PHYSICOCHEMICAL ENVIRONMENT OF PEDOGENIC CARBONATE FORMATION IN DEVONIAN VERTIC PALEOSOLS, CENTRAL APPALACHIANS, USA

The morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the Devonian Catskill Formation in central Pe nnsylvania preserve a record of the physical and chemical environment of carbonate precipitation. The carbonate is characterized by three di stinct petrographic generations. Pedogenic rhizoliths and nodules are the earliest precipitated generation, and typically consist of dull re d-brown luminescent micrite. Clear, equant calcite spar cement fills v oids in the centres of rhizoliths, as well as circumgranular cracks an d septarian voids in nodules. Early spar cements are nonluminescent to dull luminescent, whereas later spar cements exhibit bright yellow-or ange luminescence. Late stage pedogenic fractures are always occluded with very bright yellow-orange luminescent spar cements. The incorpora tion of progressively higher concentrations of Mn (up to 34 000 ppm) i nto successively younger calcite spar cements, without concomitant inc reases in Fe, suggests carbonate precipitation from an evolving meteor ic water in which Mn2+ became increasingly mobile over time. The incre ased mobility is possibly due to decreasing Eh, resulting from oxidati on of organic matter after rapid soil burial on the floodplain. The am ount of Fe2+ available for incorporation into calcite was limited beca use most iron was immobile, having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxy hydroxide mineral. Carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation. Rhizolit hs preserved below the lowest stratigraphic occurrences of pedogenic s lickensides are consistently depleted in C-13 relative to nodules, whi ch formed stratigraphically higher, within the zone of active soil shr ink and swell processes. Nodular carbonate, precipitated in proximity to deep cracks in the soil, is enriched due to increased gas exchange with isotopically heavy atmospheric CO2. Accordingly, rhizolith compos itions will most accurately estimate palaeoatmospheric levels of CO2; the use of nodule compositions may result in overestimation of P(CO2) by as much as 30%.