Geochemical analysis of weathering zones from Clear Creek watershed: implications for modeling Quaternary landscape evolution

Soil development on upland landscapes in east-central Iowa Peoria Loess deposits has been occurring for approximately the last 12,500 years. Weathering zone and pedogenic processes depend on environmental factors such as climate, precipitation, time, parent material, biota, and topography, among others. Analyzing the weathering zones of modern and paleosol profiles provides insight into current and paleo-environmental processes. This study employs several bulk geochemical analytic techniques (XRF, pXRF, LIBS, ICP-MS) to examine the weathering profiles formed in modern Peoria Loess deposits and underlying weathering profiles formed during the Farmdale Interstadial and the Sangamon Interglacial. Results indicate advanced weathering occurred in the paleosol sequences of the Farmdale and Sangamon compared to the modern weathering zone, based on depletion and enrichment of elemental concentrations. The interstadial/last interglacial paleosol weathering profiles exhibit increased depletion in CaO, MgO, Na2O, and K2O compared to the Holocene weathering profile formed in Peoria Loess. Enrichment of CaO and MgO in non-pedogenically altered Peoria Loess deposits is a possible indication of rapid loess accumulation, representing insufficient weathering of deposited material synchronous with deposition. Post-depositional weathering and hydrogeological mechanisms may also account for this mid-profile enrichment, providing for some complexity for interpretation. Regional comparison between three sediment cores - an agricultural field, a restored prairie and a pioneer cemetery - exhibit minor land-use influence on geochemical evolution with the agricultural field core exhibiting greater relative depletion in most oxides in the upper one meter, compared to the other sediment cores. However, slight regional heterogeneity in parent material, vegetation cover, and slope position may also account for geochemical variations. Therefore, it is difficult to conclude how the last 150 years of extensive land-use from human activity has impacted weathering and pedogenesis in this region. Additionally, this study validates using pXRF technology on Quaternary weathering profiles, and documents its technological shortcomings which provides essential information for drawing interpretations from these data.