Oxygen isotope fractionation in the ostracod Eucypris mareotica: results from a culture experiment and implications for paleoclimate reconstruction

Oxygen isotope analysis of the adult ostracod Eucypris mareotica cultured at controlled temperatures (10, 15, and 19 degrees C) was used to measure isotopic fractionation during shell calcification. The ostracod shells that precipitated at experimental temperatures are almost in isotopic equilibrium with the culture water as compared to the oxygen isotope fractionation of inorganic carbonates. Moreover, they had almost constant offsets from equilibrium based on the oxygen isotope fractionation of inorganic carbonates. The delta O-18 values of ostracod shells from the 10 degrees C cultures were higher than those of the 15 and 19 degrees C cultures by 1.6 and 2.7%, respectively. The observed fractionations are shown by the regression equations: 10 degrees C : delta O-18(ostracod) = 1.17 + 0.57 delta O-18(water) 15 degrees C : delta O-18(ostracod) = -0.48 + 0.6 delta O-18(water) 19 degrees C : delta O-18(ostracod) = -1.6 + 0.6 delta O-18(water) The fractionation factors (a) are slightly lower for the 15 and 19 degrees C cultures, but slightly higher for the 10 degrees C culture, as compared to inorganic carbonates (O'Neil et al. in J Chem Phys 51:5547-5558, 1969). The oxygen fractionation factors of E. mareotica are very close to those of synthetic calcite formed in isotopic equilibrium. The 'vital offsets' of valve-delta O-18 for E. mareotica is so small that we can neglect its effect when using the delta O-18 of E. mareotica living in lake waters with high pH and salinity to reconstruct the paleoenvironment. The paleotemperature or paleoisotopic composition of lake water interpreted from a core of lacustrine sediment may be closer to the true values when the delta O-18 data for E. mareotica are used.