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

在地质时间尺度上，碳通过大气、生物圈、海洋循环，甚至被纳入沉积岩；碳在储存库之间的这种运动通俗地称为“碳循环”。岩石记录中的碳可以保存为有机（生物）分子或无机碳酸钙分子，也称为石灰石。通过研究这些有机碳和无机碳化合物中碳的化学差异，我们可以重建生命史的各个方面，过去的气候变化，甚至地球大气的氧化史。最近，我们认识到，我们解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构成，但由分解的有机碳形成。因此，它的碳在化学上与构成它的有机碳更相似。更好地了解地球历史上以成岩碳酸盐形式储存的碳量，可以从根本上改变与过去氧气水平和过去气候变化有关的问题的答案。的确，成岩碳酸盐在岩石记录中比在现代海洋中更常见——例如，在英国最受欢迎的化石旅游景点，包括多塞特、萨默塞特、约克郡和格拉摩根海岸线的部分地区，大部分石灰石都是成岩碳酸盐。不幸的是，我们对不同地质时期成岩碳酸盐岩生产的认识还处于起步阶段。成岩碳酸盐的形成是由海洋沉积物中许多不同化学过程的相互作用决定的。因此，为了解释在不同的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型来回答这些问题。根据得出的计算结果，我将能够重新解释过去气候的各个方面，气候突变和地球大气氧化的历史。除此之外，我还将在富含成岩碳酸盐的地点（包括萨默塞特郡和约克郡海岸）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回溯沉积记录，从而破译哪种环境因素导致了这些成岩碳酸盐的形成。萨默塞特海岸有大灭绝的记录，约克郡海岸有海洋缺氧时期的记录。成岩碳酸盐提供的关于过去环境的信息将帮助我们解开过去这些快速气候变化事件的原因和后果。这种令人兴奋的新方法可以从成岩碳酸盐中了解过去，这是一个尚未开发的过去气候档案，可以应用于地球历史上各种其他气候变化事件。该奖学金响应了古气候科学领域跨学科的号召。通过将建模的力量与令人兴奋的新领域和实验室观察相结合，我们可以学到更多。我设计了一个奖学金研究计划，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，并召集了一批在各自领域处于世界领先地位的合作者。这个项目将为研究生命和气候历史的地球科学家提供一个模型，告诉他们如何将模型和数据方法以一种新的、强大的方式结合起来。

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.