Carbon dioxide from crumbling coasts: Rock weathering as a carbon source

This CASE project is partnered by PP Systems

Key Questions

What controls the leak of carbon dioxide to the atmosphere from the weathering of sedimentary rocks?

Background

A major CO2 emission occurs when sedimentary rocks are weathered, and this plays a central role in the geological carbon cycle, releasing as much CO2 as volcanism. The rates of these reactions and the controls on them have remained challenging to study. Using new methods, this project will unravel the physical, chemical and biological controls on sedimentary rock weathering and CO2 release, in the case of rock coastlines which erode rapidly.

Aims of the Project

Establish the rates of rock weathering and CO2 release on rocky shorelines; Constrain how physical properties and mechanical erosion enhance weathering rates; Understand the role of temperature in promoting a positive feedback through rock weathering on the carbon cycle.

Project Description

The global carbon cycle controls Earth’s climate by the release and removal of carbon dioxide (CO2) from the atmosphere. A major CO2 emission occurs when sedimentary rocks are weathered, and this plays a central role in the geological carbon cycle, releasing as much CO2 as volcanism (Hilton and West, 2020). The CO2 source can come from two processes, first through the oxidation of organic carbon in rocks, a so-called “geo-respiration”. Second, sulfide minerals can oxidise and produce sulfuric acid which can release CO2 from carbon minerals. The rates of these reactions and the controls on them have remained challenging to study. However, new methods allow us to measure the release of CO2 during sedimentary rock weathering directly in the field (Soulet et al., 2021), using carbon isotopes to track the carbon source and pathway.

Previous work has shown that physical processes that fracture and grind up rock can enhance sedimentary rock weathering (Hilton and West, 2020). This makes regions of active tectonic uplift hotspots of CO2 release, where landslides and erosion processes rapidly expose minerals to oxygen (Bufe et al., 2021). In these settings, the weathering of sulfides and rock organic matter may also increase with temperature (Soulet et al., 2021). Overall, this process could act as a positive feedback in the carbon cycle.

At the intercept of these physical and climatic controls on weathering are rocky coastlines. There, rising sea level can enhance erosion and expose sedimentary rocks. These processes can occur in the absence of tectonic uplift, potentially increasing CO2 release from rock weathering in landscapes where otherwise the fluxes are low.

To date, no measurements of CO2 release during sedimentary rock weathering exist for rocky cliff sections. This project will untangle the physical, chemical and microbiological controls on rock weathering using a combination of field and laboratory incubations, isotope geochemistry and geomicrobiology.

Methods to be used

Fieldwork will be undertaken to directly measure the fluxes of CO2 release during oxidative weathering (Soulet et al., 2021). These will be coupled with measurements of the environment of oxidation (temperature, humidity). Field locations will be selected based on capturing a range of physical and geochemical characteristics that are representative of wider areas, and have a preliminary focus on the rocky coasts of the UK. Field work in regions where tectonic uplift and rocky coast erosion combine will also be targeted (e.g. Taiwan, New Zealand). Samples will be collected to provide key insight from bulk geochemistry (%OC, %IC, %S), organic matter reactivity (ramped oxidation), and microbial community structure (16s RNA approaches). Geochemical analyses of trapped CO2 (stable isotopes, radiocarbon) will be undertaken to quantify its source. Laboratory-based incubations will further explore the drivers on this CO2 release during weathering.

Specialised skills required

Interest in fieldwork and analytical geochemistry

Please contact Robert Hilton on robert.hilton@earth.ox.ac.uk if you are interested in this project