Past and current geochemical conditions influence silicon isotope signatures of pedogenic clay minerals at the soil profile scale, Ethiopia

Abstract

Abstract Soil processes strongly govern silicon (Si) mobility in terrestrial environments and its relation to other global biogeochemical cycles. The nature of inherited soil clay minerals can be highly diverse given the variability of their weathering environment. The influence of Si isotope fractionation factor in the initial geochemical conditions of clay precipitation can therefore be still expressed in inherited clay minerals in their new environment. We studied top- and subsoil of an Ethiopian Vertic Planosol derived from parent materials of similar sources of volcanism. The selected Planosol has an abrupt textural change at a depth of ~40\u202fcm separating a bleached, silty (25\u202f±\u202f1.8% clay) ash-derived soil horizon with a crumby structure from a heavy clayey (68\u202f±\u202f3.4% clay) lacustrine-derived vertic soil horizon. The mineralogical assemblage of the clay fraction in top- and subsoil is characterized by similar proportions of 1:1 (kaolinite) and 2:1 (illite and smectite) layer-type clay minerals. This specific soil profile provides a unique opportunity to elucidate the influence of clay formation processes (inheritance versus neoformation) on the Si isotope signature of pedogenic clay minerals. Minerals of the clay fraction in the clayey vertic horizon are significantly enriched in light Si isotope (δ30Si\u202f=\u202f−1.41\u202f±\u202f0.02‰) compared to the bleached, silty horizon (δ30Si\u202f=\u202f−0.69\u202f±\u202f0.03‰). These results are corroborated by the preferential enrichment in Ge, relative to Si, in the clay fraction of the clayey subsoil compared to the silty topsoil (Ge/Si\u202f=\u202f6.3\u202f±\u202f0.14 and 4.0\u202f±\u202f0.10\u202fμmol\u202fmol−1, respectively). Our results demonstrate that geochemical conditions in lacustrine environment favor kinetically-driven Si isotope fractionation factor leading to lower Si isotope ratios in 2:1 clay minerals inherited in the new soil profile environment. The inherited soil textural conditions in the soil profile also contribute to on-going processes that result in larger Si isotope differences between the soil solution (CaCl2 extractable) and clay minerals. This implies that Si isotope signatures of clay minerals in the studied soil profile are influenced by a combination of inheritance processes in lacustrine environment and on-going neoformation processes in the soil profile. This finding has important implications for environmental studies using geochemical and Si stable isotope tracers to better understand current soil processes, to model elemental cycling in soil-plant systems and to quantify land-ocean element mass-balances.