Alkenones and alkenes in surface waters and sediments of the Southern Ocean: Implications for paleotemperature estimation in polar regions

The concentration of C₃₇-C₃₉ long-chain alkenones and alkenes were determined in surface water and surface sediment samples from the subpolar waters of the Southern Ocean. Distributions of these compounds were similar in both sample sets indicating little differential degradation between or within compound classes. The relative amounts of the tri- to tetra-unsaturated C₃₇ alkenones increased with increasing temperature for temperatures below 6°C similar to the di- and tri-unsaturated C₃₇ alkenones. The C₃₇ di-, tri-, and tetra-unsaturated methyl alkenones are used in paleotemperature calculations via the U^K₃₇ and the U^K′₃₇ ratios. In these datasets, the relative abundances of the C_37:2 and the C_37:3 alkenones as a proportion of the total C₃₇ alkenones were opposite and strongly related to temperature (the latter with more scatter), but the abundance of the C_37:4 alkenone showed no relationship with temperature. The original definition of U^K₃₇ includes the abundance of 37:4 in both the numerator and denominator, and thus it is perhaps not surprising that there is considerable scatter in the values obtained for U^K₃₇ at low temperatures. Of the two, we suggest that U^K′₃₇ is the better parameter for use in paleotemperature estimations, even in cold locations. U^K′₃₇ values in the sediments fall on virtually the same regression line obtained for the water column samples of Sikes and Volkman (1993), indicating that their calibration is suitable for use in Southern Ocean sediments. The comparison of water column data with sedimentary temperature estimates suggests that the alkenone distributions are dominated by contributions from the summer when the biomass of Emiliania huxleyi and presumably flux to the sediment, is expected to be high.