Effects of CO2 and nutrient concentration on the stable carbon isotope composition of C37:2 alkenones in sediments of the South Atlantic Ocean.

The aim of this study was to evaluate the applicability of long-chain alkenones in sediments from the South Atlantic for the reconstruction of past sea-surface temperatures and of CO2- and nutrient concentrations. The investigations focused on both the alkenone unsaturation index) and the stable carbon isotope composition of alkenones. The use of alkenone unsaturation index as paleotemperature proxy requires a firm calibration of the index. Already existing calibrations, based on culture experiments and on recent sediments, have confirmed the general validity of the index. Uncertainties exist, however, regarding the season and the water depth at which the alkenone signal is produced. Moreover, it is suggested that a single calibration may not be applicable in all oceanic regions. For this purpose, the alkenone unsaturation index of 87 surface sediment sampies from the western South Atlantic was converted into temperature and compared with modem atlas sea-surface temperatures.