Neotropical ostracode oxygen and carbon isotope signatures: implications for calcification conditions

Calcitic valves of non-marine ostracodes are important geochemical archives. Investigations of the relationship between the ranges of oxygen and carbon isotope values of modern ostracode populations and their host water provide important information on local or regional conditions and influences. Here we present the first δ¹⁸O_ostracode and δ¹³C of the freshwater ostracode species Cytheridella ilosvayi along with the isotopic composition of the waters in which the ostracodes calcified, δD_water, δ¹⁸O_water, δ¹³C_DIC values-covering a large geographical range (Florida to Brazil). With this data we extended a newly developed approach based on the estimation of δ¹⁸O values of monthly equilibrium calcites as references for the interpretation of δ¹⁸O_ostracode values. The expected apparent oxygen isotope fractionation between CaCO₃ and H₂O is correlated with temperature with smaller values occurring at higher temperatures as valid at isotope equilibrium (δ¹⁸O_calcite__eq). Uncertainties about the expected equilibrium calcites derive from incomplete knowledge of high-frequency variations of the water bodies caused by interplay of mixing, evaporation, and temperature. Coincidence between δ¹⁸O_ostracode and δ¹⁸O_calcite__eq is restricted to few months indicating a seasonal calcification of Cytheridella. There is a characteristic pattern in its difference between mean δ¹⁸O_ostracode and δ¹⁸O_calcite__eq which implies that Cytheridella provides a synchronous life cycle in its geographical range with two calcification periods in spring (May, June) and autumn (October). This ubiquitous life cycle of Cytheridella in the entire study area is considered to be phylogenetically inherited. It might have originally been adapted to environmental conditions but has been conserved during the migration and radiation of the group over the Neotropical realm.

The online version contains supplementary material available at 10.1007/s10533-022-00917-9.