Sedimentary aggregates in the Peoria Loess of Nebraska, USA

Abstract

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. this loess were transported as primary particles, were thoroughly exposed to sunlight and are potentially well suited for luminescence dating. D

Holocene loess deposition and soil formation as competing processes, Matanuska Valley, Southern Alaska

“…Both procedures were preceded by destruction of organic matter with hydrogen peroxide and dispersion with sodium hexametaphosphate. Similar to results reported by Mason et al (2003), we found that the laser method gives results that differ from those of the sieve/pipette method. Because of the greater number of published analyses of loesses and soils using sieve and pipette, we report only results by that method here.…”

Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry

“…12B and 12C). The particle size trends are consistent with data from Mason et al (2003b) and parallel the loess thickness contours of Swinehart et al (1994a) and Mason (2001).…”

Eolian dust dispersal patterns since the last glacial period in eastern Central Asia: insights from a loess-paleosol sequence in the Ili Basin

“…Fine particles can also be incorporated into silt-or sandsized aggregates which can be transported by a range of wind velocities, including dust storms (Derbyshire et al, 1998;Mason et al, 2003;Pye, 1995;Qiang et al, 2010). For example, Ujvari et al (2016) argued that ∼ 1-20 µm fractions are affected by aggregation, as shown by comparison between minimally and fully dispersed grain size distribution measurements of loess samples from southern Hungary.…”