Browsing by Author "Stephenson, J"

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    Late 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, AM
    A 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.
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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.