A Middle Pleistocene nannofossil record of sediment core MD03-2699

Changes in paleoclimate and paleoproductivity patterns have been identified by analysing, in conjunction with other available proxy data, the coccolithophore assemblages from core MD03-2699, located in the Portuguese margin in the time interval from the Marine Isotope Stage (MIS) 13/14 boundary to MIS 9 (535 to 300 ka). During the Mid-Brunhes event, the assemblages associated with the eccentricity minima are characterised by higher nannoplankton accumulation rate (NAR) values and by the blooming of the opportunistic genus Gephyrocapsa. Changes in coccolithophore abundance are also related to glacial-interglacial cycles. Higher NAR and numbers of coccoliths/g mainly occurred during the interglacial periods, while these values decreased during the glacial periods. Superimposed on the glacial/interglacial cycles, climatic and paleoceanographic variability has been observed on precessional timescales. The structure of the assemblages highlights the prevailing long-term influence of the Portugal (PC) and Iberian Poleward (IPC) Currents, following half and full precession harmonics, related to the migration of the Azores High (AH) Pressure System. Small Gephyrocapsa and Coccolithus pelagicus braarudii are regarded as good indicators for periods of prevailing PC influence. Gephyrocapsa caribbeanica, Syracosphaera spp., Rhabdosphaera spp. and Umbilicosphaera sibogae denote periods of IPC influence. Our data also highlights the increased percentages of Coccolithus pelagicus pelagicus during the occurrence of episodes of very cold and low salinity surface water, probably related to abrupt climatic events and millennial-scale oscillations of the AH/Icelandic Low (IL) System.本研究结合其他已公开的古气候代用资料，对取自葡萄牙陆缘的MD03-2699岩芯（时间跨度为海洋同位素阶段 (Marine Isotope Stage，简称MIS) 13/14边界至MIS 9，即535至300 ka）中的颗石藻 (coccolithophore) 组合开展分析，揭示了古气候与古生产力格局的演变特征。在中布容事件期间，对应偏心率极小值的颗石藻组合表现出更高的浮游植物堆积速率 (nannoplankton accumulation rate，简称NAR) 值，且机会主义类群桥石藻属 (Gephyrocapsa) 出现种群勃发。颗石藻丰度的变化同样与冰期-间冰期旋回密切相关：较高的NAR值及每克沉积物中的颗石粒数主要出现于间冰期，而冰期阶段上述指标则显著下降。叠加于冰期/间冰期旋回之上，研究还在岁差时间尺度上观测到气候与古海洋学变异性。颗石藻组合的结构特征显示，葡萄牙海流 (Portugal Current，简称PC) 与伊比利亚极向流 (Iberian Poleward Current，简称IPC) 长期占据主导地位，其变化遵循半岁差与全岁差谐波周期，这与亚速尔高压 (Azores High，简称AH) 气压系统的迁移过程紧密相关。小型桥石藻与大洋颗石藻布拉鲁德变种 (Coccolithus pelagicus braarudii) 被视为葡萄牙海流占主导时期的良好指示生物标志物。加勒比桥石藻 (Gephyrocapsa caribbeanica)、颗球藻属未定种 (Syracosphaera spp.)、棒球藻属未定种 (Rhabdosphaera spp.) 以及西博格脐球藻 (Umbilicosphaera sibogae) 则可指示伊比利亚极向流占据主导的时期。本研究数据还显示，在表层海水极度寒冷且盐度偏低的事件发生期间，大洋颗石藻大洋变种 (Coccolithus pelagicus pelagicus) 的占比出现升高，这一现象可能与亚速尔高压/冰岛低压 (Icelandic Low，简称IL) 系统的突发性气候事件及千年尺度振荡存在关联。