Quantifying phytoplankton evolution through Cretaceous Oceanic Anoxic Event 2

地质记录包含化石壳和壳化学，使我们能够重建古代地球环境。这向我们表明，地球经历过各种不同的气候，包括比现在冷得多或热得多的时期。该研究项目旨在通过利用特定变暖事件的地质记录，更好地了解生命是如何对这些变化的气候做出反应的。我们特别感兴趣的是找出驱动生物进化的因素，以及气候在其中扮演了多大的角色。了解生命如何应对不断变化的环境是很重要的，因为我们的现代气候正在迅速变暖，我们需要找出这些变化的影响。我们会看到更多的物种灭绝吗？海洋生物的数量是否会减少，从而提供更少的食物，吸收更少的二氧化碳（这将使气候更快地变暖）？为了回答这些问题，我们将研究9300万年前发生的一次气候变暖事件，也就是恐龙统治陆地的白垩纪。在这个温室气候期，南极洲出现了亚热带森林，北极出现了鳄鱼般的动物，没有冰盖，海平面比现在高70米。这一事件被称为海洋缺氧事件2 (OAE2)，因为它最初是由大量的黑色页岩沉积物发现的，这些沉积物表明海底几乎没有氧气，允许植物和动物积累大量的碳。除了所有的碳埋藏，这一事件还见证了气候的快速和极端变暖，在几万年内上升了6-9摄氏度。当时的最高气温至少为36摄氏度，而现代年平均气温很少超过30摄氏度。因此，这看起来是研究生命对变暖反应的理想事件，但也存在一些问题。在这一事件中发生的黑色页岩并没有保存我们用来测量进化的化石。为了达到这个目的，最好的化石是微小的单细胞浮游生物的壳，它们以数十亿计的数量产生，通常保存在世界各地的海底沉积物中。然而，这些贝壳是由碳酸钙制成的，在黑色页岩中，由于沉积物中的酸性流体，这些贝壳通常会溶解掉。为了解决这个问题，我们在世界各地寻找化石保存完好的岩石。我们发现了两个这样的地方，一个在坦桑尼亚（非洲东部），一个在摩洛哥（非洲西北部），不同寻常的环境很好地保存了正确年代的浮游生物化石。因为这些岩石富含粘土，而且从来没有被埋在地球深处，所以这些化石没有受到破坏，尽管它们有9000多万年的历史，但它们看起来像现代的例子。我们将利用这些化石收集信息，告诉我们浮游生物是如何应对OAE2气候变暖的。我们特别想知道进化变化的速度是否随着环境的变化而增加，以及是否有更多的物种进化或灭绝。我们还将利用这些化石来调查某些物种是否比其他物种受到的影响更大。例如，是喜欢温暖水的物种比喜欢冷水的物种受影响更大，还是那些产生更重、更厚外壳的物种比那些形成小而精致外壳的物种受影响更大？通过观察这些进化的细节，我们可以发现哪种环境变化最具影响力，例如，是水温、食物供应还是海洋化学（酸化）变化。更广泛地说，这些浮游生物进化的记录将帮助我们了解这些温暖的气候事件是如何发生的，以及地球是如何恢复到更正常的状态的。随着对生命如何应对这些过去的气候事件有了更好的了解，我们将能够更好地预测未来几十年地球生物圈将如何应对。

The geological record contains fossil shells and shell chemistry that allow us to reconstruct ancient Earth environments. This shows us that Earth has seen a wide range of different climates, including times when it was much colder or warmer than it is now. This research project aims to better understand how life responds to these changing climates by using the geological record of a specific warming event. We are especially interested in finding out what drives biological evolution and how much of a part climate plays in this. It is important to understand how life responds to changing environments, because our modern climate is warming rapidly and we need to find out what the impact of these changes will be. Will we see more extinction? Will life in the oceans become less abundant and so provide less food and absorb less carbon dioxide (which would make climate become warmer even faster)? To answer these questions, we will study a climate-warming event that occurred 93 million years ago, in the Cretaceous, when dinosaurs dominated on land. This greenhouse climate interval saw subtropical forests on Antarctica, crocodile-like animals in the Arctic, no ice-caps and sea-levels that were 70m higher than today. This event is known as Oceanic Anoxic Event 2 (OAE2) because it was first recognised by abundant black shale sediments that indicate that the ocean floors had little or no oxygen, allowing the accumulation of large amounts of carbon from plants and animals. As well as all that carbon burial, this event also saw rapid and extreme climate warming with increases of 6-9C within a few tens of thousands of years. Peak temperatures were at least 36C, compared with modern annual mean temperatures that rarely exceed 30C. So this looks like an ideal event for studying the response of life to warming, but there are some problems. The black shales that occurred during this event do not preserve the fossils that we use to measure evolution. The best fossils for this purpose are the shells of microscopic, single-celled plankton, which are produced in their billions and which are normally preserved in sea-floor sediments, worldwide. However, these shells are made from calcium carbonate and in black shales the shells are usually dissolved away because of acidic fluids in the sediment. To overcome this problem, we have searched the world to find rocks in which the fossils are preserved perfectly. We have found two such places where unusual conditions have preserved plankton fossils of the correct age very well, one in Tanzania (eastern Africa) and one in Morocco (NW Africa). Because these rocks are rich in clay and have never been buried deep in the Earth, the fossils have been protected from damage, and though they are over 90 million years old, they look like modern examples. We will use these fossils to collect information that will tell us how the plankton responded through the OAE2 climate warming. We particularly want to find out whether the rate of evolutionary change increased as the environment changed, and whether more species evolved or became extinct. We will also use these fossils to investigate whether certain types of species were more affected than others. For example, were warm-water-loving species more affected than cool-water types, or were species that produced heavier, thicker shells more affected than types that formed small, delicate shells? By looking at these details of evolution, we can find out which kinds of environmental change were most influential, for example, was it water temperature, food supply, or ocean chemistry (acidification) changes. More broadly, these records of plankton evolution will help us understand how these warm climate events operated and how Earth returned to more normal conditions. With a better understanding of how life responded to these past climate events, we will be better able to make predictions of how Earth's biosphere will respond through the coming decades.