Browsing by Author "Esat, TM"
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- ItemANSTO ECR ion source and its application to mass spectrometry(Australian Institute of Nuclear Science and Engineering (AINSE), 2007-11) Button, D; Buckley, D; Esat, TM; Hotchkis, MACAt ANSTO we have built an Electron Cyclotron Resonance (ECR) ion source to investigate new concepts for mass spectrometers [1,2] designed to measure isotopic ratios in small samples. ECR ion sources are capable of producing beams of multiply-charged atomic ions with high efficiency and are widely used as heavy ion injectors for accelerators. To meet the requirements of mass spectrometry, we have needed to adapt ECR ion source techniques to our purpose. In this presentation, these and other recent developments of our ECR ion source will be discussed.
- ItemCoral U-series ages: U in corals and in the oceans(Elsevier, 2007-07) Esat, TM; Yokoyama, YA by product of U-series dating of corals is the 234U/238U ratio in the oceans at the time the coral was growing. The expectation is that this ratio remains invariant in the oceans over the U-series time scale, of at least 500,000 years, and hence it can be used to detect itinerant U or Th through the sample over time. Often, a suite of corals from a reef section show an intriguing, approximate 1:1, correlation between the 234U/238U ratio and age. This pattern can arise by the addition of almost equal amounts of external 230Th and 234U into the coral over an extended period. Currently, the favoured explanation postulates addition of equal amounts of 230Th and 234Th. The latter has a very short half life of about 25 days but the advantage that during aqueous transport it should not fractionate from 230Th. There are at least three prescriptions for removing the extraneous added components, based on the difference between the measured and expected 234U/238U ratio, to derive a corrected or “true” age for the sample. However, there is growing evidence that 234U/238U may not have been invariant in the oceans through Glacial-Interglacial transitions. In addition, some of the correlated data sets cross over to values of 234U/238U lower than in the present ocean thus requiring both the removal and addition of 230Th and 234Th, in neighbouring corals, which is not easy to explain. We will present data from Huon Peninsula coral reefs that show much more complex behaviour than the so far recognised 1:1 linear trend. Firstly, during major sea-level transitions the 234U/238U ratio shifts from low to high values in step with rising sea-levels. There are several well documented examples of this type of behaviour. Secondly, the variability in 234U/238U is amplified during rapid changes in sea-level. Candidates for this type of behaviour occur during rapid sea-level changes associated with Heinrich-events and also during the OIS6 to OIS5 transition. At these times, there is a direct correlation between sea-level change and U as the 234U/238U ratio shifts between high and low values in step with sea-level. This type of behaviour cannot be explained by extraneous 230Th-234Th infusion or through any other plausible diagenetic process. Conversely, these results cast doubts on the 230Th-234Th addition explanation for the 1:1 234U/238U-age trend in the data and more so on the so-called corrected ages calculated from them. © 2007 Published by Elsevier Ltd.
- ItemCoupled uranium isotope and sea-level variations in the oceans(Elsevier, 2010-12-15) Esat, TM; Yokoyama, YGlobally, rivers supply uranium to the oceans with excess 234U relative to secular equilibrium and 234U taken-up by corals can be used for dating. In addition, the 234U abundance in sea water, at the time the coral was growing, can be measured independently. The veracity of U-series ages used in determining past sea-level variations is dependent on selecting pristine corals free from diagenetic alteration. A quantitative test for alteration assumes invariant 234U abundances in the oceans for at least the past half a million years and results from samples outside of a narrow range in modern ocean 234U abundance are excluded from data sets. Here, we have used previously published data to show that 234U in the oceans appears to be variable and directly related to changes in sea-level, not only over long glacial–interglacial timescales but also at very short, centennial timescales. Most of the previously discarded data can be used to provide valuable additional sea-level information. The process permits a unique insight into the interplay between sources and sinks of uranium in the oceans mediated by sea-level changes at rates far faster than previously thought possible. Similar, rapid sea-level, forcing of other trace element abundances in the oceans is likely. © 2010, Elsevier Ltd.
- ItemIODP Expedition 325 to the Great Barrier Reef: unlocking the history of reef growth and demise since the Last Glacial Maximum(Australian Geosciences Council, 2012-08-05) Webster, JM; Braga, JC; Humblet, M; Potts, DC; Iryu, Y; Hinestrosa, G; Bourillot, R; Seard, C; Camoin, G; Yokayama, Y; Thomas, AL; Thompson, B; Esat, TM; Fallon, SJ; Dutton, APredicting how the Great Barrier Reef (GBR) will respond to future global climate changes and over what time frame is crucial. Fossil reefs record critical data on geomorphic and ecological consequences of both long-term and abrupt centennial-millennial scale environmental changes. The Integrated Ocean Drilling Program (IODP) Exp. 325 investigated a succession of submerged fossil reefs on the shelf edge of the GBR to establish the course of sea-level change, define sea-surface temperature variations but also analyse the impact of these environmental changes on reef growth since the Last Glacial Maximum (LGM). Thirty-four boreholes were cored from 17 sites along four transects at three locations (Hydrographers Passage, Noggin Pass and Ribbon Reef) in water depths between 42 to 167 m. These cores record responses of the GBR to past environmental stresses similar to current scenarios of future climate change (i.e. changing sea-levels, SST’s, water quality). Initial lithologic, biologic and chronologic data document an active coral reef system that grew, drowned and backstepped up-slope as sea level rose since the LGM. We present an overview of the main Exp. 325 results, including a synthesis of the dating, paleoclimate and reef response team’s findings, in the context of the available site survey data (bathymetry, seismic, seabed imagery). Finally, we discuss the broader implications of these data for understanding how the geometry, composition and development of the GBR responded to repeated and major environmental disturbances since the LGM.
- ItemIssues in radiocarbon and U-series dating of corals from the last glacial period(Elsevier, 2008-08) Esat, TM; Yokoyama, YRadiocarbon calibration beyond the extent of tree-ring records depends on U-series dating of fossil corals or speleothem, both of which can provide independent calendar ages. Less direct methods rely on layer counting and comparison with other well-dated records. In spite of considerable effort to provide a reliable radiocarbon calibration curve beyond 25,000 years, the majority of the data show large atmospheric radiocarbon peaks which are inconsistent both in magnitude and timing between different determinations. The results of the most recent work [Chiu, T.-C., Fairbanks, R.G., Mortlock, R.A., Bloom, A.L., 2005. Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals. Quaternary Science Reviews 24 (16-17), 1797-1808], from Araki Island fossil corals, indicate a monotonic variation from about 33 to 49 ka, with no radiocarbon peaks, but with some gaps in the data. The difference between this and previous results, from fossil corals, has been attributed to selection of better-quality samples and rigorous analytical methods. However, previous results from Huon Peninsula [Yokoyama, Y., Esat, T.M., Lambeck, K., Fifield, L.K., 2000. Last ice age millennial scale climate changes recorded in Huon Peninsula corals. Radiocarbon 42 (3), 383-401; Cutler, K.B., Gray, S.C., Burr, G.S., Edwards, R.L., Taylor, F.W., Cabioch, G., Beck, J.W., Cheng, H., Moore, J., 2004. Radiocarbon calibration and comparison to 50 kyr BP with paired C-14 and Th-230 dating of corals from Vanuatu and Papua New Guinea. Radiocarbon 46 (3), 1127-1160] show radiocarbon peaks exclusively located within the gaps in the Araki data. The timing of the gaps are not random, but appear to be related to severe climate and sea-level variations associated with Heinrich events initiated in the North Atlantic. We propose that the Huon and Araki data sets are complementary rather than exclusive and that the absence of coral growth at Araki Island during Heinrich events presumably reflect local adverse conditions for coral growth. © 2007, Elsevier Ltd.
- ItemPhasing and amplitude of sea-level and climate change during the penultimate interglacial(Nature Publishing Group, 2009-05) Dutton, A; Bard, E; Antonioli, F; Esat, TM; Lambeck, K; McCulloch, MTEarth's climate has oscillated between short-lived interglacial and extended glacial periods for the past million years. Before the last interglacial, absolutely dated markers of sea level become increasingly rare; hence, our knowledge of sea-level change driven by the waxing and waning of continental ice sheets before that time is largely based on proxy records from deep-sea cores(1-3) that lack direct age control. Here we present precise U-Th ages for a remarkable collection of submerged speleothems(4,5) from Italy, which record three sea-level highstands during the penultimate interglacial period, Marine Isotope Stage 7, from 245,000 to 190,000 years ago. We find that sea level rose above-18m (relative to modern sea level) several thousand years before maximum Northern Hemisphere insolation during the first and third highstands. In contrast, the second highstand, Marine Isotope Stage 7.3, is essentially synchronous with the insolation maximum, and sea level during this highstand only peaked at about 18 m, even though the concurrent insolation forcing was the strongest of the three highstands. We attribute the different phasing and amplitude of the Marine Isotope Stage 7.3 highstand to the extensive continental glaciation that preceded it. This finding highlights the significance of cryosphere response time to the climate system. © 2009, Nature Publishing Group.
- ItemProspects for the new frontiers of earth and environmental sciences(Elsevier, 2008-08) Yokoyama, Y; Matsuzaki, H; Esat, TMOne of the major advances in environmental geochemistry, over the past two decades, has been the introduction of Accelerator-based Mass Spectrometry (AMS) for radiocarbon dating and for investigating terrestrial surface processes through trace quantities of cosmic-ray produced nuclides. During October 2006, a symposium at the University of Tokyo celebrated 50,000 hours of AMS operations at the Tokyo “Micro Analysis Laboratory, Tandem Accelerator (MALT)”. MALT is one of 10 current AMS facilities in Japan but the only one capable of analyzing multiple nuclides. More than 20 talks and over 30 posters were presented covering a diverse range of AMS studies, including, radiocarbon dating, measurements in ice cores, progress in instrumentation, and analyses of in-situ-produced nuclides. © 2008, Elsevier Ltd.
- ItemUplift rates defined by U-series and C-14 ages of serpulid-encrusted speleothems from submerged caves near Siracusa, Sicily (Italy)(Elsevier, 2009-02) Dutton, A; Scicchitano, G; Monaco, C; Desmarchelier, JM; Antonioli, F; Lambeck, K; Esat, TM; Fifield, LK; McCulloch, MT; Mortimer, GEWe have established a plausible rate of uplift near Siracusa in southeastern Sicily (Italy) over the last glacial-interglacial cycle using U-series ages of submerged speleothem calcite and C-14 ages of calcite serpulid layers that encrust the speleothems during cave submergence. The precisely determined ages of these sea level benchmarks were compared with expected relative sea level position based on glaciohydro-isostatic modeling to assess the rate of uplift in this region. When combined with the age of various late Holocene archaeological sites that have been recently described and characterized in terms of their functional position relative to sea level these data collectively define a rate of uplift <= 0.4 mm a(-1) along this portion of the Sicilian coastline. These results are consistent with an age assignment of marine isotope stage (MIS) 5.3 or 5.5 for the Akradina terrace. which in turn places temporal constraints on paleoshorelines above and below this level. © 2008, Elsevier Ltd.