Comparing Reactive Surface Area of Sediments in Hot and Cold Arid Climates

Abstract:

The reactive surface area of primary silicate phases in sediment is an important determinant in weathering rates and fluxes in the critical zone. Weathering rates in warm climates are presumed to be faster than rates in cold climates, but glacial stream systems have chemical fluxes similar to temperate climates due to the production of highly reactive fresh mineral surfaces.

To assess climate as a controller on weathering rates by comparing reactive surface area in cold arid and hot arid systems, samples were collected at the base of Denton Glacier, Wright Valley, Antarctica, and in the Anza Borrego Desert, California. Sediments were wet sieved and treated to remove carbonates and organics. Mud and very fine fractions were freeze dried. Surface areas of each size fraction were determined using the BET method. Sediment reactivity was measured through batch dissolution experiments in water buffered to pH 8.4. Samples were removed and filtered at predetermined time intervals, then refrigerated prior to ICP-OES analysis.

Antarctic glacial sediment released an order of magnitude or more cations to solution compared to alluvial sediments from the Anza Borrego Desert. The Anza Borrego sand fraction was more reactive than the mud for Ca, P, Mn, and Si, while the Antarctic mud was more reactive than the sand for all cations. The high reactivity of the glacial mud can be attributed to the high reactive surface area (10.42 m²/g) created by physical crushing. In addition, the cation exchange capacity of the mud fraction was reached more quickly in the Antarctica samples, while Anza Borrego muds produced more complicated weathering fluxes, likely due to their higher clay mineral component. These results emphasize a strong climate control and the importance of physical weathering mechanisms on the chemical weathering fluxes over geologic time scales.