Freeze-Agglomeration: An Alternative Mechanism for Clay Film Formation

Abstract

Oriented clay coatings (argillans, clay cutans, clay films, lamellae) are often interpreted to be caused by illuviation (pervection, lessivage) of fine clay particles. In montane meadow soils (Typic Humaquepts) of the northern Sierra Nevada Range, prominent clay cutans occur on ped faces of a paleosol 3Btgb horizon, which is contemporaneous or younger than 2840 ± 220 yr BP by radiocarbon dating. The unlikelihood of distinct clay cutans forming in such a short period of time via illuviation in a soil with an aquic moisture regime suggests an alternative mechanism. Laboratory studies show that silt- and sand-sized agglomerates can be produced by subjecting colloidal kaolinite (high Fe content), extracted from the montane meadow soil, to repeated freeze–thaw cycles. These agglomerates have a distinct microgranular texture and one to several birefringent domains. Thin section microscopy of the surface Oa horizon reveals that pores and root channels are coated by oriented birefringent zones resembling clay cutans. These clay cutans are similar in optical properties to laboratory-produced freeze-agglomerates. Optical microscopy of clay cutans, scraped from the paleosol 3Btgb horizon, suggests that they too may have formed via freeze-agglomeration. Freezeagglomeration is proposed as a new mechanism to produce oriented clay films.