Organic matter in Holocene paleosols at the Farwell site

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Alnsour, Najwa
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Michael Thompson
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In this work, modern and buried paleosols were sampled at the Farwell locality along the South Fork of the Big Nemaha River, southeastern Nebraska, USA. Site 1 (65-1) and site 2 (65-2) developed in Holocene alluvial deposits and are developed on a low terrace in sediments of the Late Gunder Member of the Deforest Formation. The paleosols are fine-grained and contain abundant flecks of charcoal. Soils at the third site (64-1) formed in Wisconsinan-age Peoria Loess that overlies alluvium of the Severance Formation. All these alluvial sites are dominantly fine-grained and oxidized, with reduced sections containing detrital organic matter near river level.

Field macrophological investigations indicated that soil at site 1 has A and B horizonation, including a thick dark A and a blocky or prismatic structured B horizon. At site 2, multiple dark buried soils occurred with subangular blocky structure and charcoal fragments. At site 3, the buried soil had dark colors and subangular blocky structure.

The organic carbon concentration was relatively high in the surface horizons of the two modern soils (site 1, site 3), as well as in the buried surfaces horizon of the paleosols (site 2). Organic C ranged from 3.9 to 22.7 g/kg, 3.2 to 6.5 g/kg, and from 17.5 to 10.07 g/kg in the whole soil profiles at site 1, 2, and 3 respectively. The stable carbon isotope data for the three sites indicated that mixed communities of C3 and C4 plants were predominant during the mid-Holocene period, suggesting that the climate fluctuated between colder and wetter and between warmer and drier during that period of Holocene at Farwell site.

Amino-acid N concentrations decreased with the depth in both modern and buried paleosols. Subtle increases in AA-N in the A horizons of the buried paleosols may preserve a record of biological activity in those horizons when they were near the land surface. Treatment of soil samples with HF released amino acids that were trapped between interlamellar surfaces of clay minerals. It is inferred that interaction with layer silicates has a role in stabilizing amino acids against degradation.

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Tue Jan 01 00:00:00 UTC 2013