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 in the sediments, and that large differences in δ¹⁸O of up to 6.5 C are shown for selected sediment samples.These δ¹⁸O values are highly and positively correlated with total carbonate contents (TCC) and are not effected by shifts in carbonate mineral composition.This indicates that oxygen stable-isotope fractionation among different carbonate minerals is minimum.The results suggest that marked changes in carbonate δ¹⁸O are resulted predominantly from changes in isotopic composition of the lake water.Our data support the conclusion that isotope fractionation among co-existing carbonate minerals formed in natural environmental conditions are limited to 0-1C in δ¹⁸O.The δ¹⁸O proved to be a useful environmental proxy for the study of the past changes in P-E budget of closed-basin lakes.