Glacial Mediterranean Outflow recoinstruction at IODP Site 339-U1390@en

The state of Atlantic Meridional Overturning Circulation (AMOC) is influenced by both the strength and the location of the Mediterranean Outflow (MOW) plume in the Gulf of Cadiz. To evaluate the influence of MOW on AMOC over deglaciations, precise and accurate salinity and temperature reconstructions are needed. For this purpose, we measured Mg/Ca and clumped isotopes of several benthic foraminiferal species at IODP Site U1390 in the Gulf of Cadiz. The clumped isotope results of Cibicidoides pachyderma, Uvigerina mediterranea and Pyrgo spp. are consistent between species and record no significant difference in Glacial to Holocene DWT. Over the deglaciation, the Mg/Ca-based temperatures derived from U. mediterranea indicate three periods of MOW absence at site U1390. Mg/Ca-based temperatures of Hoeglundina elegans and C. pachyderma are on average 6 °C too cold when compared to the present core-top temperature, which we explain by a carbonate ion effect on these epibenthic species related to the high alkalinity of the MOW. Combining DWT estimates with the benthic oxygen isotope data and considering different relationships between seawater oxygen isotopes and salinity, we infer a salinity decrease of MOWby 3 to 8 units over the deglaciation, and 4 units during S1, accounting for the global d18O depletion due to the decrease in ice volume. Our findings confirm that the Mediterranean Sea accumulates excess salt during a glacial low stand, and suggest that this salt surged into the Atlantic over the deglaciation, presumably during HS1.大西洋经向翻转环流（Atlantic Meridional Overturning Circulation, AMOC）的状态，同时受加的斯湾内地中海溢出流（Mediterranean Outflow, MOW）羽流的强度与位置调控。为评估冰消期内MOW对AMOC的影响，亟需获取精准可靠的盐度与温度重建数据。基于此研究目标，我们对加的斯湾国际大洋发现计划（International Ocean Discovery Program, IODP）U1390站位的多种底栖有孔虫（benthic foraminifera）开展了镁钙比（Mg/Ca）与团簇同位素（clumped isotopes）测试分析。厚皮卷转虫（Cibicidoides pachyderma）、地中海优格虫（Uvigerina mediterranea）及吡格虫属（Pyrgo spp.）的团簇同位素结果在不同物种间一致性良好，且冰期至全新世的深层水温度（Deep Water Temperature, DWT）未记录到显著差异。在冰消期阶段，源自地中海优格虫的Mg/Ca重建温度显示，U1390站位共出现三期MOW缺失事件。相较现代岩芯顶部的原位温度，优雅霍格伦虫（Hoeglundina elegans）与厚皮卷转虫的Mg/Ca重建温度平均偏低6℃，我们将该偏差归因于MOW高碱度环境对这类底表栖息类群产生的碳酸根离子效应。结合深层水温度估算结果与底栖有孔虫氧同位素数据，并考量海水氧同位素与盐度间的不同关联模式，我们推断冰消期内MOW的盐度下降了3~8个单位，在第一海因里希冷段（Heinrich Stadial 1, HS1）期间下降了4个单位，该结果同时考虑了冰量减少导致的全球δ¹⁸O贫化效应。本研究证实冰期低位期地中海会累积过量盐分，并表明这些盐分在冰消期（大概率发生于HS1阶段）涌入大西洋。