Gradients in seasonality and seawater oxygen isotopic composition along the early Permian Gondwanan coast, SE Australia

Oxygen isotope compositions of marine carbonates are commonly employed for understanding ancient temperatures, but this approach is complicated in the very distant past due to uncertainties about the effects of diagenesis and the isotopic composition of seawater, both locally and globally. Microsampled accretionary calcite from two species of the fossil bivalve Eurydesma Sowerby and Morris 1845 collected from sediments of Cisuralian age in high latitude marine sediments along the SE coast of Australia records cyclic seasonal fluctuations in shell δ¹⁸O values during growth, demonstrating the primary nature of the isotope signal and thus allowing investigation of early Permian seawater isotopic composition and water temperature in the high southern latitudes. The mean and seasonal range of δ¹⁸O_carb decreases poleward across about 10° of paleolatitude (~67°S-77°S). The presence of co-occurring dropstones and stratigraphically associated glendonites constrains winter temperatures across the region to near-freezing, thus permitting calculation of realistic estimates of water composition and summer temperatures. Summer δ¹⁸O_carb values indicate water temperatures between 5 °C and 12 °C, with warmer values at lower latitudes. The decrease in both mean sea surface temperature and seasonal amplitude with increasing latitude on the Gondwanan coast is much like that observed along high-latitude coastlines today. Calculated δ¹⁸O_water decreases toward the pole, likely associated with an increasing contribution of isotopically light fresh water derived from summer snow-melt. The gradient in δ¹⁸O_water is similar to that documented over a similar span of latitude on the modern SE Greenland coast. We infer the presence of a north-flowing coastal current of cold, O¹⁸-depleted water that entrains progressively greater amounts of more typical seawater as it moves away from the pole. δ¹⁸O values in SE Australia, however, are about 3‰ lower than those off Greenland, suggesting comparatively lower salinity water or more O¹⁸-depleted glacial ice/runoff in the Permian Gondwanan high latitudes, perhaps augmented by more depleted (negative) global average seawater. Conditions in southeastern Australia during the largest of the Permian deglaciations were warmer than present-day Antarctica at similar latitudes, but may approximate those of early-mid Miocene Antarctica, with frozen winters but summers closer to 10 °C.