Oxygen isotope fractionation in three freshwater ostracod species from early Holocene lacustrine tufa in northern Estonia

We quantified differences in oxygen isotope fractionation among three biostratigraphically important subfossil ostracod species (Metacypris cordata, Pseudocandona rostrata and Candonopsis kingsleii) from an early Holocene freshwater tufa layer in northern Estonia. Estimated mean delta O-18 values are -10.05aEuro degrees for M. cordata, -9.34aEuro degrees for C. kingsleii and -8.75aEuro degrees for P. rostrata. All three species exhibit positive offset from the weighted mean annual delta O-18 of contemporary precipitation (-10.7aEuro degrees in delta O-18(V-PDB)) and from the mean delta O-18 value of authigenic tufa carbonate (-10.64aEuro degrees) in the ostracod-bearing layer. Assuming that the known oxygen isotope fractionation in P. rostrata (+2.5aEuro degrees) and M. cordata (+1.5aEuro degrees) has remained constant over time, the theoretical delta O-18(V-SMOW) of the early Holocene lake water was calculated to have been between -11.52 and -11.92aEuro degrees, slightly less negative than the local Ordovician groundwater (-11.7 to -12.2aEuro degrees). delta O-18 values of the tufa carbonate differ by +0.6 to +1.0aEuro degrees from the calculated theoretical isotope composition (delta O-18(V-PDB)) of lake water, indicating that the tufa also did not precipitate in isotopic equilibrium with ambient waters. Results show that the greater the delta O-18 offset from the calculated, theoretical isotope composition of lake water for an ostracod species, the lower is its preferred mean July temperature. Both our data and earlier published results on delta O-18 values in Holocene lacustrine carbonates and ostracods from north-eastern Europe, display pronounced decreases in delta O-18 with an increase in latitude of the study site. This suggests that temperature-dependent, and therefore latitude-dependent isotopic composition of meteoric waters controlled the delta O-18 values in lacustrine tufa and ostracods throughout the Holocene.