Controls over the thickness and elemental enrichment patterns in microscopic weathering-zones in exposed and terra-rossa covered carbonate bedrock surfaces
Abstract:Weathering zone’s mineral and chemical compositions reflect the processes that govern denudation. We study 0-2500 μm thick weathering zones that evolve below carbonate bedrock surfaces, which experienced prolong denudation under Mediterranean climate, in the Judea Hills, Israel. Samples were collected from exposed (n=13) and terra-rossa covered (n=12) surfaces. In a previous study, denudation rates were calculated for the exposed bedrock samples. Since most of the terra-rossa volume is derived from an aeolian source, we do not consider it an integral part of the rock weathering zone. Bulk rock compositions range from limestone to dolomite, and frequently consist of a mixture of calcite and dolomite minerals. We analyzed major and trace elements across the weathering zones using a laser-ablation ICPMS system. Selected samples were further analyzed with an electron-probe. The extent of weathering zones is marked by the variation of element concentrations near the rock surface (relatively to the bulk rock), at depths of 0-500 μm and 0-2500 μm in exposed and terra-rossa covered surfaces, respectively. These zones are characterized by a relatively high porosity resulting from carbonate-mineral dissolution below the surface. Correlation analyses within each profile reveal three major elemental correlations: (1) Mg-Sr-U correlation results from variation in the abundance of the mineral dolomite. These elements are typically depleted toward the surface, due to preferential dissolution of dolomite crystals and precipitation of secondary calcite; (2) Al-Si correlation results from variations in the abundance of clay minerals, which may concentrate as an insoluble residue, or derive from the outer environment; and, (3) P-Y-REE correlation is highly enriched toward the surface, suggesting contribution from the outer environment.
In contrast to silicate rocks, the extent and intensity of the weathering zones of exposed carbonate surfaces are decoupled from denudation rates. Weathering zones in carbonates result from dissolution beneath the rock surface. In exposed bedrock, porosity is suggested to be driven by endolithic organisms. Below terra-rossa cover, porosity increases with mineral heterogeneity, as mixed calcite-dolomite rocks tend to develop thicker and more intense weathering zones.