Rhenium isotopes to track carbon dioxide emissions by oxidative weathering

Lead Research Organisation: Durham University
Department Name: Geography

Abstract

Carbon dioxide (CO2) plays a central role in controlling Earth's climate as a greenhouse gas. Atmospheric CO2 concentrations can be changed by Earth's carbon cycle, which moves carbon between the atmosphere, plant and animal life, the oceans and rocks. Atmospheric CO2 concentrations have been increasing in recent decades because of the human-induced transfer of carbon from rocks to the atmosphere by fossil fuel burning. However, natural processes can also change the rate of carbon transfer to the atmosphere. One such transfer occurs when rocks containing vast stores of organic carbon are exposed to weathering, a process that releases CO2. This process may have varied significantly over Earth's history, for instance during episodes of mountain building. However, we presently lack reliable tools to track these changes which limits our understanding of the processes controlling natural variability in atmospheric CO2 concentrations, and consequently our ability to accurately predict how the carbon cycle will evolve in the future.

To measure the oxidation of rock organic matter in the modern day, we can track the CO2 gas directly. This approach can provide a local, short term view of the rates of CO2 release. Another approach has been developed which uses the element rhenium (Re) that is hosted in rocks alongside organic carbon. When rocks are weathered, the CO2 is released as a gas, while the Re becomes dissolved in water and is carried by rivers. In this way, we can measure how much Re rivers carry to estimate CO2 emissions over larger river basins. Unfortunately, to reconstruct weathering and CO2 emissions in the past, we cannot directly use these techniques.

Instead, to look back in time at weathering of organic carbon in rocks and the associated CO2 emissions, the isotopes of Re hold much promise. This is because weathering could alter the ratio of Re isotopes released into river water, which in turn has the potential to change the global inventory of Re in seawater.

Developments in analytical geochemistry, most recently led by the research team, mean that we have been able to measure the ratio of Re isotopes in river water for the first time. Our novel, unpublished data shows that the Re isotope ratio in rivers increases with weathering rate. This observation strongly suggests that Re isotopes could be used as a proxy of past oxidative weathering and associated CO2 emissions.

In this project we will tackle the fundamental limitations of our present understanding of the Re isotope system, that hold back its current application as a weathering proxy. In particular, we must establish a deeper understanding of the relationship between Re isotope ratios in rocks, soils, the waters reacting in soils which feed rivers, and the largest rivers in the world. In parallel, we must also measure other sources of Re to the ocean from hydrothermal vents and the Re isotopes of seawater from the major ocean basins.

Our proposal is planned as a unique collaboration between two laboratories with the same analytical capabilities. This collaboration allows us to capitalise on three major benefits: (i) To ensure data accuracy by sharing analytical and method advancements; (ii) To pool the expertise of a multi-disciplinary team of scientists; and (iii) To maximise the efficient use of resources by achieving an ambitious work programme across two cutting-edge laboratories.

This project will produce the first complete assessment of the isotopic composition of Re in the bulk Earth and in weathering products being delivered to the global oceans. In doing so, we will lay the foundations for the Re isotope proxy to quantify and understand past changes in CO2 emission from rock weathering, and address a crucial shortcoming in the ability of state-of-the-art climate models to simulate the trajectory of carbon cycle changes in the past, present and future.

Planned Impact

This research proposal aims to develop a new tool to examine a key component of the carbon cycle: the carbon dioxide emissions from weathering of organic matter in rocks. Grounded on our preliminary data, our focus is on the rhenium isotope system. To make progress, we must understand the rhenium isotope system in soils, rivers, hydrothermal waters and seawater which requires extensive fieldwork using novel and cutting-edge techniques. We have identified three potential beneficiaries of the research, and designed a set of activities to engage with them to produce impact.

1) Future environmental scientists & the environmental science sector
NERC's Skills Review (2010 & 2012) identified skill gaps in the environmental sciences sector. Of these, "field research", "freshwater science" and the "translation of research into easily understandable knowledge" were identified as those in short supply.
We will contribute to specialised training in skills and methods which we use that are immediately transferable to broader questions of river water quality. This benefits both the students we train and the sectors seeking to hire these individuals. Students will join our Iceland field trip tasked with developing transparent documentation of the journey 'From the field to the lab'. While gaining training, they will make short videos. To address the skills gap associated with science communication, we will task the students with compiling their videos for non-academic users and collaborate with Dr Viv Cumming, a professional science writer, to highlight the best ways to communicate science and document the scientific process.
To create longer-term impact and significantly widen the reach, we will use our experiences to produce a set of training resources: method-based vlogs which combine the Iceland field trip with lab based videos and instruction. In addition to being posted freely online, these resources will feed directly into research-led teaching at the undergraduate and postgraduate level at DUR and RHUL.

2) School age children and the general public
Our research in the theme of Earth's natural carbon cycle is of broader interest to school children and to the general public. The link between the vastly accelerated CO2 emissions from burning of fossil fuels, and the slower reactions which occur by oxidation of organic matter in rocks are not widely documented or discussed.
To engage, we will contribute exhibits to Durham University's flagship public science festival, "Celebrate Science", to inspire young people to study science in the future. The target is Key Stage 2 children and their families, with a reach of >7,000 visitors. We will set up a demonstration of how different rocks can to lead to a CO2 source or sink. We will use the vlogs produced by the students to highlight the type of work we do in the field. We will contribute the same exhibit and activity at the 'Fete de la science - 2021', an event in the heart of Paris, run from the Institut de Physique du Globe.
The results will be disseminated by a dedicated project website and linked to social media. These outlets will also be used to provide updates on field work in Iceland and beyond, preliminary findings, lay summaries, links to publications/interviews/blogs and will provide an opportunity to evaluate the impact of our engagement.

3) Industry
Royal Dutch Shell (Amsterdam/The Hague) have expressed an interest in our proposed research, as it may directly impact their business. The behaviour of rhenium underpins Rhenium-Osmium dating, used as a direct geochronometer by industry to constrain petroleum system dynamics. Co-PI Dickson has worked with Shell for ~6 years, over which time they have become increasingly interested in the use of metals such as rhenium as petroleum system tracers. We will visit Shell over the course of the project to meet with the geochemistry and exploration teams, to investigate potential industrial applications of our research.

Publications

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