CHEMICAL WEATHERING IN LOESS SOILS OF THE MATANUSKA VALLEY, ALASKA

On geologic timescales, atmospheric carbon dioxide levels are controlled by the balance between volcanic out-gassing and consumption of CO2 through silicate chemical weathering. However, the relationship between increased weathering associated with uplift, physical erosion, and climate remains uncertain. Uplift may incite mountain glaciation, which raises the question what is the relationship between glacial activity and silicate weathering rates?

We hypothesize that in periglacial areas glacial loess-derived soils enhance chemical weathering fluxes in three ways. First, small grain size of loess increases the surface area for weathering reactions. Second, ongoing loess deposition supplies fresh mineral material for weathering. Last, loess is deposited in vegetated areas, and this vegetation provides organic acids that promote silicate weathering. We will test this hypothesis by calculating chemical weathering fluxes using a mass-balance approach along a 50 km transect of loess-derived soils in the Matanuska valley of south-central Alaska. Loess deposited during the last 6500 years of the Holocene is >20 meters deep close to river sources but depth declines exponentially downwind. Estimates of particle surface areas reveal that loess soils contain about twice the surface area of till-derived soils of the same thickness hence increasing the potential for chemical weathering.

Along our transect, podzolic soils appear where loess thickness falls below 1 m corresponding to loess rain of about 0.15 mm/yr. This suggests that organic acids are buffered by weathering loess where deposition rates are greater than 0.15 mm/yr. We suspect that mass-balance analyses along the transect will show that weathering fluxes are limited by available mineral surface area in thin loess, and are limited by organic acid availability in thicker deposits. If correct, loess rains of 0.15 mm/yr will yield the maximum consumption of CO2 by weathering. This hypothesis has two interesting implications. First, ice-age loess rain may have been responsible for maintaining high levels of soil fertility in soils distant from ice sheet margins. Second, the soils whose chemical weathering was the most influential in drawing down atmospheric CO2 during past ice ages may have been those located relatively far from ice margins.