Cesium‐137 Distribution in a California Pelloxerert: Evidence of Pedoturbation

Abstract

AbstractThe distribution of 137Cs activity in soils provides a means of assessing physical transport of soil particles. We measured the activity of 137Cs along cracks and within peds of a Chromic Pelloxerert on the flood plain of an ephemeral stream in the Central Valley of California to assess the role of pedoturbation in the formation of a Vertisol. The 137Cs activity is highest in the upper 10 cm of the soil and generally decreases with increasing depth, although 137Cs activity is also high locally along cracks and at the bottom of some cracks near the average depth of cracking (base of Bss horizon, 72‐cm depth). Laterally (along cracks vs. ped interiors), the 137Cs activity varies greatly and shows no consistent pattern. Micromorphological evidence of stress‐oriented clay in soil fabric and locally high 137Cs activity below 72 cm indicate incorporation of surface materials into deeper cracks during drier‐than‐average years. Organic carbon (OC) content decreases regularly with depth, whereas CaCO3 content is uniform throughout the upper 72 cm. The nonhomogeneous 137Cs distribution demonstrates that pedoturbation has occurred. The rate of homogenization of soil material in the upper 72 cm exceeds 25 yr and is slower than the rate of OC accumulation and humification, but faster than the rate of carbonate redistribution. We attribute relatively high 137Cs activity throughout the upper 20 to 30 cm of the soil to incorporation of surface material into minor cracks that open during the dry season. The incorporation of sloughed materials at the base of larger cracks, where evidence of shearing (slickensides) is most strongly developed, is only locally significant and is probably not the dominant mechanism of pedoturbation.