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

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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.