Oxygen isotope analysis of diatom silica and authigenic calcite from Lake Pinarbasi, Turkey

There is increasing interest in the O-18/O-16 ratio of diatom silica, parti cularly for lakes where carbonates are absent. Here we compare the O-18/O-1 6 ratios preserved in diatom silica and authigenic calcite from an open, sp ring-fed, freshwater lake core from Turkey which spans marine oxygen isotop e stage 3. The two sets of isotope data show contrasting trends in spite of their mutual dependence on the water O-18/O-16 ratio and lake-water temper ature. The most likely explanation for this divergence is difference in sea sonality of biological productivity mediated by the strongly continental cl imate of the Anatolian plateau. Diatom silica and authigenic calcite are pr ecipitated from solutes in the lake-water at different times of the year. D iatom productivity follows a well-defined seasonal cycle, peaking first and most importantly in the spring and then in the autumn. The precipitation o f calcite follows productivity by all forms of photosynthetic organisms tha t deplete CO2 but in most lakes this occurs during the summer months. The d elta O-18(calcite) curve shows mean summer temperature maxima at ca. 30-35 and ca. 58 ka BP while the intervening data represent a period of relativel y cool summers. The delta O-18(diatom) curve shows bipolar results (15-20 a nd 29-33 parts per thousand), which suggests that at least two discrete sou rces or processes contributed to the oxygen composition of the diatoms but probably involved a dilution mechanism to shift the isotopic values. The mo st likely source of depleted water is from snow entering the lake during th e spring thaw. We infer that many authigenic calcite curves from regions wi th markedly seasonal climates may be temporally limited to a few summer mon ths and that diatom silica provides complementary data on seasonally-specif ic water isotopic composition rather than a substitute for analyses based o n carbonate.