Quantifying seawater redox variations and continental weathering rates through the Paleocene-Eocene Thermal Maximum

我们现在知道，由于人类活动，地球正在变暖。但全球变暖将持续多久，地球将如何恢复？还会发生什么变化，它们将如何影响大陆和海洋中的生命？这些是科学家、政治家和公众越来越迫切地提出的一些问题。然而，虽然我们可以通过建模预测20年或50年后地球气候的状态，但我们对未来几百年或几千年的环境变化过程的理解却不那么确定。解决长期环境变化问题的另一种方法，也是本提案所采用的方法，涉及对遥远过去发生的环境变化的研究。我们从地质记录中了解到，有几个时期全球变暖非常突然和非常严重，当时气温上升的速度似乎与今天相似。因此，我们可以通过研究这些过去的事件来了解地球在超热(即异常炎热)条件下的行为。这一提议的焦点是地球历史上最严重的高温事件之一。它发生在5500万年前，与海洋和陆地动植物的重大变化有关，被称为古新世-始新世最热时期(PETM)。这项研究将涉及对PETM期间积累在海底的沉积物进行详细检查和化学分析。随着时间的推移，海洋沉积物的化学成分会随着沉积海水的化学成分的变化而变化。反过来，海水的化学成分又受到环境条件的控制，如温度和风化，因为它们也会随着时间的推移而波动。通过对横跨PETM的海洋沉积物进行适当的分析，我们拥有了追踪不断变化的环境条件的强大手段。与比利时和俄罗斯的同事合作，我们将从横跨俄罗斯、埃及和亚洲PETM的关键海洋沉积物中获得新的样本集。这些特殊的层序是在大陆架上积累起来的，对我们的研究来说是理想的，因为它们相对较厚和完整，使我们能够以非常高的分辨率对它们进行采样。这些沉积物还含有丰富的海洋来源的有机碳，这确保了它们是我们研究的理想选择。我们将开展新的钼同位素分析(海水含氧量的间接测量或“替代物”)、Os同位素分析(大陆风化速率的替代物)和碳同位素分析(全球碳循环状况的指示器)。我们还将使用美国一位同事提供的样品，对横跨大洋大洋的深海岩心中的硫酸钡进行锶同位素分析(大陆风化速率的另一个替代指标)。观察结果表明，除了地球最高温带突然出现严重的全球变暖外，海洋中的物种灭绝程度很高，陆地上的物种分布也发生了重大变化。值得注意的是，PETM标志着陆地哺乳动物开始蓬勃发展的紧接着的时间点。伴随着这些重大环境变化的是一种非常独特的碳同位素异常，它影响了陆地和海洋中的所有生物圈水库，这不仅告诉我们关于这一事件的许多信息，而且使我们能够将世界各地的PETM剖面联系起来。我们项目的总体目标是确定全球碳循环、大陆风化和海水含氧量水平在PETM发生的主要变化是如何以及在什么时间尺度上发生的。这些新信息将向我们展示地球系统过去如何应对严重的环境压力并从中恢复，并可能帮助我们了解未来变化的进程。

We now know that the Earth is warming because of human activity. But how long will warming last, and how will the Earth recover? What other changes will occur, and how will they affect life on the continents and in the oceans? These are some of the questions that are asked with increasing urgency by scientists, politicians and the public. However, whilst we can predict through modelling the state of Earth's climate in 20 or 50 years time with reasonable confidence, our understanding of the course of environmental change in the next few hundred or few thousand years is much less certain. A different approach to tackling the questions about longer-term environmental change, and the one adopted in this proposal, involves the study of environmental change that occurred in the distant past. We know from the geological record that there have been a few periods when global warming was extremely sudden and very severe, when temperatures increased at rates that appear to have been similar to those of today. We can therefore learn about how the Earth behaves under 'hyperthermal' (i.e. unusually hot) conditions by studying these past events. The focus of this proposal is one of the most severe hyperthermal episodes in Earth's history. It occurred 55 million years ago, was associated with major changes in marine and terrestrial flora and fauna, and is known as the Paleocene-Eocene Thermal Maximum (PETM). This study will involve the detailed examination and chemical analysis of sediments that accumulated on the seafloor during the PETM. The chemical composition of marine sediments, as they accumulate over time, varies in response to the fluctuating chemical composition of the seawater in which they are deposited. In turn, the chemical composition of seawater is controlled by environmental conditions, such as temperature and weathering, as they also fluctuate over time. By making appropriate analyses of marine deposits that span the PETM, we have a powerful means of tracking changing environmental conditions. Working with colleagues from Belgium and Russia, we will obtain new sample sets from key marine deposits that span the PETM in Russia, Egypt and Asia. These particular successions accumulated on the continental shelf and are ideal for our study because they are relatively thick and complete, enabling us to sample them at very high resolution. The deposits also contain abundant organic carbon of marine origin, which ensures that they are ideal for our study. We will carry out new molybdenum-isotope analyses (an indirect measure or 'proxy' for the level of seawater oxygenation), osmium-isotope analyses (a proxy for continental weathering rate), and carbon-isotope analyses (an indicator of the state of the global carbon cycle). We will also perform strontium-isotope analyses (another proxy for continental weathering rate) on barium sulphate from deep-sea cores that traverse the PETM, using samples provided by a colleague in the USA. Observations show that in addition to the sudden onset of severe global warming at the PETM, there were high levels of species extinctions in the oceans and significant changes in species distributions on land. Notably, the PETM marks the point immediately after which land mammals started to flourish. These major environmental changes were accompanied by a very distinctive carbon isotope anomaly that affected all biospheric reservoirs, both on land and in the oceans, which not only tells us much about the event but also enables us to correlate PETM sections across the world. The overall objective of our project is to establish how, and over what timescale, the major changes in the global carbon cycle, continental weathering, and seawater oxygenation levels occurred at the PETM. This new information will show us how the Earth system responded to and recovered from severe environmental stress in the past, and may help us to understand the course of future change.