Browsing by Author "Wust, RA"

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    Low latitude moisture regime changes over the last 14,000 years recorded by diatomaceous sediments from dry tropical Australia.
    (Elsevier, 2007-07) Wust, RA; Shemesh, A; Ridd, P; Stephenson, J; Jacobsen, GE; Smith, AM
    A diatomaceous sediment deposit from Long Pocket, NE Queensland, Australia, provides new evidence for rapid climatic changes. Here we present a rare sediment record from the dry tropics that spans the last ~14,000 cal yrs. The 5.5 m thick deposit is composed of purely diatomaceous debris with little atmospheric influx. The site is unique as a basaltic flow isolated and formed the area ~13,600 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 ~6000 years ago. Oxygen isotope data of the diatoms indicates further that changes in moisture source took place progressively. However, trace elemental data shows that marked changes in atmospheric flux composition occurred ~3800 yrs BP, most likely representing the timing of the onset (strengthening) of the monsoon system. Interestingly, the timing of some of the changes expressed in our record by the geochemical data coincides with Bond-cycles described from the North Atlantic. With our data, we hypothesise that the monsoon system was ineffective for latitudes 17º S during the early Holocene and that moisture was primarily derived from short range trajectories, such as the Coral Sea. Around ~3800 yrs BP abrupt climate changes led to the present system that is dominated by a wet summer monsoon. 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 possibly an intensification of ENSO during the latter part of the Holocene.
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    Taphonomic bias and time-averaging in tropical molluscan death assemblages: differential shelf half-lives in Great Barrier Reef sediments
    (The Paleontological Society, 2009-11) Kosnik, MA; Hua, Q; Kaufman, DS; Wust, RA
    Radiocarbon-calibrated amino acid racemization ages of 428 individually dated shells representing four molluscan taxa are used to quantify time-averaging and shell half-lives with increasing burial depth in the shallow-water carbonate lagoon of Rib Reef, central Great Barrier Reef, Australia. The top 20 cm of sediment contains a distinct, essentially modern assemblage. Shells recovered at depths from 25 to 125 cm are age-homogeneous and significantly older than the surface layer. Taxon age distributions within sedimentary layers indicate that the top 125 cm of lagoonal sediment is thoroughly mixed on a sub-century scale. The age distributions and shell half-lives of four taxa (Ethalia, Natica, Tellina, and Turbo) are found to be largely distinct. Shell half-lives do not coincide with any single morphological characteristic thought to infer greater durability, but they are strongly related to a combined durability score based on shell density, thickness, and shape. These results illustrate the importance of bioturbation in tropical sedimentary environments, indicate that age estimates in this depositional setting are sensitive to taxon choice, and quantify a taxon-dependent bias in shell longevity and death assemblage formation. © 2009, The Paleontological Society