Stable oxygen isotope record from Globigerina bulloides and sea surface temperature of sediment core BOFS31/1K (Table 1)@en

Two techniques for estimating past variations in sea surface temperature (SST) have been used to investigate climatic change in Biogeochemical Oceanic Flux Study (BOFS) core 31K (19°N, 20°W) from the eastern subtropical Atlantic. High-resolution SST records for the last 28 kyr have been produced using planktonic foraminiferal assemblages, based on the Imbrie-Kipp transfer function technique, and the UK' 37 index derived from abundances of C37 alkenones biosynthesized by prymnesiophyte algae. Modern observations suggest that these indices reflect particular hydrographic conditions in the upper ocean: the UK? 37 index corresponds to the temperature at the time of maximum coccolith productivity, typically late spring-early summer in the study area today, whereas the faunal transfer function is calibrated for seasonal maximum and minimum temperatures. In general, the faunal and biomarker paleotemperature records display comparable SST variations during the last glacial and deglacial, but although the overall trends are similar, differences exist in the magnitude and timing of these temperature changes. Most notably, the faunal T warm and UK' 37 SST estimates diverge by 3°C between 8 ka and 6 ka, and this offset persists through the late Holocene. This difference cannot be adequately explained by uncertainties associated with either the calibration data sets or fluctuating preservation levels. We therefore propose that the deviation in SST estimates is linked to a switch in the seasonal timing of maximum coccolith production from the summer in the glacial ocean to the late spring-early summer in the modern ocean. Our results suggest that a dual approach to SST estimation based on faunal and biomarker proxies can provide a valuable means of evaluating mixed layer and productivity changes associated with the movement of oceanographic frontal zones during the late Quaternary.本研究采用两种古海表温度（sea surface temperature, SST）变化重建技术，对大西洋东部亚热带海域的生物地球化学海洋通量研究（Biogeochemical Oceanic Flux Study, BOFS）31K岩芯（19°N，20°W）的气候变化开展探究。研究基于Imbrie-Kipp转移函数技术，利用浮游有孔虫组合（planktonic foraminiferal assemblages）重建了过去28千年的高分辨率海表温度记录；同时通过定鞭金藻（prymnesiophyte algae）生物合成的C37烯酮（C37 alkenones）的丰度，推导得到UK'37指标。现代观测表明，这两类指标分别反映上层海洋的特定水文条件：UK'37指标对应颗石藻（coccolith）生产力峰值时期的温度，现今研究区的该时段通常为晚春至初夏；而浮游有孔虫群落转移函数则针对季节最高与最低温度进行校准。晚冰期与冰消期内，浮游有孔虫群落与生物标志物古温度记录所呈现的海表温度变化趋势整体相近，但二者在温度变化的幅度与时间节点上仍存在差异。最为显著的是，基于浮游有孔虫群落的古温度估算值与UK'37指标得到的海表温度估算值在8 ka至6 ka期间相差达3℃，且这一偏差在全新世（Holocene）晚期持续存在。该偏差无法通过校准数据集或化石保存程度波动带来的不确定性得到合理解释。因此我们提出，海表温度估算结果的差异与颗石藻生产力峰值的季节时序转变存在关联：冰期海洋中该峰值出现在夏季，而现代海洋中则为晚春至初夏。本研究结果表明，结合浮游有孔虫群落与生物标志物代用指标（proxy）的双轨法海表温度重建，可为评估晚第四纪（late Quaternary）海洋锋面带（oceanographic frontal zones）移动引发的混合层（mixed layer）与生产力变化提供有效手段。