Climate change, seawater chemistry and planktic foraminiferal calcification in the Middle Eocene

从海底沉积物中恢复的地质档案详细记录了地球如何应对数百万年的气候变化，可以成为研究地球系统将如何对我们预测的温室未来做出反应的宝贵手段。在5000万年前，地球的气候比现在要热几度，两极的冰很少，全球海平面要高得多。在50到34 Ma之间，气候开始急剧冷却，南极洲开始形成大面积的冰盖，向现代两极“冰库”世界迈出了第一步。然而，这不是一个单向的气候冷却，而是在这个时候，地球的气候在短暂的温暖间隔和两个半球可能的冰碛覆盖之间迅速波动，特别是在38和42 Ma之间（始新世中期）。这些快速波动可能是由温室气体二氧化碳的变化驱动的。我们要解决的问题是，我们缺乏一个详细的记录，提供一个准确的时间和重建这些快速的气候事件，这是必要的，以了解全球和当地的气候条件，在这一关键的过渡时间间隔。在这里，我们将研究有孔虫的化石遗骸：微观浮游动物，分泌复杂的方解石（碳酸钙）壳围绕其细胞，这些细胞作为微小的化石大量保存在海底泥中。这些微生物要么生活在海洋的上部，要么生活在海底，通过观察它们外壳的化学成分，我们可以确定它们活着时海洋的温度、酸度和盐度。为了研究始新世中期的气候，我们首先要确定记录气候变化的沉积物的确切年龄。我们将通过观察沉积物的磁性来做到这一点，结合有孔虫的化学特征和存在的物种，就像一个指纹，表明一个特定的年龄。其次，我们希望记录有孔虫方解石地球化学中记录的环境条件的详细变化，以绘制一幅随时间变化的全球气候图。第三，我们希望利用这些信息来了解大气中二氧化碳的快速变化（就像今天发生的那样）对海洋酸度的影响，进而对产生碳酸盐的浮游生物产生影响。碳酸钙溶于酸，鉴于钙化浮游生物作为全球海洋食物链基础的重要性，我们需要知道它们将如何反应。这项研究将设在南安普顿的国家海洋学中心，作为古海洋学和古气候研究小组的一部分，该小组正在积极调查气候变化的关键间隔。我们将与位于圣地亚哥的加州大学斯克里普斯海洋学研究所合作，该研究所的同事在始新世有孔虫的生物学和钙化方面具有特定的专业知识。

The geological archive recovered from seafloor sediments documents in detail how the Earth has responded to millions of years of climate change and can be an invaluable means to investigate how the Earth system will react to our projected greenhouse future. Before 50 million years ago the Earth's climate was several degrees hotter than present, there was very little ice at the poles, and global sea levels were much higher. Between 50 and 34 Ma, the climate began to cool dramatically and extensive ice-sheets began to grow on Antarctica, taking the first steps towards the modern bipolar 'icehouse' world. However, this was not a unidirectional climatic cooling, but rather at this time Earth's climate fluctuated rapidly between short-lived warm intervals and times of possible icesheet cover on both hemispheres, in particular between 38 and 42 Ma (middle Eocene). These rapid fluctuations were likely driven by changes in the greenhouse gas CO2. The problem that we will address is that we lack a detailed record providing an accurate timing and reconstruction of these rapid climatic events which are necessary to understand the global and local climate conditions during this key transitional time interval. Here, we will look at the fossil remains of foraminifera: microscopic zooplankton that secrete intricate calcite (calcium carbonate) shells surrounding their cells which are preserved as minute fossils in huge numbers in seafloor mud. These microorganisms either live in the upper part of the ocean or on the seafloor and by looking at the chemical composition of their shells we can determine how warm, how acidic and how salty the ocean was when they were alive. To investigate Middle Eocene climate we will, firstly, determine exactly how old the sediments are that record climate change. We will do this by looking at the magnetic properties of the sediments which, combined with the chemical character of the foraminifera and the species present, act like a fingerprint indicating a specific age. Secondly, we want to document the detailed changes in environmental conditions recorded in the foraminiferal calcite geochemistry to produce a picture of the global climate through time. Third, we want to use this information to understand the effect rapid changes in atmospheric CO2, like that occurring today, have on ocean acidity and in turn on carbonate-producing plankton. Calcium carbonate dissolves in acid and given the importance of calcifying plankton as the base of the global marine foodchain, we need to know how they will respond. The research will be based at the National Oceanography Centre, Southampton, as part of the Palaeoceanography and Palaeoclimate research group, which is actively investigating critical intervals of climate change. We will collaborate with Scripps Institution of Oceanography, University of California at San Diego, where colleagues have specific expertise in the biology and calcification of foraminifera from the Eocene.