Deglacial Atlantic Ocean Ventilation Rates

这个研究项目解决了一个最重要的科学问题，即冰川融化的海洋通风率。海洋通风率描述了自水团最后一次“看到”大气以来所经过的时间。这一年龄信息很重要，因为海洋通风率与高纬度地区深水的形成与气候变化密切相关。海洋-大气耦合系统中的碳大部分位于深海，通过温室气体连接对气候产生重要影响。深水形成速率的微小变化可能对大气碳收支产生重大影响。对海洋沉积物岩心的地球化学分析表明，在过去的地质时期，海洋环流发生了许多这样的变化。最近的大幅度变化发生在最后一次冰川消融期间（约2万至1万年前），当时北美和北欧的大冰盖正在退缩。对深海珊瑚放射性碳含量的测量表明，在最后一次冰川消退期间，海洋在不同的运作模式（即不同的水质量分布和不同的流速）之间来回翻转。然而，如果我们不知道来自南北高纬度来源的水团的混合比例，则不可能将水团的放射性碳含量直接转换为通风率。这一障碍可以通过测量北大西洋西部深海珊瑚中的钕（Nd）同位素组成来克服。深海珊瑚是它们生长的水体中Nd同位素组成的可靠记录者。水团的Nd同位素组成反过来又与形成区域大陆的年龄密切相关，导致大西洋高纬度水域与南纬水域的Nd同位素特征非常不同。这些独特的特征使我们能够在最后一次冰川消融的突发气候事件中“分解”北大西洋西部水域的组成。将这些关于水团混合物的知识应用到现有的放射性碳数据集上，这些数据集是在相同的、绝对有年代的样本上，我们可以揭示在最后一次冰川消退期间，大西洋的通风发生得有多快，从哪里开始的。这样的信息以前从未获得过，并且有可能彻底改变我们对海洋在快速气候变化中的作用的理解。

This research project addresses a scientific issue that is of first-order importance, deglacial ocean ventilation rates. Ocean ventilation rates describe the time elapsed since a water mass last 'saw' the atmosphere. This age information is important because ocean ventilation rates are intimately linked to climate change through the formation of deep waters at high latitudes. Most of the carbon in the coupled ocean-atmosphere system is located in the deep ocean, which exerts an important influence on climate via the greenhouse gas connection. Small changes in the rate of deep water formation are likely to have a large impact on the atmospheric carbon budget. Geochemical analyses of marine sediments cores show that many such changes in ocean circulation have happened in the geological past. The most recent large amplitude changes occurred during the last deglaciation (~20 to 10 thousand years ago), a time when the large ice sheets in North America and Northern Europe were retreating. Measurements of the radiocarbon content of deep-sea corals indicate that during the last deglaciation, the ocean was flipping back and forth between different modes of operation (i.e., different water mass distributions and different flow rates). However, it is impossible to convert the radiocarbon contents of water mass masses directly into ventilation rates, if we do not know the mixing proportions of water masses derived from high northern and southern latitude sources. This hurdle can be overcome by measuring the neodymium (Nd) isotopic composition in deep-sea corals from the western North Atlantic Ocean. Deep-sea corals are reliable recorders of the Nd isotopic composition of the water mass in which they grow. The Nd isotopic composition of the water mass in turn, is closely tied to the age of the continents in its formation area, leading to very different Nd isotopic signatures for high northern versus southern latitude waters in the Atlantic Ocean. These distinct signatures enable us to 'un-mix' the composition of waters in the western North Atlantic during abrupt climate events of the last deglaciation. Applying this knowledge of water mass mixtures to the existing radiocarbon data set on the same, absolutely dated samples, we can unravel how rapid and from where ventilation of the Atlantic Ocean occurred during the last deglaciation. Information such as this has not been obtained before, and has the potential to revolutionise our understanding of the ocean's role in rapid climate change.