Pedogenesis in a wetland meadow and surrounding serpentinitic landslide terrain, northern California, USA

Landslide deposits in serpentinitic terrain of northern California are common, and often support wet meadows, which are strikingly different from the surrounding xeric landscape. These landslide deposits provide an opportunity to observe pedogenic processes across differing moisture conditions and depositional environments in ultramafic terrain. The objective of this study was to assess predominant pedogenic processes by landscape position for a stabilized landslide bench and the surrounding area in serpentinitic terrain. Distributions of dithionite extractable metals and exchangeable cations were compared to interpret processes involved in element redistribution. Landscape positions were defined as the scarp, flanks, foot, and bulge. The foot, which supports a wet meadow, is inundated during a significant part of the year due to water ponding behind the bulge. The forested scarp and flank positions are subject to episodes of erosion, probably following wildfires, thereby delivering sediments to the lower landscape positions. The major pedogenic processes on the slopes of the scarp and flanks include mineral weathering, oxidation, and illuviation of clay, as evidenced by argillic horizons in which Cr- and Al-bearing Fe-oxyhydroxides are concentrated. Serpentine-weathering products are transported from the scarp and flanks to the poorly drained foot where fine sediments and neoformed smectite yield clayey soils. On the foot, cycles of vegetation growth and deposition of sediments, coupled with reducing conditions, result in deep, organic-rich soils.