ORIGIN AND SIGNIFICANCE OF COLLOIDAL SEDIMENT IN SOILS AND LAKES OF THE UINTA RANGE, UTAH

Alpine uplands of the Uinta Mountains in northeastern Utah are mantled by notably well-developed soils. Grain size analysis with laser scattering reveals that submicron material is abundant in these soils, as well as in lakes located below the uplands. This project evaluated the theories that the submicron material in the soils is pedogenic, and that the submicron material in the lakes was derived from the alpine soils by erosion. Centrifuging was utilized to isolate submicron material from 7 samples collected from soil B horizons, and from sediments that accumulated in 8 lakes approximately 10 ka BP, according to existing radiocarbon-constrained depth-age models. XRD analysis reveals that the submicron component of lake sediment samples is strikingly monomineralic, containing only illite. In contrast, samples from alpine soils contain both illite and smectite in varying amounts. SEM analysis demonstrates the presence of platy crystals in soils, which are consistent with a pedogenic origin. Variation in K content from 0.3 to 0.7 atoms per unit cell reflects smectite and illite end-members. Geochemical analysis with ICP-MS demonstrates that ratios of immobile trace elements, and rare earth element abundances, are very similar between the submicron material from the soils and lakes. Together these results support the hypothesis that submicron material in the lakes is delivered by erosion from the alpine upland. Peak abundances of submicron material in lakes ca. 10 ka BP may reflect landscape instability and enhanced erosion during the latter part of the glacial-interglacial transition. The presence of smectite in the alpine soils indicates that chemical weathering is capable of forming secondary minerals in the Uinta alpine zone despite the periglacial climate. The monomineralic (illite-dominated) nature of the lake sediment from 10 ka BP could indicate that (deeper?) smectite-bearing soil horizons were not eroding at this time. Alternatively, the smectite may have formed post-10 ka BP through chemical reactions involving components derived from the underlying bedrock and inputs of atmospheric dust.