PALEOZOIC OCEAN TEMPERATURES: BRACHIOPOD AND CONODONT δ18O RECORDS COMPARED (Invited Presentation)

We present a compilation of oxygen isotope data based on analyses of Paleozoic brachiopods (N = 4883) and conodonts (N = 4019) from the Paleozoic Era. The compilation represents six continents and all climate zones, but is dominated by European and North American sites from tropical to subtropical latitudes. Carbonate fossil preservation was evaluated based on microtexture and Mn content when possible, whereas conodont exposure to higher burial temperatures was evaluated using the conodont color alteration index (CAI). High Mn contents in shells are often, but not always, accompanied by lower δ¹⁸O values. In contrast, there is no evidence of systematic oxygen isotope exchange in conodonts with CAI values up to 5. The tropical-subtropical δ¹⁸O records for brachiopods and conodonts show broad coherence for the Paleozoic, with lower average values for the late Ordovician through earliest Carboniferous (-4.7 and 20.2‰ for brachiopods and conodonts respectively), and higher average values for the early Carboniferous through late Permian (-2.4 and 18.1‰ respectively). Brachiopods and conodonts record essentially the same δ¹⁸O shift, ~2‰, confirming the integrity of the records. Divergences occur on timescales of 10-20 myr, especially during the late Early Devonian, early and mid Mississippian, and early Permian. These divergences reflect and inform upon age uncertainties, depth differences, regional climate patterns, and water mass differences amplified by restricted circulation on epicontinental seas. Remarkably, the average paleotemperature difference between the brachiopod and conodont records (based on locfit regressions) is only 0.4 °C. Assuming a seawater δ¹⁸O of -1‰ VSMOW (“ice-free” conditions), these records yield average temperatures of 25-35 °C for the early Paleozoic, and 15-25 °C for the late Paleozoic. This temperature pattern is supported by clumped isotope paleothermometry, which yields similar temperatures and argues for constant hydrospheric δ¹⁸O through time. The high temperatures of the early Paleozoic match those of warm intervals in Earth history (e.g., Early Triassic, mid Late Cretaceous, early Eocene) and indicate warm temperatures during much of the early evolutionary history of metazoans.