Genesis and relationship of macromorphology and micromorphology to contemporary hydrological conditions of a welded Argixeroll from the Palouse in Idaho

Soils formed in distal loess of the Palouse region have welded profiles that make assessment of their genesis and pedostratigraphic relationships difficult. These soils typically have a mollic-cambic horizon sequence that is underlain by a well-expressed albic E horizon and a strongly developed argillic horizon. In this study, we utilized hydrological monitoring combined with micromorphological analysis and new stratigraphic data to help establish the genesis of a representative Argixeroll of the region and the relationship of its morphology to ancient and contemporary hydrological conditions. Monitoring results indicate that the Argixeroll is seasonally episaturated for approximately 5-6 months per year. Ferrimanganiferous nodules, segregations, hypocoatings, and depletion mottles are observed in thin section throughout the pedon, but dominate within, just above, and below the present-day E horizon. Measured soil Eh is sufficiently low for Fe and Mn reduction to occur for 1-3 months during the period of episaturation, suggesting that at least some of these redoximorphic features are likely to be contemporary. Other compound redoximorphic features are clearly relict, however, as they are coated by illuvial clay which is unaffected by Fe/Mn redistribution processes. Thick illuvial clay coatings on interpedal fissures of the argillic horizon postdate pockets of Holocene tephra within deeply bioturbated aggregates, while other channel argillans in the same part of the pedon are clearly older. This indicates that at least two distinct phases of clay accumulation have occurred, the most recent being contemporaneous with loess aggradation during the Holocene. Our results suggest that these welded profiles consist of a mollic-cambic horizon sequence that has formed primarily in Holocene loess and an albic-argillic sequence that has formed in Wisconsinan loess. The modem-day E horizon was initiated as the clay-eluvial surface horizon of the regionally recognized Washtucna paleosol that continued forming into the late-Wisconsinan cold phase and, upon burial by Holocene loess, it has been transformed into an albic horizon and even enlarged as a result of redoximorphic processes associated with seasonal saturation. (C) 1998 Elsevier Science B.V.