Minerals in rock fragments weather in sync with the soil fine fraction across a lateral hydrochemical gradient

Abstract:

Weathering affects minerals in the fine fraction (<2 mm) of soils, massive bedrock, and all sizes in between. Much field-scale work has focused on the reactive fine fraction and on the transformation of bedrock to regolith, ignoring weathering inputs from rock fragments. However, in areas mantled by glacial drift, the shallow solum may contain an abundance of fresh rock fragments deposited by glacial activity, leading rock fragments to participate in weathering reactions to a greater degree than previously thought. Soil morphologies defined along gradients in topographic position, saturation frequency, and acidity, common in the Northeast, were used to explore the influence of weathering controls on glacially-embedded rock fragments in the solum. Weathering action is expected to be highest at the top of the hydrochemical gradient, where glacial deposits are thin, highly leached, and subject to frequent flushing with waters rich in organic acids, in contrast to deeper downslope deposits where the water table rarely rises into the solum. Soil pits were dug at three positions along three transects traversing this gradient and characterized and described using NRCS methods. Using a whole-regolith approach that considers all primary minerals regardless of source, soil, coarse fragments, and deeper deposits were collected by horizon within pits. Total elemental analysis was used in conjunction with optical petrography, SEM/EDS, and EPMA to achieve a quantitative characterization of mineral and elemental composition by depth at each location. The patterns of increased porosity in conjunction with elemental and mineralogical depletion are hypothesized to reflect the influence of lateral weathering gradients expected from soil morphology. Weathering observed in rock fragment mineral sources demonstrates the importance of incorporating >2 mm mineral fractions into ecosystem weathering calculations and global models. This work highlights the significance of rock fragments as weatherable mineral sources in soils, especially where rock fragments may harbor nutrients that are useful for forest productivity.