Analyses of benthic and planktonic foraminifera during Marine Isotope Stage 5 of ODP Hole 172-1058C

Subtropical Gyres are an important constituent of the ocean-atmosphere system due to their capacity to store vast amounts of warm and saline waters. Here we decipher the sensitivity of the (sub)surface North Atlantic Subtropical Gyre with respect to orbital and millennial scale climate variability between ~140 and 70 ka, Marine Isotope Stage (MIS) 5. Using (isotope)geochemical proxy data from surface and thermocline dwelling foraminifers from Blake Ridge off the west coast of North America (ODP Site 1058) we show that the oceanographic development at subsurface (thermocline) level is substantially different from the surface ocean. Most notably, surface temperatures and salinities peak during the penultimate deglaciation (Termination II) and early MIS 5e, implying that subtropical surface ocean heat and salt accumulation might have resulted from a sluggish northward heat transport. In contrast, maximum thermocline temperatures are reached during late MIS 5e when surface temperatures are already declining. We argue that the subsurface warming originated from intensified Ekman downwelling in the Subtropical Gyre due to enhanced wind stress. During MIS 5a-d a tight interplay of the subtropical upper ocean hydrography to high latitude millennial-scale cold events can be observed. At Blake Ridge, the most pronounced of these high latitude cold events are related to surface warming and salt accumulation in the (sub)surface. Similar to Termination II, heat accumulated in the Subtropical Gyre probably due to a reduced Atlantic Meridional Overturning Circulation. Additionally, a southward shift and intensification of the subtropical wind belts lead to a decrease of on-site precipitation and enhanced evaporation, coupled to intensified gyre circulation. Subsequently, the northward advection of these warm and saline water likely contributed to the fast resumption of the overturning circulation at the end of these high latitude cold events.副热带环流（Subtropical Gyres）是海-气系统的重要组成部分，因其能够储存巨量暖水与高盐水体。本研究解析了海洋同位素阶段（Marine Isotope Stage, MIS）5期间（约140~70 ka），北大西洋副热带环流（次表层/表层）对轨道尺度与千年尺度气候变率的响应敏感性。本研究利用北美西海岸布雷克海岭（大洋钻探计划（Ocean Drilling Program, ODP）1058站位）中表层与温跃层有孔虫的（同位素）地球化学代用资料，证实次表层（温跃层）的海洋演化过程与表层海洋存在显著差异。 尤为关键的是，表层温度与盐度在倒数第二次冰消期（第二冰消阶，Termination II）及MIS 5e早期达到峰值，这表明副热带表层海洋的热盐聚集可能源于北向热输送的减缓。与之相反，温跃层最高温出现在MIS 5e晚期，此时表层温度已开始下降。本研究认为，副热带环流内埃克曼下沉流（Ekman downwelling）因风力应力增强而加剧，是次表层增温的成因。 在MIS 5a至5d期间，副热带上层海洋水文状况与高纬度千年尺度冷事件之间存在紧密的相互作用。在布雷克海岭，这些高纬度冷事件中最为显著的几例，与（次）表层的增温和盐聚集相关。与第二冰消阶类似，副热带环流中聚集的热量可能源于大西洋经向翻转环流（Atlantic Meridional Overturning Circulation, AMOC）的减弱。此外，副热带风带的南移与增强，导致区域降水减少、蒸发加剧，并伴随环流的强化。随后，这些暖高盐水体的北向平流，可能推动了这些高纬度冷事件结束时翻转环流的快速恢复。