Unusual distributions of long-chain alkenones and tetrahymanol from the highly alkaline Lake Van, Turkey

Abstract

Long-chain C37 to C40 alkenones with di-, tri-, and tetra-unsaturation are very abundant in sediment trap material and Holocene to Late Pleistocene core samples from the Earth's largest soda lake, Lake Van (Turkey). 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 found in open marine haptophytes and sediments by an enhanced relative abundance of the tetra-unsaturated compounds, especially the C37:4 methyl ketone. Their preponderance is suggested to be a facies marker pattern for lacustrine and marginal marine areas of sedimentation. Using published U37K calibrations, no reliable absolute temperatures were obtained for the Lake Van samples. Accordingly, marine sea surface temperature determinations based on long-chain alkenones should be applied with caution when a contribution of these compounds from coastal or nonmarine sources can not be excluded. The presence of tetrahymanol and gammacer-3-one in the Lake Van materials is attributed to organic matter contributions of ciliates. The relative abundance of long-chain alkenones and of tetrahymanol/gammacer-3-one is considered to reflect changes in the environmental conditions, in particular in the hydrological setting. We suggest that times of pronounced stagnation are recognised by very high tetrahymanol/gammacer-3-one concentrations together with drastically increased stanol/stenol ratios, and intervals of enhanced convection or of high freshwater input are characterised by high alkenone contributions.