Mountains as a source of CO2: a global model of erosion, weathering and fossil organic carbon oxidation

For over a century, geologists have vigorously debated the influence of mountains on global climate via links among rock uplift, erosion, chemical weathering, and the geological carbon cycle. For decades, the focus has been on the role of mountain building in drawing down atmospheric carbon dioxide (CO₂) via silicate weathering. However, it is now recognized that mountain building and the exhumation of sedimentary rocks can release CO₂ through the oxidation of organic carbon in rocks (rock OC). We quantify this flux at a global scale and show that over geological timescales this source is as important as CO₂ emissions from volcanism.

We explore the controls of mountain erosion on CO₂ release to the atmosphere with a spatially explicit global simulation model that uses empirical constraints on rock OC oxidation flux. We know that erosion is a major control on this flux: rock OC oxidation increases with erosion, up to and greater than erosion rates of ~ 2 mm yr^-1. This contrasts with silicate weathering, where rates are limited by reaction kinetics at high erosion rates. We here constrain the spatial distribution of high erosion rates and their overlap with OC-rich bedrock lithologies. The effect of erodibility of such lithologies means that these are predisposed to high rates of CO₂ release through weathering. Hence our model relies on lithological mapping to constrain the relationship between topography and exhumation rates, and global rock OC stock. We produce a probabilistic rock OC stock map by combining global lithological maps with the USGS Rock Geochemical Database, which includes over 167,000 samples for our analysis. We consider the role of erosion and chemical weathering by using a probabilistic approach that is built on catchment-scale ¹⁰Be denudation rates, while rhenium-based estimates of oxidative weathering intensity and flux from river catchments around the world are used to constrain patterns in rock OC oxidation. To extrapolate the major controls on erosion and weathering we use local slopes derived from 90 m resolution digital elevation model (DEM) data and lithological maps. We combine the erosion, rock chemistry data and weathering intensity estimates to simulate global rock OC weathering rates at a 1 km grid scale via a statistical probability ensemble (Monte Carlo).

We will present the results of our model compilation, including the effect of lithology on erosion, weathering and CO₂ emission rates. We demonstrate that the size of the organic carbon stock in the first 1 m of bedrock is of a similar magnitude to the carbon stock of global soils, and that the emissions of CO₂ from this geological source are as large as the emissions from volcanic degassing. We identify regions of the Earth’s surface where rock OC could emit substantial amounts of CO₂ and provide new constraints on a major natural CO₂ flux derived from the erosion of mountains.