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

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.

Who and How?
Public: The public will benefit from outreach events including Festival of Nature and Discover, expert-guided field excursions highlighting how this research deciphers the history behind some of the most popular British fossil hunting localities, and activity and quiz sheets to bring students in the field via the Geologist's Association Junior Arm "Rockwatch". All of the field guides and activity/quiz sheets I generate will be hosted on my website for general use as will information describing what some of the most prominent features at some of Britain's best-loved rocky seaside outcrops can tell us about the history of life and climate.

Postgraduate students: Postgraduates will benefit from a 3-day carbonate diagenesis workshop that I will conduct twice over the course of this fellowship in conjunction with Sandra Arndt and Fiona Whitaker. This workshop has been conceived as a short course that will be conducted every three years in perpetuity. All palaeoclimate students grapple with diagenesis in some form during their postgraduate work (either directly or indirectly), yet it is an oft-neglected topic. This hands-on workshop will introduce students to 1) chemical changes and proxy fidelity; 2) authigenic minerals as proxies; 3) reaction-transport modelling. It will also establish a network of early-career researchers with a core focus but varied backgrounds, fostering future collaborations within this community. I will also invite motivated postgraduates to serve as field assistants for my field work, giving them valuable field geology skills.

Industry: The hydrocarbon industry will benefit from a new palaeoenvironmental proxy and an improved, integrative understanding regarding the precipitation of diagenetic carbonates. This fellowship will shed light on the controls on organic matter preservation and hence on petroleum source rock formation. Improved understanding of cementation, a key control on mechanical strength, will contribute to more effective prediction in tight carbonate systems, which are important hosts for shale oil and gas.

Scientific community: The scientific community will benefit from a diversification of the community. Women are traditionally underrepresented in the modelling subfield of geosciences. This fellowship not only funds a woman for continued modelling-based research, but two of the collaborators on the project (SA, FW) are women modellers. A strong mentoring network of women modellers will help mitigate gender imbalance in the field. Additionally, I will actively encourage female students to participate in the diagenesis training workshop and as field assistants. This project also benefits the scientific community by promoting ever tighter collaboration between data and modeling experts and integrating these approaches seamlessly from its inception, helping to bridge the 'data-model divide.'

Practical strategies to ensure the benefit:
1. Become a highly visible research leader and establish new international working groups to promote the multidisciplinary techniques I will employ.
2. Publish results, methodologies, and model code in high profile open access journals
3. Present work at 2 conferences per year (1 EU, 1 international), at the University of Bristol, and the departments of my external collaborators
4. Convene sessions at international meetings (min. 1 each at Goldschmidt and AGU)
5. Lead field trips for amateur rock hounds and students to local outcrops
6. Publish activity worksheets and field guides on my website
7. Design experiments for Festival of Nature and Discover and train as a STEM ambassador to bring these activities to classrooms
8. Issue press releases targeting newspapers, television and other popular media outlets, e.g. NERC Planet Earth publication, New Scientist
9. Organise a diagenesis workshop for postgraduates every 3 years
10. Involve students in my research (as field assistants plus teaching and research project supervision)