Ostracod calcite records the 18O/16O ratio of the bicarbonate and carbonate ions in water

The δ18O of ostracod valves is widely used to infer water δ18O and temperature. However, ostracod δ18O appears sensitive to other environmental variables. In addition, there is species-dependent ostracod calcite 18O enrichment, relative to slowly precipitated inorganic calcite under the same conditions. Together these uncertainties complicate ostracod paleoclimate reconstructions. This study presents a new understanding of the causes of ostracod δ18O variations based on a global database of published ostracod δ18O values in lake, marine and coastal environments, and from culture experiments. The database includes associated field/experiment host water parameters including temperature (−1 to 32 °C), water δ18O (−13.2‰ to 4.3‰ VSMOW), pH (6.9-10.4), salinity (0-72 g/kg), calcite saturation states (0.6-26), and dissolved inorganic carbon concentration [DIC] (0.9-54.3 mmol/kg). The data show that: (1) the δ18O of marine and non-marine ostracods reflects the 18O/16O of the sum of host water CO32− and HCO3− ions. For example, at a given temperature, the δ18O of non-marine ostracods decreases by 4‰ to 6‰ as [CO32−]/[DIC] reaches 70%, depending on the ostracod species. In low [CO32−]/[DIC] settings (i.e. high HCO3−/CO32−), ostracod 18O/16O is close to the 18O/16O of HCO3− ions, which explains why on average ostracod δ18O is higher than the δ18O of inorganic calcite precipitated slowly under the same conditions. (2) Taxonomic offsets in ostracod δ18O vary with the host water [CO32−]/[DIC]. In environments where HCO3− ≫ CO32− (i.e. most freshwater lakes), the 18O/16O of Candonids is indistinguishable from the 18O/16O of HCO3− ions (difference of 0.10 ± 0.16‰) while the 18O/16O of Cyprids is lower than the 18O/16O of HCO3− ions by −0.77‰ to −0.32‰, Cytherids by −0.88 ± 0.29‰, and Limnocytherids by −1.12 ± 0.05‰. (3) The sensitivity of ostracod δ18O to [CO32−]/[DIC] also varies with taxonomy. For each percent increase in [CO32−]/[DIC], Candonids δ18O decreases by −0.098 ± 0.024‰, Cyprids by −0.075 ± 0.004‰, Cytherids by −0.057 ± 0.012‰, and Limnocytherids by −0.058 ± 0.005‰. (4) The similarity in 18O/16O between ostracod calcite and the sum of host water 'HCO3− and CO32−' (despite species-specific offsets) suggests rapid precipitation of a finite DIC pool in the ostracod calcifying fluid. We propose an ostracod δ18O model whereby the more negative the 18O/16O of a given species relative to the sum of CO32− and HCO3−, the greater the isotopic equilibrium between the DIC pool and H2O in the ostracod calcifying fluid. (5) Higher host water salinities and pH induce higher [CO32−]/[DIC], resulting in lower ostracod δ18O, which explains the variable salinity and pH effects on ostracod δ18O. Our database and ostracod δ18O model shows that ostracod δ18O records from closed basin environments likely reflect high or variable [CO32−]/[DIC], rather than water temperature and δ18O alone. Our study reconciles contradictory observations of controls on ostracod δ18O and paves the way for improved paleo-environmental interpretations and reconstructions of past water [CO32−]/[DIC].