OXYGEN AND CARBON ISOTOPE FRACTIONATION IN BIOGENIC ARAGONITE - TEMPERATURE EFFECTS
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abstract
To better interpret the isotopic composition of ancient aragonitic fossils, stable-isotopic analyses have been performed on live and modern specimens of aragonitic foraminifera, gastropods and scaphopods. Samples were collected from the continental margins off southern California and Texas, U.S.A., and Mexico, and provide a range in ambient temperature of 2.6-22.0C. We observed a strong covariance between the 18O of the aragonitic foraminifera Hoeglundina elegans and that of coeval aragonitic mollusks. On the average, Hoeglundina was 0.2 0.2 depleted in 18O relative to the mollusks, and 0.6 0.3 enriched relative to the calcitic foraminifera Uvigerina. This enrichment in 18O of aragonite relative to calcite is similar to that observed in previous experimental and theoretical studies. The temperature dependences of mollusk and Hoeglundina 18O-values were not notably different from that previously determined for inorganically precipitated calcite, and no significant temperature dependence in Hoeglundina-Uvigerina18O fractionation was observed. Of note is the temperature dependence of the 13C of the biogenic aragonite. Relative to the dissolved inorganic carbon (DIC), the 13C of Hoeglundina and the mollusks decreased by 0.11 and 0.13, respectively, per C increase in temperature. The temperature dependence in Hoeglundina-DIC 13C enrichment, and the lack of it in Uvigerina-DIC enrichment, accounts for the temperature dependence in Hoeglundina-Uvigerina (calcitic) fractionation noted by us and previous workers. Isotopic differences between coeval specimens of these genera provide a rough measure of paleotemperature without requiring a knowledge of the isotopic composition of the paleo-ocean. 1986.
