COMPARISON OF PHYSICAL AND GEOCHEMICAL CHARACTERISTICS IN TWO PALEOSOLS IN LAWRENCE LIVERMORE NATIONAL LABORATORY ALLUVIAL DEPOSITS

Physical and geochemical characteristics of paleosols formed in alluvial deposits at Lawrence Livermore National Laboratory were compared in an attempt to quantify differences in paleosol maturity. Core samples from four wells were described for this study. At this site, paleosols surround channel and overbank deposits from intermittent streams, and consist of preserved B- and C-horizons. The B-horizon is typically argillic (Bt). Paleosols were observed at approximately 30 meters and 36 meters depth. The “30m–paleosol” appeared to be more strongly developed than the “36m–paleosol” since, on average, the “30m-paleosol” was 0.9-1.5 meters thick, red (10YR to 7.5YR), with moderate to very thick clay coats on blocky to prismatic structures, and contained varying amounts of carbonates while the “36m-paleosol” was 0.3-0.9 meters thick, yellow to brown (2.5Y to 10YR) with thin to moderate thick clay coats on slightly blocky to slightly prismatic structures, and contained little to no carbonates. The chemical index of alteration (CIA) indicates that the paleosols are very similar. The “30m-paleosol” CIA ranges between 68.84 to 72.53, while the “36m-paleosol” CIA ranges between 70.60 to 71.90. The major elements can be divided into two groups. The first group consists of elements that have similar medians in the two paleosols (e.g., Al2O3, Fe2O3, TiO2, SiO2, and MgO). The second group consists of major elements that show distinct medians between the two paleosols (K2O and CaO), with higher median concentrations in the “30m-paleosol”. Similar element groupings can be made for trace elements. The concentrations for Y and Nb are about the same in both paleosols. Strontium behaves similarly to K2O and CaO. Elemental concentrations did not appear to vary with depth in either paleosol, even though some physical trends were observed with depth (e.g., thickness of clay coats, and decreased redness downward). The decoupling of the physical and chemical characteristics will be explored with higher resolution sampling (to include the parent material), narrow grain size ranges, and identification of clay minerals to help define trends not apparent in bulk sample analysis.