Rethinking carbonate diagenesis: clues to past carbon cycling from an overlooked carbon sink

在地质时间尺度上，碳在大气、生物圈、海洋中循环，甚至被并入沉积岩中；这种碳在储集层之间的移动被俗称为“碳循环”。岩石记录中的碳可以作为有机(生物)分子或碳酸钙的无机分子保存下来，也被称为石灰石。通过研究有机和无机碳化合物中碳的化学差异，我们可以重建生命的历史、过去的气候变化，甚至地球大气的氧气历史。最近，我们认识到，我们阅读地质历史的方式可能会受到岩石记录中第三种碳的极大影响。“成岩碳酸盐”由与石灰岩相同的矿物组成，但由分解的有机碳形成。因此，它的碳在化学上可能更类似于它形成的有机碳。更好地了解地球历史上作为成岩碳酸盐储存的碳的数量，可能会从根本上改变与过去的氧气水平和过去的气候变化有关的问题的答案。事实上，成岩碳酸盐在岩石记录中肯定比在现代海洋中更常见--例如，英国最受欢迎的化石旅游景点的许多石灰岩都是成岩碳酸盐，包括多塞特河、萨默塞特河、约克郡和格拉摩根海岸线的部分地区。不幸的是，我们对不同地质时期成岩碳酸盐的生产的了解还处于初级阶段。成岩碳酸盐的形成受控于海洋沉积物中许多不同化学过程的相互作用。因此，为了破译在不同环境和海洋条件下何时、为什么以及产生多少成岩碳酸盐，我将使用最先进的计算机模拟来回答这些问题。根据计算结果，我将能够重新解释过去气候、突然的气候变化和地球大气氧化的历史。除此之外，我还将在成岩碳酸盐丰富的地点(包括萨默塞特和约克郡海岸)进行实地工作，并测量成岩碳酸盐的化学成分。有了这些信息，我使用的计算机模型将能够倒回沉积记录，以破译是什么环境因素导致了这些成岩碳酸盐的形成。萨默塞特海岸记录了一次大灭绝，约克郡海岸记录了海洋氧气极低的一段时间。成岩碳酸盐提供的有关过去环境的信息将有助于我们揭开这些过去快速气候变化事件的原因和后果。这种新的、令人兴奋的方法从成岩碳酸盐中了解过去，这是对过去气候的未开发档案，然后可以应用于地球历史上各种其他气候变化事件。这种奖学金响应了古气候科学领域对跨学科的呼吁。通过将建模的力量与令人兴奋的新的现场和实验室观察结合起来，我们可以了解更多。我设计了一项奖学金研究计划，整合并增强了我在野外和实验室地质学以及气候建模方面的技能，并组建了一批在各自领域处于世界领先地位的合作者。这个项目将作为地球科学家研究生命和气候历史的模型，研究如何以一种新的、强大的方式将建模和数据方法结合在一起。

Over geological timescales carbon is recycled through the atmosphere, the biosphere, the ocean, and even gets incorporated into sedimentary rock; this movement of carbon between reservoirs is colloquially called the 'carbon cycle.' Carbon in the rock record can be preserved as organic (biological) molecules or inorganic molecules of calcium carbonate, also known as limestone. By studying the chemical differences between the carbon in these organic and inorganic carbon compounds, we can reconstruct aspects of the history of life, past changes in climate, and even the history of the oxygenation of Earth's atmosphere.Recently, we have recognised that the way we read geologic history may be vastly influenced by a third type of carbon in the rock record. 'Diagenetic carbonate' is made of the same mineral as limestone, but forms from decomposed organic carbon. Its carbon, therefore, can be more chemically similar to the organic carbon from which it forms. A better understanding of the amount of carbon stored as diagenetic carbonate during earth's history could fundamentally change the answers to questions relating to past oxygen levels and past climate changes. Indeed, diagenetic carbonate certainly seems to be more common in the rock record than it is in the modern ocean - for example much of the limestone at Britain's most popular fossil tourism sites is diagenetic carbonate, including parts of the Dorset, Somerset, Yorkshire, and Glamorgan coastlines. Unfortunately, our understanding of the production of diagenetic carbonate during various geologic periods is in its infancy. The formation of diagenetic carbonate is governed by the interactions of many different chemical processes in the ocean sediment. Therefore, to decipher when, why, and how much diagenetic carbonate is produced under different environmental and oceanic conditions, I will use state of the art computer modelling to answer these questions. In light of the resulting calculations, I will be able to reinterpret aspects of past climates, abrupt climate change and the history of the oxygenation of Earth's atmosphere.In addition to this, I will also be conducting field work at sites with abundant diagenetic carbonate (including those on the Somerset and Yorkshire coasts) and measuring the chemistry of the diagenetic carbonates. With this information, the computer models I use will then be able to rewind the sedimentary record to decipher what kinds of environmental factors led to the formation of these diagenetic carbonates. The Somerset coast contains the records of a mass extinction and the Yorkshire coast the records of a time when the ocean was extremely low in oxygen. The information that the diagenetic carbonates provide about the past environments will helps us unravel the causes and consequences of these past episodes of rapid climate change. This new and exciting method to learn about the past from diagenetic carbonate, an untapped archive of past climates, can then be applied to all sorts of other episodes of climate change in Earth history.This fellowship answers a call in the field of palaeoclimate science for interdisciplinarity. We can learn much more by combining the power of modelling with exciting new field and laboratory observations. I have designed a fellowship research plan which integrates and augments my skills in field and lab geology as well as climate modelling, and assembled a cast of collaborators who are world leaders in their respective fields. This project will serve as a model to geoscientists studying the history of life and climate for how modelling and data approaches can be combined in a new, powerful way.