River dissolved inorganic carbon losses from the Andes to lowland Amazon via cryptic sedimentary exchange processes

Robert G Hilton, A. Joshua West , Mark Garnett, Kirsty Harrington, Victoria Alcock, Mathieu Dellinger , Emily Burt, Philippa Ascough, Josh Dean, Yinxue Liu

River carbon transfers and associated greenhouse gas emissions are a major component of the carbon cycle. Proposals to use rivers and their catchments to increase carbon dioxide (CO2) drawdown, by enhanced rock weathering and river alkalinity enhancement, require secure delivery of dissolved inorganic carbon (DIC) and increased cation loads (e.g. Ca2+) to the ocean. Here we explore the fate of river DIC across hundreds of kilometres on an Andes to Amazon floodplain transect, where natural weathering fluxes are high. The holistic transfer of dissolved cations, DIC and CO2 release were determined, alongside radiocarbon-based insight on carbon source, in two seasonal campaigns in 2019. In the wet season, river CO2 release was high and sustained: equivalent to 30-100 % of the instantaneous river DIC flux. A reach-scale weathering budget showed ancient DIC from carbonate minerals mixing with decadal-aged CO2. Across the river reach during the wet season, instantaneous fluxes revealed that 21 ± 6 % of DIC and 13 ± 7 % of Ca2+ were lost across the lowland floodplain from upstream to downstream. These losses are attributed to a set of inorganic processes dominated by cation exchange, alongside a potential role for addition of other weathering acids and secondary carbonate formation. Not often considered in models of river DIC export, these cryptic sedimentary processes could reduce the CO2 drawdown efficiency achieved by carbon cycle management of river geochemistry by 10s of percent or more, particularly in catchments where sediment loads have been perturbed by human activities.