Rates of consumption of atmospheric CO2 through the weathering of loess during the next 100 yr of climate change

[摘要] Quantifying how C fluxes will change in the future is a complex task formodels because of the coupling between climate, hydrology, andbiogeochemical reactions. Here we investigate how pedogenesis of the Peorialoess, which has been weathering for the last 13 kyr, will respond over thenext 100 yr of climate change. Using a cascade of numerical models forclimate (ARPEGE), vegetation (CARAIB) and weathering (WITCH), we explore theeffect of an increase in CO₂ of 315 ppmv (1950) to 700 ppmv (2100projection). The increasing CO₂ results in an increase in temperaturealong the entire transect. In contrast, drainage increases slightly for afocus pedon in the south but decreases strongly in the north. These twovariables largely determine the behavior of weathering. In addition,although CO₂ production rate increases in the soils in response toglobal warming, the rate of diffusion back to the atmosphere also increases,maintaining a roughly constant or even decreasing CO₂ concentration inthe soil gas phase. Our simulations predict that temperature increasing inthe next 100 yr causes the weathering rates of the silicates to increaseinto the future. In contrast, the weathering rate of dolomite – whichconsumes most of the CO₂ – decreases in both end members (south andnorth) of the transect due to its retrograde solubility. We thus inferslower rates of advance of the dolomite reaction front into the subsurface,and faster rates of advance of the silicate reaction front. However,additional simulations for 9 pedons located along the north–south transectshow that the dolomite weathering advance rate will increase in the centralpart of the Mississippi Valley, owing to a maximum in the response ofvertical drainage to the ongoing climate change.

The carbonate reaction front can be likened to a terrestrial lysoclinebecause it represents a depth interval over which carbonate dissolutionrates increase drastically. However, in contrast to the lower pH andshallower lysocline expected in the oceans with increasing atmosphericCO₂, we predict a deeper lysocline in future soils. Furthermore, in thecentral Mississippi Valley, soil lysocline deepening accelerates but in thesouth and north the deepening rate slows. This result illustrates thecomplex behavior of carbonate weathering facing short term global climatechange. Predicting the global response of terrestrial weathering toincreased atmospheric CO₂ and temperature in the future will mostlydepend upon our ability to make precise assessments of which areas of theglobe increase or decrease in precipitation and soil drainage.

[学科分类] 地球化学与岩石