Abstract

Chemical weathering of soils and rocks during pedogenesis is significant because it provides many essential elements for life and because it is coupled with the rise and fall of atmospheric CO₂. We used geochemical mass-balance equations to quantify the net result of pedogenic weathering, i.e. elemental loss and gain, in three residual soil–bedrock profiles on the Piedmont of North Carolina. Soils are located on interfluves and derived directly from the bedrock below: a Kanhapludult (Tarrus series) from phyllite, a Kanhapludult (Cecil series) from granitic gneiss, and a Hapludalf (Enon series) from diabase.

Bulk density ratios of soils and bedrock as well as elemental concentrations referenced to Zr, Ti, Y, and V were used to estimate strain and open-system mass-transport functions through the soil profiles. Estimated strains of the three soils indicated substantial volumetric changes during C horizon and saprolite formation. Overall, desilication was the most predominant pedogenic process removing chemical elements from the three soils. Losses of Si were about 50% of total elemental molar losses in the 8.5-m deep Tarrus profile, 75% of total losses in the 3.8-m deep Cecil profile, and 39% of total losses in the 4-m deep Enon profile. Base cations were also lost in great amounts following desilication. Losses of base cations accounted for about 50% of the total elemental losses in the Tarrus, 20% of the total losses in the Cecil, and 37% of the total losses in the Enon profiles. The specific base cations lost in greatest amounts differed among the three soils and depended on bedrock mineralogy. Sodium and Mg accounted for 24% and 16% of total elemental loss from Tarrus profiles, Na and K accounted for 14% and 4% of total elemental loss from Cecil profiles, and Ca and Mg accounted for 19% and 12% of total elemental loss from Enon profiles.

The vertical pattern of loss of base cations was not always gradual from surface soil horizons to saprolite to bedrock. For example, almost 100% of Ca in the bedrock had been lost from throughout the upper 4.5-m deep Cecil and 8-m deep Tarrus profiles. Aluminum and iron were lost from A and E horizons but were accumulated in B and C horizons due to translocation as well as secondary clay and sesquioxide formation at depth. Physical and chemical data from all three soils and geologic substrata indicate that the entire regolith profile (A through C horizons or solum plus saprolite) is formed by pedogenic processes of elemental inputs, transformations, translocations, and removals.

Citation

Oh, N-H, and D. D. Richter (2005): Elemental translocation and loss from three highly weathered soil–bedrock profiles in the southeastern United States. Geoderma 126 (1-2): 5-25. DOI: 10.1016/j.geoderma.2004.11.005