Bob Hilton

I am a geochemist who studies the exchange of carbon between the atmosphere and rocks, and how these carbon transfers respond to and drive climate change.

Our research quantifies how erosion and weathering processes act as CO₂ sinks, and CO₂ sources, and transfer CO₂ between the atmosphere, hydrosphere and oceans, and long-term, geological storage in sedimentary deposits. To do this, we’ve developed several geochemical approaches, which include trace element proxies of weathering and their isotopes (e.g. rhenium), while also tracking carbon as CO­2, dissolved and particulate organic carbon, and their radiocarbon and stable C isotope composition. I mostly work on modern river catchments to constrain fluxes and their controls, including work on small catchments in the European Alps, New Zealand (East Cape and Southern Alps), Taiwan, Sichuan, Andes, alongside over a decade of work in the Mackenzie River basin.

My research currently tackles four main themes:

1. Greenhouses gases release from Arctic Rivers in a warming world:

Rivers around the world emit large quantities of greenhouse gas, with an estimated 1.8 – 2.9 PgC yr^-1 released as CO₂, and ~20-25 TgC yr^-1 as methane (CH₄). The river CO₂ and CH₄ emissions act to counter the net transfer of greenhouse gases from the atmosphere to the land, estimated to be 1.6 ± 0.5 PgC yr^-1. Despite this recognition, the age and source of the CO₂ and CH₄ that is degassed from rivers is poorly constrained. Only with knowledge of these key features can we constrain how riverine CO₂ and CH₄ emissions will change with ongoing and future warming, unmasking potential feedbacks to warming by the natural carbon cycle. This research theme is being tackled by the RIV-ESACPE project (https://riv-escape.weebly.com/), an ERC Consolidator Grant running from June 2022. This aims to address these significant gaps in our knowledge of how lateral transfers of carbon in sediments and water of Arctic catchments impact the carbon cycle. Dr. Sanjeev Dasari is exploring new approaches to river greenhouse gas flux measurements and CO₂ and CH₄ isotope composition. Dr. Leonardo Mena-Rivera is quantifying the age and reactivity of organic matter in high latitude river systems.

2. Environmental controls on CO₂ emissions from weathering of sedimentary rocks:

Oxidation of organic carbon and sulfide minerals in sedimentary rocks is a major, natural CO₂ emission to the atmosphere and O₂ sink, thought to emit between ~70 – 140 x10¹² g C yr^-1, and has been greatly accelerated by burning fossil fuels. Despite this recognition, the natural flux is poorly constrained by measurements and the environmental controls on this major CO₂ emission are not well known.

This topic formed the theme of the ERC Starting Grant – ROC-CO₂ (2016-2022, http://roc-co2.weebly.com/) which developed novel tracers of oxidative weathering (rhenium) and new ways to directly measure CO₂ emissions. Our research has demonstrated for the first time a link between temperature and CO₂ release from sedimentary rock weathering (Soulet et al., 2021). Research funded by a Philip Leverhulme Prize led by post doc Dr. Ella Walsh is now exploring how widespread this positive feedback may be. We are also seeking to understand how this relates to microbial activity in the shallow weathering zone, and whether a temperature-feedback operates more widely.

3. Oxidative weathering in the geological past:

We lack information on how this CO₂ source has changed in the past. This research will address this knowledge gap with the trace element rhenium (Re) and its isotope system. Rhenium concentrations in river waters have been used to quantify rock-organic carbon oxidation in the modern day. The isotopic composition of Re could record global changes in oxidative weathering fluxes in the geological record, due to the potential for changes in the Re isotopic composition of seawater driven by inputs of Re from rivers. We’ve made the first measurements of the Re isotope composition of rivers, showing fractionation that could help trace oxidative weathering (Dellinger et al., 2021). Victoria Alcock is continuing this theme as part of her NERC-funded PhD. This theme is associated with an ongoing NERC Standard Grant (Dec 2019 – May 2024) (in collaboration with Dr. Alex Dickson, Dr. Mathieu Dellinger and Dr. Julie Prytulak) and is led by post-doc Dr. Madeleine Stow.

4. Geomorphic drivers of enhanced organic carbon cycling:

High magnitude geomorphic events are capable of eroding large amounts of carbon at the same time as large volumes of sediment, exporting materials quickly through river systems. Past research has used modern-day rivers to show the potential importance of infrequent, but widespread landsliding associated with earthquakes for this carbon flux. A NERC Standard Grant has funded research using New Zealand lake records, whose sediments record multiple earthquake events and associated widespread landsliding events (Wang et al., 2020). These new empirical data will inform process based models that can be used to explore erosion and carbon cycling over longer-timescales, while providing numerical experiments of how different climatic and tectonic settings influence carbon export by erosion.

Sedimentary rocks undergoing weathering at the Draix Critical Zone Observatory