Brachiopod δ18O values do reflect ambient oceanography: Lacepede Shelf, southern Australia

Although commonly used as proxies for attributes of ancient ocean waters, the δ 18 O values of brachiopods from modern seas are little studied. To evaluate the utility of brachiopods as recorders of regional oceanography, modern shells from the Lacepede Shelf (25 000 km 2 ) of southern Australia were analyzed for δ 18 O, and the results were compared to the values of ambient seawater. Southern Ocean waters cover this area of extensive cool-water carbonate deposition, but there are distinct sectors of seasonal upwelling and lesser fluvial outflow. δ 18 O values of brachiopods across the environmental spectrum from 40 to 300 m water depth are in general isotopic equilibrium with surrounding seawater. Nevertheless, δ 18 O values from individual sample sites vary as much as 0.60‰. The area of cold-water upwelling in particular is clearly delimited by a group of high δ 18 O values. The range of values across this one shelf, on the order of 2.5‰, is similar to the range of values postulated on the basis of similar results for secular changes in many ancient oceans.