Enhanced oxidative weathering in glaciated mountain catchments: A stabilising feedback on atmospheric carbon dioxide?

Abstract:

Mountain belts act as sources of carbon dioxide (CO₂) to the atmosphere if physical erosion and exhumation expose rock-derived organic carbon (‘petrogenic’ organic carbon, OC_petro) to chemical weathering. Estimates suggest 15x10²¹g of carbon is stored in rocks globally as OC_petro, ~25,000 times the amount of carbon in the pre-industrial atmosphere. Alongside volcanic and metamorphic degassing, OC_petro weathering is thought to be the main source of CO₂ to the atmosphere over geological timescales. Erosion in mountain river catchments has been shown to enhance oxidative weathering and CO₂ release. However, we still lack studies which quantify this process. In addition, it is not clear how glaciation may impact OC_petro oxidation. In analogy with silicate weathering, large amounts of fine sediment in glacial catchments may enhance oxidative weathering.

Here we quantify oxidative weathering in nine catchments draining OC_petro bearing rocks in the western Southern Alps, New Zealand. Using rhenium (Re) as a tracer of oxidative weathering, we develop techniques to precisely measure Re concentration at sub-ppt levels in river waters. Using [Re]_water/[Re]_rock as a weathering tracer, we estimate that the weathering efficiency in glacial catchments is >4 times that of non-glacial catchments. Combining this with the OC_petro content of rocks and dissolved Re flux, we estimate the CO₂ release by OC_petro oxidation. The analysis suggests that non-glacial catchments in the western Southern Alps release similar amounts of CO₂ as catchments in Taiwan where erosion rates are comparable. In this mountain belt, the CO₂ release does not negate CO₂ drawdown by silicate weathering and by riverine transfer of organic matter. Based on our results, we propose that mountain glaciation may greatly enhance OC_petro oxidation rates. Depending on the global fluxes involved, this provides a feedback to damp low atmospheric CO₂ levels and global cooling. During glacial periods (low CO₂, low global temperatures), increased CO₂ emissions by OC_petro oxidation in mountain belts may act to counter global cooling.