SEASONALITY AND A LATITUDINAL GRADIENT IN THE EARLY PERMIAN FROM THE δ18O RECORD OF FOSSIL BIVALVES

Constraining paleoclimate conditions over the transition from a glaciated world to an ice-free one during the Permian is a topic of great interest due to the many ways it mirrors the present. The method commonly employed for understanding ancient temperatures is the use of the oxygen isotope values of marine carbonate, but this approaches is complicated when used on specimens older than the Cretaceous, due to uncertainties surrounding the potential for diagenesis and the uncertain isotopic composition of seawater prior to the Cenozoic. Microsampled accretionary calcite from fossil bivalves in SE Australia record the seasonal cycle of water temperature variation, thus demonstrating the primary nature of the isotope signal and allowing for evaluation of Permian seawater isotope composition and water temperature. The presence of co-occurring dropstones, diamicts, and glendonites constrain winter temperatures to near-freezing and allow for calculations of water composition. These records recovered from the bivalve Eurydesma span roughly 11° of paleolatitude (North Sydney Basin, New South Wales to Hobart, Tasmania) and reveal cyclic seasonal fluctuations in δ¹⁸O_carb­ that vary with latitude. δ¹³C_carb values exhibit ~1‰ of seasonal variation and are in agreement with characteristically positive values for the early Permian of ~5.5‰. The δ¹⁸O_carb values vary seasonally in single specimens by up to 3.3‰ around a mean that decreases poleward from -1.2‰ to -1.8‰; more enriched isotope values correspond to dark growth bands within the shells, suggesting slower growth in the winter months. A decrease in both mean δ¹⁸O and seasonal amplitude with increasing latitude is similar to that observed off the coast of Greenland today. The reduction in amplitude is a result of decreasing summer temperatures, but relatively stable winter minima because of freezing conditions. The decrease in δ¹⁸O_carb with latitude reflects a regional decrease in δ¹⁸O_water associated with the input of isotopically depleted fresh water, similar to that observed over a similar span of latitude in Greenland today. The magnitude of depletion, ~3‰ more than that seen in Greenland today, suggests either much fresher water or, potentially, more negative global average seawater.