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Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ROC-CO2 (Carbon dioxide (CO2) emissions by rock-derived organic carbon oxidation)

Teaser

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 (the remains of past life from the oceans and on land) are returned to the surface...

Summary

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 (the remains of past life from the oceans and on land) are returned to the surface and exposed to oxygen in the atmosphere. These rocks can weather and breakdown, and during this process chemical reactions act to release CO2. Since the industrial revolution, this flux has been greatly accelerated by burning fossil fuels. However, the natural rates of CO2 release from the weathering of organic carbon in rocks are poorly constrained, yet we know they are likely to play an important role over thousands to millions of years. It remains the only geological CO2 source that has not been properly quantified. As a result, we do not fully understand what controls this CO2 emission.

The CO2 release is likely to be ~100 terra grams of carbon per year (TgC/yr), which is similar to the amount of CO2 released by volcanoes around the world. Without knowing the rate of this CO2 emission and what controls it, 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 proposal has three main objectives:

Objective 1: Assess which factors govern rock-derived organic carbon oxidation.

Objective 2: Determine how environmental changes impact oxidation rates and CO2 release.

Objective 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.

By quantifying this major CO2 emission for the first time, this project will provide a step change in our understanding of the geological, as opposed to the anthropogenically-modified, carbon cycle. Measurement of rock-derived organic carbon oxidation requires the development new approaches which harness state-of-the-art geochemical proxies and new field and laboratory methods. Data from river catchments around the world spanning different erosion rates, erosion processes, hydrology and climate, will reveal the main factors governing this process for the first time. These new measurements will enable 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.

Work performed

The first half of the ROC-CO2 project have focused on Objective 1. To do this has required extensive field campaigns by the project team. The main field approaches are designed to: i) track the weathering and breakdown of rock organic carbon in soils; ii) directly measure carbon dioxide (CO2) emissions; and iii) track metals released by these reactions in rivers to help establish proxies which allow us to upscale CO2 emissions. To do this we require very clean samples of river water, which must be collected carefully, and filtered soon after so that we can measure the metals within them. For the CO2 measurements, we must trap the gases onto sieves, using a material that sucks up the CO2 in the field, but that allows us to release it in a controlled way in the laboratory.

Together, the field campaigns have focused on the Draix-Bleone Observatory in France (10 fieldtrips). There water samples are being collected alongside our field installations to directly measure CO2 emissions. On North Island, New Zealand (2 fieldtrips), samples were collected from rivers draining similar rocks as in France, but that are being eroded much more quickly. In Canada (2 fieldtrips) samples were collected from the Arctic tributaries of the Mackenzie River, where shale rocks are very common. Most recently, we visited the Shale Hills Critical Zone Research Observatory in the Eastern USA where we benefit from complimentary measurements made by other scientists.

At these locations, we are collaborating with local organisations (including scientific researchers and local government) to collect time series river samples, which are subsequently filtered for preservation. The overall aim is to collect samples over an 18 to 24 month period. This will ensure we understand seasonal variability in river chemistry, and capture low and high river flow. We have just completed this objective in Draix, but it is still ongoing for New Zealand, and recently started for Shale Hills.

In the laboratories, a large number of analyses are underway. For the river samples, this is focused on trace metal analysis of river water, sediment and soil by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in Durham. Over the last few months, we have also made progress on the measurement of rhenium isotopes in natural samples by multi-collector ICP-MS. In addition to the primary ROC-CO2 field locations, we are also using samples from other rivers around the world from completed and ongoing collaborative projects.

For the CO2 gas samples, we measure their radiocarbon content and stable isotope ratios. The isotope measurements allow us to measure directly carbon emissions from weathering of organic matter in rocks. We are also soon to start laboratory incubation experiments to examine rock-organic carbon oxidation under controlled environmental conditions.

The major results so far can be linked to the outputs and dissemination of the results so far. These include:
- Measurements of un-weathered rock-organic carbon exported by mountain rivers (e.g. Hilton, 2017, Geomorphology).
- Identifying the important role of microbes in the oxidation or rock organic carbon in mountain soils (Hemingway, Hilton et al., 2018, Science).
- Estimation of CO2 emissions during rock weathering in the western Southern Alps, New Zealand (Horan, Hilton et al., 2017, Science Advances).
- Development of a new method to direct measure CO2 emissions during weathering of sedimentary rocks (Soulet, Hilton et al., 2018, Biogeosciences).
- Preliminary measurements from the ROC-CO2 field catchments, across space and time using multiple methods, have been presented by team members (Hilton, Ogric, Soulet) at international scientific conferences, including several invited talks.

Final results

ROC-CO2 research so far has led to several novel developments which provide the grounding for outputs that can 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 and break down. To do this, we have adapted methods previously used to measure CO2 fluxes from soils. We drill chambers into rock faces that are undergoing weathering. The amount of CO2 building up in the chamber can be measured repeatedly, telling us how fast CO2 is released over days to weeks to months. To establish where the CO2 has come from, we trap the gas in the field and take it back to the laboratory where we measure isotopes (including radiocarbon) to fingerprint the carbon source. By doing so we can separate the CO2 produced from organic matter in rocks (e.g. fossil plants), from that produced from carbonate minerals (e.g. fossil shells). We have published a scientific paper which outlines these new methods that ROC-CO2 research has developed (Soulet et al., 2018, Biogeosciences). By applying it to multiple locations, and by measuring at different times (or seasons), we now have the potential to deliver a step change in our understanding of rates of CO2 production, and what controls these rates.

A second major development has come from looking at what is left behind in soils after weathering. Collaborating with a team of international scientists from the US and EU, we have used a novel approach to look at organic matter in mountain soils in Taiwan. This characterised how reactive the carbon was and used radiocarbon to track the rock-derived organic carbon. We found evidence that some of the microbes in the soils had been using (or eating) the rock carbon. Previous work had shown this happening in the laboratory, but our measurements show that it happens in the field, and is common in mountain soils of Taiwan. These findings highlight the importance of microbes in the weathering of carbon in rocks and the CO2 release and have been published in Science (Hemingway et al., 2018, Science). Building from this research, we are now developing laboratory experiments that examine how microbes enhance weathering of rock-organic carbon.

A third direction of ROC-CO2 research has been to use the trace element rhenium in river water. This gives us a way to track rock-organic carbon weathering over large spatial scales. Using this approach, we’ve found that erosion increases CO2 emissions by this process (Ogric, Goldschmidt 2017 presentation). We have also found evidence that glacial erosion may be particularly important, linking climate change to CO2 emissions (Horan et al., 2018, Science Advances). These results feed into ongoing ROC-CO2 research in river catchments around the world.

The expected results until the end of the project are closely linked to the proposed outputs from the Description of Action, and can be broadly summarised as follows:

Research that is underway, with some results presented at conferences and/or published in scientific papers:
- Quantification of unweathered fluxes of rock-organic carbon by rivers to the oceans
- The source and fluxes of rhenium in river catchments
- Measurement of rock organic carbon oxidation rates in shale catchments around the world, based on the rhenium proxy in rivers
- Field measurement of CO2 emissions across seasonal gradients in temperature

Research that has recently started:
- Quantification of rhenium isotope behaviour during weathering
- Establishment of the role of microbial communities in driving CO2 emissions

Research that is yet to start:
- Laboratory measurements of reaction kinetics of OCpetro as a function of temperature and substrate
- Numerical modelling approach to quantify rock-organic carbon oxidation and global rates and patterns of their CO2 emissions

Website & more info

More info: https://roc-co2.weebly.com/.