Comparison of U37k′ and diatom assemblage sea surface temperature estimates with atlas derived data in Holocene sediments from the Southern West Indian Ocean

Holocene surface sediments, taken with interface multicorers (cruise ANTARES 1) on two transects located on longitudes 56°E and 58°E, and from 41°S to 52°S, i.e., across the Subtropical, Subantarctic and Polar Front Zones, are studied. Based on micropalaeontologic data (diatoms, coccoliths and foraminifera), these samples are considered to represent the last 2000 years. Phytoplankton (coccoliths and diatoms) abundance and species composition are determined and quantitatively related to the February sea-surface temperature (SST) derived from the NOAA Atlas 1994 (between 15.7 and 3.7°C). The coccolithophorid Emiliania huxleyi and the diatom Nitzschia kerguelensis are the dominant species in each microfossil group. Di- and tri-unsaturated long chain C37 alkenones (produced by Prymnesiophyceae phytoplankton) were extracted from 13 surface sediments. Comparison of alkenone unsaturation ratios (U37k′) with present day February SST revealed an apparently linear relationship, however the data scatter increases between 7°C and 4°C. The temperatures calculated using the U37k′-temperature calibration of Sikes and Volkman [Sikes, E.L., Volkman, J.K., 1993. Calibration of alkenone unsaturation ratios (U3k′) for paleotemperatures estimation in cold waters. Geochim. Cosmochim. Acta 57, 1883–1889.] show that temperature estimates are accurate within the range 16°C–8°C, but less reliable between 8°C and 3.7°C. The estimated U37k′-based temperatures were compared to those obtained from Antarctic diatom distributions using the Modern Analog Technique (MAT). The usefulness of both techniques as indicators of past SST records is demonstrated.