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 -8‰ VPDB in the Cambrian to approximately -1‰ VPDB 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 clumped isotope thermometry on Late Ordovician brachiopod shells and internal cements from the Cincinnati Arch in North America. \nThe brachiopod shells yield Δ47 temperatures of 25-61°C, and δ18OH2O values calculated from Δ47 temperatures and the 18O paleotemperature equation of Kim and O’Neil (1997) provide values of -2.9 to 4.8‰ VSMOW. The quasi-lognormal data distribution suggests partial reordering of some of the shells; however, interpretation of brachiopod temperature data was limited to the modal data (30-35 °C), representing the original temperature distribution. Cement Δ47 temperatures (17-85° C) average higher than those of brachiopod shells, as are calculated δ18OH2O values (-5.7 to 4.4‰ VSMOW). Calculated δ18OH2O 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. \nClumped isotope analyses of cements, aided by cathodoluminescence microscopy, can provide the progression from sea surface to shallow burial to deep burial temperatures. Cements show uniform cathodoluminescence and δ18O values mostly 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 100 °C necessary to begin reordering clumped carbonate bonds. However, the higher Δ47 temperatures and δ18OH2O values and similar δ18Ocarb values of cements compared with brachiopods indicate greater solid-state reordering in the cements. The brachiopod shell results support the contention of warm early Paleozoic oceans while the cement results imply that the chemistry of different calcitic materials such as calcites rich in Mn, can have different reordering rates.