Oxidation of fossil organic matter as a source of atmospheric CO2

Project Rationale

All living organisms contain organic carbon. When an organism dies, organic carbon can become trapped within ancient sedimentary rocks (fossil organic carbon). This is a major carbon reservoir and is a sink for atmospheric carbon dioxide (CO₂) over millennial- and million-year timescales (Hilton and West, 2020). However, uplift, exhumation and erosion of ancient sedimentary rocks has the potential to oxidize fossil organic carbon and release this CO₂ back into the atmosphere (Hemingway et al., 2019). This may act as a positive climate feedback and increase global temperatures. However, the mechanisms responsible for fossil carbon oxidation remain a major gap in our understanding. Fossil carbon oxidation is likely controlled by a variety of (i) abiotic (e.g., temperature, oxygen availability, erosion rates) and (ii) biotic factors. Disentangling the relative importance of these different processes is challenging but can be assessed in controlled laboratory and natural field experiments. This project will employ a suite of laboratory and field-based approaches to study the fate of fossil organic carbon in modern environments. The findings of this project will help constrain whether fossil organic carbon oxidation might serve as a large source of atmospheric CO₂ in both past and future climates.

Methodology

Field-based approaches - the student will obtain soil, bedrock and suspended river sediments from a fossil OC-rich catchment (target location: the French Alps). This will be combined with data collected from local catchments to study the fate of fossil OC in river catchments with different proportions of fossil OC. Gas Chromatography-Mass Spectrometry (GC-MS) will be used to identify and quantify fossil OC biomarkers.

Laboratory experiments –Sedimentary rocks obtained from previous sampling expeditions will be powdered & incubated at different temperatures (5°C, 15°C, 25°C, and 35°C) and the percentage of organic carbon will be determined at different time points (daily, moving to weekly) over a 6-month interval. This will be used to isolate the influence of temperature upon fossil OC oxidation. An identical experiment will be carried out but will add ¹³C-labelled substrates to the incubations. This will help determine much OC is consumed by the active microbial pool. The student will assess this by measuring the carbon isotopic composition (δ¹³C) of phospholipid fatty acids (PLFAs) using GC-isotope ratio mass spectrometry (GC-IRMS) (Petsch et al., 2003). The student will also use flume experiments to assess how transport, resuspension and erosion rates govern fossil OC oxidation rates under a range of transport conditions.

Training

The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at the School of Ocean and Earth Science. The student will receive expert training in:

 [1] organic geochemical techniques, including ¹³C-labelling (Inglis),

[2] bulk and compound specific stable isotope techniques (Inglis; Hilton), and

[3] sediment transport dynamics (Thompson).

 The student will participate in a fieldwork campaign, likely in the French Southern Alps (https://www.czen.org/content/draix-bleone-observatory) but also local river catchments. The student will present at national and international conferences, write peer-reviewed publications and a PhD thesis. The research training addresses field, numerical, statistical and laboratory skills, equipping the student for a career across a range of professions.