HIGH-LATITUDE ANNUAL TEMPERATURE CYCLES FROM BIVALVE CALCITE AND THE ISOTOPIC COMPOSITION OF EARLY PERMIAN SEAWATER

Stable oxygen isotope values of Paleozoic carbonates have been difficult to interpret due to the difficulty of unambiguously establishing the primary, unaltered, nature of the materials being sampled. Even non-luminescent samples displaying what appears to be original shell microstructure can show a range of δ¹⁸O values, indicating some degree of otherwise undetected diagenetic alteration. Here, we report stable oxygen isotope values of sequential, growth-band-parallel microsamples from the Australian Early Permian calcitic bivalve Eurydesma, that we argue offer some of the best evidence to date for primary shell compositions. Data reveal 6 years of well-constrained, regular, sinusoidal, seasonal variations that range between -2 and +1 ‰. Carbon isotope values lie near +6, consistent with other Permian datasets, and do not covary with oxygen. The pattern of seasonal compositional variation with growth provides strong evidence that these values are original to the bivalve during its lifetime, and hence reflect temperature and isotopic composition of seawater on the Australian coast 285 million years ago.

Paleotemperatures calculated assuming that δ¹⁸O of seawater is 0‰ range from ~13-26°C, too warm for this setting and time. Seawater would have to be at least -3‰ to yield winter temperatures approaching freezing, as indicated by the presence of glendonites and glacial dropstones in approximately coeval facies. This negative value might reflect seasonal mixing of isotopically negative glacial meltwater into the marine realm, or could provide evidence for a controversial decrease in seawater composition back in time. The facies from which this fossil comes is quite shallow, but from an offshore volcanic island, possibly removed from local sources of significant fresh water. The associated fauna is consistent with, but not diagnostic of, normal marine conditions. The ⁸⁷Sr/⁸⁶Sr ratio (.70755) is slightly less radiogenic than expected for the Sakmarian, and might reflect the weathering of nearby basalts and mixing with freshwater runoff. Future work, including clumped isotope analysis, on this and other large, well-preserved, originally calcitic bivalves may be able to contribute to questions about paleoclimate and seawater composition during the Paleozoic.