Relationship Between Oceanography and Brachiopod Geochemistry; eastern Great Australian Bight

Abstract

Determination of water masses and circulation in the eastern Great Australian Bight, in conjunction with analysis of δ18O values and trace element concentrations in the articulated brachiopod Anakinetica cumingi, indicate that brachiopods faithfully record ambient temperatures and oceanographic conditions. Five water masses are identified on the shelf during March 1998, on the basis of temperature, salinity, δ2H and δ18O values. The warm, high salinity Great Australian Bight Plume occurs in the western part of the study area both at the surface and at depth, and the cool, low salinity Flinders Current flows west along the continental slope. The Flinders Current provides upwelling water along the Eyre Peninsula and in the central Great Australian Bight, and mixing of this water with Great Australian Bight Plume and shelf waters forms the combined water masses Flinders Current + Great Australian Bight outflow, Modified Flinders Current and Mixed waters. The Great Australian Bight Plume and Flinders Current are isotopically distinct, with Great Australian Bight Plume waters having lower δ18O values and higher δ2H values than Flinders Current waters.

Using mean annual temperatures and δ18O values of seawater, brachiopod temperatures calculated from their δ18O values are within 1°C of measured temperatures. δ18O values in the brachiopods, in conjunction with seasonally variable temperatures and δ18O values of seawater, provide evidence for brachiopod major growth periods. Calculated brachiopod temperatures are most accurate assuming 70% of brachiopod growth occurs during summer when upwelling is occurring and 30% of growth occurs during the rest of the year. Brachiopods may be growing more during upwelling periods in response to increased food supply.

Ba, Ni and V concentrations in the brachiopod shells show spatial variation that matches with water mass distribution on the shelf. Ba positively correlates with nitrate contents in the water and is enriched in brachiopods growing in upwelling areas. Ni and V positively correlate with seawater temperatures, suggesting that incorporation of these two elements into brachiopod calcite is temperature dependant. All three elements reflect the anticlockwise circulation pattern present in the eastern Great Australian Bight during the summer major growth period.