Pedogenic Implications of Opaline Pendants in Some California Late‐Pleistocene Palexeralfs

Abstract

Hypothesized mechanisms of opal-CT authigenesis in soils, either by direct precipitation or solid-state transformation of opal-A, imply that time is an essential component of the process. Few studies have tested the proposed mechanisms, and none of those studies have conclusively demonstrated opal-CT authigenesis in soils. Pedogenic opal-A coats the underside of basal coarse fragments in some late-Pleistocene Palexeralfs in California. Opaline pendants are not found in lithologically similar, but younger, Palexeralfs. The pendants lack birefringence and have a refractive index between 1.452 and 1.456. Their x-ray diffraction pattern consists of a broad hump centered near 0.41 nm. The pendants occur in the lower solum and below the zone of clay accumulation in well-sorted gravel lenses. We consider them to represent the ultimate form of silica accumulations in these soils, and not precursors to duripans, because the large voids and low surface area in the gravel lenses probably preclude plugging and induration of the horizon. The absolute age of the pendants is unknown, but they are apparently relatively new, and possibly transient features that develop once these soils on old landscapes transcend a pedochemical threshold. The pendants represent a source of pedogenic opal not contaminated by accessory minerals that can be used to study silica diagenesis in the soil environment. The lack of opal-CT in the pendants demonstrates that either opal-CT has not formed or does not persist in this pedogenic environment. In addition, we find no evidence to support the hypothesis that solid-state opal-A to opal-CT transformations occur in soils. Our ability to test a time-dependent opal-A to opal-CT transformation is hindered by our inability to constrain the absolute age of the pendants. If solid-state transformation does occur, the pendants in these soils may be too recent to have been affected. Alternatively, it is possible that the pedochemical environment determines which opal polymorph forms and persists, regardless of the passage of time. We conclude from this study that there is no evidence for opal-CT authigenesis in these soils.