we analyze the absorption components of the Inherent Optical Properties (IOPs) for 84 study points that are located at outfall discharging areas, river estuaries and at different distances from the shoreline, reaching the open sea area at a bottom depth of more than 2000 m. The calculations of IOPs take into account all available Sentinel 2 cloudless acquisitions for three years from 2017 until 2019 and are performed using the Case-2 Regional CoastColour (C2RCC) processor, implemented in the Sentinel Application Platform (SNAP). The predominant absorber for each study area and its temporal variation is determined, deriving the specific characteristics of the different areas and tracking their seasonal and annual course. Optical data from the Galata AERONET-OC site are used for validating the absorption coefficient of phytoplankton pigment. A conclusion regarding the possibility of distinguishing riverine, marine and coastal water is derived. The study provides a sound basis for estimating the advantages and drawbacks of optical satellite data for tracking the extent of effluent and fluvial plumes with unknown concentrations of optically significant seawater constituents.

Styles APA, Harvard, Vancouver, ISO, etc.

50

Mackey, DJ. « Copper-complexing capacity of South Pacific waters ». Marine and Freshwater Research 37, n^o 4 (1986) : 437. http://dx.doi.org/10.1071/mf9860437.

Texte intégral

Résumé :

The strong copper-complexing capacity of seawater from the western tropical Pacific Ocean and the Coral Sea has been measured by a method based on competition between naturally occurring ligands and Chelex-100 ion-exchange resin. In the nutrient-depleted surface waters, the values ranged from 4 to 72 nM. For deeper waters there were larger variations with values ranging from 2 to 136 nM in the region of the salinity minimum at approximately 800-1000 m. There was no evidence for increased complexing capacity due to the sediment-water interface. Since changes in complexing capacity are ultimately of biological origin, the variability must have arisen when the waters were in the euphotic zone at polar latitudes. The samples were unfiltered and the complexing capacity could be due to either dissolved organic compounds or to particles so small that their settling velocities are negligible compared with the time elapsed since the water parcel was last in the euphotic zone. There is no direct correlation between complexing capacity and water density or concentration of nutrients. The results suggest that parcels of water up to 100 km wide can travel distances up to 10 000 km and maintain a chemical signature different from the surrounding water.

Styles APA, Harvard, Vancouver, ISO, etc.

Articles de revues sur le sujet « Seawater signature »

Créez une référence correcte selon les styles APA, MLA, Chicago, Harvard et plusieurs autres