Abstract:

Carbon and oxygen isotope compositions of pedogenic carbonate preserved in paleosols have been used extensively to reconstruct ancient environmental conditions. One concern is that pedogenic carbonate precipitated in association with a limestone parent material may include a contribution from inherited detrital (lithogenic) marine calcite, thus compromising interpretations of stable-isotope compositions. To investigate the impact of lithogenic calcite on environmental interpretations using paleosols, we measured stable-isotope ratios in pedogenic carbonates and coexisting soil organic matter (SOM) in modern Vertisols (fine, smectitic, thermic, Udic Haplusterts) from Riesel, Texas (Heiden series) and Zabcikville, Texas (Houston Black series) forming on Cretaceous chalk deposits. The pedogenic and lithogenic components in the Heiden series soil at Riesel were identified using micromorphology and cathodoluminescence (CL), which showed incorporation of luminescent marine allochems (i.e., foraminifera, mollusks) into hard nodules and soft powdery pedogenic masses. Additionally, micromorphology showed evidence of meteoric recrystallization of the allochems.

Despite the presence of these inherited carbonates into hard nodules and soft powdery masses, the stable-isotope compositions recorded a partial pedogenic signal. Recrystallization of the marine allochems resulted in overprinting of primary marine isotope ratios with ratios that are more similar to those expected for pedogenic carbonates in these soils. This study demonstrates that incorporation of parent material into pedogenic carbonates does not necessarily compromise the pedogenic signal that is useful for paleoenvironmental reconstructions. However, overprinting of marine allochems, if present, can be important and needs to be identified using micromorphology and cathodoluminescence, before using stable-isotope ratios of pedogenic carbonates for paleoenvironmental reconstructions.

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