THE OXYGEN ISOTOPE RECORD FOR THE PALEOZOIC: THE CASE FOR CONSTANT SEAWATER δ18O AND WARM EARLY-MID PALEOZOIC OCEANS

Numerous studies have observed decreasing fossil δ¹⁸O with increasing sample age. This observation has been explained by higher temperatures, lower seawater δ¹⁸O, or progressive meteoric diagenesis, with age. To reexamine the δ¹⁸O record of Paleozoic paleotemperature and seawater δ¹⁸O, oxygen isotopic data for Paleozoic brachiopods and conodonts have been compiled from recent literature, building upon the δ¹⁸O_CaCO3 compilation of Prokoph et al. (2008). To minimize diagenetic effects on brachiopod δ¹⁸O, this study emphasized (1) screening by petrographic examination of thin-sections, and (2) samples from cratons and localities with excellent fossil preservation.

For Carboniferous and Permian time-slices, the δ¹⁸O records of “select” brachiopods yield average isotopic paleotemperatures of 13-34°C, assuming seawater δ¹⁸O of -1‰ VSMOW (modern, ice-free). These data argue for modern-like temperatures and seawater δ¹⁸O for this time. For the Devonian-Ordovician, δ¹⁸O values show cyclicity of roughly 3‰ with periods of tens of millions of years. Brachiopod δ¹⁸O averages as low as -5.8‰ for 4-Ma time slices. Such values can be explained by high temperatures (~40⁰C), lower seawater δ¹⁸O (-2‰; either locally or globally), or sample diagenesis. Published δ¹⁸O studies of conodonts yield isotopic temperatures similar to those of brachiopods for the Permo-Carboniferous, but lower, modern-like temperatures for Devonian-Ordovician. However, a new phosphate-water paleothermometer equation (Puceat et al., 2010) yields conodont paleotemperatures for the Devonian-Ordovician that are close to those for brachiopod shells. These results support the contention that early-mid Paleozoic brachiopod δ¹⁸O values of -5 to -6‰ are not diagenetic artifacts. Did early-mid Paleozoic seas experience extreme warmth or low seawater δ¹⁸O? Recent clumped C-O isotope studies of Silurian brachiopods have obtained paleotemperatures of ~35°C and a seawater δ¹⁸O of -1.2‰ (Came et al., 2007), consistent with conventional δ¹⁸O analyses of brachiopods and conodonts. This convergence of brachiopod δ¹⁸O, conodont δ¹⁸O, and brachiopod clumped-isotope data provides firm evidence for (1) relatively constant seawater δ¹⁸O throughout the Paleozoic, and (2) periods of extreme warmth in the early-mid Paleozoic.