New Research on CO2 Release from Newly Exposed Organic-Rich Rocks Upends Carbon Cycle Models: Watersheds with Mountain Glaciation and Higher Erosion Rates Yield More Oxidative Weathering

 

     A new paper in Nature by researchers at Oxford University challenges the long-held belief that rock weathering is always a carbon sink. The research concludes that under certain circumstances rock exposed to weathering can be a significant source of CO2. The authors did note that the amount of global CO2 being emitted from this source globally amounts to about 1% of that emitted by burning fossil fuels. Thus, the new findings suggest that the anthropogenic effects on atmospheric CO2 are only about 1% less than previously thought. That is not quite negligible but not very significant either. The new findings will likely, however, change carbon cycle modeling considerably. This carbon source is not included in many current carbon cycle models but that is changing as the net carbon source becomes better quantified on a global level.

     The chemical weathering of silicates is a known carbon sink. The weak carbonic acid in rainwater takes up CO2 from the atmosphere into several types of rocks. Enhanced chemical weathering is a CO2 mitigation solution that has been utilized in pilot projects with certain rocks like basalt and olivine and mine tailings that can take up CO2. Erosion of organic carbon in terrestrial vegetation also acts as a carbon sink by moving that organic carbon to rivers and through time burying it in sediment.

     One aspect of the geologic carbon cycle is the CO2 emissions from volcanoes. The new research suggests that the amount of annual uncounted CO2 emissions from newly exposed organic-rich rocks is equivalent to the annual CO2 emissions spewed from volcanoes. The newly exposed organic-rich rocks mainly occur high up in some of the Earth’s youngest mountain ranges, such as the Himalayas, the Andes, and the Rockies. When rocks formed on ancient seafloors that accumulated large amounts of organic matter are exposed, they react with oxygen in the air, and this results in the release of CO2. The chemical reactions are basic organic carbon oxidation reactions and oxidation reactions with sulfide minerals. The new paper notes: “Hotspots of CO2 release are found in mountain ranges with high uplift rates exposing fine-grained sedimentary rock, such as the eastern Himalayas, the Rocky Mountains and the Andes.”

     This research is not exactly new but has been better quantified. In 2017, a paper in Science Advances showed that higher rates of erosion mountain glaciation environments in New Zealand and other places are associated with these newly defined atmospheric CO2 sources: “Oxidative weathering fluxes are two to three times higher in watersheds dominated by valley glaciers and exposed to frost shattering processes, compared to those with less glacial cover; a feature that we also observe in mountain watersheds globally. Consequently, we show that mountain glaciation can result in an atmospheric carbon dioxide source during weathering and erosion, as fresh minerals are exposed for weathering in an environment with high oxygen availability.”  

 

The graph shows that percentage of glacial cover in a watershed strongly affects whether that watershed will be a net carbon sink or a net carbon source. Source: Mountain glaciation drives rapid oxidation of rock-bound organic carbon. Kate Horan, Robert Hilton, David Selby, Chris J. Ottley, Darren R. Grocke, Murray Hicks, and Kevin Burton. Science Advances. October 4, 2017. Vol 3, Issue 10.

 

     In 2020 an article in Nature Reviews Earth & Environment resulted in the following key points.

·        Erosion resulting from mountain building increases transfer of carbon between the atmosphere and storage in rocks.

·        The traditional view has focused on carbon dioxide (CO₂) drawdown by silicate weathering, and its links to climate and erosion.

·        An emerging view also considers CO₂ drawdown by organic-carbon burial and CO₂ emissions from oxidative weathering of both rock organic carbon and sulfide minerals.

·        CO₂ sources and sinks increase with erosion, and the net balance has now been quantified in a handful of locations.

·        Climate (temperature, hydrology) regulates inorganic and organic CO₂ sinks, with complex interdependency on erosion.

·        Lithology is important: a mountain range composed of sedimentary rocks may be a weak CO₂ sink (or CO₂ source), but volcanic rocks favour CO₂ drawdown.

This 2020 paper also contained a schematic of the textbook view of net carbon cycle effects of weathering in these environments compared to the new emerging view as shown below.  Unfortunately, I could not get clear images as the full paper is behind a paywall.

Source: Mountains, erosion and the carbon cycle. Robert G. Hilton & A. Joshua West. Nature Reviews Earth & Environment volume 1, pages284–299 (2020). June 9, 2020. Mountains, erosion and the carbon cycle | Nature Reviews Earth & Environment

 

Rock organic carbon oxidation CO2 release offsets silicate weathering sink. Jesse R. Zondervan, Robert G. Hilton, Mathieu Dellinger, Fiona J. Clubb, Tobias Roylands & Mateja Ogrič. Nature (2023). Rock organic carbon oxidation CO2 release offsets silicate weathering sink | Nature

Researcher Robert G. Hilton of the Department of Geography, Durham University, Durham, UK was an author of all three of the papers referenced in this post. In all three papers, the researchers utilized the dissolved levels of the introduced trace element rhenium to estimate the level of oxidation of organic carbon. Total organic carbon (TOC) contents of black shales in the USGS Rock Geochemical Database were utilized in the modeling, which was done by a supercomputer. In the discussion of limitations and uncertainties in the 2023 paper, I found the following statement to be interesting: “Anthropogenic land-use change has doubled erosion and weathering since the early 1900s (ref. 63); hence, our global scale estimates of OCpetro oxidation rates reflect the combined influence of natural and anthropogenic activities on global weathering rates, which cannot be deconvolved in this present study.” In any case, this line of research shows that modeling of the geologic carbon cycle is evolving to better account for these differing sink/source ratios associated with different geologic environments.

 

References:

Ancient carbon in rocks releases as much carbon dioxide as the world’s volcanoes. University of Oxford. October 4, 2023. Science Daily. Ancient carbon in rocks releases as much carbon dioxide as the world's volcanoes | ScienceDaily

Rock organic carbon oxidation CO2 release offsets silicate weathering sink. Jesse R. Zondervan, Robert G. Hilton, Mathieu Dellinger, Fiona J. Clubb, Tobias Roylands & Mateja Ogrič. Nature (2023). Rock organic carbon oxidation CO2 release offsets silicate weathering sink | Nature

Mountain glaciation drives rapid oxidation of rock-bound organic carbon. Kate Horan, Robert Hilton, David Selby, Chris J. Ottley, Darren R. Grocke, Murray Hicks, and Kevin Burton. Science Advances. October 4, 2017. Vol 3, Issue 10. Mountain glaciation drives rapid oxidation of rock-bound organic carbon | Science Advances

Mountains, erosion and the carbon cycle. Robert G. Hilton & A. Joshua West. Nature Reviews Earth & Environment volume 1, pages284–299 (2020). June 9, 2020. Mountains, erosion and the carbon cycle | Nature Reviews Earth & Environment