Solid-phase redistribution of rare earth elements in hillslope pedons subjected to different hydrologic fluxes

Abstract

Prior studies indicate that patterns of rare earth element (REE) depletion or enrichment in critical zone (CZ) weathering systems are sensitive to variation not only in lithology, but also in climatic and/or biological processes. Organic ligands and secondary mineral surfaces vary in complex stability with different lanthanide series metals, which can result in solid-solution fractionation during incongruent mineral dissolution. REE fractionation during precipitation of solid phase weathering products is also expected to vary with host phase affinity and aqueous geochemistry along fluid flow paths. We postulated that patterns of REE fractionation during pedogenic weathering would exhibit mass-dependent trends as a function of depth in the soil profile. We further hypothesized that REE signatures would be influenced by depth-dependent variation in water and dissolved organic carbon (DOC) fluxes resulting from topographic position of the pedon under investigation. Field-based hypothesis testing utilized instrumented pedons derived from rhyolitic bedrock overlain by mixed conifer forest in the Jemez River Basin Critical Zone Observatory (JRB-CZO). REE depletion trends correlated with topographically-induced variation in soil pore water and DOC through-fluxes occurring predominantly during winter snowmelt. Bulk regolith analyses indicated that light rare earth elements (LREE) were depleted preferentially relative to medium and heavy REE (MREE and HREE). Lateral fluxes of water and DOC through subsurface horizons in the concave hillslope pedon correlated not only with greater REE depletion, but also with greater fractionation of REE into organo-metal colloid forms (2–23%) relative to a planar site hillslope pedon (3–13%) where vertical water and DOC fluxes were predominant. MREEs were preferentially retained in secondary colloids, indicating a mechanism for their stabilization in the weathering profile. Positive Ce-anomalies in the soils were the result of Ce retention in pedogenic Fe-(oxy)hydroxides.