Oxygen isotope fractionation in the ostracod <Emphasis Type="Italic">Eucypris mareotica</Emphasis>: 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°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 δ¹⁸O values of ostracod shells from the 10°C cultures were higher than those of the 15 and 19°C cultures by 1.6 and 2.7‰, respectively. The observed fractionations are shown by the regression equations: *20c 10^° \textC:d ^{1 8} \textO_{\textostracod} = 1. 1 7+ 0. 5 7d ^{1 8} \textO_\textwater 1 5^° \textC:d ^{1 8} \textO_{\textostracod} = - 0. 4 8+ 0. 6d ^{1 8} \textO_\textwater 1 9^° \textC:d ^{1 8} \textO_{\textostracod} = - 1. 6+ 0. 6d ^{1 8} \textO_\textwater \begin{array}{*{20}c} { 10^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = 1. 1 7+ 0. 5 7\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 5^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 0. 4 8+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 9^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 1. 6+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ \end{array} The fractionation factors (α) are slightly lower for the 15 and 19°C cultures, but slightly higher for the 10°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-δ¹⁸O for E. mareotica is so small that we can neglect its effect when using the δ¹⁸O 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 δ¹⁸O data for E. mareotica are used.