Fossil Carbon in Soil Releases as Much CO2 as Volcanoes: Why?

Published by Adrien - 14 days ago

The oxidation of fossil organic matter in rocks, known as "geo-respiration," is a major process that releases CO2 into the atmosphere and affects climate over long time scales. Two recent studies, published in PNAS and Nature respectively, have determined that this geo-respiration releases as much CO2 as volcanoes on a global scale, and that the fluxes are most significant in mountain ranges and their associated floodplains. These findings thus significantly alter our understanding of the geological carbon cycle.

The main processes and fluxes of the geological carbon cycle.
© Mathieu Dellinger, from the Reference

Erosion and weathering processes play a key role in climate evolution by slowly releasing or trapping carbon over long time scales (> 100,000 years). Sedimentary rocks contain "fossil" organic matter (the same matter that can transform into oil or gas), which can oxidize in contact with air's oxygen and thus release CO2.

This "geo-respiration," along with volcanism, is one of the main natural CO2 emissions into the atmosphere. Yet, it remains poorly understood as it is very difficult to directly measure this fossil-origin CO2. However, when fossil organic matter oxidizes, it also releases a trace element, rhenium (Re), rich in it. Oxidized rhenium is soluble and found in solution in rivers. Thus, the geo-respiration of fossil carbon can be quantified on the scale of watersheds by measuring the flux of dissolved rhenium (Re) from rivers.

In a study published in the journal PNAS, an international team measured rhenium concentrations in the rivers of Madre de Dios (Peru) in the Amazon River basin. The results show that dissolved rhenium levels are much higher in Andean rivers than in rivers flowing only through the plains (elevation < 1640 ft).

Therefore, geo-respiration is significantly more important in the mountains than in the plains. They also show that the flux of rhenium from the Andean rivers continues to increase at the exit of the Andes when the rivers form wide floodplains where Andean sediments are deposited over long periods. Thus, the geo-respiration of fossil organic matter contained in the sediments from Andes erosion continues in these floodplains bordering some mountain ranges.

On the left: Aerial view of an Andean sub-basin of Madre de Dios.
On the right: Aerial view of the floodplain of Madre de Dios downstream.
© Mathieu Dellinger, 2019.

A second study, published in the journal Nature, estimated precisely, for the first time, the total amount of CO2 emitted globally by the geo-respiration of fossil carbon.

The authors used a database of Re concentrations in several major rivers and a spatial extrapolation model to quantify geo-respiration at all points on the globe. The estimated global CO2 flux from geo-respiration is 68 megatonnes per year. This is about 150 times less than the annual anthropogenic CO2 emissions, but on the same order of magnitude as the CO2 flux emitted by volcanoes.

Finally, it appears that in the mountains, geo-respiration releases more CO2 than what is sequestered in limestones via the weathering of silicate rocks. Over the long term, chemical weathering of mountain ranges is not a carbon sink, as long believed, but rather a net source of CO2 to the atmosphere.

Dellinger, M., Hilton, R. G., Baronas, J. J., Torres, M. A., Burt, E. I., Clark, K. E., Galy, V., Ccahuana Quispe, A. J., & West, A. J. "High rates of rock organic carbon oxidation sustained as Andean sediment transits the Amazon foreland-floodplain". Proceedings of the National Academy of Sciences. Published on September 19, 2023.
Zondervan, J. R., Hilton, R. G., Dellinger, M., Clubb, F. J., Roylands, T., & Ogrič, M. "Rock organic carbon oxidation CO2 release offsets silicate weathering sink". Nature. Published on October 4, 2023.
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