Browsing by Author "Wust, RAJ"
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- ItemComparison of radiocarbon ages from different organic fractions in tropical peat cores: insights from Kalimantan, Indonesia(University of Arizona, 2008-12) Wust, RAJ; Jacobsen, GE; van der Gaast, H; Smith, AMVarious organic fractions of an Indonesian tropical peat deposit were dated using radiocarbon accelerator mass spectrometry (AMS). Four different depth layers, deposited during the last 28,000 14C yr, were analyzed and the data compared to bulk sample analyses. The pollen extracts consistently produced the oldest dates. The bulk samples (<250 μm and <100 μm) often yielded the youngest dates. The age difference between the individual fractions depended on the layer depth and hence the true age of the sampled peats. The age discrepancy was highest (~16,000 14C yr) in the oldest peat material. We interpret this to be a consequence of the input of organic matter over a long period of time, with peat oxidation and/or no peat accumulation during the last glacial maximum (LGM). The age discrepancies were smaller (between 10 and 900 14C yr) for the Holocene peat samples. It was concluded that the pollen extract fraction might be the most reliable fraction for dating tropical peat deposits that are covered by deeply rooting vegetation. © 2008, University of Arizona
- ItemLate Pleistocene onset of monsoonal rain and abrupt strengthening of ENSO 3,900 cal yrs BP recorded by diatomaceous sediments from dry tropical Australia(GNS Science, 2009-05-15) Wust, RAJ; Shemesh, A; Ridd, P; Stephenson, J; Jacobsen, GE; Smith, AMA continuous diatomaceous sediment record from dry tropical Queensland, Australia, provides new evidence for rapid precipitation variations over the last 14,000 cal years. Lava flows from Toomba Volcano formed a unique runoff-isolated lake system ~13,600 cal yrs BP that contains 5.5 m thick purely diatomaceous debris. High precipitation during shortly after the development of the lake system supports other studies that the monsoonal system developed around 13,000 cal yrs BP. Geochemical data and isotope analysis of the diatoms reveal that primary productivity was high during the onset of the deposit with little changes until the mid Holocene, when abrupt moisture regime changes occurred ~6,000 years ago. Prior to that, precipitation across this present-day dry tropical site must have been plenty to sustain a perennial lake system. Furthermore, oxygen isotope data of the diatoms indicates that changes in moisture source took place progressively shortly after 9,000 years ago and lasted until about 6,500 years when abrupt shifts in source occurred until 6,000 years ago. Since then, isotopic values remained similar with to periods of rapid changes between 3,000-1,800 and ~400-200 cal yrs BP (Little Ice Age). Trace elemental composition of the record provides further evidence for marked changes of the atmospheric composition ~3,900 yrs BP and may represent the timing of the drying of the Australian continent (enhanced erosion of top soils), which most likely signifies the intensification of the El Niño Southern Oscillation (ENSO) in this region. The interpretation is supported by previous studies from corals from tropical Pacific and marine sediments from, for example, the Cariaco Basin off the Venezuelan coast. At Cariaco, the changes were interpreted to be due to the onset and intensification of ENSO. These studies suggested that over the last 4000 years, strong ENSO with increasing variability dominated the monsoonal regions. In summary, our record shows that the dry tropics received more precipitation during the late Pleistocene and early Holocene than during the late Holocene. These findings are similar to findings from the wet tropical NE-Australia. However, our record shows a marked collapse of the ocean/atmospheric system in the low latitudes and an intensification of ENSO during the latter part of the Holocene round 3,900 cal yrs Bp with several marked climatic shifts since then, with the last one occurring during the Little Ice Age.
- ItemPossible evidence for wet Heinrich phases in tropical NE Australia: the Lynch's Crater deposit(Elsevier, 2008-03) Muller, J; Kylander, M; Wust, RAJ; Weiss, D; Martinez-Cortizas, A; LeGrande, AN; Jennerjahn, T; Behling, H; Anderson, WT; Jacobsen, GEUnarguably, one of the most significant paleoclimatological discoveries of the last two decades has been that of abrupt climate events (Dansgaard-Oeschger cycles and Heinrich events). Most evidence for these events has originated from the high-latitude Northern Hemisphere.. with few records documenting the response of the low latitude Southern Hemisphere. Here we present new data from Lynch's Crater, a unique terrestrial record from NE-Australia that may show evidence for southward propagations of the Intertropical Convergence Zone (ITCZ) during abrupt climate perturbations as a result of alteration of the low latitude air masses. Proxies for precipitation/wetness indicate enhanced rainfall in the region during Heinrich events (H events 1-3) and the 8.2 ka Northern Hemisphere cold event. A fully coupled atmosphere/ocean climate model simulating a 1 Sv freshwater influx to the North Atlantic Ocean produces a scenario which agrees with the climate changes shown by the Lynch's Crater record. The model shows precipitation anomalies that include a southward migration of the ITCZ and a zonal shift in mid-latitude storm tracks over the Southern Hemisphere equatorial region. These data indicate large-scale shifts of the austral summer ITCZ position that is known to control monsoonal precipitation in NE Australia. This terrestrial record from Australia may demonstrate the involvement of the tropical western Pacific Ocean in ITCZ migrations during abrupt climate events of the last glacial period. Defining such past migrations offers insight into the importance and role of the equatorial region in global climate dynamics. © 2007, Elsevier Ltd.
- ItemSea level change over the past 2500 years from northeastern Australia(Australian Geosciences Council, 2012-08-05) Lewis, SE; Wust, RAJ; Webster, JM; Collins, J; Wright, SA; Jacobsen, GEThis study C-14 dated living and fossil oyster bed deposits (>50 cm thick) to examine when sea level fell to its present position. These thick oyster bed deposits are formed in the inter-tidal zone and are relatively long-lived features with one bed dated in this study growing for ∼1000 years. Consistent growth rates at each sampling location indicated that oyster bed accumulation over time has been continuous, although growth rates varied markedly between locations from 0.30 mm/year to 6.4 mm/year suggesting the influence of localised environmental variability in water circulation, wave exposure and boulder lithology. We will show that a rapid sea-level fall occurred between 800 and 1200 years BP where sea level fell from ∼ + 1 m to present position with rates up to 7 mm/year but no less than 2.5 mm/year. The rapid fall supports the stepped sea-level model. This rapid fall influenced the movements of indigenous people as well as coincided with the development of inshore fringing coral reefs and geomorphological changes along the coastal zone.
- ItemSediment mixing and stratigraphic disorder revealed by the age-structure of Tellina shells in Great Barrier Reef sediment(Geological Society of America, 2007-09) Kosnik, MA; Hua, Q; Jacobsen, GE; Kaufman, DS; Wust, RAJRadiocarbon-calibrated amino acid racemization ages of 250 individually dated Tellina shells from two sediment cores are used to quantify molluscan time averaging with increasing burial depth in the shallow-water carbonate lagoon of Rib Reef, central Great Barrier Reef, Australia. The top 20 cm of sediment contain a distinct, essentially modern assemblage with a median age of only 5 yr. Sediment between 20 and 125 cm are age-homogeneous and significantly older than the surface sediment (median age 189 yr). Shell age distributions within layers indicate that the top 125 cm of lagoonal sediment is thoroughly mixed on a subcentennial scale. Shell size is an important correlate of shell half-life and an important determinant of the inferred age of sedimentary layers. These results illustrate the importance of bioturbation in these environments, indicate that age estimates in this depositional setting are sensitive to specimen choice, and document a size-dependent bias in death assemblage formation. © 2007, Geological Society of America