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Earth Environment integrates a group of leading researchers and postgraduate students who have research interests in environmental change and the long-term interaction between mankind and its environment. Research is directed toward understanding globally significant processes within the themes:
climate and sea-level change
human evolution and migration
soil production and erosion
sediment and nutrient fluxes to waterways and the oceans.
Earth Environment focuses on determining the timing and rate of change of major environmental and earth surface processes, and using trace element and isotopic systems to constrain the nature of past and present environments. Emphasis is given to developing detailed records that span a few tens to several hundred thousand years of the Earth's recent history. The results are used as a basis for understanding past and present environmental change with the ultimate aim to predicting future environments. Earth Environment specialises in the reconstruction of high-resolution environmental records from growth bands preserved in fossil and modern corals, speleothems (cave deposits), and layered sedimentary deposits.
Earth Environment research activities are underpinned by world-class laboratory facilities that enable analysis of virtually any trace element or isotope system, including
radiogenic isotopes (U-series, U-Pb, Rb-Sr, Sm-Nd, Lu-Hf)
light stable isotopes (Li, B, C, N, O)
heavy stable isotopes (Mg, Ca, Fe, Cu, Zn, Mo, etc.)
cosmogenic isotopes (Be-10, C-14, Al-26, Cl-36).
The instrumentation base includes thermal ionisation mass spectrometry (TIMS), Excimer laser ablation combined with multi-collector sector-ICPMS and quadrupole-ICPMS, dual inlet and continuous flow gas-source stable isotope mass spectrometry (DI-CF-IRMS), optically stimulated luminescence (OSL), electron spin resonance (ESR), radiocarbon and gamma ray counting equipment, palaeomagnetic laboratory, as well as clean room and sample preparation facilities. Accelerator mass spectrometry (AMS) and charge collection TIMS are accessed through the neighbouring Research School of Physical Sciences and Engineering.
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Our research is led by the academic staff members Prof. J. Chappell, Dr S. Eggins, Dr T. Esat, Dr D. Fabel, Dr M. Gagan, Prof. R. Grün (Area Coordinator), Prof. M. McCulloch, Dr B. Pillans and Dr E. Rhodes. The research endeavour is greatly enhanced by our research associates and postdoctoral fellows Dr C. Alibert, Dr J. Desmarchelier, Dr G. Dunbar, Dr J. Marshall, and Dr C. Pelejero, as well as our academic visitors Dr E. Calvo, Dr J. Jia, Dr P. Montagna, Dr W. Müller, Dr A. Pike, Ms R. Pickering, Dr. B. Roark, and Dr J. Wynn. Much of our research is carried out by our dedicated PhD students: Ms N. Abram, Ms B. Ayling, Ms R. Berdin, Ms K. Dowell, Ms R. Fraser, Mr T. Fujioka, Ms E. Hendy, Ms K. Lilly, Mr I. McCulloch, Ms H. McGregor, Mr D. Qu, Mr M. Smith, Ms J. Trotter, Mr T. Wyndham and Mr Y. Zhou.
Earth Environment has been highly successful in procuring government ARC grants. In 2003, Prof. J. Chappell was the leading investigator of two large ARC grants, with Dr T. Esat on Millenial-scale instability of sea level and the climate system: new analysis of coral terraces in Papua New Guinea and with Dr M. Honda, Dr D. Fabel and Dr K. Fifield on Production and transport of soil and sediments, determined by cosmogenic radionuclides and noble gases. Dr M. Gagan was chief investigator of Quantifying the El Niño-Indian Ocean Dipole system using high-resolution coral palaeoclimate archives with W. Hantoro (Indonesian Institute of Sciences), J. Lough (AIMS), G. Meyers (CSIRO) and G. Dunbar. Prof. R. Grün was the leading investigator on Stable isotopes in marsupials: reconstruction of environmental change in Australia with Dr M. Gagan, Dr D. Bowman (NTU) and Dr R. Wells (Flinders). Prof. M. McCulloch led two ARC projects, with Dr Hearty (JCU) and Prof Halliday (ETH) on Sea levels, sea surface temperatures and El Nino variability during warm interglacials and jointly with Dr Lough (Australian Institute of Marine Sciences) on The coral record and environmental impacts in the Great Barrier Reef: quantification of anthropogenic fluxes. Dr C. Pelejero is an ARC APD fellow on Uptake of atmospheric CO2 in the oceans and implications for global change: New proxy developments.
In 2003, Dr E. Calvo was awarded an ARC APD fellowship with the grant The key role of the Southern Ocean in atmospheric CO2 sequestration, and Dr S.M. Eggins was co-Chief Investigator on newly awarded ARC Discovery-Project Uncoupling past salinity and temperature signals in the Indo-Pacific Warm Pool: implications for climate change in the Australian region with Prof. P. DeDeckker (Geology).
Several staff members, Prof. Chappell, Drs Fabel, Pillans and Rhodes, and students, Mr Bernal and Mr Smith, are members of the Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME).
During 2003 there have been a number of personnel changes. We are pleased to welcome Dr E. Rhodes (formerly University of Oxford) to lead the luminescence laboratory. Dr. G. Dunbar (formerly of Victoria University of Wellington) joined the area as Research Associate on the ARC Discovery grant: Quantifying the El Niño-Indian Ocean Dipole system using high-resolution coral palaeoclimate archives. Ms Jo Lo Presti also arrived to provide support as the new Area Administrator. During 2003 several staff members left to take on new responsibilities, Mrs P Delatorre at the ACT government and Mr A. Alimanovic at the University of Melbourne. We thank them for their dedicated work and wish them all the best for their future.
Earth Environment attracted four new PhD students in 2003: Ms S. Burgess will work on the geochemistry of Plesiastrea versipora, Ms K. Dowell on the formation of opals, Ms K. Lilley on constraints on glacial history of East Antarctica from cosmogenic dating and isostatic modeling, Ms R. Berdin Late Quaternary Climate and Typhoon Variability in the Northern Indo-Pacific Warm Pool Reconstructed from Raised Coral Reefs of the Philippines and Mr T. Fujioka on applying cosmogenic noble gases to geological processes at the Earth's surface.
The following students finished their PhDs: Mr Juan Pablo Bernal-Uruchurtu on “In situ measurement of U-series disequilibria in iron oxy/hydroxides and its application to weathering geochronology”. Dr Bernal returned to his native Mexico to work in the Geological Survey. Ms Erica Hendy submitted “Coral Reconstructions of Decadal-to-Centennial Climate Variability in the Great Barrier Reef Since 1565 AD”. Dr Hendy started a postdoctoral position at the Lamont Doherty Geological Observatory in New York. Ms Helen McGregor’s thesis was entitled “Coral Records of Climate Variability in the West-Pacific Warm Pool”. Dr McGregor took on a postdoctoral position at the Universität Bremen. Mr Paul Rustomji submitted “Holocene geomorphology of the MacDonald and Tuross Rivers” and Ms Pauline Treble “Palaeoclimate records constructed from southern Australian modern, Holocene and Pleistocene speleothems”. Drs Rustomji and Treble moved to the US to work at UCLA.
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The highlights of 2003 were Wolfgang Müller’s paper in Science on “The origin and migration of the Alpine Iceman”, presenting a reconstruction of the life of Ötzi which caused world-wide media attention and Malcolm McCulloch’s Nature paper “Coral Record of increased sediment flux to the inner Great Barrier Reef since European settlement” which threw the scientific cat amongst Queensland’s sugar cane farmer pigeons causing a very public, very heated debate. The quality of our research is documented by numerous publications in the leading earth sciences journals including Nature, Science, Earth and Planetary Science Letters, Geochimica et Cosmochimica Acta, Geology, Geophysical Research Letters, and Quaternary Science Reviews.
Our other PhD students and recent graduates also published in leading international journals, with outstanding papers by Nerilie Abram in Science on “Coral reef death during the 1997 Indian Ocean Dipole linked to Indonesian wildfires”; Helen McGregor in Geochimica et Cosmochimica Acta on “Diagenesis and geochemistry of Porites corals from Papua New Guinea: implications for paleoclimate reconstruction”; Pauline Treble in Earth and Planetary Science Letters on “Comparison of high resolution sub-annual records of trace elements in a modern (19111992) speleothem with instrumental climate data from southwest Australia”.
The breadth of the research activities of Earth Environment are documented in the projects detailed in the annual report. Involving many colleagues and Earth Environment staff members and colleagues in Australia and overseas, Nerilie Abram worked on “Holocene variability of the Indian Dipole” ; Bridget Ayling on “Modern and palaeoclimatic conditions at Oeno Atoll and Henderson Island, SE Pacific: geochemical signatures of environmental conditions and diagenetic history”; Samantha Burgess on “Plesiastrea versipora: hermatypic coral at its southern latitudinal limit”; Eva Calvo and Carlos Pelejero on “Marine Isotopic Stage 5e in the Southwest Pacific: Similarities with Antarctica and ENSO inferences” and “280-year long Sr/Ca and δ18O records from Flinders Reef, western Coral Sea”, John Chappell on “Contrasting regimes of soil production”; Stephen Eggins on “Daily banding and modulation of Mg/Ca in foraminiferal calcite by symbiont photosynthesis and respiration”; Tezer Esat on “Accurate dating of rapid rises of sea level in the last glacial cycle”; Derek Fabel on “Basal thermal regimes of former ice sheets constraints using cosmogenic nuclides”,“Estimating neotectonic movement in southern Victoria using cosmogenic burial dating” and “Tracing the post-Younger Dryas retreat of the northern Fennoscandian Ice Sheet using cosmogenic radionuclide exposure ages”; Toshi Fujioka on “Extremely low erosion rates at the gibber plain in South Australia;Rebecca Fraser on “Stable isotope analysis of bone collagen and tooth enamel of Australian marsupial faunas: a baseline study investigating the implications for palaeodiet and palaeo-environmental reconstruction”; Michael Gagan on “Coral records of surface-ocean evaporation: the “other half” of the hydrological cycle”; N.S Grumut on “Coral radiocarbon records of Indian Ocean water mass mixing and wind-induced upwelling along the coast of Sumatra, Indonesia”;Yiefei Jia on “Thorium/Uranium systematics of Precambrian deep-sea pelagic black shales: implications for redox state of the early atmosphere” and “15N-enriched Archean crust or 15N-depleted crust recycled into -6 ‰ upper Mantle?”; John Marshall on “Coral Ba/Ca Record of Runoff from the Fitzroy River”; Malcolm McCulloch on “Impact of European Settlement on Sediment and Freshwater Runoff into the Inner Great Barrier Reef” and “Geochemical Records of Bleaching Events from the Great Barrier Reef”; Helen McGregor on “Coral *18O records of late Holocene amplification of the El Niño-Southern Oscillation”; Paolo Montagna on “Climate variability of the last millennium revealed by high-resolution stable isotope record in an Italian speleothem (NW Sardinia)”; Andrew Moy on “Late Pleistocene palaeoceanographic and geochemical evolution of the South Tasman Rise”
Wolfgang Müller on “In-situ trace elemental and Sr isotopic analysis of teeth by LA-ICPMS”, Alistair Pike on “In-situ uranium series dating of the Omo Kibish I human”; Brad Pillans on “Revised ages for Miocene hominid-bearing sediments at Haritalyangar, Indian Siwaliks”;Dingchuang Quo on “A seven-year regulation of the El Niño-Southern Oscillation 3,700 years ago”; Roark on “Geochemical Records of Bleaching Events from the Great Barrier Reef”; Martin Smith on “Geochronology of Long-Term Landscape Evolution, NW NSW”; Julie Trotter on “Conodont Geochemistry proxies for understanding palaeoenvironments, bioevents and geoevents of the Palaeozoic”; Timothy Wyndham on “Biogeochemical cycling of trace metals in coastal seawater”; and Jonathan Wynn on “Soil organic carbon and carbon isotope inventories of the Australian Continent”.
None of the scientific endeavours could be carried out without the active involvement of our excellent support staff: Mr A. Alimanovic (radiocarbon, cosmogenic isotopes), Mr J. Cali (stable isotope instrumentation), Mrs J. Cowley (stable isotope measurement), Mr D. Edwards (palaeomagnetism), Mr N. Hill (luminescence), Mr D. Kelleher (radiocarbon, fieldwork management), Mr L. Kinsley (mass spectrometry instrumentation), Ms J. Lo Presti (area administrator), Dr G Mortimer (chemistry and mass spectrometry), Mr S. Robertson (IT, ESR), Mrs H. Scott-Gagan (stable isotope measurement), and Mrs L. Taylor (ICP-MS, U-series chemistry).
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Holocene variability of the Indian Ocean Dipole
N.J. Abram and M.K. Gagan
The Indian Ocean Dipole (IOD) is an ocean-atmosphere climate oscillation that has profound effects on rainfall throughout the Indian Ocean region. It is believed that IOD events can be triggered by anomalously strong Asian summer monsoon conditions and/or strong El Niño events. Detailed knowledge of these interactions is generally limited to recent decades and it is not clear how the IOD will be affected by projected future changes in monsoon or El Niño strength due to greenhouse warming. In order to develop an improved understanding of the long-term interactions of the IOD with the monsoon and El Niño systems we used modern and fossil corals from the Mentawai Islands (southwest Sumatra, Indonesia) to reconstruct eastern IOD upwelling events over the past 6000 years.
Four strong IOD upwelling events were reconstructed over the 140-year period from 1858-1998. These events are characterised by strong cool anomalies in coral Sr/Ca, along with dry anomalies in coral residual d18O. Similar cool/dry periods were reconstructed in corals dated at 1940, 4410, 4440 and 6340 cal yBP. These palaeo IOD events appear to have been particularly frequent in the ~4400 cal yBP corals, with strong events identified in at least three of the twenty years of coral record, and weaker upwelling apparent in many of the other years. To further examine the similarities/differences between IODs in the past and present, composite events were created using the modern and fossil coral IOD records (Fig. 1). IOD cooling has a similar timing in the annual cycle in the modern and palaeo composites, however the palaeo upwellings appear to have been more intense and lasted approximately two months longer than the modern IOD upwellings. Both composites also record dry conditions during IOD events, however the strength and duration of drying are again greater in the palaeo composite.
The Mentawai coral records clearly show that the IOD has been a persistent feature of Indian Ocean climate since at least the mid-Holocene. The consistent timing of the IOD within the annual cycle suggests that the seasonally reversing wind fields of the Asian monsoon have been a critical factor in driving IOD upwelling throughout this period. The influence of the Asian monsoon may also account for the enhanced strength and duration of palaeo IODs as it is well documented that the Asian summer monsoon was stronger than present during the early-middle Holocene. The monsoon was at its greatest strength around 9000 yBP and decreased progressively throughout the Holocene, however the Mentawai corals suggest that there was a maximum in IOD activity around 4400 yBP. This maximum may be due to a combination of IOD triggers, with the increasing frequency of strong El Niño events after ~6000 yBP together with the stronger monsoon producing optimum IOD conditions at around 4400 yBP.
We conclude that the Holocene variability in the IOD system recorded by the Mentawai corals appears to have been related to synergistic variations in the strength of the monsoon and El Nino systems. Given the predictions of greenhouse-gas driven increases in Asian summer monsoon strength, it is likely that IOD events will become more common and more intense over the coming century.
Figure 1. Composite coral records of the IOD. Composite modern and palaeo IOD events were reconstructed by averaging the coral Sr/Ca SST (circles) and residual δ18O (grey bars) anomaly records of four modern and five palaeo IOD upwelling events. Error bars show the standard error for the composite records. X-axis scale refers to the number of years before or after peak IOD cooling.
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Modern and palaeoclimatic conditions at Oeno Atoll and Henderson Island, SE Pacific: Geochemical signatures of environmental conditions and diagenetic history
B. Ayling, M.T. McCulloch and M.K. Gagan
This study is aimed at reconstructing climatic conditions during Marine Isotope Stage 9 (MIS 9), an interglacial period spanning 320-300ka. In particular, high resolution data illustrating seasonal processes from MIS 9 are sought, as such records are of particular interest to paleoclimatologists but none yet exist for any interglacial prior to MIS 5e. This reflects both (a) the lack of suitable fossil coral to work on due to limited existence of paleoreefs older than MIS 5e, and (b) post mortem effects of diagenesis on coral, resulting in degradation of primary skeletal geochemistry. In this study fossil Porites and Tridacna are being used to reconstruct records of recent and paleo sea surface temperature (SST) and hydrologic balance.
A 5 week field expedition to Oeno Atoll and Henderson Island, SE Pacific was completed during February and March 2003 to collect modern and fossil samples, and a range of species of fossil coral (Porites, Montastrea and Favid) believed to be of MIS 9 age (based on previous U-series analyses) were recovered. A modern Porites core was also sampled from Oeno Atoll, 180km NW of Henderson Island. These fossil carbonates have been the subject of careful screening for diagenetic effects prior to stable isotope and trace element analysis. Initial examination of the fossil Porites for diagenesis revealed excellent preservation of primary porosity, with calcite cementation absent (see Figure 1d). The modern Porities coral is displayed in figures 1a c for comparison. Limited dissolution has affected the fossil coral as evident by creation of void space in the calcification centres (Figure 1e, 1g), etched pore walls (Figure 1f) and the creation of microporosity in the bulk skeleton (see Figure 1h).
Figure 1. (a) Modern Porites petrographic thin section (plane polarized light). Arrow indicates dissepiments. (b, c) Modern Porites SEM images, arrows indicate dissepiments. (d) Fossil Porites petrographic thin section (plane polarized light). Arrow indicates discoloured centres of calcification where dissolution has occurred. (e,f) Fossil Porites SEM images, arrows indicate altered centres of calcification. (g) Fossil Porites backscattered electron image, illustrating radial aragonite crystals around centre of calcification (indicated by arrow). (h) Fossil Porites BE image illustrating microporosity of bulk coral skeleton.
Preliminary stable isotope results suggest a 0.2‰ difference in δ18O between the modern and fossil Porites, equating to less than 1°C difference in temperature between MIS 9 and present climate (see Figure 2). Skeletal Sr/Ca is more sensitive to diagenesis than δ18O, and initial TIMS results indicate a mean offset of 0.5 mmol/mol between the modern and fossil Porites, which is equivalent to a decrease of ~10°C (see Figure 2) and is inconsistent with the δ18O data. The fossil Porites Sr/Ca record could be the product of diagenetic effects, differences in temperature between modern and MIS 9 climate, or possible changes in oceanic Sr/Ca over interglacial/glacial timescales. Trace element geochemistry on multiple Porites samples affected by varying degrees of diagenesis is being investigated to resolve this problem.
Figure 2. Sr/Ca and δ18O records for modern and fossil Porites.
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Plesiastrea versipora: Hermatypic coral at its southern latitudinal limit
Corals growing in high latitude waters are sensitive to changes in climate, especially seasonal fluctuations in sea surface temperature. In the cool-water, high-energy environments of South Australia scleractinian corals record variability of temperature, salinity and ocean circulation in the Southern Ocean. Scleractinia are typically stenotypic organisms with distributions limited by relatively minor fluctuations in environmental variables. Plesiastrea versipora (Lamarck, 1816) is a unique species of scleractinia, because it occurs around the entire Australian coastline, which suggests it tolerates a wider range of climatic variation. In high latitudes, P. versipora occurs in most encrusting reef systems, with colonies of massive habit up to 3 m in diameter and growth rates of 2-4 mm per year. Therefore, individual colonies may prove to be a good sentinel organism for changes in environmental conditions in southern Australia and by extension the Southern Ocean on centennial timescales. Fieldwork was conducted in collaboration with the Great Australian Bight and Shelf Seas Program within the South Australian Research and Development Institute (SARDI). Core samples of up to 500 mm were obtained from two regions, including Spencer Gulf and Gulf St Vincent, South Australia (Figure 1). Laser ablation-inductively coupled plasma mass spectrometry (LA-ICP MS) will be used to measure climatic tracers including temperature and upwelling proxies from high-latitude corals. Skeletal extension rates of P. versipora will be compared for intra- and inter-annual differences in environmental conditions between the cool, nutrient-enriched shelf waters and the warm, low-nutrient gulf waters.
Figure 1: A Plesiastrea versipora colony at Seacliff reef, South Australia and extracting a sample core.
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280-year long Sr/Ca and d18O records from Flinders Reef, western Coral Sea
E. Calvo, J.F. Marshall, C. Pelejero, M.T. McCulloch, J.M. Lough1 and M.K. Gagan
The combination of parallel Sr/Ca and d18O records in corals allows reconstruction of past changes in sea surface temperature (SST) and seawater d18O composition (McCulloch et al., 1994). The latter provides climatic information related to changes in the hydrologic cycle and can be interpreted as a salinity proxy. Since the d18O signal is affected by both SST and seawater isotopic composition, a salinity record can be obtained by removing the temperature signal using a parallel Sr/Ca record, a proxy for SST, obtained from the same coral. Low resolution (5-year intervals) Sr/Ca and d18O analyses, going back to 1710 AD, have been performed on a Porites coral core collected from Flinders Reef, an offshore reef on the Queensland Plateau (17ºS, 149ºE), 250 km from the north-east coast of Australia. For the last 280 years, the preliminary Sr/Ca-SST record shows an increasing long-term trend towards the warm temperatures recorded during 1990, when the coral was collected. An increasing trend towards more negative d18O values (warmer and/or less saline conditions) is also observed in the isotopic record, which also reflects the 20th century warming. Despite this general common trend, interdecadal variability differences between both records suggest that temperature alone cannot explain the d18O changes observed in this site of the Coral Sea. A freshening of surface waters after 1870 has recently been reported from coral cores collected from the inshore region in the Great Barrier Reef and interpreted as indicating a weakening in trade winds and ocean circulation (Hendy, 2002). In the Flinders coral, however, an apparent freshening occurs in the early 1800s, followed by a subsequent transition to more saline conditions during the first half of the 20th century.
Hendy, E. J. et al. Science 295, 1511-1514 (2002)
McCulloch, M.T. et al. Geochimica at Cosmochimica Acta, 58, 2747-2754 (1994)
1 Australian Institute of Marine Science, Townsville, Australia
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Contrasting regimes of soil production
J. Chappell, E. Rhodes, L.K. Fifield1, N. Spooner2, A. Heimsath3, and B.J. Pillans
The evaluation of long-term landscape processes using cosmogenic nuclides has focused on contrasting regimes of soil production in agricultural landscapes in southeast NSW and southwest Western Australia. Previous determinations of soil production on soil-mantled granite hillslopes in the Bega Valley, NSW, found that the production rate ranges from about 10 to 50 mm per thousand years, and decreases as soil depth increases. These measurements were based on the concentrations of 10Be produced by cosmic rays in quartz contained in near-surface rocks. We recently confirmed these results at other sites in the Bega Valley. However, on applying the same methods in soil-mantled granite terrain near Moora, WA, unexpectedly we found production rates of only 2-3 mm per thousand years: apparently, 10 times slower than in eastern NSW.
The origin of this paradox lies in the impacts of past episodes of aridity, which occurred repeatedly in WA throughout the Quaternary. The soils at the sites near Moora were formed largely from sand that was repeatedly remobilised: cosmogenic 10Be indicates that alternation between soil- and sand-mantles has occurred throughout at least the last million years. To understand the history and to determine rates of soil regeneration at the end of each arid, sandy phase, we are now combining measurements of cosmogenic nuclides with optical dating of sand particles within the soil profiles.
2 Formerly RSES
3 Dartmouth College, Hanover NH, USA.
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Daily banding and modulation of Mg/Ca in foraminiferal calcite by symbiont photosynthesis and respiration
S. Eggins, A. Sadekov1, and P. De Deckker2
The Mg/Ca composition of tiny, calcium carbonate tests (shells) that are secreted by planktonic foraminifera, is widely assumed to be controlled by seawater temperature. This assumption underpins the increasing use of the bulk Mg/Ca composition of fossil foraminifera tests to estimate past seawater temperatures and to reconstruct palaocean and climate records spanning hundreds of thousands of years. Using two high-resolution microanalysis techniques (electron microprobe mapping and laser ablation-ICPMS depth profiling), we show that tests of the planktonic foraminifera Orbulina universa comprise mulitple low- and high-Mg growth bands see Figure 1. The number of paired bands is typically between 3 and 6, which is consistent with the number of days reported between final chamber formation and gametogenesis in laboratory culture experiments, and thus with a diurnal origin. The amplitude of Mg/Ca variation within individual tests and across many daily growth bands cannot be accounted for by the limited range of seawater temperature in the shallow, euphotic-zone habitat (0-100m depth) of O. universa. We propose that Mg/Ca composition of calcite precipitated by O. universa is modulated by changes in pH, due to the day-night, photosynthesis-respiration cycle of algal symbionts. These results indicate that the Mg/Ca composition of calcite precipitated by O. universa is sensitive to influences that modify the biological activity of foraminifera and their algal symbionts. The results challenge a fundamental assumption of Mg/Ca palaeo-seawater thermometry that the Mg/Ca composition of foraminiferal calcite is solely determined by seawater temperature.
1 Department of Palaeontology, Moscow State University, Moscow, Russia
2 Department of Geology, Australian National University
Figure 1. Mg/Ca intensity map of O. universa chamber wall cross-section measured by electron microprobe (Cameca SX100) and (2) Mg/Ca composition depth profile measured through the same chamber wall by laser ablation-ICPMS.
Accurate dating of rapid rises of sea level in the last glacial cycle
T. Esat, J. Chappell, J. Desmarchelier, L.K. Fifield1 & M.T. McCulloch
Northern hemisphere climates switched repeatedly and abruptly between cold and warm states during the ice ages. This now widely-known but unexplained finding poses uncertainties about future climate. A key to this problem is the exact chronology of past sea level and climatic changes, which we aim to establish by improved dating of millenial-scale sea level changes during the last 100,000 years that are recorded in coral terraces at Huon Peninsula, PNG.
We have focused on precise dating of rapid sea level excursions that occurred between 30,000 and 65,000 years ago. U-series disequilibrium ages of some 50 previously-dated samples from key stratigraphic points were remeasured using ICP-multicollector mass spectrometry. New methods were established for identifying subtle micrometre-scale phenomena that falsify ages, using electron microscopy, laser-ablation ICP mass spectrometry, and electron microprobe trace element mapping. The new evaluation methods were combined with age remeasurements by U-series and Accelerator Mass Spectrometry (AMS) radiocarbon, and precise sets of ages were established from existing samples for two key sea level rises (32.6 and 37.8 ka). Advances overseas with the chronology of abrupt climatic shifts in Greenland ice cores and north Atlantic marine sea records indicates that the rise at 37.8 ka corresponds to the major Heinrich-4 ice break-out.
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Basal thermal regimes of former ice sheets constrained using cosmogenic nuclides
D. Fabel, J.Harbor1, A. Stroeven2, C. Clarhäl2, J. Kleman2, D. Elmore3 and D. Fink4
The motion of ice sheets occurs as a result of internal deformation and basal sliding, which includes direct sliding of ice over its substrate, internal deformation in the substrate, and enhanced deformation in basal ice. Basal sliding is ineffective where basal ice is below the pressure melting point (cold based, or frozen bed conditions) and most effective where basal ice is at the pressure melting point (warm based, or thawed bed conditions), so that water is present. Reconstructions of the last glacial maximum Fennoscandian and Laurentide ice sheets are extremely sensitive to assumed basal thermal patterns, resulting in estimates of ice thickness that vary on the order of 1 km.
Recognition of subglacial boundaries between sliding and frozen-bed areas for former ice sheets is typically based on distinct morphological contrasts between areas with glacial landform assemblages and relict areas showing little alteration of pre-existing features. However some of these boundaries, especially on continental shield areas, are clearly visible from air photos but have minimal topographic expression (Fig. 1). Understanding the chronology and erosional development of such boundaries is important to provide insight into the pattern and persistence of basal conditions under ice sheets.
Geomorphic evidence and cosmogenic radionuclide concentrations of bedrock outcrops on either side of sliding boundaries on low-relief upland plateaus in northern Sweden (Fig. 1) are consistent with negligible erosion in relict landscape (frozen bed) areas due to the last glaciation, but also indicate very limited erosion in the sliding areas. This pattern and magnitude of landscape modification indicates that sliding was short lived in these areas, likely as a transient phase during deglaciation. These sites demonstrate that short periods of sliding are in some cases sufficient to produce landscapes that are recognized as ‘glacial’ from air photos.
Glacial landforms attributed to sliding in areas formerly covered by ice sheets must be viewed as the cumulative total area that has experienced sliding at any time during a glaciation. At this stage it is not possible to determine during which phase of ice sheet glaciation the sliding occurs, but it is unlikely to happen simultaneously in all sliding areas. Therefore, the actual extent of sliding areas during different ice sheet phases is presumably considerably less than the cumulative total area, which has important implications for establishing appropriate basal boundary conditions for ice sheet reconstructions at different times during a glaciation.
1 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana, U.S.A.
1 Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden
3 Department of Physics, Purdue University, West Lafayette, Indiana, U.S.A.
4 AMS-ANTARES, Environment Division, ANSTO, RMB 1, Menai, NSW 2234, Australia.
Fig. 1. Infra-red air photo (a), geomorphological map (b) and oblique airphoto (c) of Mt. Tjuolmma and Juovvakielas on Ultevis. The oblique air photo (c) is taken from the point shown by the eye symbol in (b). Pre-Late Weichselian lateral meltwater channels are crosscut by till lineations and the lateral sliding boundary. The channels can be traced on the glacially eroded side of the boundary as water-filled depressions aligned with the down-stream continuation of the lateral channels. The lateral sliding boundary itself appears as a sharp feature on air photos and terminates in transverse lee-side scarps. Sample locations are marked by open squares (erratic samples) and closed squares (bedrock samples). 10Be and 26Al (italics) apparent exposure ages with uncertainties are given in thousands of years. An asterisk indicates a weighted mean age.
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Estimating neotectonic movement in southern Victoria using cosmogenic burial dating
The aim of this project is to determine the age of several tectonically displaced sedimentary deposits in the Cape Liptrap area of southern Victoria using cosmogenic burial dating.
The geomorphology of this region testifies to the profound influence of faulting in shaping the landscape. While quantitative constraints on the displacement history of these faults are not yet available, dramatic evidence for recent movement is indicated by kink-bands in ~125,000 year old cemented dune limestone near Cape Liptrap. There are no numerical ages for tectonically offset older sedimentary deposits that could provide further constraints on the age and rate of neotectonic movement. We are using a relatively new chronological technique, cosmogenic burial dating, utilising the radioactive decay of cosmic-ray produced Be-10 and Al-26 in buried quartz, to estimate these ages. Based on the differential decay of the in-situ produced cosmogenic nuclides Be-10 (radioactive half-life = 1.51 ± 0.03 Ma) and Al-26 (0.705 ± 0.02 Ma) in quartz, the technique is widely applicable to dating up to approximately 5 million year old quartz-rich sediments. The ratio Al-26/Be-10 in quartz is dominated by the nuclide production rate ratio for all but the very slowest erosion rates where radioactive decay starts to become significant. Because Al-26 and Be-10 are produced at a constant ratio, independent of absolute production rates, the Al-26/Be-10 ratio in quartz is robust against temporal production rate variations. In a steadily eroding landscape, quartz grains within the soil and sediment contain Al-26 and Be-10 concentrations in this predictable ratio. If these quartz grains are subsequently buried, for example deep within a sedimentary deposit, then cosmogenic nuclide production within those grains is attenuated by the overburden and inherited Al-26 and Be-10 concentrations diminish by radioactive decay. Because Al-26 decays more rapidly than Be-10, the Al-26/Be-10 ratio decreases exponentially with time. By measuring the Be-10 and Al-26 concentrations using accelerator mass spectrometry, the current Al-26/Be-10 ratio in the sample can be determined, and the burial time calculated.
Preliminary results indicate that coarse sand and gravel overlying horizontally cut marine platforms at elevations ranging from 4 18 m above present mean sea level were deposited between 630 ka and 1.15 Ma. Although the results are intriguing, they are complicated by post burial cosmogenic nuclide production. We are currently evaluating methods of obtaining reliable ages for samples with complex burial histories.
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Tracing the post-Younger Dryas retreat of the northern Fennoscandian Ice Sheet using cosmogenic radionuclide exposure ages
D. Fabel, A. Stroeven1, T. Dahlgren2, J. Harbor3, C. Hättestrand1, J. Kleman1 and D. Fink4
The aim of this collaborative project is to determine the rate of retreat of the Fennoscandian ice sheet from the Younger Dryas limits in northern Norway to the terminal limits in the northern Swedish mountains (Fig. 1). The north to south retreat history is poorly constrained due to a lack of datable material. We are working to provide new constraints on the timing and pattern of deglaciation using cosmogenic nuclide apparent exposure ages.
The work involves mapping and dating depositional and erosional geomorphological features related to the former ice sheet margin. Because the ice sheet initially had warm-based conditions close to its margin, the dominant morphology is one of eskers and aligned lineation systems, such as crag-and-tails. Abundant meltwater eroded bedrock locally to considerable depth and deposited fans or deltas perched above current local base levels (Fig. 1). However, subglacial conditions during final deglaciation were generally cold-based, inhibiting the formation of eskers and lineation systems, although there are widespread (lateral) meltwater channel erosional imprints and occasional plucking scars.
Each geomorphological setting was examined for its value in providing deglaciation ages, testing the initial assumption that, (i) abundant erosion on crags of crag-and-tails, across transverse erosional scarps, and in meltwater channels has exposed bedrock surfaces without a prior exposure history and (ii) depositional features contain embedded boulders without a prior exposure history (on the surfaces of eskers and deltas, and erratics). Preliminary results indicate that meltwater channels, transverse erosional scarps, and erratics yield deglaciation ages that are consistent with the limited ages provided by other methods, but that crag-and-tails yield apparent exposure ages that are too old, presumably because of a prior exposure history that was not fully removed by glacial erosion.
1 Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden
2 Department of Geology, University of Tromsø, Tromsø, Norway
3 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana, U.S.A.
4 AMS-ANTARES, Environment Division, ANSTO, RMB 1, Menai, NSW 2234, Australia.
Fig. 1. (A) Crag and tail. Ice flow was from right to left. (B) Perched glaciofluvial delta dissected by meltwater channels. (C) Esker
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Stable isotope analysis of bone collagen and tooth enamel of Australian marsupial faunas: a baseline study investigating the implications for palaeodiet and palaeo-environmental reconstructions
R.A. Fraser, R. Grün, and M.K. Gagan
Fossil mammal faunas are a major source of palaeoecological information: the diversity and abundance of species from well-dated deposits can provide proxy data for past vegetation and palaeoclimates. The stable isotopes of carbon, nitrogen and oxygen within fossil bone and teeth are an additional source of diet and climate information. Whilst the use of stable isotopes in palaeodiet research is employed extensively overseas in the archaeological and ecological fields, it remains vastly under used in Australian palaeoecology.
There remains uncertainty about how stable isotopes reflect animals’ diet and climate in the Australian context, therefore this project has begun by measuring the d13C and d18O tooth enamel from modern marsupial herbivores; kangaroos, wombats and koalas. By analysing these tissues from modern species with known dietary preferences, from distinct geographic, floristic and climatic regions, we can investigate the existing relationships between diet, environment and the isotopes measured.
d13C in tooth enamel analysed in 2003.
Kangaroo specimens: initial isotopic analysis of enamel CO3, (using the Kiel Carbonate device/ MAT 251) from the incisor and four molars within an individuals’ tooth row revealed significant differences in d13C between teeth. The average difference for 28 specimens gathered from south eastern Australia and Queensland was 4.06 ‰, the largest difference was 7.21‰ within a Macropus rufus jaw from Wilcannia in arid far-western New South Wales. The most noticeable pattern observed in the enamel CO3 isotope data shows that the early formed incisor and first molar are predominantly more negative in d13C than the other molar teeth, which formed later in adulthood. This is shown in figure 1. It is unclear whether this variation in d13C values reflects plant diet variability, perhaps due to greater seasonality in the environment, or physiological effects that may be age dependent. Ongoing investigation involves sampling kangaroos that have inhabited regions of varying percentages of C4 and C3 grasses to ascertain if grass diet availability can explain these differences.
Fig 1: Pattern of isotopic values within different teeth of Kangaroos from regions of varying percentages of C4 grasses.
Ultimately, these modern baseline data will aid the interpretation of isotopic values found in fossil faunas. Gaining an insight into the diets of Australia’s extinct megafauna, and the changing diets of extant fauna (such as kangaroos) over time in response to changing climate and vegetation regimes, will increase our understanding of ecological change.
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Extremely low erosion rates at the gibber plain in South Australia
T. Fujioka, J. Chappell, M. Honda, I. Yatsevich, K. Fifield and D. Fabel
The Australian continent has been considered to be the most stable continent. There is, however, only a few quantitative data to assess this hypothesis, although low erosion rates are expected in the most of arid area in Australia. Cosmogenic surface exposure dating has been utilized for quantitative estimation of erosion rates near the ground surface. In an attempt to provide evidence of Australian continental stability, we have measured cosmogenic 21Ne and 10Be in three silcretes from Orapinna and CooberPedi in semi-arid South Australia.
In order to accurately determine the amount of cosmogenic 21Ne in the samples, a new method was developed to evaluate nucleogenic neon, which is a critical interference in estimating cosmogenic neon. Nucleogenic neon is produced by a reaction of oxygen in the crust and subsequently incorporated into minerals ("trapped" nucleogenic), and/or it is produced "in-situ" where a particles are provided from the decay of U and Th within minerals ("in-situ" nucleogenic). We estimated the amount of "in-situ" nuleogenic 21Ne using production rates derived from U and Th contents in the samples, and silcrete formation ages at the site (55±5 Ma). We calculated the amounts of "trapped" nucleogenic 21Ne from the amounts of fissiogenic 136Xe in the samples using the crustal production ratio (nucleogenic 21Ne/fissiogenic 136Xe). Using this method, it was determined that 10-20% of excess 21Ne was nucleogenic in origin.
Minimum exposure ages and maximum erosion rates were calculated from the amounts of cosmogenic 21Ne, after correction for nucleogenic 21Ne. All samples studied showed long exposure ages (>4 Ma) and low erosion rates (<0.1 m/Ma). The erosion rates in this study appear to be extremely low compared to commonly quoted erosion rates estimated in Australia (5 to 50 m/Ma) and they are among the lowest ever measured globally.
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Coral records of surface-ocean evaporation: the “other half” of the hydrological cycle
M.K. Gagan, L.K. Ayliffe* and M.T. McCulloch
Evaporation is more prevalent than precipitation over the oceans and is the fundamental driver of the global hydrological cycle. Yet little is known about marine evaporation in the past, nor its role in global climate change. Changes in tropical surface-ocean evaporation rates in the past could induce atmospheric feedbacks that amplify the global climate response to minor forcings. For example water vapour, the premier atmospheric greenhouse gas, is thought to be an important factor driving glacial-interglacial climate change, but its potential role remains unclear because the past record of marine evaporation in the tropics is largely unknown.
We show here that season-specific measurements of skeletal Sr/Ca and d18O in corals offer the opportunity to reconstruct rates of tropical marine evaporation in the past. A significant attraction of the coral Sr/Ca and d18O technique is its ability to estimate sea-surface temperature (SST) and seawater d18O simultaneously. Precise determinations of coral Sr/Ca allow us to subtract the temperature component from the coral d18O signal and monitor seasonal changes in seawater d18O. Coupled measurements on the same coral sample avoid the problem of using separate time series to study coupled oceanic and atmospheric phenomena. Surface-dwelling Porites corals from the Dampier Archipelago, northwest Australia are ideal for quantifying the relationship between surface-ocean evaporation, seawater d18O, and sea-surface salinity (SSS). Mean annual solar radiation and evaporation rates on the hot, subtropical coast of northwest Australia are among the highest on Earth. In such a setting, salinity and seawater d18O vary together because evaporation, which increases salinity, also concentrates the heavy isotope of d18O.
Previous work has demonstrated that precise, high-resolution measurements of coral Sr/Ca at Dampier track seasonal variations in SST. Figure 1A shows that subtle differences between the Dampier coral Sr/Ca-SST and d18O-SST curves (defined as residual Dd18O), mimic the timing and magnitude of strong seasonal changes in SSS. 18O enrichments in the coral record become progressively greater in austral spring and reach maxima in November-December. This pattern coincides with the sharp austral spring rise in solar radiation, air temperature, and SST, all of which would serve to increase surface-ocean evaporation and the concentration of Dd18O and salt in seawater.
The veracity of the linear correlation between coral Dd18O and SSS can be confirmed by comparison with the relation for equilibrium evaporation of water at 26 oC, the mean SST at Dampier (Fig. 1B). The slopes of the d18O/SSS regression lines are in close agreement, 0.23 ‰ psu-1 for the coral data versus the theoretical slope of 0.25 ‰ psu-1. Also, equilibrium evaporation at 26 oC should yield water vapour depleted in d18O by 8.7 ‰, relative to seawater. This is in good agreement with the depletion of 8.1 ‰ for evaporated water given by the intercept of the regression line for coral Dd18O and SSS.
Based on these results, we conclude that high-resolution coral Dd18O records have the potential to yield insights into marine evaporation rates in the past. The technique is being applied to improve our understanding of the specific mechanisms by which subtle changes in past insolation seasonality were converted to significant changes in the global climate.
Figure 1: Relationship between surface-ocean evaporation, coral residual d18O (Dd18O), and salinity. (A) Comparison of Sr/Ca-SSTs (red curve) and d18O -SSTs (blue curve) for Porites lutea from the Dampier Archipelago, Western Australia (20º36'S, 116º45'E). The Sr/Ca-SST and d18O -SST relations are: TSr = 167.8 16013 * (Sr/Ca) and T18O = 0.82 - 5.59 * d18O. Lower curves show seasonal changes in coral Dd18O (blue) and monthly salinity (orange). Residual δ18O is obtained using: Dd18O = ∂d18O/∂T * [T18O - TSr], where ∂d18O/∂T is the temperature-dependent oxygen isotope fractionation (-0.18 ‰ per ºC) determined for Porites. (B) Comparison of linear regression lines for coral Dd18O and salinity (blue line) with the theoretical line for equilibrium evaporation of water at 26ºC (black line). The regression equations are: Dd18O coral = 0.225 * DSSS 7.4 and Δd18O theoretical = 0.246 * DSSS 8.1.
* Present address: Department of Geology and Geophysics, The University of Utah, USA.
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Coral radiocarbon records of Indian Ocean water mass mixing and wind-induced upwelling along the coast of Sumatra, Indonesia
N.S. Grumet1, N.J. Abram, J.W. Beck2, R.B. Dunbar1, M.K. Gagan, T.P. Guilderson3,4, W.S. Hantoro5 and B.W. Suwargadi5
Radiocarbon (14C) in the skeletal aragonite of annually banded corals track radiocarbon concentrations in dissolved inorganic carbon (DIC) in surface seawater. As a result of nuclear weapons testing in the 1950s, oceanic uptake of excess 14C in the atmosphere has increased the contrast between surface and deep ocean 14C concentrations. We have used accelerator mass spectrometric (AMS) measurements of 14C/12C ratios (D14C) in Porites corals from the Mentawai Islands, Sumatra (0ºS, 98ºE) and Watamu, Kenya (3ºS, 39ºE) to document the temporal and spatial evolution of the 14C zonal gradient in the tropical Indian Ocean. Our intrabasin comparison of coral D14C records from the coasts of Sumatra and Kenya reveals a distinct difference in water mass mixing processes across the equatorial Indian Ocean basin.
The rise in D14C in the Sumatra coral, in response to the maximum in nuclear weapons testing, is delayed by 2-3 years relative to the rise in coral D14C from the coast of Kenya (Figure 1). Kenya coral D14C values rise quickly because surface waters are in prolonged contact with the atmosphere. In contrast, wind-induced upwelling and rapid mixing along the coast of Sumatra entrains 14C-depleted water from the subsurface, which dilutes the effect of the uptake of bomb-produced 14C by the surface-ocean. Differences during the steady state pre-bomb period also suggest that rapid mixing at the Mentawai site leads to a greater influence of deeper water depleted in 14C. Convergence of the Mentawai and Watamu D14C records later in the bomb produced 14C rise is attributed to a water mass renewal rate of thermocline waters off the coast of Sumatra of between 2 and 3 years. Results from a box model confirm that a water mass renewal rate of approximately 2.5 years can account for the observed lead-lag relationship between the Watamu and Mentawai coral D14C records.
Bimonthly AMS D14C measurements on the Mentawai coral reveal mainly interannual variability with minor seasonal variability. Singular spectrum analysis of the Sumatra coral D14C record reveals a significant 3-year periodicity. These results lend support to the concept that interannual variability in Indian Ocean upwelling and sea-surface temperatures is related to ENSO-like teleconnections over the Indo-Pacific basin.
Figure 1: Bimonthly Mentawai (thick black line) D14C time-series superimposed on the long-term Mentawai trend (thin black line) from 1945 to 1990. The long-term trend was calculated using singular spectrum analysis. Coral radiocarbon levels respond to atmospheric testing of nuclear weapons in the mid 1950s. The increase in D14C recorded at Watamu (thin and thick grey lines) leads the increase at the Mentawai Islands by 2-3 years during the initial rise of bomb 14C between 1954 and 1963.
1 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305
2 NSF AMS Facility, Department of Physics, University of Arizona, Tucson, Arizona, 85721
3 Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94551
4 Institute of Marine Sciences, University of California at Santa Cruz, Santa Cruz, CA 65064
5 Research and Development Center for Geotechnology, Indonesian Institute of Sciences (LIPI), Bandung 40135, Indonesia.
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Thorium/Uranium systematics of Precambrian deep-sea pelagic black shales: implications for redox state of the early atmosphere
Y. Jia, M.T. McCulloch and C. Allen
To address the question of the redox state of the Precambrian atmosphere-hydrosphere system via sediments requires measurement of redox sensitive trace elements, and inter-element ratios, in deep water black shales with a chemical sedimentary “hydrogenic” component. This approach is endorsed by recent progress in research of redox-sensitive trace metals records in late Proterozoic and Phanerozoic sedimentary rocks, which has provided important clues to how the redox state of depositional environments has changed over time. Many conventional studies, in contrast, have been on first cycle volcanogenic turbidites with a minimal hydrogenic input (Taylor and McLennan, 1995). Accordingly, we have analysed the redox-sensitive, trace element compositions of the 2.1 Ga black shales in Birimian Blet, West Africa, and the 2.7 Ga Archean counterparts in Timmins, Canada, Tati Belt, Botswana, and Kanowna District, Western Australia. These pyrite-bearing black shales, which were originally argillaceous sediments containing organic matter and low in thermal maturity, were primarily deposited in the deep-sea pelagic environments. Th/U ratios are lower in the Proterozoic shales (0.38-0.82, average 0.67), and Archean shales (0.47-3.65, average 2.43) relative to “conventional” Archean upper crust (3.8), PAAS (4.7), or average upper continental crust (3.8). Calculated U concentrations from hydrogenic component are between 0.90 and 2.45 in the Proterozoic shales, and range from 0.06 to 0.96 for the Archean black shales. Given the conservative behavior of Th in the sedimentary cycle, variably low Th/U ratios in these Precambrian black shales signify that U6+, soluble in oxidized surface waters, was reduced to insoluble U4+ in reducing bottom waters, as in the contemporary Black Sea. The results are consistent with a locally to globally oxidized atmosphere-shallow hydrosphere pre-2.0 Ga.
Taylor S.R. and McLennan S.M. (1995) Reviews of Geophysics, Vol 33, pp 241-265.
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15N-enriched Archean crust or 15N-depleted crust recycled into -6 ‰ upper Mantle?
R. Kerrich1 and Y. Jia
To investigate the origin and evolution of nitrogen in the atmosphere, crust, and mantle systems during Earth’s early history. Nitrogen concentrations and isotopic compositions have been measured on 2.7 to 1.0 Ga Precambrian metasedimentary rocks from Botswana, Canada and Ghana; pre-metamorphic 2.7 Ga VMS deposits in Canada; and on post-metamorphic hydrothermal micas from 2.7 to 1.8 Ga quartz vein systems in Zimbabwe, Canada, and Ghana, . Archean sedimentary kerogens, K-silicates in VMS deposits, and K-micas in hydrothermal systems that sample average crust, yield d15N values of 15-24 ‰, compared to 2 to 6 ‰ in Phanerozoic counterparts. Palaeoproterozoic equivalents have intermediate d15N of 7 to 12 ‰, implying a secular decrease in crustal d15N. In parallel, N contents increase from tens to hundreds of ppm in the Archean, up to hundreds to thousands of ppm since the Palaeoproterozoic. The 15N-enriched nitrogen in both Archean sedimentary rocks and hydrothermal vein systems cannot be caused either by long-term diffusional loss of 14N, or by N-isotopic shifts due to metamorphism; such fractionations are £2‰ as established from empirical studies of Phanerozoic terranes with progressive metamorphism, and experiments. Moreover, pre- and post-metamorphic Archean samples have 15N enriched values in common. Retention of near-primary 15N-enriched values is endorsed by the lack of covariation of C/N with d13C or d15N, or with metamorphic grade. It is possible that the 15N-enriched values stem from a different N-cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations observed exceeds any presently known. The 15N-enriched nitrogen does not robustly constrain Archean redox-state. We attribute the 15N-enrichment to a secondary atmosphere derived from CI-chondrite-like material and comets with d15N of +30 to +42‰. Shifts of atmospheric d15N to its present values of 0‰ can be accounted for by a combination of early growth of the continents and sequestering of atmospheric N2 into crustal rocks, recycling into the mantle, and mantle degassing. Consequently, these shifts are tracked by the secular change of d15N in continental crust. If Earth’s surface environment became oxygenated at ~ 2 Ga, then there were no associated large N-isotope excursions. Based on a few 15N depleted Archean cherts Marty and Dauphas (2003) proposed that recycling of 15N depleted Archean crust could shift the upper mantle from a primordial value of +6 to +8 ‰ to the observed value of -5 ‰. Our new data rule out this model.
Marty B., Dauphas N., (2003) Earth Planetary Science Letters, Vol 216, pp 433-439.
1 University of Saskatchewan, Canada
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Coral Ba/Ca Record of Runoff from the Fitzroy River
J. Marshall, M.T. McCulloch and G. Brunskill1
A 250 year record of Ba/Ca in a Porites sp. coral was obtained from Humpy Island, located on the inner shelf some 30 km north of the mouth of the Fitzroy River. The Ba/Ca record is similar to that obtained for the Burdekin catchment, in that there are numerous peaks from 1850 to the present, but prior to 1850 there are much fewer Ba/Ca peaks with the exception of periods related to drought-breaking floods. While the relationship between increased Ba/Ca and European settlement appears to hold for the Fitzroy as well as the Burdekin, there are some significant differences. Firstly, the periods of runoff for the Burdekin and Fitzroy are not always coeval, as evidenced by the modern instrumental record. However, this can be seen as independent evidence for the relationship between Ba/Ca and river discharge. Secondly, there does not appear to be a relationship between the height of the Ba/Ca peak and the discharge volume; for example, the highest Ba/Ca peak is in 1947, but the Fitzroy discharge at this time is only moderate. Thirdly, the Ba/Ca peaks for the Humpy Island coral are not as large and the background levels are more noisy compared to the Havannah coral. This could be a reflection of the interaction between the Fitzroy estuary and the increased tidal range in this region.
1Australian Institute of Marine Science, PMB3, Townsville MC, Qld 4810, Australia
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Late Pleistocene palaeoceanographic and geochemical evolution of the South Tasman Rise
A. D. Moy, W.R. Howard & M.K. Gagan
This study uses a series of sediment cores recovered from the South Tasman Rise (STR), to reconstruct the palaeoceanographic and geochemical evolution in the Pacific sector of the Southern Ocean. Sediment cores MD972106 (45.2ºS, 146.3ºE, 3300m) and RS147GC34 (45.1ºS, 147.7ºE, 4001m) preserve records covering the last 160,000 years, with chronology controlled by AMS radiocarbon dates and benthic d18O (tied to SPECMAP). Collaborative research between the IASOS/ACE CRC - University of Tasmania and RSES - The Australian National University have enabled palaeoceanographic variables to be measured allowing us to estimate changes in past deep-water ocean circulation, carbonate chemistry and pCO2.
In water masses close to and below the calcite saturation horizon, planktonic foraminifera shell weights have been used as an index of dissolution at the seafloor and thus, used to estimate past deepwater [CO3=] in the Atlantic and Pacific Oceans (Broecker and Clark, 2001) . In waters situated well above the calcite saturation horizon, Barker and Elderfield (2002) , have shown planktonic foraminiferal shell weight variations over glacial-interglacial periods are related to surface water [CO3=] through time in response to changing atmosphere pCO2.
At the STR, the core sites are located close to the calcite saturation horizon and past changes in carbonate chemistry should be reflected in the sediment records at these sites. Planktonic foraminiferal shell weight results show increasing values during glacial periods (Termination I and Termination II), suggesting an increase in deepwater [CO3=] during these times (Figure 1). The most interesting aspect about the planktonic foraminiferal shell weights at the STR is the excellent correlation of planktonic (G. bulloides) d18O and planktonic shell weights over the past 160,000 years (Figure 1). These results suggest a couple of possible scenarios:
Planktonic foraminiferal shell weights measured in cores sites close to the calcite saturation horizon have been used as an index of dissolution. The tight correlation between shell weight and d18O, suggest the d18O values reflect selective dissolution of d18O enriched outer surfaces of the planktonic foraminifera measured.
(2) If marine calcification is sensitive to the concentration of atmospheric CO2, its effects should be seen in surface water pCO2 and [CO3=]. Planktonic foraminifera shell weights at the STR, show glacial-interglacial variations that may record calcification rates of planktonic foraminifera in surface water. If this is the case, then the d18O reflects changes in ice volume and SST that are tightly linked to changes in atmospheric CO2 (and planktonic foraminifera shell weight).
Figure 1. Oxygen isotope and shell weight data covering the last 160,000 years from core MD972106
at the South Tasman Rise
Barker, S. and Elderfield, H., 2002. Foraminiferal calcification response to Glacial-Interglacial changes in atmospheric CO2. Science, 297: 833-836.
Broecker, W.S. and Clark, E., 2001. Glacial to Holocene redistribution of carbonate ion in the deep sea. Science, 294: 2152-2155.
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In-situ trace elemental and Sr isotopic analysis of teeth by LA-ICPMS
Fossil biominerals, as the commonly solely preserved constituents of past organisms, are a source of valuable information in anthropology, archaeology, and palaeontology. However, their chemical composition is often altered by interaction with their surrounding depositional environment making it difficult to retrieve accurate in-vivo compositional signatures by bulk sample analysis.
Using a new approach, high-spatial resolution compositional profiling of teeth by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS and LA-MC-ICPMS), it is possible to: 1) detect domains in dental enamel that have escaped alteration through analysis of alteration-prone elements (e.g. U, Y, Ce, La) and comparison to levels found in modern enamel; 2) extract time-series information stored in sequentially-grown tooth enamel (Figure 1); and 3) perform essentially non-destructive analysis of rare and thus precious (e.g. hominid) fossil specimens with minimal damage.
This approach is being used to reconstruct human subsistence patterns (migration and palaeodiet) at seasonal resolution by combining Sr isotope (migration) and Sr/Ca, Ba/Ca, Zn/Ca trace elemental analyses (palaeodiet) measured by laser-ablation-ICPMS (Fig. 1). Another application is the accurate reconstruction of palaeo-exposure of humans to toxic metals (such as Pb) during e.g. ancient mining operations (Figure 2).
Figure 1. In-situ Sr isotopic analysis by LA-MC-ICPMS of human enamel (incisor, Uzbekistan; ~3rd century A.D.), performed in order to distinguish between sedentary and nomadic lifestyles. The variations may suggest annual migration (red, 87Sr/86Sr), and correlated changes in palaeodiet (blue; lower Sr/Ca implies more animal proteins relative to plant food), which appear to be antiphase. (Sr intensity only is preliminarily used as proxy for Sr/Ca ratio; in the future, Sr/Ca, Ba/Ca, Zn/Ca ratios and Sr isotopes will be simultaneously analyzed by ICPMS and MC-ICPMS.) Enamel growth occurs over ~5 years (0.5-6 years).
Figure 2. Trace elemental profile by LA-ICPMS of a human tooth that has lain in Pb-contaminated soils for ~800 years from an early medieval Pb-Ag mining town (Germany). Inner left and right enamel is characterised by extremely low U, La, Y levels and constant, albeit comparatively high Pb concentrations (~8 ppm; interpreted to be in-vivo), whereas dentine (centre) and outer enamel shows various degrees of Pb alteration/contamination.
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Marine Isotopic Stage 5e in the Southwest Pacific: Similarities with Antarctica and ENSO inferences.
C. Pelejero, E. Calvo, G.A. Logan1 and P. De Deckker2
A detailed record of alkenone-derived sea-surface temperatures (SSTs) offshore western New Zealand has been generated. It is based on data from marine deep sea core SO136-GC3 (42º18'S, 169º53'E, 958m water depth) and covers the penultimate deglaciation and Last Interglacial. SSTs were 3.5 to 4.5ºC warmer than present, peaking 4.5 thousand years ahead of ice volume minima. The short duration of Marine Isotopic Stage 5e off New Zealand exhibits a striking parallelism with the record of air temperatures at Vostok, Antarctica. Changes in latitudinal SST gradients for the Southwest Pacific, from New Zealand to the equator, are also assessed, showing values consistently lower than today. In this region, this situation usually occurs during periods with positive values of the Southern Oscillation Index and thus, La Niña conditions. By inference, we suggest that our assessed low thermal gradients might be indicative of a prevalence of either persistent or more frequent La Niña like conditions, particularly during early Stage 5e.
1 Petroleum and Marine Division, Geoscience Australia, Canberra, Australia
2 Department of Geology, The Australian National University, Canberra, Australia
Figure 1: (A) Globigerina bulloides d18O and U K'37 -SSTs for core SO136-GC3 (vertical numbers correspond to age model pointers). Arrow and dashed line mark the U K'37 -SST for the uppermost Holocene sample and the modern annual mean SST, respectively. (B) U K'37 -SSTs for core SO136-GC3 compared to dD from Vostok, a proxy for air temperature in Antarctica.
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In-situ uranium series dating of the Omo Kibish I human
A.W.G. Pike1, S. Eggins, R. Grün, C. Stringer2, M.H. Day2, A. Bartsiokas3, R.E.M. Hedges1
We have made the first application of laser ablation plasma mass spectrometry (LA-ICP-MS) to the U-series dating of a fossil hominid.
Until now, the most successful 230Th/U chronologies have been obtained where archaeological material is inter-stratified with calcium carbonate (speleothem) deposits which remain closed to the migration of U after deposition. Co-occurrence of such deposits with hominid fossils is rare, thus attempts have been made to directly date the fossils, despite difficulties in understanding post depositional U migration into bones and teeth. Sample size requirements for conventional mass spectrometry can prove too destructive for dating valuable specimens, especially given that modelling uranium uptake requires measurement of the spatial distribution of U-series isotopes across entire bone sections.
LA-ICP-MS provides high spatial resolution, and rapid measurement of U-series isotopes with minimal sample destruction. Previous studies were successful on zircon samples which typically have >100 ppm U, whereas bones typically have 1-50 ppm U. We have developed a method that provides accurate U-series isotope ratios on fossil bones containing >3ppm U, using an ArF 193nm excimer laser and a Finnigan Neptune ICP mass spectrometer. The laser is focussed to a spot 200mm x 20mm on the sample in a He atmosphere to maximise sampling efficiency and pulsed at 20Hz, giving a crater depth of approximately 10mm. Results are calibrated using bones for which U-series isotopes have been measured using conventional mass spectrometry. The 95% confidence for calibration is typically ± 1.5% for 234U/238U and ± 2.0% for 230Th/238U, which for a typical bone gives an overall 2s precision on measurements of 2-4%. While this is less precise than conventional mass-spectrometry the advantages of high spatial resolution outweigh the disadvantages of diminished precision.
We applied LA-ICP-MS to a fragment of the Omo Kibish I skull and used the diffusion-adsorption (D-A) model to calculate a date of 93 +8-5 ka (Figure 1). Omo Kibish I has been classified as morphologically modern human and has never been directly dated, but is thought to be 130±5 ka from U-series dates on associated shells. Shells, however, are notoriously open systems and U-series has been shown in some cases to over estimate the age due to leaching of U or underestimate due to uptake of U. The diffusion-adsorption model predicts characteristic U and U-series isotope distributions across a bone section as U is gained or lost in response to geochemical changes in the burial environment. Leaching or recent uptake of U lead to characteristic profiles which can be used to reject the bone as unsuitable for dating. In this case, the U profile is close to uniform, and the U-series date profile is È-shaped, confirming that uptake has happened under relatively constant conditions, giving us confidence in the date.
Our date of 93.0+8-5 ka is the first direct date of this human, confirming that Omo I is indeed one of the oldest examples of modern H. sapiens from Africa. This date also demonstrates the suitability of the technique to the direct dating of hominid fossils, and we expect that laser ablation U-series dating will lead to further critical insights into the timing of modern human evolution.
Figure 1: Laser ablation U and U-series date profiles measured across a section of the Omo Kibish I skull fragment. The bold curve represents the maximum likelihood date calculated using the D-A model, giving a date of 93+8-5 ka (at 95% confidence)3. The dates from the void in the centre of the bone have been excluded. Mean 230Th/232Th activity is 4800 across the bone and > 70 at all points in the bone. Error bars are at 1s.
1 Research Laboratory for Archaeology, University of Oxford, OX1 3QJ, UK.
2 Department of Palaeontology, The Natural History Museum, London, SW7 5BD, UK
3 Department of History and Ethnology, Democritus University of Thrace, P.O.Box 217, 69100 Komotini, Greece.
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New ages for Miocene hominid-bearing sediments at Haritalyangar, Indian Siwaliks
B. Pillans, M. Williams1, D. Cameron2 and R. Patnaik3
The Haritalyangar area (31º32’N, 76º38’E), in Himachal Pradesh, northern India, has yielded fossil hominid specimens for almost 100 years, but new dating of fossil-bearing localities significantly changes our perception of hominid evolution within this region. Previous work at Haritalyangar suggested that hominid fossils were between 6.3 and 7.7 million years old. However, subsequent paleomagnetic studies, and revisions to the Geomagnetic Polarity Time Scale (GPTS) have increased the ages to between 8.6 and 9.23 Myr. For the first time we provide an accurate age of 8.85 Myr for the rare, large ape, Indopithecus (more than 2 million years older then previously thought), while the primitive primate genus Sivaladapis occurs in deposits dating to 9.1 Myr. Specimens of Sivapithecus and Sivalhippus,believed to be an early, archaic member of the modern orang-utan lineage, are also revised to around 9 Myr. We also document the first appearance of ostrich in south Asia.
1 Research School of Earth Sciences, The Australian National University.
2 Mawson Graduate Centre for Environmental Studies, The University of Adelaide.
3 Department of Anatomy & Histology, The University of Sydney.
Upper second molar of a rare, Upper Miocene ape, Indopithecus giganteus von Koenigswald 1950, discovered at Haritalyangar in 2002, and dated to 8.85 Myr by magnetostratigraphy.
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A seven-year regulation of the El Niño-Southern Oscillation 3,700 years ago
D.Qu, M. K. Gagan, W.S.Hantoro1, B.W. Suwargadi1
The El Niño-Southern Oscillation (ENSO) is the dominant coupled atmosphere-ocean mode of interannual climate variability, affecting most of the tropics and subtropics and many mid-latitude regions of North America, South America, and eastern Asia. Recent research indicates that El Niño events have become more severe and more frequent in recent decades. Whether this change simply represents a mode of natural climate variability, or is a result of human-induced greenhouse forcing, is still debated. This debate highlights the importance of the need for understanding the long-term behaviour of this key climate system.
The oxygen isotopic composition of coral aragonite offers a robust proxy of ENSO variability because it is sensitive to both the oceanic and atmospheric components of ENSO. Corals from the island of Sumba, Indonesia, are well situated to record the sea-surface cooling and droughts brought about by El Niño events. A high-resolution oxygen isotope time-series for a modern coral from Sumba records all the major ENSO events between 1985 and 1996. During El Nino events, the skeletal d18O values increased both in winter and summer, in response to the cooler sea-surface temperatures (SSTs) and drier climate.
We have extended this approach to produce a high-resolution, 57-year-long coral d18O record of past ENSO variability from a fossil coral that grew within the fringing reef of Sumba 3,700 years ago (Fig. 1). The record was extracted from a 1.12 m long, high-quality Porites core (MS7) with an average annual skeletal extension rate of 1.6 cm/yr, which is typical for modern Sumba Porites. The fossil coral d18O record shows a remarkably regular interannual variability with a 7-8 year period. Each 7-8 year cycle terminates with the diagnostic signature of a strong El Niño, anomalously cool SSTs in winter accompanied by drought in the summer.
Our results suggest that El Niño events were less frequent and more regular 3,700 years ago, relative to the unpredictable 3-8 year periodicity for modern El Niño events. The seven-year regulation of El Niño is fascinating because it suggests that, under certain background climate states, El Niño events may be predicted years in advance. We are in the process of investigating the Holocene evolution of the Indo-Pacific Warm Pool climate for clues about the processes regulating ENSO.
1 Research and Development Center for Geotechnology, Indonesian Institute of Sciences.
Figure 1: High-resolution skeletal d18O record for Holocene coral MS7. This coral has a uranium-series age of 3,700 years. The shaded areas indicate the diagnostic ocean-atmosphere signature of El Nino events, when anomalously cool SSTs in winter and the reduction of 18O-depleted rainfall in summer both produce higher skeletal d18O values. The Y-axis values are reversed to show warmer/wetter conditions toward the top.
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Geochemical Records of Bleaching Events from the Great Barrier Reef
B. Roark1, M.T. McCulloch, L. Ingram2 and J. Marshall
The health of coral reefs world-wide is increasingly threatened by a wide array of stressors. On the Great Barrier Reef (GBR) these stressors include increased sediment flux associated with land use changes, increased sea surface temperatures (SST) and salinity changes due to large floods, the latter two of which are factors in an increased number of bleaching events. The ability to document long-term change in these stressors along with changes in the number of bleaching events would help discern between natural and anthropogenic changes to this ecosystem. Here we present results of an initial calibration effort aimed at identifying bleaching events and the associated stressors using stable isotopic and trace element analysis in coral cores. Three ~15-year time series of geochemical tracers (d13C, d18O, and Sr/Ca) on Porites coral cores obtained from Pandora Reef and the Keppel Islands on the GBR have been developed at near weekly resolution. Since the d13C of the coral skeletal carbonate is known to be affected by both environmental factors (e.g. insolation and temperature) and physiological factors (e.g. photosynthesis, calcification, and the statues of the symbiotic relationship between corals and zooxanthellae) it is the most promising proxy for reconstructing past bleaching events. The first record (PAN-98) comes from a coral head that had undergone bleaching and died shortly after the large-scale bleaching events on Pandora Reef in 1998. A second core (PAN-02) was collected from a living coral within 10m of PAN-98 in 2002. Sr/Ca ratios in both cores track even the smallest details of an in situ SST record. The increase in SST that occurred three to four weeks prior to bleaching was faithfully recorded by a similar decrease in the Sr/Ca ratio in PAN-98, indicating that calcification continued despite the high SST of 30-31°C. The d13C values decreased by about 5 ‰ one week after the SST increase, and remained at this value for about 4 weeks until the coral died. In 1994 and 1995, there are decreases in the d13C values of 3 ‰. In 1994, a flood plume from the Burdekin River reached Pandora Reef and bleaching was reported. In 1995 we note a 4-5 week period of elevated SST based on the Sr/Ca results, which may have been sufficient to cause stress or bleaching of the coral. No clear decreases in d13C values associated with any bleaching event was evident in the PAN-02 record, however there is a clear growth hiatus that lasted several months during the 1998 bleaching event. d18O results in both records show many of the same details as the Sr/Ca and SST record, suggesting temperature changes as the dominant control. However, during flooding events (1996, 1997, and 1998), the d18O values were decreased by increased freshwater input to the reef. The associated salinity changes were determined by subtracting the temperature component from the d18O signal using Sr/Ca ratios and compared with the weekly average flow records from the Burdekin River and a Ba/Ca record (McCulloch et al. 2003) of sediment flux to the reef. Similar results were obtained in a third record from the Keppel Islands which included one of the largest floods of the century and a bleaching event in 1991.
1 Department of Geography, University of California, Berkeley, CA
2 Department of Earth and Planetary Science, University of California, Berkeley
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Geochronology of long-term landscape evolution, NW NSW
M. Smith, B. Pillans, S. Eggins, and J. Dunlap
The regolith geochronology and long-term landscape development history of north-western New South Wales is being investigated by dating regolith materials from a variety of locations, using an array of dating techniques. This region includes several distinct geological regions, the Broken Hill block, the Cobar Basin, the Eromanga and Surat Basins and the northern Murray Basin, from which several areas have been targeted for investigation, including Bourke, Broken Hill, Cobar, Parkes, Peak Hill, Tibooburra and White Cliffs. The use of multiple techniques is a key aspect of this study as each technique is capable of dating the products of different weathering and landscape forming processes, such as silcretes (Figure 1).
The main geochronological methods being used are palaeomagnetic dating, d18O dating of clay minerals, (U Th)/He dating and U Pb methods. Pilot palaeomagnetic analyses of samples from several sites have been carried out, and several iron oxides and apatites have been (U Th)/He dated. Clay minerals have been analysed by XRD to determine the clay content and suitability for d18O dating. Work is being undertaken to determine the applicability of U Pb geochronology to late stage anatase found in silcretes throughout the field area and in related sites from South Australia. Laser Ablation ICP-MS analysis of silcrete samples has been performed to determine their suitability for conventional high-precision U Pb dating, and to obtain preliminary U Pb dates (Figure 2).
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Late Quaternary evolution of the Leeuwin Current, Western Australia
M. Spooner1, P. De Deckker1, M. K. Gagan
Australia is largely an arid continent and has a poor record of environmental change. Palaeoenvironmental histories extracted from Australian lake sediments are incomplete due to the lack of water and surface-sediment deflation. Therefore, the use of deep-sea cores to reconstruct past environmental change is advantageous as the deposition of sediment and microfossils is almost continuous.
We have recently completed a foraminifer faunal analysis and high-resolution d18O and d13C record for deep ocean core (MD002361) spanning at least the last 700,000 years from offshore north-western WA to investigate the late Quaternary evolution of the Leeuwin Current. This current is a warm, low salinity poleward flow originating from the Throughflow of water from the Pacific into the Indian Ocean. Today it runs southward along the western coast of Australia and has a significant impact on the climate of Western Australia. Past investigations of the Leeuwin Current so far have been inconclusive, especially in regard to the occurrence of this warm surface current during glacial periods. There is some evidence that the Leeuwin Current was absent during glacials, which may have resulted in the cold northbound Western Australian Current reaching the surface.
Initial investigations to ~ 425,000 years BP reveal that the Leeuwin Current was prevalent during all the marine isotopic stages (Fig.1). Tropical and subtropical foraminifer species, such as G. ruber and G. sacculifer, live in surface waters (0-75m). Comparison of all interglacial periods, such as the Holocene and Isotope Stages 7,9 and 11, shows that the relative abundance of G. ruber and G. sacculifer was equivalent to modern day abundances. However, the high abundance of the tropical species G. sacculifer indicates that the Leeuwin Current may have been enhanced during Isotope Stage 5. In addition, other tropical species such as P. obliquiloculata indicate warmer conditions and possible enhancement of the Leeuwin Current during Isotopic Stages 5 and 7.
There appears to be no evidence of dramatic upwelling events during glacial periods as indicated by the relatively low abundances of G. bulloides, G. glutinata and N. dutertrei. The only indication of nutrient-rich, cooler water is during Isotope Stages 12 and 10, given by the greater abundance of G. bulloides.
The species G. inflata resides in transitional waters between tropical and polar water masses. The relative abundance of G. inflata indicates a change in the water column during the glacial periods, with its greatest abundance during the last glacial maximum. This may indicate cooler waters during glacial periods and hence a reduced Leeuwin Current. This is also confirmed by the reduction of tropical and subtropical species, especially during the last glacial maximum.
We plan to reconstruct sea-surface temperatures using Mg/Ca ratios of G. ruber tests and possibly a transfer function using foraminifer assemblages developed by Dr T.T Barrows (RSPhySE).
1Department of Earth and Marine Sciences, ANU.
Figure 1: Diagram showing variations in core MD 002361 against depth for the relative abundance of planktonic foraminifera and skeletal d18O and d13C values. G. ruber, G. sacculifer, G. menardii and P. obliquiloculata are species which favour tropical-subtropical water conditions. G. bulloides, G. glutinata and N. dutertrei are species, which prefer cooler and nutrient rich water conditions. G. inflata prefers transitional water masses between subtropical and polar water temperatures.
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Conodont geochemistry proxies for understanding palaeoenvironments, bioevents and geoevents of the Palaeozoic
A resurgence in studies using conodont apatite as a proxy for seawater composition has prompted a re-assessment of their suitability as geochemical tracers. This has not been adequately addressed in the past and requires clarification; an effective sampling protocol is also needed to ensure data integrity. Sample integrity is critical for determining primary geochemical signatures to evaluate environmental conditions and processes in Earth history.
This study is characterising the trace element & isotopic compositions of conodont apatite using high-resolution, in-situ, micro-analytical techniques (eg. laser ablation ICPMS). Compositional relationships to ultrastructure, taxonomy, and histology, have been recognised and are assessed in the context of sample integrity & diagenesis. Temporal variations are also recognised from conodonts extracted from continuous stratigraphic sections throughout the Ordovician and Early Silurian, which provide insights into ambient ocean conditions & environmental processes likely operating during this period.
The ubiquity and biostratigraphic significance of conodonts underscore their potential importance for geochemical studies, especially as potential recorders of ocean chemistry and environmental change during the Palaeozoic and early Mesozoic. Understanding extinction events and the processes controlling life during Earth history has immediate implications for present and future life on Earth.
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Biogeochemical cycling of trace metals in coastal seawater
T. Wyndham and M.T. McCulloch
The aim of this project is to use proxy records of seawater composition, incorporated in coral aragonite, to investigate the biogeochemical cycling of a selection of reactive trace metals (rare earth elements and bioactive transition metals) in the coastal waters of the Great Barrier Reef. Coral records of dissolved metal concentrations may reveal features of coastal marine processes that are difficult or impossible to identify using periodic sampling methods. In combination with direct measurement of dissolved and particulate metal concentrations, which can be used to aid in identifying specific cycling processes, this approach may provide a unique perspective upon the biogeochemical cycling of reactive trace metals. Based on these investigations, the application of trace element records in corals as proxies for biological, climate or anthropogenic changes in coastal seawater can also be explored.
Initial investigations of coral records of REE and Mn have produced a number of interesting results. The REE composition of coastal seawater inferred from the coral record at several coastal sites on the GBR appears to be dependant on seasonal factors. REE fractionation displays a strong seasonal cycle that correlates closely with Mn concentration. Higher Nd/Yb ratios and higher Mn concentrations in summer result from scavenging of heavy REE by particulate organic ligands and Mn reductive dissolution respectively, both processes occurring at higher rates during periods of high primary productivity. The Ce anomaly also displays a strong seasonal cycle showing an enhanced anomaly during summer and during flood events. This is consistent with the Ce anomaly being primarily controlled by the abundance of Ce oxidising bacteria. Based on these arguments, we suggest that the coral record of dissolved REE and Mn may be a useful proxy for biological activity in coastal seawater. Further investigations will be aimed at better characterising these changes in seawater composition and exploring the behaviour of other reactive trace metals using this approach.
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Soil Organic Carbon and Carbon Isotope Inventories of the Australian Continent
J.G. Wynn, M.I. Bird, L. Vellen, and E. Grand-Clement
Research during 2003 has focused on extending the SOC inventories to soils of other textures along gradients in particle size (soil texture) within 4 climatic and ecological regions of Australia: desert shrublands near Birdsville, QLD; semi-arid subtropical savannas near Winton, QLD; tropical rainforests and woodlands near Cardwell, QLD; and temperate rainforests and heathlands in western TAS. With the extension to clay soils, SOC inventory and stable carbon isotope ratio values can be regressed with respect to two primary controlling factors, the annual water deficit and percent fine fraction (Figure 1). Annual water deficit is defined as the negative of mean annual availability of water (Berry and Roderick, 2002):
-W = -(MAP - Q / r L)
(where MAP is Mean Annual Precipitation, Q is annual solar radation, and r and L are the density and latent heat of evaporation of water at 25°C.
Each of the 31 sand soil sites were analysed for 14C activity in the lab of Prof. John Chappell, RSES. 14C activity shows no pronounced relationship the annual deficit of water (-W). These 14C data explain much of the excess carbon in sites that are “overmeasured,” or “underpredicted.” These data indicate some proportion of stabilized pre-1950 carbon (pM, percent modern < 100), with a more dominant pool of labile carbon fixed since 1950 (pM between 100 and 165).
Organic matter from a selected set of sandy soil sites was respired and collected for AMS 14C analysis at the ANSTO Lucas Heights AMS facility (AINSE Grant 03/130). Samples were selected at sites along 2 climatic gradients of mean annual temperature and precipitation: two primary controls on carbon turnover. These results can be used to constrain the mixing ratio of stable pre-bomb carbon in “overmeasured.”
Figure 1. Surface regression of SOC Inventory and stable carbon isotope ratio to annual water deficit and percent fine fraction (<63 µm). Centre point is mean of 5 transects with gray “error bars” of variance between transects and vertical black line showing deviation of data from surface fit.
Berry S.L., Roderick M.L. (2002) Aust. J. of Botany, Vol 50, pp511-531.
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