Oxygen isotope analysis of multiple, single ostracod valves as a proxy for combined variability in seasonal temperature and lake water oxygen isotopes

摘要

Paleoclimate studies in lakes typically use oxygen isotopic ratios in samples that consist of multiple ostracod specimens, to obtain an average delta O-18 value that reflects the mean temperature and delta O-18 of lake water over the life spans of the combined individuals measured. This approach overlooks potential information on seasonal climate variability that is recorded in the single valves of short-lived ostracods. Here we estimate seasonal variability in ostracod delta O-18 by measuring 10-30 individual carapaces of Cyprideis torosa in selected stratigraphic levels of a sediment core from paleolake Riwasa in Haryana, India. The mean delta O-18 values of ostracod populations show a general decrease from 9.6 to 8.3 kyr BP, which was interpreted previously as resulting from strengthening of the Indian summer monsoon during the early Holocene. The delta O-18 measurements of single ostracods within samples show a large range (up to 15 aEuro degrees) and standard deviation (up to +/- 3.3), suggesting high seasonal variability in the hydrology of this playa lake. The great variability is ascribed to changes in both seasonal temperature (16 A degrees C) and delta O-18 of lake water in a drying water body. The latter is attributable to the Rayleigh distillation process, described using a Craig-Gordon model for isotopic fractionation during evaporation from an open water body. Our results suggest that the range of delta O-18 values measured in single ostracod carapaces is useful to evaluate seasonal changes in lake temperature and hydrology. Even with great intra-sample delta O-18 variability, however, the mean delta O-18 of multiple (more than 10) ostracods can be used to infer long-term climate trends.