Letteratura scientifica selezionata sul tema "Paleoclimatology - Australia"

Assigning accurate dates to hypersaline sediments opens important terrestrial records of local and regional paleoecologies and paleoclimatology. However, as of yet no conventional method of dating hypersaline systems has been widely adopted. Biomarker, mineralogical, and radiocarbon analyses of sediments and organic extracts from a shallow (13 cm) core from a hypersaline playa, Lake Tyrrell, southeastern Australia, produce a coherent age-depth curve beginning with modern microbial mats and extending to ~ 7500 cal yr BP. These analyses are furthermore used to identify and constrain the timing of the most recent change in hydrological regime at Lake Tyrrell, a shift from a clay deposit to the precipitation of evaporitic sands occurring at some time between ~ 4500 and 7000 yr. These analyses show the potential for widespread dating of hypersaline systems integrating the biomarker approach, reinforce the value of the radiocarbon content of biomarkers in understanding the flow of carbon in modern ecologies, and validate the temporal dimension of data provided by biomarkers when dating late Quaternary sediments.

The analysis of rodent middens, principally deposited by packrats (Neotoma sp), has rapidly become the most important paleoecologic and paleoclimatologic tool in the southwestern United States. The recent discovery of rodent middens created by stick-nest rats (Leporillus sp) and rock wallabies (Petrogale sp) in Australia (Green et al, 1983; P S Martin, oral commun, 1984) and by dassie rats (Petromus typicus) in South Africa (L Scott, oral commun, 1984) portends the use of midden analysis in arid regions worldwide. Several recent reviews of southwestern paleoecology (eg, Spaulding et al, 1983) rely heavily on rodent middens for ecologic and climatic reconstructions.

The taxonomical consideration, probable phylogeny and stratigraphic significance of twenty-eight middle Eocene (Bartonian) planktic foraminiferal species from the eastern limb of Jabal Hafit, Al Ain area, United Arab Emirates (UAE), Northern Oman Mountains (NOM) are presented, and twenty one of them are illustrated. Identification of these twenty-eight species belonging to ten genera Globoturborotalia, Subbotina, Globigerinatheka, Inordinatosphaera, Orbulinoides, Hantkenina, Acarinina, Morozovelloides, Pseudohastigerina and Turborotalia has led to the recognition of three biostratigraphic zones, in ascending order: Morozovelloides lehneri PRZ (E11), Orbulinoides beckmanni TRZ (E12) and Morozovelloides crassata HOZ (E13). Eight out of the identified species are recorded, in this study, for the first time from Jabal Hafit: Globoturborotalia martini, Subbotina gortanii, S. jacksonensis, S. senni, Globigerinatheca barri, Acarinina praetopilensis, A. punctocarinata and Morozovelloides bandyi. The second or third record of three species from J. Hafit outside its original records are recently documented by the present author: Inordinatosphaera indica, Hantkenina australis and H. compressa. The paleontology, paleoclimatology and paleogeographic distribution of the identified taxa at Jabal Hafit and other Paleogene outcrops in the UAE and Tethys are presented and discussed. The identified fauna emphasis the wide geographic areas in the Tethys, from Atlantic to Indian-Pacific Oceans via Mediterranean.

Abstract. Sea surface temperature (SST) reconstructions based on isoprenoid glycerol dialkyl glycerol tetraether (isoGDGT) distributions from the Eocene southwest (SW) Pacific Ocean are unequivocally warmer than can be reconciled with state-of-the-art fully coupled climate models. However, the SST signal preserved in sedimentary archives can be affected by contributions of additional isoGDGT sources. Methods now exist to identify and possibly correct for overprinting effects on the isoGDGT distribution in marine sediments. Here, we use the current proxy insights to (re-)assess the reliability of the isoGDGT-based SST signal in 69 newly analyzed and 242 reanalyzed sediments at Ocean Drilling Program (ODP) Site 1172 (East Tasman Plateau, Australia) following state-of-the-art chromatographic techniques. We compare our results with paleoenvironmental and paleoclimatologic reconstructions based on dinoflagellate cysts. The resulting ∼ 130 kyr resolution Maastrichtian–Oligocene SST record based on the TetraEther indeX of tetraethers with 86 carbon atoms (TEX86) confirms previous conclusions of anomalous warmth in the early Eocene SW Pacific and remarkably cool conditions during the mid-Paleocene. Dinocyst diversity and assemblages show a strong response to the local SST evolution, supporting the robustness of the TEX86 record. Soil-derived branched GDGTs stored in the same sediments are used to reconstruct mean annual air temperature (MAAT) of the nearby land using the Methylation index of Branched Tetraethers with 5-methyl bonds (MBT'5me) proxy. MAAT is consistently lower than SST during the early Eocene, independent of the calibration chosen. General trends in SST and MAAT are similar, except for (1) an enigmatic absence of MAAT rise during the Paleocene–Eocene Thermal Maximum and Middle Eocene Climatic Optimum, and (2) a subdued middle–late Eocene MAAT cooling relative to SST. Both dinocysts and GDGT signals suggest a mid-shelf depositional environment with strong river runoff during the Paleocene–early Eocene progressively becoming more marine thereafter. This trend reflects gradual subsidence and more pronounced wet/dry seasons in the northward-drifting Australian hinterland, which may also explain the subdued middle Eocene MAAT cooling relative to that of SST. The overall correlation between dinocyst assemblages, marine biodiversity and SST changes suggests that temperature exerted a strong influence on the surface-water ecosystem. Finally, we find support for a potential temperature control on compositional changes of branched glycerol monoalkyl glycerol tetraethers (brGMGTs) in marine sediments. It is encouraging that a critical evaluation of the GDGT signals confirms that most of the generated data are reliable. However, this also implies that the high TEX86-based SSTs for the Eocene SW Pacific and the systematic offset between absolute TEX86-based SST and MBT'5me-based MAAT estimates remain without definitive explanation.

Paleoflood data are restrictive reflections of climatic conditions, representing one component of a region's climate; high rainfall intensity storms. In regions with a fairly simplistic, uniform hydroclimatological setting (floods above a given magnitude threshold are caused by predominantly one type of atmospheric circulation pattern), the temporal distribution of floods reflects that of the causal circulation pattern. Slackwater-deposit-based paleoflood reconstructions for three rivers in northern Australia cover an aggregate of 1200 years. Slackwater deposits (SWD) are fine-grained sediments which settle from suspension in low velocity areas during floods. These deposits approximate the flood's high water level, and allow reasonably accurate estimation of discharge. Radiocarbon dating of associated organics, and thermoluminescence (TL) dating of the 90-125 μm quartz fraction of the sediments, produce a paleoflood chronology. In this study, radiocarbon ages on SWD ranged from 1200 yr BP to modern, while TL ages on SWD and other fluvial sediments ranged from 2.6 to 60 ka. TL dating appears to have a large temporal range (1-100 ka) and a restricted spatial range (the lower reaches of a basin), while radiocarbon dating has a more restricted temporal range (0-35,000 yr BP) and a large spatial range (anywhere in the basin). The northern Australian paleoflood data formed clusters at 300-440 yr BP and 160 yr BP-present. This distribution is attributed to variations in the intensity of the El Nino/Southern Oscillation (ENSO) circulation (which prevents floods from occurring in northern Australia), and the anti-ENSO circulation (which is associated with large floods).

Measurements of surface uplift rate potentially hold the key to understanding the tectonics of mountain belts and areas of uplift. Wolfe (1993) compiled a multicharacter data set (CLAMP) of the physiognomy of woody dicotyledon leaves with a view to using it to extract climatic information from fossil floras. The limited extent of this data set, along with the anomalous behaviour of some outliers cause me to question the global relationship between physiognomy and climate which has been implicitly assumed in all analyses of the data conducted to date. Additional data collected from native vegetation in New Zealand and Australia are compared to the CLAMP data set. These data include samples along altitudinal transects and from different forest types growing in the same climatic regime. In addition taphonomic samples were collected from lake bottom sediments and their physiognomic signals compared to those of the adjacent living vegetation. The possibility that the relationship between climate and physiognomy is sufficiently non-linear that only local relationships should be sought is investigated. To estimate the climate at a certain flora, resemblance functions are used to select physiognomically similar sites. Estimations of climate are formed using only these sites. The power of this approach to estimate mean annual temperature, mean annual precipitation, mean growing season precipitation, and moist enthalpy is investigated using the modern sites, and varying the number of nearest neighbours and dimensions used as well as the type of ordination. The collection of altitudinal transects has allowed the study of physiognomic change with altitude. Because these transects were collected over a very restricted area it was possible to observe this change without the superimposed effects of changing continentality and variation in latitude. In addition studies made of adjacent floras in similar climatic regimes and taphonomic studies of leaves in lacustrine sediments has allowed the beginning of a realistic assessment of possible errors in climate estimation for fossil sites. Fossil sites examined using CLAMP and related methods are re-examined using the nearest neighbour approach.

It is widely believed that Australian Aboriginals utilised fire to manage various landscapes however to what extent this impacted on Australia???s ecosystems remains uncertain. The late Pleistocene/Holocene fire history from three sites within the Sydney Basin, Gooches Swamp, Lake Baraba and Kings Waterhole, were compared with archaeological and palaeoclimatic data using a novel method of quantifying macroscopic charcoal, which is presented in this study. The palynology and other palaeoecological proxies were also investigated at the three sites. The Gooches Swamp fire record appeared to be most influenced by climate and there was an abrupt increase in fire activity from the mid-Holocene perhaps associated with the onset of modern El Ni??o dominated conditions. The Kings Waterhole site also displayed an abrupt increase in charcoal at this time however there was a marked decrease in charcoal from ~3 ka. Lake Baraba similarly had displayed low levels of charcoal in the late Holocene. At both Kings Waterhole and Lake Baraba archaeological evidence suggests intensified human activity in the late Holocene during this period of lower and less variable charcoal. It is hence likely that at these sites Aboriginal people controlled fire activity in the late Holocene perhaps in response to the increased risk of large intense fires under an ENSO-dominated climate. The fire history of the Sydney Basin varies temporally and spatially and therefore it is not possible to make generalisations about pre-historic fire regimes. It is also not possible to use ideas about Aboriginal fire regimes or pre-historic activity as a management objective. The study demonstrates that increased fire activity is related to climatic variation and this is likely to be of significance under various enhanced Greenhouse scenarios. There were no major changes in the composition of the flora at all sites throughout late Pleistocene/Holocene although there were some changes in the relative abundance of different taxa. It is suggested that the Sydney Sandstone flora, which surrounds the sites, is relatively resistant to environmental changes. Casuarinaceae was present at Lake Baraba during the Last Glacial Maximum and therefore the site may have acted as a potential refugium for more mesic communities. There was a notable decline in Casuarinaceae during the Holocene at Lake Baraba and Kings Waterhole, a trend that has been found at a number of sites from southeastern Australia.

Thesis (Ph.D.) - James Cook University, 2004.
"Met een samenvatting in het Nederlands" -- T.p. "Ter verkrijging van der graad van doctor aan de Universiteit Utrecht, op gezang van de Rector Magnificus, Prof. Dr. W. H. Gispen, ingevolge het besluit van het College voor Promoties in het openbaar te verdedigen op donderdag 30 septembber 2004 des morgens om 10:30 uur" -- T.p. Typescript (photocopy) Bibliography: leaves 175-181.

Scope and outline of the thesis: With the aim set for this study, the European sequence chosen as reference for global correlation is the Barremian type section at Angles, SE France (Birkelund et al., 1984). Various studies on this section, which contains a Barremian-Aptian transition interval, have already described its ammonite stratigraphy (Delanoy, 1995), δ13C isotope stratigraphy (Wissler et al., 2002), and dinocyst content (e.g. De Renéville and Raynaud, 1981; Srivastava, 1984; Wilpshaar, 1995). A corresponding stratigraphic interval was sampled at Deep Sea Drilling Project (DSDP) site 263 located on the NW Australian continental margin. To test the established bio-stratigraphic framework for Australia, further drillholes were chosen from two other depositional basins, i.e., from the Carpentaria Basin the Bureau of Mineral Resources (BMR) Mossman-1 core, and from the Eromanga Basin: Geological Survey of Queensland (GSQ) Hughenden-7, GSQ Manuka-1, and GSQ Eromanga-1, drillcores. In Chapter 2 the direct correlation between an Australian Barremian-Aptian marine section with European ammonite controlled standard successions, based on corresponding stratigraphic events in the compositional development of the dinoflagellate assemblages is presented. It outlines the correlation between the Barremian-Aptian part of DSDP site 263 in NW Australia, and the equivalent interval in the Barremian stratotype section at Angles, SE France. The correlation provides a suit of nine dinoflagellate cyst (dinocyst) index events, which are useful for cross- hemisphere comparisons. These dinocyst events further enable age-assessment for the Australian dinocyst zonation scheme traditionally used for Austral biostratigraphic correlation. Palaeoenvironmental evaluation of the dinoflagellate cyst assemblages in DSDP site 263 indicates that the NW Australian margin was affected by cool water-currents during the Barremian-Aptian. In Chapter 3 the established timeframe from chapter 2 is applied to BMR Mossman-1 located in northeastern Australia. Comparison of the palynostratigraphic framework, elaborated with δ13Corg data, with time equivalent Tethyan and Atlantic records shows identical bio- and geochemical stratigraphic changes and reveals an interval in BMR Mossman-1, which most likely corresponds to oceanic anoxic event (OAE) 1a. Palaeoenvironmental reconstructions are based on groups distinguished within the dinocyst and spore/pollen assemblages. These groups indicate warmer and wetter conditions prior to OAE 1a changing to cooler and drier conditions starting at the onset of OAE 1a. The δ13Corg results add new detail to global stratigraphic changes in Barremian-Aptian carbon isotope records and confirm the assumption that OAE 1a was of global extent. In comparison with the global sea-level chart of Hardenbol et al. (1998), the changes in palynology, carbon isotope stratigraphy, and lithology allow identification of variations within a third order cycle. In Chapter 4 the dinoflagellate cyst zonation as presented in chapter 2 and 3 is further tested by applying it to three cores from the Eromanga Basin in eastern Australia; GSQ Hughenden-7, GSQ Manuka-1, and GSQ Eromanga-1, respectively. The dinocyst events combined with the δ13Corg records inferred an Early Aptian to Albian age for the studied sections. The changes in palynology, isotope stratigraphy and lithology are deduced to reflect third order sea-level fluctuations. Comparison of the data with the BMR Mossman-1 core from the Carpentaria Basin reveals that the start of the mid-Cretaceous marine incursion into the Eromanga Basin is related with the earliest Aptian sea-level rise. Further comparison with time-equivalent Tethyan records shows that the Toolebuc Formation relates to OAE 1c and correlates with the inflexion point of a relative maximum sea-level fall in the earliest Late Albian. The same palaeoenvironmental groups as in the previous chapter have been used, they indicate cooler and drier conditions at the onset, changing to warmer and more humid conditions within the upper part of the Toolebuc Formation. In Chapter 5 comparison of Barremian to Albian sequences from the three Early Cretaceous depositional basins in Australia show similar variations within the δ13Corg stratigraphy as in timeequivalent Atlantic and Pacific sequences. The observed excursions in the isotope records from the European and Australian sequences can be correlated by biostratigraphic events. For the Barremian-Albian time interval eleven successive dinocyst events proof to be useful for global comparisons. Correlation of the selected results reveals similar patterns and responses in the δ13Corg records for the Tethyan, Boreal and Austral realms, and that two periods of oceanic anoxic events, representing OAE 1a and 1c, can be recognized in Australia as well. Furthermore, it enabled recognition of mid-Cretaceous stage and substage boundaries in Australia, and as such provides age assessment for the Australian dinocyst zonation scheme. The similar patterns in the carbon isotope records, and the response of the palynological assemblages from Australia and other areas to palaeoenvironmental and climatic changes during the mid-Cretaceous indicate that OAE 1a and 1c were of global extent. The first anoxic event lies within the transgressive systems tract of cycle T13, and the latter correlates to the inflexion point of the regressive R14 and following transgressive systems tract of cycle T15, indicating that the underlying driving forces for the various OAEs differ.