Foraminiferal, leachate and detrial neodymium isotopes from the South Atlantic

Antarctic Intermediate Water is an essential limb of the Atlantic meridional overturning circulation that redistributes heat and nutrients within the Atlantic Ocean. Existing reconstructions have yielded conflicting results on the history of Antarctic Intermediate Water penetration into the Atlantic across the most recent glacial termination. In this study we present leachate, foraminiferal, and detrital neodymium isotope data from three intermediate-depth cores collected from the southern Brazil margin in the South Atlantic covering the past 25?kyr. These results reveal that strong chemical leaching following decarbonation does not extract past seawater neodymium composition in this location. The new foraminiferal records reveal no changes in seawater Nd isotopes during abrupt Northern Hemisphere cold events at these sites. We therefore conclude that there is no evidence for greater incursion of Antarctic Intermediate Water into the South Atlantic during either the Younger Dryas or Heinrich Stadial 1. We do, however, observe more radiogenic Nd isotope values in the intermediate-depth South Atlantic during the mid-Holocene. This radiogenic excursion coincides with evidence for a southward shift in the Southern Hemisphere westerlies that may have resulted in a greater entrainment of radiogenic Pacific-sourced water during intermediate water production in the Atlantic sector of the Southern Ocean. Our intermediate-depth records show similar values to a deglacial foraminiferal Nd isotope record from the deep South Atlantic during the Younger Dryas but are clearly distinct during the Last Glacial Maximum and Heinrich Stadial 1, demonstrating that the South Atlantic remained chemically stratified during Heinrich Stadial 1.

南极中层水（Antarctic Intermediate Water）是大西洋经向翻转环流（Atlantic Meridional Overturning Circulation）的核心分支，其功能是在大西洋海域内重新分配热量与营养盐。现有古气候重建研究针对末次冰消期以来南极中层水侵入大西洋的演化历史，得到了相互矛盾的结果。本研究报道了采自南大西洋巴西南缘3支中层水深岩芯的浸出液、有孔虫及碎屑钕同位素数据，所有岩芯的时间跨度覆盖过去2.5万年。研究结果显示，经脱碳酸处理后的强化学浸提步骤，无法提取该区域的古海水钕同位素组成。新获得的有孔虫记录表明，在北半球快速冷事件发生期间，研究站位的海水钕同位素组成未出现显著变化。据此我们得出结论：无论是新仙女木事件还是海因里希1期，均无证据显示南极中层水对南大西洋的侵入强度有所增强。然而，我们观测到中全新世期间，南大西洋中层海水的钕同位素值呈现更强的放射成因特征。这一放射成因偏移与南半球西风带南移的相关证据相契合，该环流系统的南移可能使得南大西洋海域在中层水形成过程中，更多携带放射成因信号的太平洋源水被卷携进入。本研究的中层水深记录在新仙女木事件期间，与南大西洋深水有孔虫钕同位素记录的数值相近，但在末次盛冰期与海因里希1期则存在显著差异，这表明海因里希1期南大西洋仍维持化学分层状态。