Periodic Reporting for period 5 - ROC-CO2 (Carbon dioxide (CO2) emissions by rock-derived organic carbon oxidation)
Reporting period: 2021-09-01 to 2022-05-31
The carbon cycle controls Earth’s climate by adding and removing carbon dioxide (CO2) from the atmosphere. A major pathway of CO2 release happens when rocks containing ancient organic matter are returned to the surface and exposed to oxygen in the atmosphere. These rocks can weather and breakdown and chemical reactions release CO2. Since the industrial revolution, this flux has been accelerated by burning fossil fuels. However, the natural rates of CO2 release from the weathering of organic carbon in rocks are poorly constrained. This means we cannot fully understand how and why atmospheric CO2 and global climate change over thousands to millions of years. It also makes it challenging to predict how long human-made CO2 emissions will persist in the atmosphere and oceans over the coming centuries. We don’t know yet whether this natural CO2 emissions from weathering of rock organic carbon may be increasing due to anthropogenic activities.
To address this knowledge gap and quantify a major geological CO2 source, the ROC-CO2 project had three main objectives:
1: Assess which factors govern rock-derived organic carbon oxidation.
2: Determine how environmental changes impact oxidation rates and CO2 release.
3: Quantify the global CO2 emissions by rock-derived organic carbon oxidation during chemical weathering, and assess how they may have varied both over Earth history and via anthropogenic change.
To deliver these objectives required us to measure rock organic carbon oxidation. To do this, we developed new approaches to harness state-of-the-art geochemical proxies and new field and laboratory methods. Sample and data from river catchments around the world spanning different erosion rates, erosion processes, hydrology and climate, were used to reveal the main factors governing this process for the first time. These new measurements enabled us to construction data-driven numerical model to provide the first quantification of CO2 emissions by this process, and assessment of how it might change in the future.
The projects main outcomes and conclusions of ROC-CO2 are:
- Development and testing of new methods to track and quantify oxidative weathering at the rock outcrop scale (direct CO2 measurements) and at the scale of landscapes (rhenium and its isotopes).
- Demonstrating that erosion plays a major role in driving oxidative weathering, making mountains "hotspots" of CO2 release from rocks.
- Finding that increased temperature can increase CO2 release during rock weathering, meaning this process acts as a previously unrecognised "positive feedback", where warming causes more CO2 release.
- The present-day global flux is significant and may have changed spatially and through geological time and the more recent past.
To address this knowledge gap and quantify a major geological CO2 source, the ROC-CO2 project had three main objectives:
1: Assess which factors govern rock-derived organic carbon oxidation.
2: Determine how environmental changes impact oxidation rates and CO2 release.
3: Quantify the global CO2 emissions by rock-derived organic carbon oxidation during chemical weathering, and assess how they may have varied both over Earth history and via anthropogenic change.
To deliver these objectives required us to measure rock organic carbon oxidation. To do this, we developed new approaches to harness state-of-the-art geochemical proxies and new field and laboratory methods. Sample and data from river catchments around the world spanning different erosion rates, erosion processes, hydrology and climate, were used to reveal the main factors governing this process for the first time. These new measurements enabled us to construction data-driven numerical model to provide the first quantification of CO2 emissions by this process, and assessment of how it might change in the future.
The projects main outcomes and conclusions of ROC-CO2 are:
- Development and testing of new methods to track and quantify oxidative weathering at the rock outcrop scale (direct CO2 measurements) and at the scale of landscapes (rhenium and its isotopes).
- Demonstrating that erosion plays a major role in driving oxidative weathering, making mountains "hotspots" of CO2 release from rocks.
- Finding that increased temperature can increase CO2 release during rock weathering, meaning this process acts as a previously unrecognised "positive feedback", where warming causes more CO2 release.
- The present-day global flux is significant and may have changed spatially and through geological time and the more recent past.
ROC-CO2 developed new geochemical techniques to study rock weathering, testing and applying them to field areas around the world. These included shale catchments: the Draix-Bleone Observatory in France; North Island, New Zealand; Arctic tributaries of the Mackenzie River; Shale Hills Critical Zone Research Observatory in the Eastern USA; and a mountains to floodplain transect of the RIo Madre de Dios in Peru. At these locations we collaborated with local organisations (including scientific researchers and local government).
Returning these samples to the laboratories resulted in a large number of analyses. For the river samples, this focused on trace metal analysis of river water, sediment and soil by Inductively Coupled Plasma Mass Spectrometry. We were also the first to make measurements of rhenium isotopes in river water samples by multi-collector ICP-MS. For the CO2 gas samples, we measured their radiocarbon content and stable isotope ratios. The isotope measurements allow us to measure directly carbon emissions from weathering of organic matter in rocks.
The major results can be linked to the outputs and dissemination of the results. These include:
- Measurements of un-weathered rock-organic carbon exported by mountain rivers (e.g. Hilton, 2017, Clark et al., 2017).
- Identifying the important role of microbes in the oxidation or rock organic carbon in mountain soils (Hemingway et al., 2018) and ongoing work by PDRA Georgiadis (presented at AGU 2021).
- Estimation of CO2 emissions during rock weathering catchments around the world using the trace element rhenium (e.g. Horan, et al., 2019, Hilton et al., 2021).
- Development of a new method to direct measure CO2 emissions during weathering of sedimentary rocks (Soulet, Hilton et al., 2018, Biogeosciences).
- Development and refinement of methods to measure the isotopes of rhenium in river waters, soils and sediments (Dellinger et al., 2020, JAAS) which allow the first measurements of Re isotopes on river waters as a weathering proxy (Dellinger et al., 2021, EPSL).
- Demonstrating a previously unknown feedback in the geological carbon cycle, showing CO2 emissions from rock weathering increase with temperature (Soulet et al., 2021, Nature Geoscience).
- Ongoing research on themes of microbial communities and global upscaling of measurements. These have been presented by team members (Hilton, Ogric, Soulet, Grant, Roylands, Zondervan, Georgiadis) at international scientific conferences, including several invited talks (EGU, AGU, Goldschmidt).
Returning these samples to the laboratories resulted in a large number of analyses. For the river samples, this focused on trace metal analysis of river water, sediment and soil by Inductively Coupled Plasma Mass Spectrometry. We were also the first to make measurements of rhenium isotopes in river water samples by multi-collector ICP-MS. For the CO2 gas samples, we measured their radiocarbon content and stable isotope ratios. The isotope measurements allow us to measure directly carbon emissions from weathering of organic matter in rocks.
The major results can be linked to the outputs and dissemination of the results. These include:
- Measurements of un-weathered rock-organic carbon exported by mountain rivers (e.g. Hilton, 2017, Clark et al., 2017).
- Identifying the important role of microbes in the oxidation or rock organic carbon in mountain soils (Hemingway et al., 2018) and ongoing work by PDRA Georgiadis (presented at AGU 2021).
- Estimation of CO2 emissions during rock weathering catchments around the world using the trace element rhenium (e.g. Horan, et al., 2019, Hilton et al., 2021).
- Development of a new method to direct measure CO2 emissions during weathering of sedimentary rocks (Soulet, Hilton et al., 2018, Biogeosciences).
- Development and refinement of methods to measure the isotopes of rhenium in river waters, soils and sediments (Dellinger et al., 2020, JAAS) which allow the first measurements of Re isotopes on river waters as a weathering proxy (Dellinger et al., 2021, EPSL).
- Demonstrating a previously unknown feedback in the geological carbon cycle, showing CO2 emissions from rock weathering increase with temperature (Soulet et al., 2021, Nature Geoscience).
- Ongoing research on themes of microbial communities and global upscaling of measurements. These have been presented by team members (Hilton, Ogric, Soulet, Grant, Roylands, Zondervan, Georgiadis) at international scientific conferences, including several invited talks (EGU, AGU, Goldschmidt).
ROC-CO2 research so far has led to several novel developments which go beyond the state of the art. We have led the design and development of the first method to measure directly CO2 release from sedimentary rocks as they weather. To establish where the CO2 has come from we measured carbon isotopes to fingerprint the carbon source. By doing so we separate the CO2 produced from organic matter in rocks (e.g. fossil plants), from that produced from carbonate minerals (e.g. fossil shells) (Soulet et al., 2018, Biogeosciences). By applying this new method to multiple locations, and by measuring at different times (or seasons), we have delivered a step change in our understanding of rates of CO2 production, finding that temperature increases the rates of CO2 release from rock weathering (Soulet et al., 2021, Nature Geoscience; Roylands et al., in press, Chemical Geology).
A second major development of ROC-CO2 research has been to use the trace element rhenium and its isotopes in river water. This tracks rock-organic carbon weathering over large spatial scales. Using this approach, we’ve found that erosion increases CO2 emissions by this process (e.g. Horan et al., 2019; Hilton et al., 2021) and used these measurements to scale up to estimate global fluxes (Zondervan et al., EGU 2022). We've also been the first to measure rhenium isotopes in river waters, providing a promising proxy of weathering, and one which may be applied to reconstruct past rates of sedimentary rock weathering (Dellinger et al., 2021). This work led to a spin-out project funded by a NERC, UK, Standard Grant (2021-2024).
Finally, we have worked at the interface of geochemistry and microbiology, examining how microbes in soil may use (or eat) the rock carbon (Hemingway et al., 2018). Ongoing work is using microbial biomarkers to examine how oxidative weathering fluxes relate to the size and make up of microbial communities (Georgiadis et al., AGU 2021).
A second major development of ROC-CO2 research has been to use the trace element rhenium and its isotopes in river water. This tracks rock-organic carbon weathering over large spatial scales. Using this approach, we’ve found that erosion increases CO2 emissions by this process (e.g. Horan et al., 2019; Hilton et al., 2021) and used these measurements to scale up to estimate global fluxes (Zondervan et al., EGU 2022). We've also been the first to measure rhenium isotopes in river waters, providing a promising proxy of weathering, and one which may be applied to reconstruct past rates of sedimentary rock weathering (Dellinger et al., 2021). This work led to a spin-out project funded by a NERC, UK, Standard Grant (2021-2024).
Finally, we have worked at the interface of geochemistry and microbiology, examining how microbes in soil may use (or eat) the rock carbon (Hemingway et al., 2018). Ongoing work is using microbial biomarkers to examine how oxidative weathering fluxes relate to the size and make up of microbial communities (Georgiadis et al., AGU 2021).