OCEAN TEMPERATURES IN THE LATE ORDOVICIAN: A CLUMPED ISOTOPE STUDY OF BRACHIOPODS AND CEMENTS

Throughout the Phanerozoic, δ¹⁸O of carbonate fossils displays an increasing trend of -7 to -8‰ in the Cambrian to approximately -1‰ in the modern. This trend has led to three different hypotheses: 1) sea surface temperatures decreased through the Phanerozoic; 2) δ¹⁸O of seawater rose through the Phanerozoic, or 3) diagenesis has altered older samples more than younger samples. To test these hypotheses, we used cathodoluminescence and clumped carbonate isotope thermometry on Late Ordovician brachiopod shells and internal calcite cements from the Cincinnati Arch in North America.

The brachiopod shells yield Δ₄₇ temperatures of 30-67°C, and δ¹⁸O_H2O values calculated from Δ₄₇ temperatures and the ¹⁸O paleotemperature equation of Kim and O’Neil (1997, Geochim. Cosmochim. Acta) provide values of -2.2 to 5.5‰. These data suggest partial reordering of some of the shells, however we have limited interpretation of brachiopod temperature data to those that do not exceed the metazoan population thermal limit of 42° C put forward by Pörtner (2002, Comp. Biochem. Physiol. A). Cement Δ₄₇ temperatures (40-75° C) are higher than those of brachiopod shells, as are calculated δ¹⁸O_H2O values (0.9 to 6.3‰). Calculated δ¹⁸O_H2O of the brachiopods echo previous studies suggesting that seawater δ¹⁸O has not changed more than ± 1‰ throughout the Phanerozoic and that the trend seen in the δ¹⁸O of carbonate fossils is a product of higher temperatures during the early Paleozoic with cooling towards the modern.

Cements show uniform cathodoluminescence and yield d¹⁸O values similar to those of the best-preserved brachiopod fossils indicating that these cements formed in a marine setting. Based on the burial history of the units, the brachiopods and cements did not reach the threshold of 80° C to begin reordering the clumped carbonate bonds. However, the higher Δ₄₇ temperatures and δ¹⁸O_H2O values for the cements, while having δ¹⁸O_carb values similar to the brachiopods, are indicative of greater solid-state reordering in the cements. This suggests that the calcite cements began reordering at a lower temperature than brachiopod shells. Overall, these results support the contention of warm early Paleozoic oceans and imply that different calcitic materials can have different rates of reordering.