Core-top calibration of the alkenone index vs sea surface temperature in the Indian Ocean

The alkenone index (UK′37) has been calibrated against sea surface temperature (SST) using a suite of 54 surface sediments from the Indian Ocean (20°N to 45°S), spanning temperatures between 6 and 29°C (0.2 < UK′37 < 1.0). A computerized oceanographic atlas has been used to calculate modern SST for the coring sites. We have constructed two sets of UK′37−SST calibrations for various water depths between 10 and 50m. The first set is based on the comparison of UK′37 with annual mean temperatures for all cores. In the second set, we assume that UK′37 values in surface sediments are linked to seasonal productivity patterns, and UK′37 is compared with mean temperatures at times of yearly production maximum. In both calibrations, the UK′37 values are strongly correlated with SST, with a best fit obtained using temperatures from 10 m and seasonal productivity patterns. The regression line is similar to that found with laboratory cultures of E. huxleyi, and with other calibrations based on recent sediments from other oceans. Above 24°C the slope of our UK′37−SST relationship is smaller (approximately 0.020 UK′37 unit °C−1) than the general slope (approximately 0.033).