(Appendix) Th, U and Pa chemistry of North Atlantic deep-sea sediments

The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the climate system. However, its role during climate change is still poorly constrained particularly during an Interglacial to Glacial climate transition and the associated global cooling. We present here the first reconstruction of the evolution of the vertical structure of the rate of the AMOC from the Last Interglaciation to the subsequent glaciation (128,000-60,000 years ago) based on sedimentary (231Pa/230Th) records. We show a deep AMOC during the interglacial warmth Marine Isotope Stage (MIS) 5.5 and a shallower glacial one during glacial MIS 4. The change between these two patterns occurred mostly during the glacial inception, i.e. the transition from MIS 5.5 to MIS 5.4. Our data show that AMOC was enhanced during this latter transition as a consequence of a large increase of the overturning rate of the Intermediate Waters, above 2500 m. We suggest that this AMOC pattern required a reinforced Gulf Stream-North Atlantic Current system that ultimately supported ice-sheet growth by providing heat and moisture to the Northern high latitudes. From MIS 5.4 to MIS 5.1, the AMOC was broadly continuous below 2000 m and supported periods of ice-sheet growth. As a result, a glacial AMOC is triggered at the beginning of MIS 4 due to the extension of ice-sheet and the subsequent reorganization of deep-water formation. This study highlights the role of intermediate waters as a major player during climate change.

大西洋经向翻转环流（Atlantic Meridional Overturning Circulation，AMOC）是气候系统的关键特征。然而，其在气候变化过程中的作用仍未得到充分约束，尤其是在间冰期向冰期的气候转型及伴随的全球降温阶段。本研究基于沉积（231Pa/230Th）记录，首次重建了末次间冰期至后续冰期（12.8万至6万年前）期间大西洋经向翻转环流速率的垂直结构演化历史。研究结果显示，在间冰暖期海洋同位素阶段（Marine Isotope Stage，MIS）5.5时，大西洋经向翻转环流处于深层环流状态；而在冰期MIS4期间，其转为较浅层环流状态。这两种环流模式的转变主要发生在冰期起始期，即从MIS5.5到MIS5.4的转型阶段。本研究数据表明，在这一转型期内，由于2500米以上中层水的翻转速率大幅提升，大西洋经向翻转环流得以增强。我们提出，该环流模式依赖于增强的湾流-北大西洋流系统，该系统最终通过向北半球高纬度地区输送热量与水汽，促进了冰盖的发育。从MIS5.4至MIS5.1，2000米以深的大西洋经向翻转环流整体保持稳定，并支撑了多段冰盖发育时期。受冰盖扩张及随后深水形成过程重组的影响，MIS4初期触发了冰期状态的大西洋经向翻转环流。本研究强调了中层水在气候变化过程中的核心作用。