Stable isotope and geochemical analyses of sediment core SO164-17-2 from the southern Florida Straits

Modeling and proxy studies indicate that a reduction of Atlantic Meridional Overturning Circulation (AMOC) strength profoundly impacts temperatures and salinities in the (sub)tropical Atlantic, especially on subsurface levels. While previous studies focused on prominent periods of AMOC reduction during the last deglaciation, we aim to test whether similar reconfigurations of the subtropical hydrography occurred during the moderate climatic alterations punctuating the last interglacial, Marine Isotope Stage (MIS) 5. Here, we present temperature and salinity records from a Florida Straits core by combining d18O and Mg/Ca analyses on surface (Globigerinoides ruber, white) and deep-dwelling (Globorotalia crassaformis) foraminifera, covering MIS 5 in high resolution. The data reveal increasing salinities at intermediate depths during interglacial cooling episodes, decoupled from relatively stable surface conditions. This probably indicates the spatial expansion of saline Subtropical Gyre waters due to enhanced Ekman downwelling and might also point to a changed density structure and altered geostrophic balance in Florida Straits. Notably, these oceanographic alterations are not consistently occurring during periods of AMOC reduction. The data suggest that the expansion of gyre waters into Florida Straits was impeded by the increasing influence of Antarctic Intermediate Water (AAIW) from MIS 5.5 to ~107 kyr BP. Afterwards, increasingly positive benthic d13C values imply a recession of AAIW, allowing the temporary expansion of Gyre waters into Florida Straits. We argue that the inferred transient subtropical salt accumulation and warm pool expansion might have played a pivotal role in reinvigorating meridional overturning and dampen the severity of interglacial cold phases.模式模拟与替代指标研究表明，大西洋经向翻转环流（Atlantic Meridional Overturning Circulation, AMOC）强度减弱会对（副）热带大西洋的温度与盐度产生显著影响，尤其在次表层水体中。既往研究多聚焦于末次冰消期内大西洋经向翻转环流显著减弱的时段，而本研究旨在检验：在经历多轮温和气候波动的末次间冰期——海洋同位素阶段（Marine Isotope Stage, MIS）5——内，亚热带水文环流是否发生过类似的重构。本研究通过对表层栖息的白氏红拟球虫（Globigerinoides ruber, white）与深水栖息的厚壳抱球虫（Globorotalia crassaformis）开展氧同位素（d18O）与镁钙比（Mg/Ca）分析，基于佛罗里达海峡的一支岩芯，重建了覆盖海洋同位素阶段5的高分辨率温度与盐度记录。数据显示，在间冰期降温事件期间，中层水体盐度呈上升趋势，且这一变化与相对稳定的表层水体环境脱耦。这一现象或反映出盐度较高的亚热带环流水体因埃克曼下沉增强发生了空间扩张，同时也可能指示佛罗里达海峡的密度结构发生改变、地转平衡遭到破坏。值得注意的是，这些海洋学变化并非始终与大西洋经向翻转环流减弱时段同步。数据表明，自海洋同位素阶段5.5至约107 kyr BP前，南极中层水（Antarctic Intermediate Water, AAIW）的影响逐渐增强，阻碍了环流水体向佛罗里达海峡的扩张。在此之后，底栖碳同位素（d13C）值持续偏正，指示南极中层水出现收缩，使得环流水体得以暂时性地向佛罗里达海峡扩张。本研究认为，本次推断出的亚热带瞬时盐度积累与暖池扩张，可能在重振经向翻转环流、缓解间冰期冷期强度方面发挥了关键作用。