Comparison of U-37(k') 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 ANTARE S 1) on two transects located on longitudes 56 degrees E and 58 degrees E, and from 41 degrees S to 52 degrees S, i.e., across the Subtropical, Subant arctic and Polar Front Zones, are studied. Based on micropalaeontologic dat a (diatoms, coccoliths and foraminifera), these samples are considered to r epresent the last 2000 years. Phytoplankton (coccoliths and diatoms) abunda nce and species composition are determined and quantitatively related to th e February sea-surface temperature (SST) derived from the NOAA Atlas 1994 ( between 15.7 and 3.7 degrees C). The coccolithophorid Emiliania huxleyi and the diatom Nitzschia kerguelensis are the dominant species in each microfo ssil group. Di- and tri-unsaturated long chain C-37 alkenones (produced by Prymnesiophyceae phytoplankton) were extracted from 13 surface sediments. C omparison of alkenone unsaturation ratios (U-37(k')) with present day Febru ary SST revealed an apparently linear relationship, however the data scatte r increases between 7 degrees C and 4 degrees C. The temperatures calculate d using the U-37(k')-temperature calibration of Sikes and Volkman [Sikes, E .L., Volkman, J.K., 1993. Calibration of alkenone unsaturation ratios (U-3( k')) for paleotemperatures estimation in cold waters. Geochim. Cosmochim. A cta 57, 1883-1889.] show that temperature estimates are accurate within the range 16 degrees C-8 degrees C, but less reliable between 8 degrees C and 3.7 degrees C. The estimated U-37(k')-based temperatures were compared to t hose obtained from Antarctic diatom distributions using the Modem Analog Te chnique (MAT). The usefulness of both techniques as indicators of past SST records is demonstrated.