Resolving Past Changes in Ocean Oxygenation: Utility of Chromium Isotopes

越来越多的人担心，最近世界部分海洋中溶解氧浓度的下降预示着地球气候的变化。这是因为地质记录中有大量证据将海水氧气与气候变化联系在一起--例如，二叠纪末(2.51亿年前)的缺氧海洋与全球变暖和大规模物种灭绝有关。然而，我们不知道的是，究竟是什么控制了溶解氧浓度和气候之间的联系？回答这个问题极其困难，因为对过去海水氧气水平的直接测量，以及海洋表面温度等气候参数的直接测量，只能追溯到几十年前。相反，我们必须依赖保存在地质记录中的间接测量或所谓的“代理”测量。在过去的海水含氧量中，有很多种“替代物”。其中大多数依赖于对海洋沉积物中金属浓度的测量。然而，现在很明显，金属浓度可能会受到许多不同变量的影响(例如，上覆水柱中初级生产力的水平)，所有这些变量都可以独立于海水氧气而变化。绕过其中一些问题的一种方法是通过分析这些金属的同位素。最近，研究集中在铁和钼的同位素分析上，但一个关键问题是，铁和钼对海水氧的微小变化都不敏感。他们只能区分有氧(氧气充足)或正辛酸(没有氧气，硫化氢含量高)的情况。出于所有这些原因，显然有必要寻找新的海水氧气替代品。铬是一个理想的候选者，原因有很多。首先，它在有氧条件下是可溶的，但在轻微缺氧条件下不能溶解(因此会在海洋沉积物中积累)。其次，它的同位素在氧气和缺氧条件下的表现非常不同，现在可以使用最先进的仪器来检测这一点。第三，铬及其同位素的化学组成相对简单。该项目的首要目标是测试铬同位素作为海水氧气示踪剂的效用。为此，我们将：(1)测量在各种氧气条件下沉积的海洋沉积物的铬同位素组成。(2)确定海洋沉积物中铬吸收的精确机制，以及铬在埋藏后的迁移潜力。(3)获取气候剧烈变化时期的铬同位素记录。我们希望我们的研究结果能更好地理解海水、氧气和全球气候变化之间的关系。这些知识对于预测和缓解未来的地球气候将是必不可少的。

There is growing concern that a recent reduction in the concentration of dissolved oxygen in parts of the world's oceans signals a change in the Earth's climate. This is because there is lots of evidence in the geological record that links seawater oxygen to climate change- the anoxic (oxygen deficient) oceans at the end of the Permian (251 million years ago), for example, are associated with global warming and massive extinctions. Nevertheless, what we don't know, is exactly what controls the link between the concentration of dissolved oxygen and climate? Answering this question is extremely difficult because direct measurements of past levels of seawater oxygen, as well as climate parameters such as sea-surface temperature, extend back only as far as a few tens of years. Instead, we must rely on indirect, or so-called 'proxy' measurements that are preserved in the geological record. There are a number of 'proxies' for past levels of seawater oxygen. Most of them rely on measurements of the concentration of metals in marine sediments. However, it is now clear that metal concentrations can be affected by a number of different variables (for example, the level of primary productivity in the overlying water column), all of which can vary independently of seawater oxygen. One way to circumvent some of these problems is by analysis of the isotopes of these metals. In recent times, research has focused on the analysis of iron and molybdenum isotopes, but a key problem is that both iron and molybdenum are insensitive to small changes in seawater oxygen. They are only able to distinguish between oxic (oxygen-replete) or euxinic (no oxygen, high levels of hydrogen sulphide) conditions. For all of these reasons, there is a clear need to find new proxies for seawater oxygen. Chromium is an ideal candidate, for a number of reasons. Firstly, it is soluble under oxic conditions, but insoluble (and therefore accumulates in marine sediments) under slightly oxygen deficient conditions. Second, its isotopes behave very differently under oxic vs oxygen deficient conditions, and this can now be detected using state-of-the-art instrumentation. Third, the chemistry of chromium and its isotopes is relatively simple. The overarching aim of this project is to test the utility of chromium isotopes as a tracer of seawater oxygen. To this end, we will: (1) Measure the chromium isotope composition of marine sediments deposited in a range of oxygen conditions. (2) Determine the precise mechanism of chromium uptake into marine sediments, and the potential for movement of chromium after it has been buried. (3) Obtain records of chromium isotopes for a period of dramatic climate change. We expect the results of our research to provide an improved understanding of the relationships between seawater oxygen and global climate change. Such knowledge will be imperative for both the prediction and mitigation of Earth's climate in the future.