STABLE ISOTOPE VARIATION IN ORDOVICIAN BRACHIOPODS FROM THE TRENTON GROUP OF CENTRAL NEW YORK; IMPLICATIONS FOR GEOCHEMISTRY OF ORDOVICIAN SEAWATER

Stable isotope ratios of oxygen in brachiopod shell carbonate have been used widely as proxies for seawater δ¹⁸O through the Phanerozoic (e.g. Veizer, et al, 1997). In this study, stable isotope ratios of carbon and oxygen were obtained on well-preserved brachiopod shells from the Napanee and Kings Falls Formations of the Ordovician (Chatfieldian) Trenton Group in central New York. Our sample set includes multiple specimens of orthid and strophomenid brachiopod genera with excellent preservation of primary shell microfabrics and ultrastructure when imaged using SEM. With the exception the punctate genus Rafinesquina, all brachiopod samples have a narrow range of δ ¹⁸O_PDB from -6.2 to -5.8 per mil. δ ¹³C_PDB values are somewhat more variable (+1.0 to +2.1 per mil) and clustered by genus. Calcite spar cement in shelter voids within brachipod packstones has δ ¹⁸O_PDB of -8.3 to -8.7 and δ ¹³C_PDB of +1.0 to +0.9, suggesting precipitation of cement from warm water during early burial diagensis. Late hydrothermal vein calcite has δ ¹⁸O_PDB of -11.2 and δ ¹³C_PDB of -0.2. We interpret the brachiopod data as recording equilibrium precipitation of shell calcite from the Ordovician seawater of the Trenton shelf, based upon studies of modern brachipods that support their use as records of seawater stable isotopic signatures (e.g. Parkinson, et al 2005). Using the isotope paleothermomenter of Anderson and Arthur (1983), our data suggest that Trenton shelf seawater had δ ¹⁸O_SMOW of -3 to -5 per mil, assuming that paleotemperatures were in the range of 20-35^oC. These data are consistent with the Phanerozoic data set of Veizer, et al (1999) and support the hypothesis of secular change in the stable isotope chemistry of the global oceans through the Phanerozoic.