Year

2005

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences - Faculty of Science

Abstract

This thesis presents a palaeoenvironmental study of the Gulf of Carpentaria, northern Australia, from around the Last Glacial Maximum (LGM) to the present. Foraminifers, microscopic unicellular aquatic organisms, occur throughout the sediment in the time frame studied. Data on the species composition and preservation of the microfossils found in the Gulf of Carpentaria cores are utilised to reconstruct past environments by comparison to the known assemblages of living foraminifers in various modern environments. The Gulf of Carpentaria is a shallow epicontinental sea, situated between Australia and Papua New Guinea, and is a maximum of 70m deep. It is separated from the Pacific Ocean to the east by Torres Strait, which is 12m deep at its shallowest, and from the Indian Ocean and Arafura Sea to the west by the Arafura Sill, which is 53m below sea-level (bpsl) at its shallowest. For at least ten thousand years in the lead up to the LGM (which reached its peak about twenty thousand years ago), and for about ten thousand years after, sea levels were lower than the 53m-deep Arafura Sill. The continental shelf in the Gulf of Carpentaria area between Australia and Papua New Guinea was exposed, creating a land bridge between the two islands, and a lake developed in the Carpentaria Basin. This palaeolake is termed Lake Carpentaria (named by Torgersen et al., 1983). Documentation of the timing in fluctuations in the extent and salinity of Lake Carpentaria provides information on local and regional climatic systems, such as the Australian summer monsoon. Constraining the nature and timing of the postglacial rise in sea-level which flooded the lake provides evidence for global eustatic sea-level reconstructions. Analysis of sediment cores from the Gulf of Carpentaria, beginning around 40ka cal BP (forty thousand calendar years before present), shows the existence of Lake Carpentaria (a large, non-marine water body of fluctuating extent) until sealevel rose over the Arafura Sill and inundated the palaeolake around 10.5ka cal BP. The earliest studied phase dates to around 40ka cal BP which is a marineinfluenced brackish water lacustrine facies where Lake Carpentaria is briefly at its maximum extent: 12m deep in its deepest section. The existence of such a large body of water (around 150,000km2) supports the existence of a strong Walker Circulation in the region enhancing precipitation. Between 40ka and 18.8ka cal BP the non-marine, increasingly saline, Lake Carpentaria decreased to 7m maximum water depth, adding to the evidence of aridity around the LGM in northern Australia. At 18.8ka cal BP the lake freshened and monospecific bivalve, foraminiferal and ostracod populations dominated the still shallow (around 8m deep) lake. The lake was expanding, and from around 15±2ka cal BP, fluctuations are noted in the general trend of increasing precipitation. The recorded variations in precipitation intensity may result from stronger seasonality (i.e. monsoons) and/or interdecadal variability (e.g. El Niño Southern Oscillation). At 12.7ka cal BP Lake Carpentaria was at around 12m maximum water depth – the maximum documented extent in the studied period. At this stage there was some exchange of waters with the Arafura Sea via tidal outlet channels in the Arafura Sill (indicating sea-level around 60m below present), seen as a marine influence beginning in the western margins at 12.7ka cal BP. At 12.4ka cal BP the sealevel had risen to the same height as water levels within the lake (58m bpsl). By 12.2.ka cal BP sea-level was up to 2m higher than the previous lake level, and flowed into the lagoonal Lake Carpentaria via channels in the Arafura Sill. By 10.5ka cal BP the sea-level had overtopped the highest surface of the 53m-deep Arafura Sill and the transition to marine conditions began in the Gulf of Carpentaria, confirming the accepted models of sea-level rise.

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