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

Quantifying how C fluxes will change in the future
is a complex task for models because of the coupling between
climate, hydrology, and biogeochemical reactions. Here we
investigate how pedogenesis of the Peoria loess, which has
been weathering for the last 13 kyr, will respond over the next
100 yr of climate change. Using a cascade of numerical models
for climate (ARPEGE), vegetation (CARAIB) and weathering
(WITCH), we explore the effect of an increase in CO2
of 315 ppmv (1950) to 700 ppmv (2100 projection). The increasing
CO2 results in an increase in temperature along the
entire transect. In contrast, drainage increases slightly for a
focus pedon in the south but decreases strongly in the north.
These two variables largely determine the behavior of weathering.
In addition, although CO2 production rate increases in
the soils in response to global warming, the rate of diffusion
back to the atmosphere also increases, maintaining a roughly
constant or even decreasing CO2 concentration in the soil gas
phase. Our simulations predict that temperature increasing in
the next 100 yr causes the weathering rates of the silicates to
increase into the future. In contrast, the weathering rate of
dolomite – which consumes most of the CO2 – decreases in
both end members (south and north) of the transect due to its
retrograde solubility. We thus infer slower 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
transect show that the dolomite weathering advance rate will
increase in the central part of the Mississippi Valley, owing
to a maximum in the response of vertical drainage to the ongoing
climate change. The carbonate reaction front can be likened to a terrestrial
lysocline because it represents a depth interval over which
carbonate dissolution rates increase drastically. However, in
contrast to the lower pH and shallower lysocline expected in
the oceans with increasing atmospheric CO2, we predict a
deeper lysocline in future soils. Furthermore, in the central
Mississippi Valley, soil lysocline deepening accelerates but
in the south and north the deepening rate slows. This result
illustrates the complex behavior of carbonate weathering facing
short term global climate change. Predicting the global
response of terrestrial weathering to increased atmospheric
CO2 and temperature in the future will mostly depend upon
our ability to make precise assessments of which areas of the
globe increase or decrease in precipitation and soil drainage.