The evolution of Chalk Sea ecosystems: biodiversity, resilience and ecological function in a warming world

我们大气中的二氧化碳含量已经达到了数百万年前的水平，而且还在稳步上升。随着地球持续变暖，科学家们越来越多地转向化石记录，以帮助了解海洋生态系统是什么样子的，以及它们如何在极端气候变化下发挥作用。过去的一次变暖事件发生在大约9400万年前的白垩纪晚期，记录在英国白垩中。它被称为Cenomanian-Turonian边界事件，导致了全球物种灭绝和过去2.5亿年来最高的海平面。我们将利用自然历史博物馆大量未开发的白垩化石，结合新的实地调查和测量过去温度的新方法，以前所未有的细节研究海洋生态系统是如何对这一事件做出反应的，以及它是如何被塑造的。从多佛的白色悬崖到英格兰南部连绵起伏的丘陵，西至德文郡，北至约克郡，白垩是英国景观的标志性和重要组成部分。我们今天看到的岩石是由生活在晚白垩纪白垩海的浮游生物化石（纳米化石）的微观骨骼组成的。当它们死后，它们的小骨架漂流到海底，形成一种软泥，随着时间的推移逐渐积累，变成了岩石。其他白垩海物种的骨骼和洞穴也成为化石，为整个生态系统提供了独特的记录；从最小的浮游生物到最大的顶级捕食者，如鲨鱼和海洋爬行动物。由于它在建筑和其他工业中的使用，英国各地挖掘了许多白垩矿和采石场。这些提供了一个密集的研究站点网络，使我们能够比以往任何其他变暖事件更详细地了解白垩海生态系统在空间和时间上的变化。这些遗址的化石已经被收集了200多年，大部分都被收藏在自然历史博物馆。这一“整个生态系统”档案之所以没有被研究过，其中一个原因是，19世纪收集的大多数标本缺乏关于它们来自白垩的确切部分的细节，以及它们是在变暖事件之前、期间还是之后存活的细节。然而，我们最近已经证明，从白垩岩中提取灰尘大小的纳米化石是可能的，这些化石仍然附着在较大的化石上，并利用这些浮游生物来确定标本的年代。这使化石收藏以一种以前不可能的方式进行研究。我们还将对英国各地的关键地点进行新的实地研究，以研究详细的逐层变化；计算和识别所有现存的化石，以帮助我们了解整个生态系统。通常，这样的研究只关注一个化石群，比如菊石或有孔虫，但我们将收集所有的信息，这样我们就可以展示整个白垩海生态系统是如何在变暖事件中发生变化的。测定白垩海的温度是我们最后的挑战。传统的技术需要对过去海水的化学成分进行假设，而这是无法确定的。相反，我们将采用一种最新开发的化学技术，称为块状同位素古温学，来测量保存完好的动物化石壳中不同的、稀有的、重同位素之间的键。当温度下降时，这些同位素倾向于“聚集”在一起，因此它们之间的键提供了壳形成时温度的直接测量。通过分析贝壳内的单个生长带，我们将重建几年来的季节变化，显示当地冬季和夏季温度如何随着全球变暖而变化。除了对白垩海生态系统如何响应过去的变暖进行首次详细研究外，我们还将把我们的发现与对当前海洋生态系统如何响应当今变暖的预测进行比较；用过去来检验对未来变化的预测。

The amount of carbon dioxide in our atmosphere is already at a level last recorded millions of years ago, and is steadily rising. As our planet continues to warm, scientists are increasingly turning to the fossil record to help understand what marine ecosystems look like and how they were able to function under extreme climate change. One past warming event occurred in the Late Cretaceous about 94 million years ago and is recorded in the British Chalk. Known as the Cenomanian-Turonian Boundary Event, it led to global extinctions and the highest sea levels of the past 250 million years. We will study how marine ecosystems responded to, and were shaped by, this event in unprecedented detail, using the vast and untapped chalk fossil collections of the Natural History Museum, coupled with new fieldwork and a novel method of measuring past temperatures.From the white cliffs of Dover and the rolling downs of southern England, west to Devon and north to Yorkshire, the chalk is an iconic and important part of the British landscape. The rock we see today is made up of the microscopic skeletons of fossil plankton (nannofossils) that lived in the Late Cretaceous Chalk Sea. When they died, their tiny skeletons drifted down to the seafloor forming an ooze that gradually accumulated over time and turned into rock. The skeletons and burrows of other Chalk Sea species were also fossilised, providing a unique record of the entire ecosystem; from the tiniest plankton to the largest apex predators such as sharks and marine reptiles. Owing to its use in building and other industries, numerous chalk pits and quarries were excavated across the UK. These provide a dense network of study sites, enabling us to see how Chalk Sea ecosystems changed in space and time in far more detail than for any other past warming event.Fossils from these sites have been collected for over 200 years and most are housed in the Natural History Museum. One reason why this 'whole ecosystem' archive has not been studied before is that most specimens collected in the 1800s lack details of exactly which part of the chalk they came from, and whether they were alive before, during or after the warming event. We have shown recently, however, that it is possible to extract dust-sized nannofossils from the chalk rock that still adheres to the larger fossils, and to use these plankton to date the specimens. This opens up the fossil collections for study in a way that has not been possible before.We will also undertake new field studies of key sites around the UK to study detailed bed-by-bed changes; counting and identifying all the fossils present to help us understand the whole ecosystem. Usually, such studies only focus on one fossil group, such as ammonites or foraminifera, but we will collect information on everything so we can show how the entire Chalk Sea ecosystem changed through the warming event.Determining the temperature of the Chalk Sea is our final challenge. Traditional techniques require assumptions about the chemical composition of past seawater - something that cannot be known for certain. Instead, we will apply a recently developed chemical technique, called clumped isotope palaeothermometry, to measure the bonds between different, rare, heavy isotopes within the well-preserved shells of fossil animals. These isotopes tend to 'clump' together as temperature falls, and so the bonds between them provide a direct measurement of temperature at the time the shell was formed. By analysing individual growth bands within the shells we will reconstruct seasonal changes across several years, showing how local winter and summer temperatures change with global warming.As well as having the first, detailed study of how Chalk Sea ecosystems changed in response to past warming, we will also compare our findings to projections of how current marine ecosystems might change in response to present-day warming; using the past to test predictions of future change.