Alkenones and coccolithophorids in late Quaternary sediments from the Walvis Ridge

Sea surface temperatures (SSTs) derived from the alkenone UK'37) record of Quaternary sediments may be subject to bias if algae with different temperature sensitivities have contributed to the sedimentary alkenone record. The alkenone-derived SST records are usually based on a UK'37-temperature relationship which was measured in culture experiments using the coccolithophorid Emiliania huxleyi (F.G. Prahl, L.A. Muehlhausen and D.L. Zahnle, 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochim. Cosmochim. Acta 52, 2303-2310). To assess possible effects of past species changes on the UK'37-temperature signal, we have analyzed long-chain alkenones and coccolithophorids in a late Quaternary sediment core from the Walvis Ridge and compared the results to SST estimates extracted from the d18O record of the planktonic foraminifer Globigerinoides ruber. Alkenones and isotopes were determined over the entire 400-kyr core record while the coccolithophorid study was confined to the last 200 kyr when the most pronounced changes in alkenone content occurred. Throughout oxygen-isotope stages 6 and 5, species of the genus Gephyrocapsa were the predominating coccolithophorids. E. huxleyi began 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 alkenone concentrations are inversely related to the relative abundances of E. huxleyi, and directly related to that of Gephyrocapsa spp., suggesting that species of this genus were the principal alkenone contributors to the sediments. Nevertheless, SST values obtained from the UK'37-temperature relationship for E. huxleyi compare favourably to the isotope-derived temperatures. The recently reported UK'37-temperature relationship for a single strain of Gephyrocapsa oceanica (J.K. Volkman. S.M. Barrett, S.I. Blackburn and E.L. Sikes, 1995. Alkenones in Gephyrocapsa oceanica: Implications for studies of paleoclimate. Geochim. Cosmochim. Acta 59, 513-520) produces unrealistically high SST values indicating that the temperature response of the examined strain is not typical for the genus Gephyrocapsa. This is supported by the C37:C38, alkenone ratios of the sediments which are comparable to average ratios reported for E. huxleyi, but significantly higher than for the G. oceanica strain. Most notably, the general accordance of the alkenone characteristics between sediments and E. huxleyi persists through stages 8 to 5 and even in times that predate the first appearance of this species (268 ka; H.R. Thierstein, K.R. Geitzenauer and B. Molfino, 1977. Global synchroneity of late Quaternary coccolith datum levels: Validation by oxygen isotopes. Geology 5, 400-404). Our results suggest that UK'37-temperature relationships based on E. huxleyi produce reasonable paleo-SST estimates even for late Quaternary periods when this species was scarce or absent because other alkenone-synthesizing algae, e.g. of the genus Gephyrocapsa.

基于第四纪沉积物中烯酮（alkenone）UK'37记录重建的海表温度（Sea Surface Temperatures, SST），若具有不同温度敏感性的藻类参与了沉积烯酮记录的形成，则可能引入偏差。烯酮反演得到的海表温度记录通常基于UK'37-温度校准关系，该关系由F.G. Prahl、L.A. Muehlhausen与D.L. Zahnle于1988年在颗石藻（coccolithophorid）艾氏扁球藻（Emiliania huxleyi）的培养实验中测得（F.G. Prahl, L.A. Muehlhausen and D.L. Zahnle, 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochim. Cosmochim. Acta 52, 2303-2310）。为评估地质历史上颗石藻物种更替对UK'37-温度信号的潜在影响，我们对沃尔维斯脊（Walvis Ridge）晚第四纪沉积岩芯中的长链烯酮与颗石藻进行了分析，并将结果与浮游有孔虫（planktonic foraminifer）红拟抱球虫（Globigerinoides ruber）的δ18O记录反演得到的海表温度估算值进行对比。本研究在整个40万年的岩芯记录中测定了烯酮与同位素组成，而颗石藻分析仅局限于烯酮含量变化最显著的近20万年时段。在氧同位素6期至5期期间，桥石藻属（Gephyrocapsa）物种为优势颗石藻类群。艾氏扁球藻自5/4期界面起相对丰度逐步升高，在3期时取代桥石藻属成为优势类群，并在全新世（Holocene）达到最高丰度。经碳标准化的烯酮浓度与艾氏扁球藻的相对丰度呈负相关，而与桥石藻属物种的相对丰度呈正相关，表明该属藻类是沉积物中烯酮的主要贡献者。尽管如此，基于艾氏扁球藻的UK'37-温度关系得到的海表温度估算值，与同位素反演的温度结果吻合良好。此前有研究基于大洋桥石藻（Gephyrocapsa oceanica）单一株系建立了UK'37-温度校准关系（J.K. Volkman, S.M. Barrett, S.I. Blackburn and E.L. Sikes, 1995. Alkenones in Gephyrocapsa oceanica: Implications for studies of paleoclimate. Geochim. Cosmochim. Acta 59, 513-520），该关系得到的海表温度值过高且不符合实际，表明所测试株系的温度响应特征并不代表桥石藻属的整体特征。沉积物的C37:C38烯酮比值与已报道的艾氏扁球藻平均比值相近，且显著高于大洋桥石藻株系的比值，这一结果佐证了上述结论。尤为关键的是，沉积物与艾氏扁球藻的烯酮特征一致性，贯穿了氧同位素8期至5期，甚至早于该物种首次出现的时期（26.8万年；H.R. Thierstein, K.R. Geitzenauer and B. Molfino, 1977. Global synchroneity of late Quaternary coccolith datum levels: Validation by oxygen isotopes. Geology 5, 400-404）。本研究结果表明，即便在晚第四纪艾氏扁球藻稀缺或缺失的时期，基于艾氏扁球藻建立的UK'37-温度关系仍可得到合理的古海表温度估算值，因为彼时存在其他能够合成烯酮的藻类类群，例如桥石藻属类群。