Relating the spatial variability of chemical weathering and erosion to geological and topographical zones

Abstract

The relationship between the spatial variability of soil elements, weathering and erosion rates can be very complex. Topography, abruptly-changing elevation gradients and slopes of varying geological composition exert an important influence on weathering trajectories and erosion. This complex interconnectivity is only rarely addressed in weathering studies.The main objectives of this investigation are, therefore, to relate weathering and erosion to various and geomorphic units in a dry-alpine and threshold landscape having steep slopes (Kan catchment, Tehran Province, Iran). A number of common weathering indices were tested using a genetic algorithm. The best indices are the (K + Na)/Ti ratio, the WIP (weathering index according to Parker), and the PI (product index according to Ruxton). However, the recently suggested 4Si-M+-R2 and M-F-Wmin systems discriminate weathering trends more accurately. Our results show that these soils have a low to moderate weathering stage. Weathering trajectories point to the active formation of kaolinite and oxyhydroxides. River sediments are slightly more weathered and contain the most-weathered topsoils and suggest erosion in the catchment is predominantly due to topsoil removal. Chemical weathering is influenced by the dominant geomorphic units (unconsolidated deposits of the Quaternary and solid bedrock) and landform features (combination of altitude and slope). Aspect does not appear to have a significant impact on weathering. Denudation (~erosion) rates were determined using meteoric 10Be since the fluvial sediments did not contain enough quartz for in situ 10Be analyses. This procedure has higher uncertainties due to difficulties in estimating the depositional flux of meteoric 10Be. Due to tectonic uplift, the erosion rates are very high. To maintain a soil layer, soil production rates must be also high. Although annual precipitation is relatively low, slope and soil dynamics seem particularly high, which leads to a fast turnover of the soil material and maintains the weathering intensity at a rather low to intermediate level.