Devriendt, LS; Mcgregor, HV; Chivas, AR
The delta O-18 of ostracod valves is widely used to infer water delta O-18 and temperature. However, ostracod delta O-18 appears sensitive to other environmental variables. In addition, there is species-dependent ostracod calcite O-18 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 delta O-18 variations based on a global database of published ostracod delta O-18 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 degrees C), water delta O-18 (-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 delta O-18 of marine and non-marine ostracods reflects the O-18/O-16 of the sum of host water CO32- and HCO3- ions. For example, at a given temperature, the delta O-18 of non-marine ostracods decreases by 4% to 6% as [CO32-]/[DIC] reaches 70%, depending on the ostracod species. In low [CO2-3]/[DIC] settings (i.e. high HCO3- / CO32-), ostracod O-18/O-16 is close to the O-18/O-16 of HCO3- ions, which explains why on average ostracod delta O-18 is higher than the delta O-18 of inorganic calcite precipitated slowly under the same conditions. (2) Taxonomic offsets in ostracod delta O-18 vary with the host water [CO32-]/[DIC]. In environments where HCO3 >> CO32- (i.e. most freshwater lakes), the O-18/O-16 of Candonids is indistinguishable from the O-18/O-16 of HCO3- ions (difference of 0.10 +/- 0.16%) while the O-18/O-16 of Cyprids is lower than the O-18/O-16 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 delta O-18 to [CO32-]/[DIC] also varies with taxonomy. For each percent increase in [CO32-]/[DIC], Candonids delta O-18 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 O-18/O-16 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 delta O-18 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 delta O-18, which explains the variable salinity and pH effects on ostracod delta O-18. Our database and ostracod delta O-18 model shows that ostracod delta O-18 records from closed basin environments likely reflect high or variable [CO32-]/[DIC], rather than water temperature and delta O-18 alone. Our study reconciles contradictory observations of controls on ostracod delta O-18 and paves the way for improved paleo-environmental interpretations and reconstructions of past water [CO32-]/[DIC]. (C) 2017 Elsevier Ltd. All rights reserved.