Abstract
Inorganic precipitation of aragonite is a common process within tropical carbonate environments. Across the Northwest Shelf of Australia (NWS) such precipitates were abundant in the late Pleistocene, whereas present-day sedimentation is dominated by calcitic bioclasts. This study presents sedimentological and geochemical analyses of core data retrieved from the upper 13 meters of IODP Site U1461 that provide a high-resolution sedimentary record of the last ~15 thousand years. Sediments that formed from 15 to 10.1 ka BP are aragonitic and characterised by small needles (<5 µm) and ooids. XRF elemental proxy data indicate that these sediments developed under arid conditions in which high marine alkalinity favoured carbonate precipitation. A pronounced change of XRF-proxy values around 10.1 ka BP indicates a transition to a more humid climate and elevated fluvial runoff. This climatic change coincides with a shelf-wide cessation of inorganic aragonite production and a switch to carbonate sedimentation dominated by skeletal calcite. High ocean water alkalinity due to an arid climate and low fluvial runoff therefore seems to be a prerequisite for the formation of shallow water aragonite-rich sediments on the NWS. These conditions are not necessarily synchronous to interglacial periods, but are linked to the regional hydrological cycle.
Highlights
Holocene sedimentation across the Northwest Shelf of Australia (NWS) is limited and primarily occurs at the inner ramp below 50 metres of water depth and along a small pelagic ridge at approx. 140 mwd[1]
Core samples from industry wells do not resolve this stratigraphic interval sufficiently[7], whereas sediment cores off the NW Australian margin do not record sedimentary processes on the shelf (Fig. 1)[8,9,10,11]
This study focuses on the upper 13 meters of IODP Site U1461 applying a combination of petrographical methods and X-ray fluorescence (XRF) analyses derived proxy records
Summary
Holocene sedimentation across the NWS is limited and primarily occurs at the inner ramp below 50 metres of water depth (mwd) and along a small pelagic ridge at approx. 140 mwd[1]. Holocene sedimentation across the NWS is limited and primarily occurs at the inner ramp below 50 metres of water depth (mwd) and along a small pelagic ridge at approx. Across the mid to outer ramp sediments are dominated by ooids and aragonite needle mud[1,2,5] Both were dated to have formed shortly after the Last Glacial Maximum (LGM) and are currently stranded at the sea-floor in water depths from 50 to 100 meters and 120 to 200 meters respectively[1,5]. Zirconium is transported by aeolian processes[11,14] while aluminium and potassium are primarily transported by fluvial processes[10,11] The ratio between those elements represents a proxy of fluvial runoff and dust flux.
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