Contour-crossing calculations quantify decadal changes in the pCO2 impacts of inorganic chemical weathering in the North American Arctic

Preston Cosslett Kemeny 

Degradation of Arctic permafrost is altering biogeochemical dynamics by exposing previously frozen sediment to water, acids, and oxidants. While thaw-driven enhancement of carbonate and silicate mineral weathering results in drawdown of atmospheric carbon dioxide (pCO2), acting as a negative feedback on modern climate change, enhancement of sulfide mineral oxidation increases pCO2 and acts as a positive feedback. The net balance of these chemical weathering reactions sets the sign of the permafrost mineral weathering feedback and partially underlies predictions for the impacts of Arctic landscape change on pCO2 over decadal timescales. This work inverts historical river observations using the MEANDIR model (v1.4) and interprets those results with a new polar conceptual framework to demonstrate that increases in carbonate weathering and silicate weathering since 1980 have fully compensated for increases in sulfide oxidation within the Mackenzie River catchment (sampled upstream of its confluence with the Arctic Red River). Despite declines in the ratio of weathering-derived alkalinity to weathering-derived carbon, permafrost degradation has not yet significantly modified the impact of inorganic chemical weathering on pCO2 within this major Arctic catchment. Within smaller and steeper sedimentary systems, such as the Peel River, enhanced sulfide oxidation has reduced CO2 drawdown. Likewise, increases in carbonate weathering and silicate weathering within the Yukon River have not been sufficient to compensate for increases in sulfide mineral oxidation through time. The relative importance of thaw-driven mineral exposure, couplings among weathering reactions, and changes in the length of flow paths collectively determine how each weathering system responds to decadal permafrost thaw; such processes are expected to vary across geomorphic setting and ecosystem structure. Non-linearity in the magnitude of chemical weathering required to compensate for changes in the ratio of weathering-derived alkalinity to weathering-derived carbon suggests that it will be increasingly difficult to avoid further reductions in CO2 drawdown over the coming decades.