浮游有孔虫δ18O和Mg/Ca比值示踪过去45万年以来西北大西洋混合层和温跃层的温度和密度梯度在千年尺度上的变化

The millennial-scale Dansgaard-Oeschger warming and cooling events in Greenland ice-cores were terminated by Heinrich massive iceberg-rafting events in which the North American Laurentide Ice Sheet periodically released armadas of icebergs into the North Atlantic. The release of cold and fresh meltwater from icebergs is hypothesized to weaken the northward heat transport by the northern branch of warm and salty Gulf Stream into the higher northern latitudes, resulting in colder and dryer climate. It is known that the meltwater was released into the North Atlantic during the glacial periods, but the extent to which the meltwater impacted the stratification (i.e., the vertical temperature and density gradients) of upper water masses is poorly known. Given the pivotal importance of the Gulf Stream and the overlying westerlies in redistributing heat and moisture surrounding the North Atlantic and impacting the global climate, reliable estimates of the temperature and density gradients of upper water masses are crucial for understanding the circulation. However, it is unknown if these abrupt climate events during last glacial are pervasive on a longer timescale. In this project, we propose to reconstruct the frequency of iceberg-rafting, changes in the biological productivity, and stratification of the upper water masses for the last 450 ka. This project will use sediments from the IODP Site U1313 from the northwest Atlantic. We have obtained continuous samples at 2 cm intervals between 0 and 22 mcd covering the last 450 ka. We propose to measure the oxygen and carbon isotopes in several planktonic foraminifers and iceberg-rafted detritus (IRD) of a selected few cold intervals. It is also proposed to determine the Mg/Ca in several planktonic foraminifera to assess changes in the temperature and density gradients, especially during the cold periods. By examining several planktonic foraminiferal species, frequency of the IRD-grains, and applying multiple geochemical proxies, this project will shed light on the “impact of iceberg released meltwater” and thus allow us to assess the status of the ocean circulation and ensuing global change.

格陵兰冰芯记录了一系列的D/O（暖/冷）事件，这些千年尺度的气候事件均终止于大规模的冰筏事件, 即H事件。在H事件中，冰川融水阻止了温暖高盐度的Gulf Stream向北部高纬海域的流动，进而减弱了向北的热输送，使得高纬地区气候变得寒冷干燥，故准确评估D/O和H事件期间上层水体的温度和盐度梯度对于理解全球环流至关重要。然而，由于缺乏古海洋记录，我们还不清楚是否在更长时间尺度上也存在着类似于末次冰期的极端气候事件。本项目旨在重建45万年以来冰筏事件发生的频率，生产力变化和上层水体分层情况。拟利用西北大西洋IODP U1313站位获得的间隔2 cm的连续沉积物样品，通过分析多种浮游有孔虫的碳氧同位素和Mg/Ca比值，以及冷期冰筏碎屑的碳氧同位素，揭示上层水体温度和盐度梯度的变化。同时，通过识别多种浮游有孔虫种类和冰筏碎屑出现的频率，阐明45万年冰融水对洋流循环和全球气候变化的影响。

西北大西洋被认为是气候热点之一，因为它在供应冰山和融水方面的作用改变了北大西洋的海面温度和盐度。相比之下，位于次极地环流内的两个深水翻转点由西北大西洋释放的淡水量控制。淡水输入的变化改变了控制深水翻转强度的上层水结构，从而控制了副极地和副热带环流的强度。尽管在过去的五年中对北大西洋进行了许多研究；然而，融水改变上层水结构（即温度和盐度）的程度在最后一次冰期循环中并不为人所知。此外，对于较旧的时间尺度，数据甚至变得稀缺。我们对活塞岩心进行了地球化学和微古生物学测量，其中考虑了沉积物输送、融水供应和冰山排放，从而改变了上部水柱结构。我们还使用地球物理（即高分辨率地震、多波束测深等，无需项目成本）来评估原位沉积的真实性，从而评估地层的完整性。在四种浮游有孔虫中进行了地球化学测量，即氧和碳同位素、放射成因 14C-AMS 测年、Mg/Ca、Mn/Ca、Fe/Ca 和 Al/Ca 比率。此外，冰筏碎屑 (IRD) 和有孔虫普查数据是在亚极地和亚热带环流之间延伸的三个沉积岩芯中获得的。研究发现，起源于哈德逊海峡冰流的冰山仅在 H 事件期间改变了上层水结构，并在副极地和亚热带环流中经历了最后四个冰消期。此外，我们的结果表明，温暖时期的轻微 IRD 事件并未显着改变上层水结构，这意味着缺乏足够的融水排放。这些融水“量”的变化可能为大西洋经向翻转环流的强度奠定了背景。通过这个项目，我们对北大西洋上层水体浮游有孔虫丰度的来源、运输、强度和变化率有了一些新的认识。我们有进一步的证据表明副极地和亚热带环流之间盐度条件的变化（目前无法提供）。该项目将有助于推动北大西洋气候突变及其与全球遥相关的研究进展。

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