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的变化，对大气的长期组成和地球环境对扰动的反应起着重要的控制作用，但在过去很难量化。在这里，我们建议在单细胞生物（称为有孔虫）的微小碳酸钙壳中开发两个新的古代海洋化学代用物。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碳酸盐的浓度，因为壳中的比例是由海水中硫酸盐与碳酸盐的比例控制的。在短时间尺度上，硫酸盐浓度保持不变，但碳酸盐离子的浓度对各种参数有反应，包括海洋的pH值和大气中的二氧化碳量。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含量做出反应，因为硫酸钡是非常不溶的矿物，所以随着硫酸盐浓度的降低，钡的浓度就会增加。这两种代用物是相互联系的，因为我们需要知道硫酸盐浓度在很长一段时间内是如何变化的，这样才能计算出溶解的碳酸盐代用物。已发表的研究表明，对于有孔虫中钡钙的比例，唯一的控制因素是海水中钡的含量。然而，对于硫钙比，可能有其他因素影响这一比例。因此，我们首先提出分析来自四个现代遗址的有孔虫壳材料的硫钙比，这些遗址具有不同的特征，如温度和一系列有孔虫物种，以了解控制这些化学比的因素。然后，我们将在最后两个冰期-间冰期期间测试这种溶解的碳酸盐代用品，在那里我们可以从冰芯记录中知道大气二氧化碳的浓度，也可以应用一个已建立的海洋pH值代用品，硼同位素。我们可以使用硼同位素和大气二氧化碳测量来独立计算碳酸盐离子浓度，以验证硫钙比计算碳酸盐离子浓度的准确性。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理。我们还将测试每种代用物在地质记录中的保存情况，方法是将已知的具有特殊“玻璃状”保存的样品与同时代的具有所谓“霜状”保存的样品进行比较，这表明壳在沉积在沉积物中后发生了变化。最后，我们将应用这些代理来生成过去6500万年海洋化学的长期记录。我们还将研究另外两个与大气二氧化碳变化有关的快速事件：大约5500万年前的快速变暖事件（古新世-始新世热极大期）被认为是由大气二氧化碳的快速增加引起的，以及向具有永久极地冰盖的地球的过渡（始新世-渐新世边界）可能是由大气二氧化碳的减少引起的。如果成功的话，这些替代物的发展将代表着我们在理解海洋化学如何随时间变化的能力上的一个巨大飞跃。支配这些代用物的原则也可能适用于由其他生物形成的其他种类的碳酸盐矿物，因此这项资助的工作将为它们在更长的地球历史范围内进行测试和应用做好准备。这将代表着我们对海水化学演变和控制它的生物地球化学循环的理解发生了一步的变化，无论是在诸如大灭绝这样的快速事件中，还是在诸如板块构造所造成的长期变化中。

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.