Neotropical Ostracode Oxygen and Carbon Isotope Signatures - Implications for Calcification Conditions

Abstract

Calcitic valves of non-marine ostracodes are important geochemical archives. Investigations of the relationship between the isotopic ranges of modern ostracode populations and their host water provide important information on local or regional conditions and influences. Here we present the first δ18Oostracode and δ13Costracode dataset of the freshwater ostracode species Cytheridella ilosvayi along with characterization of the precipitating water - chemical composition, δDwater, δ18Owater, δ13CDIC 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 δ18Oostracode 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 (δ18Ocalcite_eq). Additional to a species-specific offset, unusual high deviations of δ18Oostracode vs. δ18Owater values most likely reflect the time lag between valve calcification and sampling. Coincidence between δ18Oostracode and δ18Ocalcite_eq is restricted to few months indicating a seasonal calcification of Cytheridella. There is a characteristic pattern in its difference between mean δ18Oostracode and δ18Ocalcite_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.