Alasdair C. G. Knight

Cryospheric retreat associated with deglaciation and permafrost thaw exposes previously stable carbon reservoirs to active biogeochemical cycling. While substantial effort has focused on constraining greenhouse gas emissions from thawing soil organic matter, many cryospheric environments also host large carbon stocks within bedrock and regolith, whose contribution to carbon-climate feedbacks remains poorly quantified. Mobilisation of these carbon stocks can enhance CO2 release to the atmosphere via an acceleration of sulphide mineral and rock-derived organic carbon oxidation. Multi-decadal increases in riverine sulphate concentrations across the Arctic, together with growing reports of “rusting rivers”, provide compelling evidence for a direct link between permafrost degradation and enhanced oxidative weathering. However, the magnitude, spatial distribution, and climatic significance of this feedback remain poorly constrained.Here, we quantify the impact of cryospheric retreat on oxidative weathering through a field investigation spanning five valleys in Svalbard underlain by contrasting lithologies. Surface water geochemistry and discharge measurements collected from glacier termini to valley bottoms reveal that CO2 release from geological carbon stocks is sustained—and in some cases amplified—downstream of glacier margins. These observations indicate that enhanced oxidative weathering is not confined to subglacial environments but persists across regions subject to paraglacial processes. This improved spatial understanding of oxidative weathering is used to inform a forward model of carbon release from geological reservoirs resulting from permafrost thaw and and ice-sheet retreat, focused on the last deglaciation. Together, our results demonstrate that cryospheric retreat can enhance carbon release from geological reservoirs, representing a previously under-constrained positive feedback on the Earth system with implications for both past climate transitions and ongoing cryospheric retreat.

Enhanced weathering is a carbon dioxide (CO₂) mitigation strategy that promises large scale atmospheric CO₂ removal. The main challenge associated with enhanced weathering is monitoring, reporting, and verifying (MRV) the amount of carbon removed as a result of enhanced weathering reactions. Here, we study a CO₂ mineralization site in Consett, Co. Durham, UK, where steel slags have been weathered in a landscaped deposit for over 40 years. We provide new radiocarbon, δ¹³C, ⁸⁷Sr/⁸⁶Sr, and major element data in waters, calcite precipitates, and soils to quantify the rate of carbon removal. We demonstrate that measuring the radiocarbon activity of CaCO₃ deposited in waters draining the slag deposit provides a robust constraint on the carbon source being sequestered (80% from the atmosphere, 2σ = 8%) and use downstream alkalinity measurements to determine the proportion of carbon exported to the ocean. The main phases dissolving in the slag are hydroxide minerals (e.g., portlandite) with minor contributions (<3%) from silicate minerals. We propose a novel method for quantifying carbon removal rates at enhanced weathering sites, which is a function of the radiocarbon-apportioned sources of carbon being sequestered, and the proportion of carbon being exported from the catchment to the oceans.