Reconstructing Intermediate Water Temperature Response to Pliocene-Pleistocene Climates

在大气中二氧化碳浓度较高的情况下对未来气候变化的预测取决于我们对不同气候组成部分之间的关系和反馈的理解，这些气候组成部分可以是过程（海洋环流、冰盖行为），也可以是区域（例如低纬度和高纬度）。现代气候系统中的一些关键变量与过去的重大气候转变有关，但它们是否驱动、放大、或对气候事件的反应仍然是一个激烈争论的话题。在这个项目中，我们建议调查海洋中间沃茨对大冰生长的反应-在过去的四百万年里，北方的冰盖和大气中二氧化碳含量的下降。中间沃茨位于水深约500 - 1500米处。它们对于连接大面积的海洋很重要，因为它们首先是由高纬度地区（例如南大洋）的表面沃茨下沉形成的，但延伸到赤道，然后它们可能通过上升流的过程返回表面海洋。这意味着温度、盐度和营养物质的变化可以从极地地区传递到热带地区。人们认为，在冰盖退缩期间，这些水团对于将二氧化碳释放到大气中以及作为快速气候变化的“反馈”至关重要。尽管中间沃茨对全球气候的重要性，但它们对400万年前气候系统中重大事件的反应尚不清楚。我们还知道，在过去的400万年里，生活在中间沃茨的有孔虫生物发生了大量灭绝事件。虽然这些灭绝事件是全球性的，但其原因尚不清楚，但可能与中间沃茨的温度有关。我们建议对过去400万年的中间水温和有孔虫组合进行彻底调查，以评估灭绝事件的原因。我们将从西南太平洋产生第一个中间水温记录，可追溯到400万年前。我们选择的地点位于当今最重要的中间水团（南极中间水）。我们将使用一种关键的有孔虫物种的微量金属成分来计算过去的温度，这种有孔虫物种没有灭绝，生活在中间沃茨。我们将通过生成反映南大洋中层地层区域表层海洋环流的海面温度记录，评估中层水性质发生任何变化的原因。我们将使用海藻中的特定有机残留物（烯酮）来确定过去的海面温度值。因此，我们的目标不仅是重建中间水温，而且还调查是什么导致了观察到的任何变化。目前，有孔虫灭绝记录中的样本之间存在很大的时间间隔。通过减少这些差距和重建有孔虫组合，我们将更精确地确定有孔虫的繁殖和不断变化的中层水性质之间的联系。由于这种彻底的方法，这项研究方案将回答有关连接低纬度和高纬度并在过去400万年中造成广泛海洋灭绝事件的过程的关键问题。

Projections of future climate change under higher atmospheric carbon dioxide concentrations rely upon our understanding of the relationships and feedbacks that operate between different climate components, either as processes (ocean circulation, ice-sheet behaviour) or regions (e.g. low versus high latitudes). A number of key variables within the modern climate system have been linked to major climate transitions in the past, but whether they drove, amplified, or responded to climate events remains a topic of intense debate.In this project we propose to investigate the response of the intermediate waters of the ocean to the growth of large ice-sheets in the northern hemisphere and falling atmospheric CO2 conditions over the last 4 million years. Intermediate waters are found at around 500 - 1500 m water depth. They are important for connecting large areas of the ocean, because they are formed first by the sinking of surface waters in the high-latitudes (e.g. the Southern Ocean), but extend to reach the equator where they may then return to the surface ocean by the process of upwelling. This means that changes in temperature, salinity, and nutrients can be transported from the polar regions to the tropics. It is thought that during the retreat of the ice-sheets, these water masses were extremely important for releasing carbon dioxide into the atmosphere and acting as a "feedback" for rapid climate change. Despite the importance of intermediate waters for global climate, their response to significant events in the climate system since 4 million years ago is unclear. We also know that over the last 4 million years there have been a large number of extinction events in organisms called foraminifera that live in intermediate waters. Although these extinction events are global in extent, their cause is not known but may have been linked to the temperature of the intermediate waters. We propose a thorough investigation of intermediate water temperatures and foraminifera assemblages over the last 4 million years to assess the cause of the extinction events. We will generate the first record of intermediate water temperature from the SW Pacific that extends back to 4 million years ago. Our chosen site sits within the most important intermediate water mass of the present day (Antarctic Intermediate Water). We will use the trace metal composition of a key species of foraminifera that did not become extinct and which lives in intermediate waters, to calculate past temperatures. We will assess the causes of any changes in intermediate water properties by generating records of sea-surface temperature which reflect the surface ocean circulation in the area of intermediate formation in the Southern Ocean. We will use specific organic remains (alkenones) from marine algae to determine past sea-surface temperature values. We therefore aim to not only reconstruct intermediate water temperatures, but also investigate what caused any changes that are observed. At present, there are large gaps in time between samples in the foraminifera extinction record. By reducing these gaps and reconstructing foraminifera assemblages, we will more precisely determine the links between foraminifera extinctions and changing intermediate water properties. As a result of this thorough approach, this research programme will answer key questions about the processes that connected the low- and high-latitudes and caused widespread marine extinction events over the last 4 million years.