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DIVISION S-10—WETLAND SOILS

Saturation, Reduction, and the Formation of Iron–Manganese Concretions in the Jackson-Frazier Wetland, Oregon

^a USDA Forest Service, Pacific Northwest Research Station, 2770 Sherwood Lane, Suite 2A, Juneau, AK 99801
^b L.C. Lee and Associates, Seattle, WA 98119
^c Dep. of Crop and Soil Sci., Oregon State Univ., Corvallis, OR 97331

^* Corresponding author (ddamore{at}fs.fed.us).

The Jackson-Frazier wetland is a rare, relatively undisturbed wetland in the Willamette Valley of Oregon that experiences repetitive annual flooding and drying cycles. Redoximorphic features formed by these wetting and drying cycles have not been calibrated with saturation and reduction. Long-term measurements of soil saturation and soil redox potential (E_H) were combined with measurements of Fe and Mn concentrations to understand the formation of these redoximorphic features in the wetland soils. The Jackson-Frazier wetland is underlain by three stratigraphic units that control its hydrology: Holocene alluvium (A and Bss horizons), Malpass clay (2Bt horizon), and Irish Bend silts (3BC horizon). All three units are saturated for up to 9 mo each year, leading to E_H values that fall below the Fe reduction threshold. Extractable Fe and Mn in nodules, concretions, and soft masses document substantial depletions from the matrix soil in all three units. In the 3BC, diffuse halos of Fe accumulation and matrix chromas from two to four indicate a source of Fe that can be reduced, translocated, and concentrated in nodular form. In the Bss and 3BC horizons, soft masses surrounding concentrations and higher ratios of oxalate to dithionite-citrate-bicarbonate Fe (Fe_Ox/Fe_DCB) in the larger concentrations suggest that nodules and concretions are currently forming by accretion of Fe. In the 2Bt, virtually all of the Fe and Mn is gone from the matrix, and concretion boundaries are sharp. These are not relict features, as the current conditions would favor continued formation had the supply of Fe and Mn not been exhausted.

Abbreviations: DCB, dithionite-citrate-bicarbonate • DO, dissolved oxygen • E_H, redox potential • Fe_DCB, dithionite-citrate-bicarbonate iron • Fe_Ox, acid ammonium oxalate iron • Mn_DCB, dithionite-citrate-bicarbonate manganese