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

Abstract

AbstractCalcitic 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 δ18Oostracodeand δ13C of the freshwater ostracode speciesCytheridella ilosvayialong with the isotopic composition of the waters in which the ostracodes calcified, δDwater, δ18Owater, δ13CDICvalues—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 δ18Oostracodevalues. The expected apparent oxygen isotope fractionation between CaCO3and H2O is correlated with temperature with smaller values occurring at higher temperatures as valid at isotope equilibrium (δ18Ocalcite_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 δ18Oostracodeand δ18Ocalcite_eqis restricted to few months indicating a seasonal calcification ofCytheridella. There is a characteristic pattern in its difference between mean δ18Oostracodeand δ18Ocalcite_eqwhich implies thatCytheridellaprovides a synchronous life cycle in its geographical range with two calcification periods in spring (May, June) and autumn (October). This ubiquitous life cycle ofCytheridellain 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.