GEOCHEMISTRY OF OSTRACODE CALCITE .1. AN EXPERIMENTAL-DETERMINATION OF OXYGEN-ISOTOPE FRACTIONATION

Late-instar juveniles of the North American ostracode, Candona rawsoni , were cultured to maturity under controlled oxygen isotope compositio n and temperature (15 degrees C and 25 degrees C) to measure isotopic fractionation during shell calcification. The ostracode shells formed at experimental temperatures are not in isotopic equilibrium with wate r but had a constant offset from equilibrium based on the oxygen isoto pe fractionation of inorganic carbonates. The oxygen isotope compositi on of ostracode shells from 15 degrees C cultures were higher by about 2 parts per thousand compared to that from 25 degrees C cultures, a d ifference similar to that expected for inorganic calcite across a 10 d egrees C temperature range. The observed fractionations are expressed by the regression equations 25 degrees C: delta(18)O(shell)(PDB) = -0. 47 + 0.97 delta(18)O(water)(VSMOW) 15 degrees C: delta(18)O(shell)(PDB ) = 1.12 + 1.07 delta(18)O(water)(VSMOW) The fractionation factors (al pha) are: 1.0305 at 25 degrees C and 1.0322 at 15 degrees C. The shell s of C rawsoni are elevated by about 0.8-1 parts per thousand in delta (18)O compared to inorganic carbonates (O'Neil et al., 1969), but are close to marine biogenic aragonites (Grossman and Ku, 1986) with a sli ghtly different temperature dependence. Copyright (C) 1997 Elsevier Sc ience Ltd.