Evaluation of Oxygen Stable-Isotope Fractionation Among Co-existed Carbonates in the Sediments of Lake Qinghai

Mineralogical examination and stable isotope analyses for the Lake Qinghai sediments indicate that authigenic carbonate minerals are present either in the form of one type or several types co-existed in the sediments, and that large differences inδ18O of up to 6.5‰ are shown for selected sediment samples. Theseδ18O values are highly and positively correlated with total carbonate contents (TCC) and are not affected by shifts in carbonate mineral composition, indicating that oxygen stable-isotope fractionation among different carbonate minerals is minimum. The results suggest that marked changes in carbonateδ18O are resulted predominantly from changes in isotopic composition of the lake water, and that differentiatedδ18O values resulted possibly from isotope fractionation among co-existed carbonates formed in natural environmental conditions are limited to 0-1‰. Theδ18O proved to be a useful environmental proxy for the study of the past changes in P-E budget of closed-basin lakes.