Global silicate weathering flux overestimated because of sediment–water cation exchange

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 118; no. 1; pp. 1 - 6
• Main Authors: Tipper, Edward T., Stevenson, Emily I., Alcock, Victoria, Knight, Alasdair C. G., Baronas, J. Jotautas, Hilton, Robert G., Bickle, Mike J., Larkin, Christina S., Feng, Linshu, Relph, Katy E., Hughes, Genevieve
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 05-01-2021
• Subjects: Bonding strength; Carbon dioxide; Catchments; Cation exchange; Cation exchanging; Chemical reactions; Degassing; Feedback; Fluxes; Isotopes; Marine sediments; Minerals; Particulate matter; Physical Sciences; River systems; Rivers; Sediment load; Sediments; Sodium; Strontium isotopes; Water chemistry; Weathering; global biogeochemical cycles; silicate weathering; suspended particulate matter; cation exchange
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.2016430118

Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes CO₂ from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na⁺) from silicate minerals. The large exchange pool supplies Na⁺ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net CO₂ consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth CO₂ degassing fluxes need to be further investigated.