Reconstructing Intermediate Water Temperature Response to Pliocene-Pleistocene Climates

在大气二氧化碳浓度较高的情况下，对未来气候变化的预估依赖于我们对不同气候成分之间的关系和反馈的理解，这些关系和反馈可能是过程（海洋环流、冰盖行为），也可能是区域（例如低纬度与高纬度）。现代气候系统中的一些关键变量与过去的重大气候变化有关，但它们是驱动、放大还是响应气候事件，仍然是一个激烈争论的话题。在这个项目中，我们建议研究在过去的400万年里，海洋的中间水对北半球大冰盖的增长和大气二氧化碳水平下降的反应。在水深约500 - 1500米处发现中间水域。它们对于连接大片海洋很重要，因为它们首先是由高纬度地区（如南大洋）的地表水下沉形成的，然后延伸到赤道，在那里它们可能通过上升流的过程返回表层海洋。这意味着温度、盐度和营养物质的变化可以从极地地区转移到热带地区。据认为，在冰盖退缩期间，这些水团对于向大气中释放二氧化碳和作为快速气候变化的“反馈”起着极其重要的作用。尽管中间水域对全球气候很重要，但它们对400万年前以来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，生活在中间水域的有孔虫生物发生了大量灭绝事件。虽然这些灭绝事件的范围是全球性的，但其原因尚不清楚，但可能与中间水域的温度有关。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的调查，以评估灭绝事件的原因。我们将生成首个来自西南太平洋的中等水温记录，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域（南极中间水域）。我们将利用一种没有灭绝、生活在中间水域的有孔虫的微量金属成分来计算过去的温度。我们将通过生成反映南大洋中间地层区域表面海洋环流的海表温度记录来评估中间水性质变化的原因。我们将使用来自海藻的特定有机残留物（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，而且还要调查导致观察到的任何变化的原因。目前，在有孔虫灭绝记录中，样品之间存在较大的时间差距。通过缩小这些间隙和重建有孔虫组合，我们将更精确地确定有孔虫灭绝与中间水性质变化之间的联系。由于这种彻底的方法，这项研究计划将回答有关连接低纬度和高纬度并导致过去400万年中广泛的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.