UNUSUAL DISTRIBUTIONS OF LONG-CHAIN ALKENONES AND TETRAHYMANOL FROM THE HIGHLY ALKALINE LAKE VAN, TURKEY

Long-chain C-37 to C-40 alkenones with di-, tri-, and tetra-unsaturati on are very abundant in sediment trap material and Holocene to Late Pl eistocene core samples from the Earth's largest soda lake, Lake Van (T urkey). Thus, the known distribution range of these typical biomarkers for haptophyte microalgae is extended to highly alkaline environments . The observed unsaturation patterns differ strikingly from those foun d in open marine haptophytes and sediments by an enhanced relative abu ndance of the tetra-unsaturated compounds, especially the C-37:4 methy l ketone. Their preponderance is suggested to be a facies marker patte rn for lacustrine and marginal marine areas of sedimentation. Using pu blished U-37(K) calibrations, no reliable absolute temperatures were o btained for the Lake Van samples. Accordingly, marine sea surface temp erature determinations based on long-chain alkenones should be applied with caution when a contribution of these compounds from coastal or n onmarine sources can not be excluded. The presence of tetrahymanol and gammacer-3-one in the Lake Van materials is attributed to organic mat ter contributions of ciliates. The relative abundance of long-chain al kenones and of tetrahymanol/ gammacer-3-one is considered to reflect c hanges in the environmental conditions, in particular in the hydrologi cal setting. We suggest that times of pronounced stagnation are recogn ised by very high tetrahymanol/gammacer-3-one concentrations together with drastically increased stanol/stenol ratios, and intervals of enha nced convection or of high freshwater input are characterised by high alkenone contributions. Copyright (C) 1997 Elsevier Science Ltd.