Silicon Isotope Signatures of Si Pools in Soils On Paragneiss and Sandstone, Black Forest, Germany.

Silicon is a main component of soils and release of Si from soil by weathering plays an important role in the global Si cycle. Silicon is an important nutrient for carbon-sequestering organisms and its supply to the oceans directly influences the global carbon cycle.
The release of Si from soils to the weathering solution is controlled by solubilization and precipitation reactions at low temperatures which are associated with significant mass-dependent stable isotope fractionation. Silicon isotope fractionation therefore might be a useful tool for studying weathering processes.

In order to better understand the Si cycle in soils, we determined the Si isotope composition (δ³⁰Si = [(^30/28Si_Sample/^30/28Si_NBS28) -1]*1000) of the principle Si pools in soil profiles. We focused our study on relatively young soil profiles developed after the LGM on sandstone and paragneiss in the Black Forest (Germany). Multicollector ICP-MS coupled to an UV-femtosecond laser ablation system provide a method to investigate the Si isotope composition of solutions, bulk materials and at high spatial resolution on the mineral scale by using various sample preparation techniques.

Bulk soils show no differences between various mineral soil horizons and locations and Si isotope compositions are close to those of primary minerals. In both catchments, primary minerals show a narrow range with δ³⁰Si values of around -0.3‰. However, internal silicate cycling has caused significant isotope fractionation as clay fractions reveal significant lower δ³⁰Si values of -0.5 to -1.5‰. Adsorption processes seems to enrich secondary minerals in light isotopes, while isotopically heavy Si is lost into solution (e.g. Ziegler et al., GCA, 2005). The most negative value is found in a subsoil horizon enriched in iron oxides by lateral water flow, which is likely caused by preferential adsorption of isotopically light Si onto iron oxides (Delstanche et al., GCA, 2009). Compared to the underlying subsoil horizons some bulk organic topsoils exhibit lower δ³⁰Si values, which might reflect a biological signature due to cycling of Si in the forest vegetation. These results demonstrate that the study of Si isotopes provides a promising tool to gain a better understanding of the Si cycle in soil systems.