Saturday, 15 July 2006

A Model of Silicate Replacement of Carbonate on Dolomitic Landscapes.

Cynthia Stiles and Krista Stensvold. Univ of Wisconsin - Madison, Dept of Soil Science, 1525 Observatory Dr, Madison, WI 53706-1299

Silicate replacement of dissolving carbonates is a significant soil formation process. Clay-rich soils formed on carbonatic bedrock (Terra Rosa) are presumed to be derived from a combination of dissolution and replacement, with varying amounts of materials originating mainly from the carbonate source. Windborne additions contribute significantly to soil formation in these settings. Congruent dissolution of the carbonatic material elevates pH and enrichs the soil solution at the weathering interface, inducing precipitation and strong aggregation of neoformed phyllosilicates and oxyhydroxide phases, derived from loess, to build argillic horizons from the interface upward. This augments normal “top-down” argilluviation happening in the upper loess-derived solum. Geochemical and micromorphological assessment of soils forming on dolomite-rich bedrock in the southern Driftless Area (sDA) of Wisconsin reveals a pattern of complex pedogenesis dominated by surficial additions/transformations of wind-blown materials (loess) and dolostone degradation. Total elemental compositional data show strong differentiation between the upper portion of these soils (quartz-rich silt loams) and underlying redder hued clay-rich subsoils (also called the Rountree Formation) abruptly contacting bedrock. The high clay contents of these subsoils (41-83%) suggests that the relatively high purity dolomite cannot be the sole parent material for sDA soils, but rather serve as a foundation for silicate replacement through geochemical accumulation zones with high pH and oxidizing Eh conditions. Conservative tracers in the soil profiles with depth indicate that both parent materials contributed to soil formation. In the deeper horizons, ‘dilution' of loess materials by selective translocation of Si and Al into dissolving carbonate matrix may have been as great as 95%, as Zr levels approach that of the bedrock parent and the assumption is made that the loess contains significantly greater amounts of Zr than the carbonates (up to 100x). Micromorphology supports this concept by showing pervasive intergranular clay intercalation between dolomite grains of the saprolite, suggesting replacement of calcite cements from the original bedrock by pedogenic clays (sandy loam horizons with clay component dominated by fine to very fine clays) derived from the overlying silty materials. The thick sub-surface clays derived from combined parent material processing indicate that they are the product of long-term inputs into the epikarst interface with subsequent dissolution over long episodes of weathering. Based on this observed geochemical and physical behavior, we propose a six-stage model for silicate replacement of carbonatic rock in humid environments with abundant available soil organic matter: 1) intergranular cements infiltrated and replaced by mixed Mn carbonates and oxyhydroxides; 2) pedogenic phyllosillicates (smectites and illites) replace Mn phases buffered by carbonate dissolution; 3) laminar phyllosilicate replacement with hydrated Fe phases along intergranular voids; 4) carbonate crystallite boundaries effected by acid hydrolysis and replaced by Mn/Fe phases and phyllosilicates; 5) intragranular replacement continues with some remaining carbonate within the crystallite boundary; and finally 6) complete dissolution and isovolumetric replacement by phyllosilicates and Fe oxyhydroxides. This creates stable and persistent argillic horizons built from alkaline bedrock degradation which differ from argillic horizons developed closer to the surface through organic acid dominated argilluviation. Generations of bedrock-derived and cumulative argillic horizons may occur in soil profiled and tend to have gradual to diffuse boundaries as they agglomerate into thick sequences.

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