Reconstructing Intermediate Water Temperature Response to Pliocene-Pleistocene Climates

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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.