The response of the Agulhas Leakage system to Pliocene-Pleistocene climate evolution

大西洋的海洋环流由“传送带”的运行所主导，传送带将温暖的海水向北输送到欧洲并经过欧洲，然后冷却并下沉到冰岛北部，然后在深海中回流到南大西洋。这种传送带的强度与向北流动的大西洋水域的温度和盐度有关，这些温度和盐度控制着水的密度和地表水在北部下沉的能力。大量研究表明，向北大西洋添加淡水可能会使输送系统变得更弱，因为水的密度降低了，更难下沉。在南大西洋发现了对大西洋地表水性质的另一种控制：在开普敦以南，印度洋和大西洋交汇，印度洋的一些温暖和咸水被转移到大西洋，在那里它们被向北输送并进入传送带。这一过程被称为“阿古拉斯泄漏”，并不是一成不变的，而是根据西风的位置和强度以及与南极绕极洋流相关的海洋锋面的位置而变化。在过去几个冰盖扩张和消退的周期(“冰川-间冰期周期”)中，我们观察到了阿古勒斯渗漏强度的变化，但目前还不清楚是什么导致了这些变化，也不清楚它们是如何在过去500万年里发展起来的，因为我们知道气候系统中其他地方发生了其他重大转变(例如，更大的冰盖的扩大、风系统的增强、南极绕极洋流位置的变化)。在这个项目中，我们利用第一个连续的沉积物序列来延伸到过去500万年，这些沉积序列位于阿古勒斯渗漏的直接途径内。这项新记录(地点U1479)是在2016年春天钻探的，它跨越了更新世的冰河时代(0-270万年前)和全球气候变暖的上新世(270-530万年前)。此前，我们已经找到证据表明，在冰盖比现在更大的时候(我们原本预计会导致更多的降温)，南大西洋的渗漏增加(变暖更多)，但我们的记录在它们所覆盖的时间窗口中是有限的。我们将重建U1479站点的海面温度，将其追溯到500万年前的今天。我们将把我们的结果与南大西洋现有的数据进行比较，以考虑是否有证据表明，随着风力和位置的变化，以及南极绕极流位置的变化，阿古拉渗漏发生了变化。作为第一次从阿古利斯泄漏走廊连续重建跨越过去500万年的表层海洋属性，我们的新数据也可能揭示海洋传送带如何以及为什么也在这一时间窗口进化。

Ocean circulation in the Atlantic Ocean is dominated by the operation of the 'conveyor belt', which transports warm waters northward towards and past Europe, which then cool and sink to the north of Iceland before flowing back to the South Atlantic in the deep ocean. The strength of this conveyor is linked to both the temperature and the salinity of the northward flowing Atlantic waters, which control water density and the ability of the surface waters to sink in the north. A lot of research has demonstrated that by adding freshwater to the North Atlantic the conveyor system can become weaker, because the water density is reduced and more difficult to sink. An additional control of Atlantic surface water properties is found in the South Atlantic: to the south of Cape Town, the Indian and Atlantic Oceans meet, and some of the warm and salty waters of the Indian Ocean are transferred into the Atlantic, where they are transported northward and into the conveyor. This process, termed 'Agulhas leakage', is not constant, but varies depending upon the position and intensity of the westerly winds as well as the position of the oceanic fronts associated with the Antarctic Circumpolar Current. Over the last few cycles of ice sheet expansion and retreat (the 'glacial-interglacial cycles') we have observed changes to the strength of Agulhas leakage, but it is not clear what caused these changes, nor how they developed over the last 5 million years when we are aware of other major transitions occurring elsewhere in the climate system (e.g. growth of larger ice sheets, intensification of wind systems, shifting Antarctic Circumpolar Current position).In this project we take advantage of the first continuous sediment sequence to extend through the last 5 million years located within the direct pathway of the Agulhas leakage. The new record (Site U1479) was drilled in spring 2016, and spans the ice ages of the Pleistocene (0-2.7 million years ago) and the globally warmer climate of the Pliocene (2.7-5.3 million years ago). Previously we have identified evidence for increasing leakage (more warming) in the South Atlantic during times where ice-sheets were larger than present (which we would have expected to cause more cooling), but our records are limited in the time window they cover. We will reconstruct sea-surface temperatures at Site U1479 from the present day back to 5 million years ago. We will compare our results to existing data from the South Atlantic, to consider whether there is evidence for changes in Agulhas leakage in response to changes in wind strength and position, and changes to the position of the Antarctic Circumpolar Current. As the first continuous reconstruction of surface ocean properties spanning the last 5 million years from the corridor of Agulhas leakage, our new data will also potentially shed light onto how and why the ocean conveyor has also evolved over this time window.