Glacial Retreat Enhances Geochemical Weathering-Driven CO2 Production in a Proglacial Indicator River (Huascarán National Park, Peru)

Abstract:

It is well established that most river systems globally are net carbon dioxide (CO₂) emitters due to inputs of terrestrial-sourced organic matter (OM), which fuels microbial respiration and CO₂ production. Glacial-fed, or proglacial, rivers can also be net CO₂ emitters, though terrestrial OM inputs into these systems are often minimal. The difference lies in the biogeochemical processes driving the CO₂ fluxes. As glaciers melt, underlying bedrock can be exposed and resultant meltwaters may transport large quantities of comminuted sediments downstream. These sediments can be highly reactive and induce geochemical weathering. For instance, sulfide oxidation can produce CO₂ – either through sulfuric acid-induced carbonate weathering or a carbonate equilibrium shift – which can then outgas to the atmosphere. As this process can influence local CO₂ budgets, geochemical weathering-driven CO₂ fluxes in proglacial systems require more scientific consideration.

Peru holds 70% of the world’s tropical glaciers, which are actively retreating as a consequence of climate change. In Huascarán National Park, we investigated the impact of glacial retreat on geochemical weathering and CO₂ production in two proglacial rivers during the 2020 wet season. Shallap Valley glacier has retreated to expose sulfide-rich bedrock, leading to the acidification of the meltwater. Here, the river’s pH is ~3.5, and in situ CO₂ concentrations were 3x higher than atmospheric equilibrium, with clear evidence that geochemical weathering was driving this trend up to 10km downstream of the glacier terminus. Comparatively, the nearby Cojup Valley glacier has not receded to expose its sulfide-rich bedrock, and the river’s chemistry reflects that. In situ CO₂ concentrations in Cojup Valley were 1.8x above the atmospheric concentration near the glacier, but decreased as we moved downstream until they reached equilibrium, and the geochemical weathering signature was markedly different from that in the Shallap Valley. As the Cojup Valley glacier retreats further and more of its underlying bedrock is exposed, we might expect a shift in the river’s geochemistry toward what was observed in Shallap Valley. This study may therefore provide a glimpse into the evolution of geochemical weathering in Peruvian proglacial rivers as glacial melt intensifies.