SEASONAL VARIATION OF CARBON AND OXYGEN ISOTOPES FROM THE PERMIAN OF SOUTHEASTERN AUSTRALIA, AS RECORDED BY A CIRCUMPOLAR GONDWANAN BIVALVE (EURYDESMA MORRIS 1845)

The early Permian transition from an icehouse to greenhouse world is archived in the sedimentary record of southeastern Australia. Stable isotope geochemistry of well-preserved fossil carbonates provides an opportunity to peer into the climate of this transitional world. Nine specimens of the bivalve Eurydesma, spanning roughly 11° of paleolatitude (North Sydney Basin, New South Wales to Hobart, Tasmania), were sequentially microsampled for stable isotope values in order to resolve seasonal variation and how it varied with latitude. δ¹³C values show only ~1‰ of seasonal variability and are in agreement with published, characteristically heavy values for the early Permian. δ¹⁸O varies regularly, and positive isotope values correspond to growth banding in the shells, suggest slower growth in the winter months. Seasonality has the greatest amplitude at the lower latitudes in the North Sydney Basin (3.2‰) as well as the least negative mean δ¹⁸O of -0.7‰, while the higher latitude samples have the lowest degree of seasonality (1.6‰) and a mean δ¹⁸O of -1.8‰. The trend toward decreasing seasonal range in water temperature approaching the poles is similar to that seen today at high latitudes and provides further support that the data are representative of primary conditions during the Permian deglaciation. However, δ¹⁸O values are more negative than expected for a high-latitude glacial climate and require further explanation. Shell microtextures and trace element geochemistry do not support an explanation based on diagenetic alteration. Warm paleotemperatures calculated assuming an ocean water composition of ~0‰ do not agree with independent cold-water indicators such as dropstones and glendonites that have led to the classic interpretation of Eurydesma as a cold-water bivalve. Mixing with isotopically negative freshwater is a possibility but seems unlikely given the positive values of, and low variation in, δ¹³C; the consistency of seasonal cycles in δ¹⁸O within shells and the paleoecology of the associated fauna, both of which are more consistent with normal marine conditions. The remaining explanation - a more isotopically negative ocean - is a contentious hypothesis. Additional sampling across latitude may help to resolve these uncertainties.