Geochemistry of Surface Sediments from the Southern Australian Continental Margin Including Offshore West Tasmania, South Australia and Victoria

Brief description

As part of Bureau of Mineral Resources (BMR) survey 67 using Rig Seismic offshore South Australia, Victoria and West Tasmania, many gravity cores were sampled and analysed for light hydrocarbons as part of the Continental Margins surface geochemistry project. Eight of those cores were analysed, both at sea and in Canberra, for a variety ofcomplementary geochemical data. This Record provides a summary of the geochemical analyses subsequently conducted on those cores. The data presented here include (i) porewater metabolites (ii) geochemical analyses of major and trace element abundances (iii) radiochemical data and select palaeoceanographic indicators. Some of these data have been used to assist in the interpretation of the light hydrocarbon data, while much of theother data has applications to a variety of environmental and resource issues in southern Australia. These data (i) provide a data-base which can be used to assess long-termenvironmental change associated with anthropogenic discharges to the southern marginsof Australia (ii) allow assessment of Late Quaternary and natural climatic change and (iii) provide clues to geochemical processes operating in the sediments which are important inidentifying formation of some seafloor minerals including marine phosphorites and manganese crusts and nodules. This Record and these data also form part of ageochemical data-base which includes data from other parts of the Australian continental margin. These data will be used to systematically develop the basis for an understanding of the controls on the geochemical compositions of sediments from around Australia. The pore water data indicate that the outer-shelf/upper-slope cores are probably sub-oxicto anoxic in character within the top few centimetres as a result of organic carbon burialand recycling in the core-tops. In contrast the lower-slope sediments are more oxic (oxicto sub-oxic) and these reflect generally lower rates of organic matter input into these sediments. Pore water silicate data in some cores suggest uptake of silica into the solid phases at depth, which may be indicative of glauconite formation in the sediments. Glauconite is often associated with modern phosphorite accumulations in sediments, although these data provide no direct clues to phosphorite formations in these sediments. The inventory of sedimentary manganese in the outer-shelf/upper-slope cores is generally significantly lower than that measured in lower-slope cores. This observation, and the pore water data which shows manganese remobilisation andrecycling between pore waters and sediments, may help explain an important processcontributing to the formation of abundant manganese crusts and nodules in the deepwaters off West Tasmania including the South Tasman Rise. The down-core distributions of some radiochemical species, including authigenicuranium and excess thorium 230 are being used to both determine sedimentation ratesand assess Late Quaternary paleoceanography in the region. These data and several other potential palaeoceanographic indicators indicate down-core trends which may beindicative of natural palaeoceanographic and palaeochemcial changes between glacialand interglacial periods, but these cannot be assessed until these sediments have been dated. This dating is in progress at the Flinders University of South Australia.

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