Peter M. Jacobs

Sedimentary aggregates in the Peoria Loess of Nebraska, USA

Loess grain size data used to infer transport direction or wind strength are generally derived from vigorously disaggregated samples. However, these data may not adequately represent the effective particle size distribution during loess transport, if the transported dust contained aggregates of finegrained material. Thin sections of minimally altered C and BC horizons in the late Pleistocene Peoria Loess of Nebraska, USA, indicate the presence of aggregates with diameters of 30–1000 Am. The larger aggregates (>250 Am) are unlikely to have been transported, and are interpreted as the result of soil faunal activity and other pedogenic processes after deposition. Aggregates smaller than 250 Am could have a similar origin, but laser diffraction particle size analysis suggests that many are sedimentary particles. Comparison of minimally and fully dispersed particle size distributions from each sampling site was used to estimate the modal diameter of aggregates. The aggregate modal diameter becomes finer with decreasing loess thickness, representing increasing distance from the source. A similar trend was observed in the modal diameter of fully dispersed particle size distributions, which represents the mode of sand and silt transported as individual grains. We interpret both trends as the result of sorting during transport, supporting the interpretation that many of the aggregates were transported rather than formed in place. Aggregate content appears to increase with distance from the source, explaining a much more rapid downwind increase in clay content than would be expected if clay were transported as particles smaller than 2 Am diameter. Although the Peoria Loess of Nebraska contains sedimentary aggregates, many of the coarse silt and sand grains in

Late Quaternary climate change, loess sedimentation, and soil profile development in the central Great Plains: A pedosedimentary model

Research on soil genesis often assumes a “top-down” model, in which the soil profi le develops downward from a stable land surface. This model is inapplicable to upland landscapes affected by frequent dust deposition, where soils grow upward as they develop. On the central Great Plains, late Quaternary loess sections proximal to immediate source areas contain the Brady Soil, a prominent marker separating late Pleistocene Peoria Loess from Holocene Bignell Loess. Farther from immediate dust source areas, the Brady Soil and Bignell Loess are not recognizable in the fi eld. On loess tablelands in these distal regions, surface soils typically contain a prominent, clay-rich B horizon below a thick silty A horizon. Assuming top-down pedogenesis, this could be interpreted as a postglacial soil profi le formed in Peoria Loess, with the B horizon produced by weathering and clay illuviation. We propose a strikingly different interpretation, in which the upper B horizon at distal sites is the Brady Soil A horizon that has been transformed by burial, organic matter loss, and modern subsoil structure formation processes. The overlying modern A horizon represents Bignell Loess. Properties of the Brady Soil at proximal sites (a distinctive burrowed zone, high clay content and TiO 2 /ZrO 2 , and low volcanic glass content) can be traced to the B horizon in distal soils. A decrease in smectite abundance above the Brady Soil at proximal sites is identifi able at the top of the clay-rich B horizon in distal soils. The spatial variation of clay content in loess and soil horizons is best explained by eolian sedimentation patterns. The higher clay content in the Brady Soil and distal B horizons defi nes a fi ne-grained zone that represents a late phase of Peoria Loess accumulation. Evidence of chemical weathering is minimal, and illuvial clay is rare to absent in the clay-rich B horizons. Illuvial clay does often occur deep in the solum and is related to the depth below the top of the Brady Soil. The depth of occurrence of illuvial clay is not related to modern climate parameters, although depth to secondary carbonate appears to be in equilibrium with modern climate. Upland soils in the central Great Plains are composite soils; their properties are the result of a pedosedimentary history linked to regional climate change that has infl uenced sedimentation and pedogenesis since the late Pleistocene.

Stable Carbon Isotope Record of Holocene Environmental Change in the Central Great Plains

Ratios of stable carbon isotopes preserved in soil organic matter record vegetation and climate change and are applicable to the dry environment of the central Great Plains. We sampled two loess sections and one sand sheet swale in southwest Nebraska to study a continuous record of late Pleistocene and Holocene climate change. Carbon isotope (δ13C) data are combined with previously reported and new optically stimulated luminescence (OSL) age control to establish the timing and magnitude of vegetation change. Stratigraphic units studied include the late-Pleistocene Peoria Loess and Brady Soil, Holocene Bignell Loess, and four buried soils within Bignell Loess. Isotope ratios and OSL ages indicate a shift from a C3 to C4 vegetation community during formation of the Brady Soil, reflecting climatic warming spanning the Pleistocene-Holocene transition. By the early Holocene, between 10,100 and 9100 yrs. ago, drought became a persistently recurring feature in the region as indicated by the deposition of Bignell Loess that originated from reworking of devegetated dunes and sand sheet deposits. Periods of rapid loess deposition are marked by an increase in C3 vegetation, which we interpret to indicate vegetation adapted to high rates of landscape disturbance. Conversely, C4 vegetation types dominated during periods of landscape stability and soil formation, when precipitation was sufficient for vegetation to grow on dune fields. We conclude that total annual precipitation and landscape stability are more important factors than seasonality of precipitation and atmospheric CO2 concentration in determining the chronology of C3-C4 vegetation change during the Holocene.

Influence of parent material grain size on genesis of the Sangamon Geosol in south-central Indiana

Abstract Few studies have attempted to evaluate the effects of parent material grain size on long-term pedogenesis in the glaciated Midwest. The Sangamon Geosol provides such an opportunity, since it formed for a minimum of 100 ka before burial during the late Wisconsinan. This study investigates the influence of parent material grain size on the genesis of a suite of well-drained profiles of the Sangamon Geosol in a small region along the sedimentologically varied Illinoian drift margin in south-central Indiana. Three measures of soil development (depth of leaching, solum thickness, and Bt clay accumulation) were regressed against commonly measured parent material grain-size parameters. The strongest parent material grain-size predictor is sand percentage, suggesting that sand content, and its influence on pore size and thus permeability to water and air, is a major facilitator in long-term pedogensis in calcareous glaciogenic sediments.