SEASONAL VARIATIONS OF CARBON AND OXYGEN ISOTOPE RATIOS IN SHELLS OF THE CIRCUMPOLAR GONDWANAN BIVALVE EURYDESMA, AND THEIR IMPACT ON INTERPRETATIONS OF EARLY PERMIAN CLIMATES

Climates in the early Permian of southeastern Australia were responding to a time of significant global change. A major shift from an icehouse to a greenhouse world has long been inferred from the sedimentary and fossil records, but quantifying the amount of change throughout the early Permian has been difficult. The biggest obstacle has been trying to obtain oxygen isotope ratios from shell materials that are representative of original conditions and not diagenesis. Here, we build on the limited study of Ivany and Runnegar (2010) to explore the potential for recovering high-resolution seasonal isotope records from the same bivalve (Eurydesma) over a wide geographic range. Specimens come from 10 localities spanning about 11° of modern latitude in exposures from the northern Sydney Basin in New South Wales to Hobart, Tasmania. Bulk samples yield carbon and oxygen isotope values that are in agreement with published data from Korte et al. (2008). High resolution sampling (6 samples per year over 5 years of shell accretion) has yielded obvious seasonal variation in δ¹⁸O values that corresponds with growth banding and little to no variation in characteristically heavy δ¹³C values (~5‰); several specimens yielded irregular profiles that will allow us to compare altered and unaltered shells. The former yielded more negative δ¹⁸O values overall and an absence of clear seasonal variation. Oxygen values of shells that appear unaltered are nevertheless more negative than expected for this high-latitude setting. Potential explanations for more negative values include warm paleotemperatures, freshwater influx, isotopically light seawater, or some combination thereof. Co-occurring glendonites and glacial sediments indicate low temperatures, consistent with the hypothesis that Eurydesma was a cold-water organism (Runnegar, 1979). Significant freshwater influx should produce brackish conditions that are not compatible with the associated marine invertebrates or the carbon isotope ratios. Isotopically negative seawater could be a global influence that might explain the low values seen in our and published data, but this is a contentious hypothesis. The geographic spread of our samples, coupled with sedimentological and paleontological evidence, provides us with a unique opportunity to explore these various scenarios.