New Foraminiferal Proxies for Cenozoic Seawater Chemistry (ForSea)

海洋的化学成分在地质时期发生了很大变化，对大气的长期组成和地球环境对扰动的反应产生了重大控制，但过去很难量化。在这里，我们建议开发两个新的代理古代海洋化学在微小的碳酸钙壳的单细胞生物称为有孔虫。第一个指标是壳中硫与钙的比例。这很可能记录了海水中溶解的碳酸盐的浓度，因为贝壳中的比例是由海水中硫酸盐与碳酸盐的比例控制的。在短时间内，硫酸盐浓度保持不变，但碳酸盐离子的浓度会对各种参数做出反应，包括海洋的pH值和大气中的二氧化碳含量。第二个指标是贝壳中钡与钙的比例。预计这将对海水中硫酸盐的量做出反应，因为硫酸钡矿物非常不溶，因此随着硫酸盐浓度降低，钡浓度能够增加。这两个代理是联系在一起的，因为我们需要知道硫酸盐浓度如何在很长一段时间内变化，以计算溶解碳酸盐代理。已发表的工作表明，有孔虫中钡钙比例的唯一控制因素是海水中钡的含量。然而，对于硫-钙比率，可能存在影响该比率的其他因素。因此，我们首先建议分析有孔虫壳材料的硫钙比从四个现代网站具有不同的特点，如温度和一系列有孔虫物种，以了解是什么控制这些化学比例。然后，我们将在过去两个冰川-间冰期期间测试这种溶解的碳酸盐代用品，我们从冰芯记录中了解大气二氧化碳的浓度，并且还可以应用海洋pH值，硼同位素的既定代用品。我们可以使用硼同位素和大气二氧化碳测量值来独立地计算碳酸根离子浓度，以测试根据硫钙比计算碳酸根离子浓度的相对性。我们将通过分析我们有独立证据表明硫酸盐浓度较低的时间段的样本来测试硫酸盐浓度代用品。我们还将测试如何以及每个代理保存在地质记录中，通过比较已知具有特殊的“玻璃状”保存的样品与同期的例子，所谓的“霜冻”保存指示发生在壳后，它被沉积在沉积物中的变化。最后，我们将应用这些替代物来生成过去6500万年来海洋化学的长期记录。我们还将研究与大气二氧化碳变化有关的两个更快速的事件：大约5500万年前的快速变暖事件（古新世-始新世热最大值）被认为是由大气二氧化碳的快速增加引起的，以及向永久极地冰盖（始新世-渐新世边界）的地球过渡可能是由大气二氧化碳减少引起的。如果成功的话，这些代理的发展将代表着我们理解海洋化学如何随着时间的推移而变化的能力的一个巨大飞跃。管理这些代理的原则也可能适用于其他生物形成的其他种类的碳酸盐矿物，因此这项研究将为它们在地球历史的更长时间内进行测试和应用铺平道路。这将代表我们对海水化学演化和控制它的地球化学循环的理解的一个步骤变化，无论是在快速事件（如质量迁移）还是长期变化（如板块构造造成的变化）期间。

The chemistry of the oceans has changed a lot over geological time and, exerts a major control on the long-term composition of the atmosphere and on how the Earth's environment responds to perturbation, and yet is hard to quantify for the past. Here we propose to develop two new proxies for ancient ocean chemistry in the tiny calcium carbonate shells of single celled organisms called foraminifera. The first proxy is the ratio of sulfur to calcium in the shell. This is likely to record the concentration of dissolved carbonate in seawater because the ratio in the shell is controlled by the ratio of sulfate to carbonate in seawater. Over short time scales sulfate concentrations stay the same, but the concentration of the carbonate ion responds to various parameters including the pH of the oceans and the amount carbon dioxide in the atmosphere. The second proxy is the ratio of barium to calcium in the shell. This is expected to respond to the amount of sulfate in the seawater because the mineral barium sulfate is very insoluble, so as the concentration of sulfate gets lower, barium concentrations are able to increase. The two proxies are linked because we need to know how sulfate concentrations change over long time periods to make the calculations for the dissolved carbonate proxy. Published work shows that for barium calcium ratios in the foraminifera the only control is the amount of barium in seawater. However, for sulfur-calcium ratios there may be other factors which affect this ratio. Therefore, we first propose to analyse foraminiferal shell material for its sulfur-calcium ratio from four modern sites with different characteristics such as temperature and for a range of foraminiferal species to understand what controls these chemical ratios. We will then test this dissolved carbonate proxy across the last two glacial-interglacial time periods where we know the concentration of atmospheric carbon dioxide from ice-core records, and can also apply an established proxy for ocean pH, boron isotopes. We can use the boron isotope and atmospheric carbon dioxide measurements to independently calculate carbonate ion concentration to test the against calculations of carbonate ion concentration from the sulfur-calcium ratios. We will test the sulfate concentration proxy by analysing samples from time periods for which we have independent evidence that sulfate concentrations were lower. We will also test how well each of the proxies is preserved in the geological record by comparing samples known to have exceptional 'glassy' preservation with contemporaneous examples with so called 'frosty' preservation indicative of changes occurring in the shell after it was deposited in the sediment. Finally, we will apply these proxies to generate long term records of ocean chemistry across the last 65 million years. We will also study two more rapid events linked to changes in atmospheric carbon dioxide: a rapid warming event ~55 million years ago (the Palaeocene-Eocene Thermal Maximum) thought to be caused by a rapid increase in atmospheric carbon dioxide, and the transition to an Earth with permanent polar ice caps (the Eocene-Oligocene boundary) which may have been caused by declining atmospheric carbon dioxide. If successful, the development of these proxies will represent a big leap forward in our ability to understand how the chemistry of the ocean has changed through time. The principles that govern these proxies are also likely to apply to other sorts of carbonate minerals formed by other organisms, so the work in this grant would prepare the way for them to be tested and applied across a much longer span of Earth's history. This would represent a step change in our understanding of the evolution of seawater chemistry and the biogeochemical cycles that control it, both during rapid events such as mass extinctions and longer term changes such as those imposed by plate tectonics.