Where did all the CO2 go? Insights from boron isotopes in deep-sea corals

在过去的250万年左右的时间里，地球的气候在温暖时期（比如今天的间冰期）和寒冷时期（冰期）之间有规律地振荡，冰期是几公里的冰覆盖了北半球。被困在冰芯中的古代空气气泡告诉我们，尽管这些循环最终是由地球绕太阳轨道的变化引发的，但它们在很大程度上是由大气中温室气体二氧化碳（CO2）浓度的增加所驱动的——CO2在冰期低，在间冰期高。在每个周期中，冷却进入冰期往往相当缓慢（需要9万至3万年），而结束每个冰期的变暖往往非常迅速（大约1万年）。由于这些变暖事件导致北半球冰盖急剧而迅速的退缩，它们被称为冰川消融。最后一次去冰期大约开始于1.8万年前，完成于1万年前。尽管这些冰期-间冰期旋回是最近全球气候变化的最显著和最重要的例子，但其确切原因尚不清楚。然而，我们所知道的是，在冰期去冰期间，二氧化碳最有可能被从冰期储存的深海转移到大气中，在那里它使地球变暖并推动冰盖的退缩，直到下一个冷却周期开始。为了确定是哪种机制导致二氧化碳像这样移动，我们需要确切地知道它在海洋中的去向。一些研究指出，它被储存在南极洲周围循环的深水中，因此表明这是该地区运行的机制造成的。尽管这与我们的许多观察结果一致，但其他一些线索表明，地球另一边的北太平洋也很重要。最近甚至有人提出，深海根本没有参与其中。在这项提案中，我们通过确定二氧化碳是否储存在南极洲周围来阐明这场辩论。1.8万年前海水中的二氧化碳含量没有实际的测量数据，因此我们必须使用间接的测量方法。我们将使用的代理是基于古代深海珊瑚骨骼中的硼。深海珊瑚，就像它们在温暖的热带海洋中发现的近亲一样，用碳酸钙制造骨骼。碳酸钙骨架中硼的同位素组成与珊瑚生长的pH值有关，海水的pH值与海水中二氧化碳的含量成正比。因此，pH值是一种非常有用的直接示踪剂，可以显示冰川深渊中储存的二氧化碳。然而，为了获得最佳的pH重建，我们首先需要比目前更好地校准代理。我们将主要通过在实验室中在已知的pH值下种植深海珊瑚，并测量它们的硼成分来做到这一点。有了更好的了解，我们不仅可以了解这些动物将如何受到未来海洋酸化的影响，而且通过测量不同年龄的古代深海珊瑚骨骼的硼同位素组成，我们可以重建整个冰川消融过程中一个地方的pH值是如何演变的。我们有一些深海珊瑚样本，它们来自西南太平洋1500米深的地方，有3万到8千年的历史。我们对这个区域很感兴趣，因为它被认为是二氧化碳的主要途径，因为它从深海混合到海洋上层，然后最终进入大气。我们将制作的pH值记录将是对我们目前关于在冰川消融期间二氧化碳如何在海洋和大气之间移动的想法的彻底测试；因此，这项研究将为冰期-间冰期二氧化碳分压变化的机制提供有价值的见解。

Over the last 2.5 million years or so the Earth's climate has regularly oscillated between warm periods, like today called interglacials, and frigid cold periods called glacials when several kms of ice blanketed the Northern Hemisphere. Bubbles of ancient air trapped in ice cores tell us that, although the cycles are ultimately triggered by changes in the Earth's orbit around the Sun, they are largely driven by increases in the atmospheric concentration of the greenhouse gas carbon dioxide (CO2) - CO2 is low during glacial periods and high during interglacial periods. During each cycle, cooling into a glacial tends to be rather slow (taking between 90 to 30 thousand years) and the warming that terminates each glacial period tends to be very rapid (~10 thousand years in length). Since these warming events caused the dramatic and rapid retreat of the northern hemisphere ice sheets they are known as deglacials. The last deglacial began around 18 thousand years ago and was completed by around 10 thousand years ago. Despite these glacial-interglacial cycles being the most dramatic and significant recent examples of global climate change, their exact cause is not known. What we do know however is that during a deglacial CO2 is most likely being moved out of the deep oceans where it is stored during glacial periods, to the atmosphere, where it warms the Earth up and drives the retreat of the ice sheets, until the next cooling cycle begins. In order to tie down which mechanisms are responsible for moving the CO2 around like this we need to know exactly where in the ocean it is going. Some studies point to it being stored in the deep abyss in water that circulates around Antarctica, therefore suggesting it is mechanisms operating in this region that are responsible. Although this agrees with many of our observations, some other clues point to the North Pacific on the other side of the globe, as being important. And it has even been recently suggested that the deep ocean isn't involved at all. In this proposal we shed light on this debate by determining whether or not CO2 was stored around Antarctica.No actual measurements exist of the CO2 of seawater 18 thousand years ago, therefore we have to use indirect measurements known as proxies. The proxy we will use is based on boron in ancient deep-sea coral skeletons. Deep-sea corals, like their cousins found in warm tropical seas, make skeletons out of calcium carbonate. The isotopic composition of boron in their calcium carbonate skeleton is related to the pH in which the coral grew and the pH of seawater is proportional to the amount of CO2 it contains. Therefore, pH is a very useful and direct tracer ofthe CO2 stored in the glacial abyss. However, in order to get the best pH reconstructions we first need to calibrate the proxy better than it is currently. We will mainly do this by growing deep-sea corals at known pH in the laboratory and measuring their boron composition. Armed with this better understanding we will not only get an idea of how these animals will be affected by future ocean acidification, but, by making measurements of the boron isotopic composition of ancient deep-sea coral skeletons of different ages we can reconstruct how pH evolved in one location through the entire deglacial. We have a number of deep-sea coral samples from around 1500 m water depth in the SW Pacific that are from 30 to 8 thousand years old. We are interested in this region because it has been put forward as a key route for CO2 as it is mixed from the deep abyss into the upper levels of the ocean and then ultimately into the atmosphere. The pH record we will produce will be a thorough test of our current ideas of how CO2 moves between ocean and atmosphere during a deglacial; this study will therefore provide valuable insights into the mechanisms responsible for glacial-interglacial pCO2 change.