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测取决于我们对不同气候成分之间的关系和反馈的理解，这些气候成分可以是过程(海洋环流、冰盖行为)，也可以是区域(例如，低纬度与高纬度)。在过去，现代气候系统中的一些关键变量与主要的气候转变有关，但它们是否驱动、放大或响应气候事件仍然是一个激烈辩论的话题。在这个项目中，我们建议调查海洋中间水域对过去400万年来北半球大冰盖的增长和大气二氧化碳条件下降的响应。中间水域位于水深约500-1500米处。它们对于连接海洋的大片区域很重要，因为它们首先由高纬度(例如南大洋)的表层水下沉形成，但延伸到赤道，在那里它们可能会通过上升流过程返回表层海洋。这意味着温度、盐度和营养物质的变化可以从极地地区输送到热带地区。人们认为，在冰盖消退期间，这些水团对向大气中释放二氧化碳并对快速气候变化起到“反馈”作用极其重要。尽管中间水对全球气候很重要，但它们对400万年前以来气候系统中的重大事件的反应尚不清楚。我们还知道，在过去的400万年里，生活在中间水域的被称为有孔虫的生物发生了大量的灭绝事件。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调查，以评估灭绝事件的原因。我们将从西南部太平洋产生第一个中等水温记录，这可以追溯到400万年前。我们选择的地点位于当今最重要的中间水团(南极中水)。我们将使用一种尚未灭绝的、生活在中间水域的有孔虫关键物种的微量金属成分来计算过去的温度。我们将通过产生反映南大洋中间地层区域表层海洋环流的海面温度记录，评估中间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)来确定过去的海表面温度值。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较大的时间间隔。通过缩小这些差距和重建有孔虫组合，我们将更准确地确定有孔虫灭绝和变化的中间水性质之间的联系。作为这种彻底方法的结果，这一研究方案将回答有关过去400万年来连接低纬度和高纬度并造成大范围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.