Date of Award
Master of Science
Michael L. Thompson
Paleosols are ancient soils that possess information about the earth’s environmental and climatic conditions in the past. Buried paleosols are formed by the burial of surface horizon soils by younger layers of sediments over time. Holocene buried paleosols in Nebraska at the Bignell Hill, Old Wauneta Roadcut (OWR), and Farwell Locality were studied to better understand the organic carbon storage for over a thousand years.
All three sites are located in Nebraska. The Bignell Hill and the OWR sites occur in an upland setting, and the Farwell Locality is in an alluvial setting. The stratigraphic sequence (top to bottom) of the upland sites consists of the Bignell Loess, which buries Peoria Loess, and a dark colored soil called Brady Soil that is developed in the upper part of the Peoria Loess. The Brady Soil is easily recognized and identified by a dark horizon in the soil profile that is thoroughly burrowed by insects, including cicada nymphs. The Farwell Locality is located near the Nebraska-Kansas border, along the South Fork of the Big Nemaha River. The Farwell Locality (site 65-2) occurs in the Late Gunder Member of the Deforest Formation, in an alluvial landscape.
At the Bignell Hill and OWR sites, the organic C content of the Brady Soil was greater than that of the overlying Bignell Loess. The organic C content ranged between 0.32% and 0.74%, and 0.43% and 0.81% for the Bignell Hill and OWR sites, respectively. In contrast to the typical depth distribution of organic C in most soils, which decreases with increasing depth, the organic C concentration of the Brady Soil was the least at the top horizon of the soil. The clay content at both sites was relatively low, with an average content between 20-25% at both sites. Smectite was the dominant layer silicate mineral, and there appeared to be no relationship between organic C content and smectite content of the paleosols. Stable carbon isotope data at both the Bignell Hill and OWR sites suggest a mixed C3/C4 plant community that occurred during the development of the Brady Soil, and shifted to C4 vegetation at the end of the Brady Soil pedogenesis.
The organic C content was relatively high in the comparable paleosols at the Farwell Locality. Organic C concentration ranged between 0.65% and 1.52%, and it increased upwards with depth. Charcoal fragments from human activities are likely to have contributed to the high organic C concentration of 1.52% in the buried paleosol horizon 2Ab1. The stable isotope carbon values indicate that when the paleosol was at the land surface, there was a C3/C4 mixed plant community, but it was dominated by C3 vegetation. Throughout the soil profile, the clay content was greater than 30%. Smectite was the dominant layer silicate, and its concentration was fairly uniform with depth. The amount of organic C in the paleosols may have been preserved due to rapid sediment deposition from alluvial sources
Long-term storage of organic C in the buried surface horizons of the paleosols of this study does not appear to be controlled by the association of soil organic matter with layer silicates or Fe oxides. Instead, it is likely that microbial degradation has been very slow since the time of burial. After burial, microbes continued to degrade plant residues such as cellulose, lignin, and proteins into low molecular weight C compounds (amino acids, other organic acids, sugars), to produce energy. The organic C content observed in the buried paleosols is likely composed of carbon that has been assimilated by microbes in the buried paleosols.
Llewin Hugh Froome
Froome, Llewin Hugh, "Organic matter in Holocene paleosols in the US Mid-Continent" (2016). Graduate Theses and Dissertations. 15701.