Current rates of CO₂ removal due to rock weathering in the UK

Chemical weathering of silicate and carbonate rocks via carbonic acid provides a natural sink for carbon dioxide, regulating climate over geological timescales. Although the magnitude of CO₂ removal via weathering has been estimated at a global scale using the geochemistry of the world's largest rivers, it has generally not been quantified at national level. In the United Kingdom, the variable bedrock geology and long-legacy of anthropogenic land use provide challenges to isolating carbonate and silicate mineral weathering, meaning we lack constraint on an important flux in the UK's carbon cycle. Here we use river chemistry data collected over 40 years for 52 catchments across the UK and apply a geochemical inversion model (MEANDIR) to assess the silicate and carbonate contributions to UK river chemistry, and to estimate the CO₂ consumption from natural weathering. Silicate-derived solutes account for 0-46 % of dissolved river chemistry by mass (median 7 %) and carbonate-derived solutes for 9-82 % (median 49 %), with the remainder attributed to evaporite and cyclic sources. The maximum present-day carbon dioxide removal (CDR) through combined silicate and carbonate weathering is 2.58 MtCO₂ yr^-1 for the studied catchments (representing 40 % of total UK area). Extrapolated to the entire UK, the maximum CO₂ consumed by silicate and carbonate weathering is 1.6 (0.8-2.8) MtCO₂ yr^-1 and 4.8 (4.2-5.4) MtCO₂ yr^-1 respectively. If sulphuric acid replaces carbonic acid in weathering, CDR may be 17 % and 28 % lower for silicate and carbonate weathering respectively. Accounting for sulphuric acid weathering, a conservative estimate suggests a net combined present-day CDR of 4.5 MtCO₂ yr^-1 for the UK. This is comparable to the lower-end estimates for potential additional CDR through Enhanced Weathering (EW) in the UK. If EW CDR targets are met, EW could more than double the natural weathering flux in rivers, with implications for river chemistry which warrants consideration before national implementation.