Soil development in weathering pits of a granitic dome (Enchanted Rock) in central Texas

Abstract Weathering pits form on various rock types worldwide and often contain unique ecosystems in periodically flooded or desiccated depressions. However, little is known about soil formation in such pits. Here, we investigated two weathering pits (WP1 and WP2) on the summit of Enchanted Rock, a granitic exfoliation dome in central Texas. We aimed at unravelling the origin of the parent material, evaluating evidence for dating the soil development, and identifying the major soil forming processes. We sampled five soil profile cores in each pit and analyzed diverse soil properties, such as C and N contents, pH, exchangeable cations, texture, and iron oxides. In addition, we performed energy dispersive X-ray spectroscopy and applied optically stimulated luminescence (OSL) and 14C dating analyses. Both pits were affected by redoximorphic features and organic matter accumulation (2.5 ± 1.4% C in the topsoil). WP1 revealed Na accumulation in the subsoil and pronounced translocation of clay minerals, which accumulated at the bottom (up to 50% clay in the subsoil, 13–34% in topsoil). Such processes were absent in WP2, in which clay contents and their variations were comparatively small (16–25%). Here, acidic and nutrient poor soil horizons (pH 4.0–4.4, 0.5–1.5% C, 4.1–4.6 mg kg−1P) alternated several times with nutrient rich and neutral soil horizons (pH 5.1–5.6, 2.0–2.5% C, 23–33 mg kg−1P). This phenomenon was observed down to the bottom of the pit. We conclude that the parent material for soil development consists of three sources: autochthonous material from in-situ weathering of the solid rock, granitic grus particles from upslope positions, and eolian sediments from the area surrounding Enchanted Rock. Phases of sedimentation may have alternated with phases of stability and topsoil formation, resulting in buried former topsoils. OSL dating revealed that sedimentation began not later than 5110 years ago in 42 cm depth of WP1 and 2290 years ago in 26 cm depth of WP2. Due to contamination by younger carbon, 14C dates underestimate the beginning of soil development. The intensity of post sedimentary soil forming processes (clay mineral translocation, sodium enrichment, ferrolysis, organic matter accumulation) depends on the age of the pit and on its morphology, i.e. on the possibility of water and dissolved substances running off.