Organic Matter in Holocene Paleosols at the Farwell Site, Nebraska.

Paleosols are archives of paleoenvironmental information. The objectives of this study were to identify mechanisms of organic matter stabilization at the millennial scale and to refine paleovegetation reconstructions of midcontinent, Late-Quaternary environments. Two sites at the Farwell locality along the South Fork of the Big Nemaha River, southeastern Nebraska were chosen to study organic matter in buried paleosols that developed in loess and alluvium. We evaluated the soils with respect to field morphology, particle size distribution, pH, dithionite-extractable Fe, clay mineralogy, total carbon and nitrogen concentrations, stable carbon isotopes, and amino acid-nitrogen. At site 1, we identified two buried paleosols, and at site 2, we identified one buried paleosol. The paleosols at site 1 were composed of fine-textured sediments of the Late Gunder Member of the alluvial Deforest Formation; they were buried by late Holocene alluvial deposits. At site 2, one paleosol was identified, formed in colluvium-alluvium of the Severance Formation, which was overlain by Peoria Loess. In most of the paleosol horizons, smectite was the dominant layer silicate present. The alluvium at site 1 was enriched in smectite and depleted in vermiculite compared with the alluvium-colluvium at site 2. These differences are likely to reflect variation in the sources of the sediments of the DeForest Formation and the Severance Formation. In general, the depth trends in organic C and total N concentrations did not indicate the presence of buried paleosol surface horizons. But abundant charcoal fragments in the paleosols at site 1 did indicate the presence of buried surfaces and human activity. Fire-cracked rocks were also observed in the buried paleosol horizons where charcoal was most abundant, indicating that the site had been used for cooking hearths. The stable carbon isotope data for paleosols at both sites indicated that mixed communities of C3 and C4 plants occurred during the mid-Holocene period. In general, amino acid N concentrations decreased with depth in both modern and buried paleosols, reflecting diminished biological activity farther from the modern soil surface horizon. Amino acid N did not appear to be preferentially preserved in the paleosol horizons.