Carbon and oxygen isotope variations among lacustrine ostracods: implications for palaeoclimatic studies

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Abstract
Large ranges of 13C/12C and 18O/ 16O ratios are found among modem ostracods from lakes in Jamaica and Mexico. If their calcite carapaces grew in isotopic equilibrium with the lake water, then the main period of carapace formation was probably in the cooler and/or wetter months. Differences in δ13C and δ18 O values between ostracods from different sites relate to spatial variations in the composition of the lake water, and differences between species at the same site probably reflect differences in organic photosynthesis or decay among the micro-habitats occupied by the species. There nevertheless remain significant variations in both δ13C and δ18O among modern valves of the same species at the same site; a consequence of the temporal and spatial variation in lake water and the fact that ostracods form their carapace over a short period. Individual valves from a 1-cm section of a sediment core show that the isotopic effects of changes in the rainfall versus evaporation status of a lake over a period of c. 102 years can be of a similar magnitude to changes recorded by bulk, fine-grained carbonate throughout the late Quaternary. These findings emphasize the need for caution when using isotopic data for single ostracod valves in palaeoclimatic studies.