The availability of elements that serve as nutrients for a subaerial biofilm and initial soil formation depends critically on the weathering state of rock surfaces. This project aims at the development of a weathering index using a new protocol for sampling and analysis of directly releasable cations from granite. The effects of metabolic activities by microorganisms, temperature, radiation, and precipitation will be validated with experiments in the lab and tested in several field campaigns.
Studies on rock weathering are often focused either on the degradation and dissolution of primary minerals or on catchment studies, where weathering rates are calculated from concentrations of dissolved mineral constituents in the runoff. In contrast, the method we propose to develop aims at the quantification of readily available elements, which emerge from biotic or abiotic weathering processes. The determination of such a weathering index provides a direct measure for the rock surfaces’ reactivities. It allows the investigation of the effects of different environmental factors. Most of the weathering indices defined in the literature are based on the elemental composition of the weathered surface layers in comparison to the parent rock. In contrast, we propose an operational weathering index that reflects the chemical availability of mineral-derived elements. The proposed weathering index is empirically based on the quantitative measurement of mineral-derived elements in a drop of aqueous solution that has been spread onto the investigated rock surface (“drop-on-stone”). Such a protocol requires analytical equipment that is suitable for analyzing very small sample volumes, and that is portable and applicable under field conditions. For this we intend to adapt capillary electrophoreses with capacitatively coupled contactless conductivity detection.
The new method will provide a fast, easy to use and inexpensive tool to quantify the mobilization of ions from rock surfaces due to environmental forcing factors and can be used for related scientific and applied investigations. The weathering index permits the interpretation of the results in a relative scale of the weathering state.