Earth Surface Geochemistry

The Earth Surface Geochemistry group at Oxford seeks to understand the drivers and dynamics of the exchange of elements between rocks, soil, sediments and the surface environment. We have a strong focus on the carbon cycle, quantifying the processes that both remove and release carbon dioxide (CO₂) as rock minerals and organic carbon interact with air, water and life.

Overall, we are motivated to quantify how climate change (e.g. permafrost thaw, deglaciation, changing hydroclimate) and earth surface dynamics (e.g. erosion processes) steer the exchange of CO₂ and methane (CH₄) between soil, sediment and river waters and the atmosphere. We consider the long-term evolution of Earth’s surface environment, and how weathering helps steer atmospheric CO₂ concentrations and global climate. We also quantify carbon transfers in the present day which are vulnerable to change as landscapes warm. These important themes are funded by NERC, ERC (RIV-ESCAPE) and the Leverhulme Trust.

We use a variety of novel methods across our research areas (see below) and employ an interdisciplinary approach across Earth Sciences, calling on isotope and elemental geochemistry, geomorphology, hydrology, and microbiology. We combine a range of field approaches – working on the geochemistry of weathered rocks alongside river waters, while we also track CO₂ and CH₄ and their isotope composition (radiocarbon, stable isotopes). Laboratory and numerical modelling experiments provide key mechanistic understanding. By doing this research, we also quantify the production, pathway and fluxes of elements in water, which can be nutrients for life and include potential contaminants.

Our current research focuses on field work in the Mackenzie River Basin; Peruvian Andes; Isle of Lewis, UK; Svalbard and the River Thames.

We meet regularly for “Carbon Coffee”, an informal and friendly research meeting open to all, where we discuss papers on this theme and hear about everyone’s research as it progresses.

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Climate Change and Carbon Dioxide Emissions from Oxidative Weathering

Sedimentary rocks lock up vast reserves of carbon for millions of years, only re-mobilised when exposed at Earth’s land surface. Some 1100 Petagrams of carbon (PgC) is present in these sedimentary rock outcrops, and can be released as CO₂ during chemical weathering via the “geo-respiration” of organic carbon in rocks, and during the oxidation of reactive sulphide minerals. These combine as a major CO₂ emission on geological timescales (10³-10⁶ years) alongside volcanism.

Our group has provided new evidence that climate warming can increase CO₂ emissions from sedimentary rock weathering: i) CO₂ emissions from sedimentary rock weathering increase with temperature (e.g. outputs of the 2015-2022 ROC-CO₂ project); and ii) deglaciation can expose sedimentary rocks, with apparent increases in oxidative weathering. These observations suggest that oxidative weathering exerts a positive feedback on Earth’s Climate System which seemingly counters the well-studied silicate weathering negative feedback.

To measure these processes, we have designed and delivered new methods. These include trace element proxies of weathering (e.g. dissolved rhenium to trace rock organic carbon oxidation) and direct measurement of CO₂ release in field and laboratory settings using radiocarbon to track geological C sources.

Ongoing work is funded by a NERC Pushing the Frontier grant ‘Accelerated carbon dioxide release from sedimentary rocks in a warming world’. This ambitious proposal will challenge the existing weathering-carbon cycle paradigm and constrain how CO₂ emissions from sedimentary rocks act as a positive feedback over 1000s to 10,000s of years. Only with this information of carbon cycle feedbacks can we establish the drivers on Earth’s long-term climate, while identifying solutions to counter natural leaks of CO₂ that would otherwise contribute to the remaining carbon budget over the coming decades.

Greenhouse gas release from river surfaces and terrestrial landscapes

Ongoing research concerns the major fluxes of greenhouse gases CO₂ and CH₄ from river surfaces, and is linked to the ERC Consolidator Grant "2022-2027: ‘RIV-ESCAPE: RIVEr emissions of greenhouse gases from warming landSCAPEs’ - see project webpage.

Large amounts of carbon dioxide (CO₂) and methane (CH₄) are released from the surface of rivers each year, with global estimates that total over 2 PgC/yr. River CO₂ and CH₄ could act as a leak of carbon (C) back to the atmosphere over the coming decades.

However, we have been unable to assess how much these fluxes could change, and if old C may be emitted from the surface of rivers to the atmosphere.

To move forward, the RIV-ESCAPE team will focus on three key questions: i) how do river flow dynamics and C supply combine to drive CO₂ and CH₄ release from rivers and link C emissions to climate? ii) do high latitude rivers draining peatlands and permafrost zones release old C as CO₂ and CH₄? and iii) how will the age, source and fluxes of CO₂ and CH₄ from rivers respond to ongoing and future warming?

Recent outputs from this work have established how permafrost-derived carbon contributes to river CO₂ fluxes in a major Arctic River. Active fieldwork campaigns are establishing the underlying processes and vectors of change.

Organic matter preservation and stability and mineral-microbe interactions

We seek to quantify the factors that enhance the persistence of organic matter accumulation in soils and sediments, while also exploring the stability and vulnerability of organic matter to climate change. This research theme has two main components.

1) The climate sensitivity of rock organic matter oxidation and role of microbes in remineralisation

Rock organic matter oxidation is a major source of CO₂ in the geological carbon cycle. It is increasingly recognised that microbial communities are present in the shallow rock weathering zone, but how they breakdown rock organic matter, and whether microbial communities drive apparent temperature sensitivity of CO₂release, are large knowledge gaps. We are tackling this using a range of field and laboratory methods, combing geochemistry and geomicrobiology approaches.

2) The stability of organic matter in high latitude and permafrost landscapes

The permafrost carbon cycle feedback involves a major natural flux of greenhouse gases which are are projected to increase over the next 100 years. Key uncertainties involve the reactivity of organic matter present across vast northern landscapes.

We are quantifying the time-dependent stability of organic matter using river sediments to capture organic matter inputs from across landscapes, mixing heterogenous sources, ages and mineralisation states. The stability of OC in river sediments is also important to assess in its own right. Sources of OC include microbial biomass, plant debris, mineral associated organic carbon, and rock derived organic carbon. We have built a new Ramped Oxidation system at Oxford to investigate the mechanisms regulating the preservation of organic carbon in soils and sediments.

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