Calibration of unsaturation patterns in long-chain ketone compositions for palaeotemperature assessment

Abstract

A series of long-chain (C₃₇, C₃₈, C₃₉), primarily di- and tri-unsaturated methyl and ethyl ketones, first identified in sediments from Walvis Ridge off West Africa and from the Black Sea¹, has been found in marine sediments throughout the world². The marine coccolithophorid Emiliania huxleyi and members of the class Prymnesiophyceae are now the recognized sources of these compounds^3,4. Experiments with laboratory cultures of algae showed the degree of unsaturation in the ketone series biosynthesized depends on growth temperature^2,5, a physiological response observed for classical membrane lipids⁶. Brassell and co-workers^2,7 thus proposed that systematic fluctuations in the unsaturation of these alkenones noted down-core in sediments from the Kane Gap region of the north-east tropical Atlantic Ocean and correlated with glacial-interglacial cycles provide an organic geochemical measure of past sea-surface water temperatures. Using laboratory cultures of E. huxleyi, we have calibrated changes in the unsaturation pattern of the long-chain ketone series versus growth temperature. The calibration curve is linear and accurately predicts unsaturation patterns observed in natural particulate materials collected from oceanic waters of known temperature. We present evidence supporting the proposed palaeotemperature hypothesis^2,7 and suggesting absolute 'sea-surface temperatures' for a given oceanic location can be estimated from an analysis of long-chain ketone compositions preserved in glacial and interglacial horizons of deep-sea sediment cores.