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

Abstract

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 δ18O values are −10.05‰ for M. cordata, −9.34‰ for C. kingsleii and −8.75‰ for P. rostrata. All three species exhibit positive offset from the weighted mean annual δ18O of contemporary precipitation (−10.7‰ in δ18OV-PDB) and from the mean δ18O value of authigenic tufa carbonate (−10.64‰) in the ostracod-bearing layer. Assuming that the known oxygen isotope fractionation in P. rostrata (+2.5‰) and M. cordata (+1.5‰) has remained constant over time, the theoretical δ18OV-SMOW of the early Holocene lake water was calculated to have been between −11.52 and −11.92‰, slightly less negative than the local Ordovician groundwater (−11.7 to −12.2‰). δ18O values of the tufa carbonate differ by +0.6 to +1.0‰ from the calculated theoretical isotope composition (δ18OV-PDB) of lake water, indicating that the tufa also did not precipitate in isotopic equilibrium with ambient waters. Results show that the greater the δ18O 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 δ18O values in Holocene lacustrine carbonates and ostracods from north-eastern Europe, display pronounced decreases in δ18O 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 δ18O values in lacustrine tufa and ostracods throughout the Holocene.