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Time-Dependent Changes in Pedogenic Processes on Marine Terraces in Coastal Oregon

Environmental Science and Policy Program, Univ. of Southern Maine, 37 College Ave., Gorham, ME 04038; and Dep. of Soil Science, Univ. of Wisconsin, Madison, WI 53706

*Corresponding author (langley{at}maine.maine.edu).

ABSTRACT

Very little is known regarding soil-forming processes on marine terraces along the Pacific Coast. This study examined a soil chronosequence on an elevated marine terrace sequence in south-coastal Oregon in order to investigate changes in soil-forming processes with time. The soils range in age from 80 000 to >500 000 yr, and with increasing age classify as Inceptisols, Spodosols, and Ultisols. Relevant pedogenic processes were studied using morphological properties; extractable Fe, Al, and Si; dissolved Fe, Al, and Si in soil solution; and micromorphology. Podzolization and in situ weathering of silt to clay are the dominant and contemporary processes on the young and intermediate-aged terraces, and argilluviation and clay neoformation are dominant processes on the older terraces. Evidence for podzolization includes organically complexed Fe and Al that peak in Bs horizons and mineral grains coated by Fe as evidenced in thin section. In addition, up to 93% of the Fe and Al in solution in the E horizons is retained in the Bs horizons. In situ weathering of fine silt in the younger soils is indicated by irregular depth trends in fine silt/total silt ratios. Argilluviation is manifested by clay films observed in the field and in thin section, which increase in thickness and abundance with time. Evidence for clay neoformation includes the retention of up to 54% of dissolved silica in the Bt horizons of the older soils. Podzolization and in situ weathering decrease in importance with time, and argilluviation and clay neoformation become increasingly more significant.

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