ALKENONE AND COCCOLITHOPHORID SPECIES CHANGES IN LATE QUATERNARY SEDIMENTS FROM THE WALVIS RIDGE - IMPLICATIONS FOR THE ALKENONE PALEOTEMPERATURE METHOD

Sea surface temperatures (SSTs) derived from the alkenone U-37(K')) re cord of Quaternary sediments may be subject to bias if algae with diff erent temperature sensitivities have contributed to the sedimentary al kenone record. The alkenone-derived SST records are usually based on a U-37(K')-temperature relationship which was measured in culture exper iments using the coccolithophorid Emiliania huxleyi (F.G. Prahl, L.A. Muehlhausen and D.L. Zahnle, 1988. Further evaluation of long-chain al kenones as indicators of paleoceanographic conditions. Geochim. Cosmoc him. Acta 52, 2303-2310). To assess possible effects of past species c hanges on the U-37(K')-temperature signal, we have analysed long-chain alkenones and coccolithophorids in a late Quaternary sediment core fr om the Walvis Ridge and compared the results to SST estimates extracte d from the (delta(18)O record of the planktonic foraminifer Globigerin oides ruber. Alkenones and isotopes were determined over the entire 40 0-kyr core record while the coccolithophorid study was confined to the last 200 kyr when the most pronounced chan es in alkenone content occ urred. Throughout oxygen-isotope stages 6 and 5, species of the genus Gephyrocapsa were the predominating coccolithophorids. E. huxleyi bega n to increase systematically in relative abundance since the stage 5/4 transition, became dominant over Gephyrocapsa spp. during stage 3 and reached the highest abundances in the Holocene. Carbon-normalized alk enone concentrations are inversely related to the relative abundances of E. huxleyi, and directly related to that of Gephyrocapsa spp., sugg esting that species of this genus were the principal alkenone contribu tors to the sediments. Nevertheless, SST values obtained from the U-37 (K')-temperature relationship for E. huxleyi compare favourably to the isotope-derived temperatures. The recently U-37(K')-temperature relat ionship for a single strain of Gephyrocapsa oceanica (J.K. Volkman, S. M. Barrett, S.I. Blackburn and E.L. Sikes, 1995. Alkenones in Gephyroc apsa oceanica: Implications for studies of paleoclimate. Geochim. Cosm ochim. Acta 59, 513-520) produces unrealistically high SST values indi cating that the temperature response of the examined strain is not typ ical for the genus Gephyrocapsa . This is supported by the C-37:C-38 a lkenone ratios of the sediments which are comparable to average ratios reported for E. huxleyi, but significantly higher than for the G. oce anica strain. Most notably, the general accordance of the alkenone cha racteristics between sediments and E. huxleyi persists through stages 8 to 5 and even in rimes that predate the first appearance of this spe cies (268 ka; H.R. Thierstein, K.R. Geitzenauer and B. Molfino, 1977. Global synchroneity of late Quaternary coccolith datum levels: Validat ion by oxygen isotopes. Geology 5, 400-404). Our results suggest that U-37(K')-temperature relationships based on E. huxleyi produce reasona ble paleo-SST estimates even for late Quaternary periods when this spe cies was scarce or absent because other alkenone-synthesizing algae, e .g. of the genus Gephyrocapsa, responded similarly to temperature chan ges. (C) 1997 Elsevier Science B.V.