Role of bedrock weathering in microaggregate formation - limestone alteration in the aeration zone

During pedogenesis, the development of the soil aggregate system may be strongly dependent on weathering of bedrock in the vadose zone. There, periodic drying and flushing by meteoric waters provides alternating hydro- and biogeochemical conditions for fluid-rock interactions and provokes the dissolution, displacement and placement of minerals as well as the release or adsorption of colloids. As a result, the seepage suspension is enriched with mobile mineral and organic matter that infiltrates into the bedrock void system thereby fueling aggregate forming materials and composite building units to exposed surfaces of the bedrock. We aim to elucidate related bedrock alteration processes in dependence on water composition, seepage vs. saturation and the fracture network during weathering.

Our study combines the investigation of the weathering rim of natural bedrocks (outcrop analogue) with the simulation of the natural conditions by column experiments in the laboratory. Study object are Triassic limestones (Upper Muschelkalk) of the Hainich area in Thuringia (central Germany). The columns were filled with fresh, unaltered material, crushed into coarse gravel fraction size and percolated with artificial rainwater or soil litter extract over a runtime of 6 months. In order to mimic natural conditions percolation periods changed with periods of drying. Geochemical data of the liquid phase resemble very well the alternating periods of drying and flushing by systematic changes of the element concentration and milieu parameters. Generally, dissolved elements in the seepage are higher in concentration when litter extract is used pointing towards a significant impact on dissolution kinetics, especially after periods of longer water-rock interaction. Weathered natural bedrock surfaces (bedrock clasts in the covering soil and fractures and voids in the bedrock) exhibit carbonate dissolution (edge pits and dissolution vugs) and the formation of clay mineral coatings, in part with iron oxides. The same holds true for rock clasts after the column experiments. The alteration is macroscopically visible by brownish and beige coatings on formerly greyish pristine surfaces. This feature seems more pronounced on clasts percolated with liquids from soil litter extracts than on clasts treated with artificial rainwater indicating the formation of organo-mineral associations during solid-liquid interaction.

Generally in both, nature and experiment, facial aging features include the dissolution of carbonates, the formation of clay minerals as well as oxides and hydroxides of iron, but also the appearance of organic constituents. Our results contribute to a better mechanistic understanding of the role of bedrock alteration during weathering for (a) the provision of microaggregate forming materials and (b) the formation of composite building units and microaggregates from pristine environments.