Добірка наукової літератури з теми "Paleo-seawater composition"

Coastal areas are characterized by considerable demographic pressure that generally leads to groundwater overexploitation. In the Mediterranean region, this situation is exacerbated by a recharge reduction enhanced by climate change. The consequence is water table drawdown that alters the freshwater/seawater interface facilitating seawater intrusion. However, the groundwater salinity may also be affected by other natural/anthropogenic sources. In this paper, water quality data gathered at 47 private and public wells in a coastal karst aquifer in Apulia (southern Italy), were interpreted by applying disparate methods to reveal the different sources of groundwater salinity. Chemical characterization, multivariate statistical analysis, and mixing calculations supplied the groundwater salinization degree. Characteristic ion ratios, strontium isotope (87Sr/86Sr), and pure mixing modelling identified the current seawater intrusion as a main salinity source, also highlighting the contribution of water–rock interaction to groundwater composition and excluding influence from Cretaceous paleo-seawater. Only the combined approach of all the methodologies allowed a clear identification of the main sources of salinization, excluding other less probable ones (e.g., paleo-seawater). The proposed approach enables effective investigation of processes governing salinity changes in coastal aquifers, to support more informed management.

The behaviour of bioavailable trace metals and their stable isotopes in the modern oceans is controlled by uptake into phototrophic organisms and adsorption on and incorporation into marine authigenic minerals. Among other bioessential metals, Cd and its stable isotopes have recently been used in carbonate lithologies as novel tracer for changes in the paleo primary productivity and (bio)geochemical cycling. However, many marine sediments that were deposited during geologically highly relevant episodes and which, thus, urgently require study for a better understanding of the paleo environment are rather composed of a mixture of organic matter (OM), and detrital and authigenic minerals. In this study, we present Cd concentrations and their isotopic compositions as well as trace metal concentrations from sequential leachates of OM-rich shales of the Cryogenian basal Datangpo Formation, Yangtze Platform (South China). Our study shows variable distribution of conservative and bioavailable trace metals as well as Cd isotope compositions between sequential leachates of carbonate, OM, sulphide, and silicate phases. We show that the Cd isotope compositions obtained from OM leachates can be used to calculate the ambient Cryogenian surface seawater of the restricted Nanhua Basin by applying mass balance calculations. By contrast, early diagenetic Mn carbonates and sulphides incorporated the residual Cd from dissolved organic matter that was in isotopic equilibrium with deep/pore waters of the Nanhua Basin. Our model suggests that the Cd isotopic composition of surface seawater at that time reached values of modern oxygenated surface oceans. However, the deep water Cd isotope composition was substantially heavier than that of modern fully oxygenated oceans and rather resembles deep waters with abundant sulphide precipitation typical for modern oxygen minimum zones. This argues for incomplete recycling of Cd and other bioavailable metals shortly after the Sturtian glaciation in the redox stratified Cryogenian Nanhua Basin. Our study highlights the importance of sequential leaching procedures when dealing with impure authigenic sediments such as OM-rich carbonates, mudstones, or shales to achieve reliable trace metal concentrations and Cd isotope compositions as proxies for (bio)geochemical metal cycling in past aquatic systems.

Abstract. The Mg/Ca ratio of Foraminifera calcium carbonate tests is used as proxy for seawater temperature and widely applied to reconstruct global paleo-climatic changes. However, the mechanisms involved in the carbonate biomineralization process are poorly understood. The current paradigm holds that calcium ions for the test are supplied primarily by endocytosis of seawater. Here, we combine confocal-laser scanning-microscopy observations of a membrane-impermeable fluorescent marker in the extant benthic species Ammonia aomoriensis with dynamic 44Ca-labeling and NanoSIMS isotopic imaging of its test. We infer that Ca for the test in A. aomoriensis is supplied primarily via trans-membrane transport, but that a small component of passively transported (e.g., by endocytosis) seawater to the site of calcification plays a key role in defining the trace-element composition of the test. Our model accounts for the full range of Mg/Ca and Sr/Ca observed for benthic Foraminifera tests and predicts the effect of changing seawater Mg/Ca ratio. This places foram-based paleoclimatology into a strong conceptual framework.

Abstract. The Mg / Ca ratio of foraminifera calcium-carbonate tests is used as proxy for seawater temperature and widely applied to reconstruct global paleo-climatic changes. However, the mechanisms involved in the carbonate biomineralization process are poorly understood. The current paradigm holds that calcium ions for the test are supplied primarily by endocytosis of seawater. Here, we combine confocal-laser scanning-microscopy observations of a membrane-impermeable fluorescent marker in the extant benthic species Ammonia aomoriensis with dynamic 44Ca-labeling and NanoSIMS isotopic imaging of its test. We infer that Ca for the test in A. aomoriensis is supplied primarily via trans-membrane transport, but that a small component of passively transported (e.g. by endocytosis) seawater to the site of calcification plays a key role in defining the trace-element composition of the test. Our model accounts for the full range of Mg / Ca and Sr / Ca observed for benthic foraminifera tests and predicts the effect of changing seawater Mg / Ca ratio. This places foram-based paleoclimatology into a strong conceptual framework.

Abstract. The chromium isotope system (53Cr ∕ 52Cr, expressed as δ53Cr relative to NIST SRM 979) in marine biogenic and non-biogenic carbonates is currently being evaluated as a proxy for the redox state of the ocean. Previous work has concentrated on using corals and foraminifera for this purpose, but investigations focusing on the behavior of Cr in bivalves as potential archives are lacking. Due to their often good preservation, fossil marine biogenic carbonates have the potential to serve as useful archives for the reconstruction of past ocean redox fluctuations and eventually link those to climatic changes throughout Earth's history. Here, we present an evaluation of the Cr isotope system in shells of some modern bivalves. Shell species from Lucidinadae, Cardiidae, Glycimerididae and Pectenidae, collected systematically from one Mediterranean location (Playa Poniente, Benidorm, Spain) over a 3-year period reveal δ53Cr values ranging from 0.15 ‰ to 0.65 ‰, values that are systematically below the local seawater δ53Cr value of 0.83±0.05 ‰. This attests to a significant reduction of dissolved seawater chromium in the process leading to calcification and thus for control of Cr isotope fractionation during biological routes. A similar, constant offset in δ53Cr values relative to surface seawater is observed in shells from Mytilius edulis from an arctic location (Godhavn, Disko Bay, Greenland). Chromium concentrations in the studied shells are significantly controlled by organic matter and typically range from 0.020 to 0.100 ppm, with some higher concentrations of up to 0.163 ppm recorded in Pectenidae. We also observe subtle, species-dependent differences in average Cr isotope signatures in the samples from Playa Poniente, particularly of Lucidinadae and Cardiidae, with considerably depressed and elevated δ53Cr values, respectively, relative to the other species investigated. Intra-species heterogeneities, both in Cr concentrations and δ53Cr values, are favorably seen to result from vital effects during shell calcification rather than from heterogeneous seawater composition. This is because we observe that the surface seawater composition in the particular Playa Poniente location remained constant during the month of July of the 3 years we collected bivalve samples. Intra-shell heterogeneities – associated with growth zones reflecting one to several years of growth, both in δ53Cr and Cr concentrations – are observed in a sample of Placuna placenta and Mimachlamys townsendi. We suspect that these variations are, at least partially, related to seasonal changes in δ53Cr of surface seawaters. Recognizing the importance of organic substances in the bivalve shells, we propose a model whereby reduction of Cr(VI) originally contained in the seawater as chromate ion and transported to the calcifying space, to Cr(III), is effectively adsorbed onto organic macromolecules which eventually get included in the growing shell carbonates. This study, with its definition of statistically sound offsets in δ53Cr values of certain bivalve species from ambient seawater, forms a base for future investigations aimed at using fossil shells as archives for the reconstruction of paleo-seawater redox fluctuations.

Abstract. The boron isotopic composition (δ11B) of marine carbonates (e.g. corals) has been established as a reliable proxy for paleo-pH, with the strong correlation between δ11B of marine calcifiers and seawater pH being now well documented. However, further investigations are needed in order to better quantify other environmental parameters potentially impacting boron isotopic composition and boron concentration into coral aragonite. To achieve this goal the tropical scleractinian coral Acropora sp. was cultured under 3 different temperature (22, 25 and 28 °C) and two light conditions (200 and 400 μmol photon m−2 s−1). The δ11B indicates an internal increase in pH from ambient seawater under both light conditions. Changes in light intensities from 200 to 400 μmol photon m−2 s−1 could bias pH reconstructions by about 0.05 units. For both light conditions, a significant impact of temperature on δ11B can be observed between 22 and 25 °C corresponding to enhancements of about 0.02 pH-units, while no further δ11B increase can be observed between 25 and 28 °C. This non-linear temperature effect complicates the determination of a correcting factor. B/Ca ratios decrease with increasing light, confirming the decrease in pH at the site of calcification under enhanced light intensities. When all the other parameters are maintained constant, boron concentrations in Acropora sp. increase with increasing temperature and increasing carbonate ions concentrations. These observations contradict previous studies where B/Ca in corals was found to vary inversely with temperature suggesting that the controlling factors driving boron concentrations have not yet been adequately identified and might be influenced by other seawater variables and species specific responses.

The causal effects among uplift, climate, and continental weathering cannot be fully addressed using presently available geochemical proxies. However, stable potassium (K) isotopes can potentially overcome the limitations of existing isotopic proxies. Here we report on a systematic investigation of K isotopes in dissolved load and sediments from major rivers and their tributaries in China, which have drainage basins with varied climate, lithology, and topography. Our results show that during silicate weathering, heavy K isotopes are preferentially partitioned into aqueous solutions. Moreover, δ41K values of riverine dissolved load vary remarkably and correlate negatively with the chemical weathering intensity of the drainage basin. This correlation allows an estimate of the average K isotope composition of global riverine runoff (δ41K = −0.22‰), as well as modeling of the global K cycle based on mass balance calculations. Modeling incorporating K isotope mass balance better constrains estimated K fluxes for modern global K cycling, and the results show that the δ41K value of seawater is sensitive to continental weathering intensity changes. Thus, it is possible to use the δ41K record of paleo-seawater to infer continental weathering intensity through Earth’s history.

AbstractA newly discoveredVerbeekinaassemblage from the Xiaoxinzhai Section in the Baoshan Block in western Yunnan, China, provides additional data for better understanding fusulinid biostratigraphy and the thermal condition of middle Permian (Guadalupian) seawater of this block. This assemblage comprises 11 species of six genera, includingVerbeekina,Pseudodoliolina,Sumatrina,Yangchienia,Xiaoxinzhaiella, and ?Rugosochusenella. The age of this assemblage is considered to be Midian (approximately Capitanian), based on combined evidence of stratigraphic position and specific composition. Furthermore, this assemblage bears two unusual attributes: overwhelming dominance ofVerbeekinaand relatively low total diversity, compared with coeval fusulinids from South China, which represents a paleo-tropical setting during the middle Permian. These features indicate that the Baoshan Block was probably located in a subtropical setting with warm water during the Midian time; however, its water temperature was still not as optimal as the tropical area (e.g., South China) for the diversification of fusulinids.

Abstract. TEX86 (TetraEther indeX of glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbon atoms) has been widely applied to reconstruct (paleo-) sea surface temperature (SST). While Marine Group I (MG I) Thaumarchaeota have been commonly believed to be the source for GDGTs, Marine Group II (MG II Euryarchaeota) have recently been suggested to contribute significantly to the GDGT pool in the ocean. However, little is known how the MG II Euryarchaeota-derived GDGTs may influence TEX86 in marine sediment record. In this study, we characterize MG II Euryarchaeota-produced GDGTs and assess the likely effect of these tetraether lipids on TEX86. Analyses of core lipid (CL-) and intact polar lipid (IPL-) based GDGTs, 454 sequencing and quantitative polymerase chain reaction (qPCR) targeting MG II Euryarchaeota were performed on suspended particulate matter (SPM) and surface sediments collected along a salinity gradient from the lower Pearl River (river water) and its estuary (mixing water) to the coastal South China Sea (seawater). The results showed that the community composition varied along the salinity gradient with MG II Euryarchaeota as the second dominant group in the mixing water and seawater. qPCR data indicated that the abundance of MG II Euryarchaeota in the mixing water was three to four orders of magnitude higher than the river water and seawater. Significant linear correlations were observed between the gene abundance ratio of MG II Euryarchaeota vs. total archaea and the relative abundance of GDGTs-1, -2, -3, or -4 as well as the ring index based on these compounds, which collectively suggest that MG II Euryarchaeota may actively produce GDGTs in the water column. These results also show strong evidence that MG II Euryarchaeota synthesizing GDGTs with 1–4 cyclopentane moieties may bias TEX86 in the water column and sediments. This study highlights that valid interpretation of TEX86 in sediment record, particularly in coastal oceans, needs to consider the contribution from MG II Euryarchaeota.

Determining paleo-seawater composition and temperature is critical for reconstructing Earth’s surface conditions through time and has major implications for understanding the origin and evolution of life on Earth. Paleo-seawater composition is inferred from the chemical and iso-topic compositions of biogenic and inorganic chemical precipitates that form out of seawater. Some of the widely used geochemical and isotopic tracers that have been measured in marine carbonates for determining the physical and chemical attributes (e.g., temperature, pH, salinity) of the past oceans include Sr/Ca, Mg/Ca, Li/Ca, Ba/Ca, 18O, 47, 11B, etc. However, each geochemical and isotopic proxy used for determining paleo-seawater temperature has its limi-tations and requires addressing four main issues: (i) inter-species variability in biogenic car-bonates, (ii) variation in past seawater composition, (iii) post-depositional alteration of sam-ples, and (iv) temperature-calibration of the proxy. Hence, an important component of paleo-climate research involves the development of the robust geochemical and isotopic proxies which are minimally affected by factors (e.g., pH, growth rate, vital effect, etc.) other than temperature. Recent improvements in mass spectrometry have resulted in high-precision Ca isotopic measurements (44/40Ca) which allows us to investigate the applicability of 44/40Ca as a paleotemperature proxy. Due to the high abundance of Ca in inorganic and biogenic car-bonates, the 44/40Ca value is expected to be unaffected by the diagenetic alteration of geolog-ical archives which makes the 44/40Ca composition of carbonates a powerful tool for paleocli-mate research. In this thesis, Ca isotopic measurements were performed on various inorganic and biogenic carbonates along with measurements of other geochemical (Sr/Ca, Mg/Ca, Li/Ca, Ba/Ca, B/Ca) and traditional isotopic proxies (18O, 13C) to understand the efficacy of these different viii proxies in reconstructing water temperatures. A novel sample-loading technique, involving a Re-Ta double filament assembly along with the use of tantalum oxide as an activator, was developed for Ca isotopic measurements using thermal ionization mass spectrometry (TIMS). Using this loading technique, the analytical reproducibility for 44/40Ca was better than 0.08‰ (2SD). Elemental ratios in selected biogenic carbonates were measured using a quadrupole inductively coupled plasma mass spectrometer (ICPMS). Using a cold-plasma technique and bracketing using synthetic and international carbonate standards, high precision data were obtained. For example, the external precision was better than ± 0.03 (SD) for Sr/Ca (mmol/mol) and ± 0.025 (SD) for Mg/Ca (mmol/mol). To understand the effect of temperature and the roles of pH and precipitation rate on Ca isotopic composition of inorganic carbonates, calcite precipitation experiments were conducted in a controlled-laboratory environment at different temperatures (5, 10, 20, 30, 40 and 50 °C). At each temperature, the effects of precipitation rate, pH and calcite dissolution rate on Ca isotopic composition of the precipitated carbonate were investigated. It was observed that the calcite dissolution rate is a function of temperature; at low temperatures (5-10 °C), the Ca isotopic fractionation was maximum (44/40Ca = -0.84 to -1.07‰; 44/40Ca = δ44/40Cacrystal - δ44/40Caparent solution) and the associated dissolution rate was low (0.5×10-7 mol/m2/h). In contrast, at relatively high temperatures (40-50 °C), the calculated dissolution rate was higher (25×10-5 mol/m2/h) while the Ca isotopic fractionation was the low (44/40Ca = -0.1 to -0.07). The results from the precipitation experiments suggest that the temperature-controlled variation in 44/40Ca is ~0.02 ‰/°C in inorganic calcites. The 44/40Ca-T relationship proposed in this study (44/40Ca = 0.024 ± 0.003 × T(°C) -1.08 ± 0.11) for inorganically precipitated carbonates can be used to investi-gate significant temperature changes in the geological past. The proposed 44/40Ca-T relation-ship was applied to published Ca isotopic data for cap carbonates to infer that the seawater ix temperature variability was between -17.9 C to 24.5 °C during Neoproterozoic snowball Earth events and these temperature estimates are consistent with published temperature estimates based on climate/ice sheet models. The second part of the thesis focuses on biogenic carbonates. To understand the efficacy of different geochemical and isotopic proxies, multi-elemental (Sr/Ca, Mg/Ca, Li/Ca, Ba/Ca, B/Ca) and isotopic (44/40Ca, 18O, 13C) measurements were performed on the same set of samples drilled from fourteen consecutive low-density and high-density bands in a Porites sp. coral which was collected from the Kavaratti island of the Lakshadweep archipelago in the Arabian Sea. For this coral, the temperature sensitivity of 44/40Ca was determined as ~0.094 ‰/°C. With the present analytical precision and proposed calibration equation, it is proposed that Ca isotopic composition of corals can be used to reconstruct paleotemperatures with a temperature resolution of ~1°C. The temperatures estimated from the geochemical and isotopic analyses were compared with satellite sea-surface temperatures over seven years. The results indicate that a combination of Sr/Ca, Li/Ca and 44/40Ca measurements in corals provide the most accurate sea-surface temperature. A new set of temperature-proxy relationships have been proposed for the Porites sp. in the Arabian Sea. The proposed proxy-SST relationships are: (i) Sr/Ca (mmol/mol) = 10.46 (±0.04) - 0.054 (±0.002) × SST (°C), (ii) Li/Ca (mol/mol) = -0.134 × SST + 9.99 and (iii) 44/40Ca SRM915a (‰) = 0.094 (±0.01) × SST -1.95 (±0.39). To further evaluate the applicability of Ca isotope ratios as a paleotemperature proxy, 44/40Ca was meas-ured in fish otolith samples. Otolith samples are commonly found in the fossil record and are important archives for paleoclimate research. Seasonal growth bands in otolith can yield high-resolution temperature and paleo-water chemistry where other archives like coral or foraminif-era are not available. Elemental ratios (Sr/Ca, Mg/Ca and Ba/Ca) along with 44/40Ca were measured in otolith samples from six different species of fish collected from different x geological locations with a wide range of temperatures from 2 to 25 °C. These samples were previously characterized for their 18O, 13C, and clumped isotope (47) compositions. The temperature sensitivity of 44/40Ca in fish otoliths (~0.02 ‰/°C) was determined for the first time in this thesis. The results suggest that Ca stable isotopes in fish otoliths can be used as a proxy for paleotemperature reconstruction. There is limited information on the temperature dependence of Ca isotopic fractionation in foraminifera. While some studies have reported a strong temperature dependency of 44/40Ca values (0.24 ‰/°C) in G. sacculifer culture samples, other studies have reported negligible temperature sensitivity of 44/40Ca values in foraminifera. The discrepancy between these stud-ies suggests a complex biomineralization pathway in foraminifera as well as the effects of additional factors on the 44/40Ca composition of foraminifera. To further evaluate the temper-ature sensitivity of 44/40Ca in foraminifera, 44/40Ca values were measured in samples which were also characterised for their 18O, 13C, Mg/Ca and Sr/Ca. Paired measurements of isotopic and elemental ratios were performed in benthic (C. wuellerstorfi), thermocline planktic (G. truncatulinoides and G. inflata) and other planktic species (G. ruber, G. bulloides, O universa, G. siphonifera) of foraminifera collected from the North Atlantic Ocean using a sediment trap. Some of these samples were earlier analysed for Mg/Ca and 18O measurements and used for the Mg/Ca thermometry calibration by Anand et al. (2003). In this study, we have analysed selected samples from Anand et al. (2003), showing a wide range in Mg/Ca values (0.99-4.62 mmol/mol), for their 44/40Ca compositions. When the species G. ruber and those with esti-mated habitat temperatures less than 3 °C are excluded, 44/40Ca values show a significant (r2 = 0.76) relationship with temperature [44/40Ca (‰) = 0.35 exp (0.055 × T (°C)], with temper-ature dependency of ~0.055 ‰/°C for 44/40Ca in foraminifera, thereby suggesting the possi-bility of using 44/40Ca in foraminifera as a paleotemperature proxy with a precision of ± 1.5 xi °C. There is a well-defined positive correlation between 44/40Ca and Sr/Ca for a few selected species, suggesting similar temperature dependent Sr partitioning and 44/40Ca fractionation in foraminifera. However, higher Sr/Ca ratio and low 44/40Ca in G. ruber suggests that the com-position of this surface-dwelling species may be affected by precipitation rate. Our study sug-gests that in addition to Mg/Ca, 44/40Ca in foraminifera can be used for accurate paleotemper-ature reconstruction. Overall the results reported in this thesis document the applicability, use-fulness, and limitations of Ca stable isotopes as a paleotemperature proxy.

In this chapter the methods and results of modeling the long-term carbon cycle are presented in terms of predictions of past levels of atmospheric CO2. The modeling results are then compared with independent determinations of paleo-CO2 by means of a variety of different methods. Results indicate that there is reasonable agreement between methods as to the general trend of CO2 over Phanerozoic time. Values of fluxes in the long-term carbon cycle can be calculated from the fundamental equations for total carbon and 13C mass balance that are stated in the introduction and are repeated here: . . . dMc/dt = Fwc + Fwg + Fmc + Fmg – Fbc – Fbg (1.10) . . . . . . d(δcMc)/dt = δwcFwc + δwgFwg + δmcFmc + δmgFmg – δbcFbc – δbgFbg (1.11) . . . where Mc = mass of carbon in the surficial system consisting of the atmosphere, oceans, biosphere, and soils Fwc = flux from weathering of Ca and Mg carbonates Fwg = flux from weathering of sedimentary organic matter Fmc = degassing flux for carbonates from volcanism, metamorphism, and diagenesis Fmg = degassing flux for organic matter from volcanism, metamorphism, and diagenesis Fbc = burial flux of carbonates in sediments Fbg = burial flux of organic matter in sediments δ = [(13C/12C)/(13C/12C)stnd – 1]1000. Variants of equations (1.10) and (1.11) have been treated in terms of non–steady-state modeling (e.g., Berner et al., 1983; Wallmann, 2001; Hansen and Wallmann, 2003; Mackenzie et al., 2003; Bergman et al., 2003), where the evolution of both oceanic and atmospheric composition, including Ca, Mg, and other elements in seawater, is tracked over time. However, since the purpose of this book is to discuss the carbon cycle with respect to CO2 and O2, and so as not to overburden the reader with too many mathematical expressions, I discuss only those aspects of the non–steady-state models that directly impact carbon. These are combined with results from steady-state strictly carbon-cycle modeling (Garrels and Lerman, 1984; Berner, 1991, 1994; Kump and Arthur, 1997; Francois and Godderis, 1998; Tajika, 1998; Berner and Kothavala, 2001; Kashiwagi and Shikazono, 2002).