Concentrations, variations and distribution of molybdenum (Mo) in catchment outlet sediments of China: Conclusions from the China geochemical baselines project

Abstract The concentrations, variations and distribution of molybdenum (Mo) in catchment outlet sediments from China were studied in this contribution based on the results of recently-finished, continental-scale, China Geochemical Baselines (CGB) project. The data from CGB were combined with other published data-sets with the purpose of understanding the spatial distribution of Mo in China and, in addition, relating the observed geochemical features to natural and/or anthropogenic processes. This geochemical mapping with a density of ca. 3000 km 2 per site involves catchment sediments in two horizons at each site: a top sample (TOS) at a depth of 0–25 cm and a bottom sample (BOS) at a depth greater than 100 cm. In total, 3382 TOS and 3380 BOS samples were collected. The analytical results show that the concentrations of Mo in the catchment outlet sediments of China range from 0.07 to 57.86 μg/g for the TOS and 0.09–32.43 μg/g for the BOS. The median and mean values of the TOS data are 0.73 and 1.01 μg/g respectively, and those of the BOS data are 0.71 and 0.94 μg/g. The variations of both the TOS and BOS are high at this continental scale. The Mo concentrations vary greatly from one geological and/or geographic unit to another. For instance, from the Cathaysia Block with the TOS and BOS median values of 1.22 and 1.46 μg/g to the North China craton with those of approximately 0.61 and 0.58 μg/g. Various processes govern the spatial distribution of Mo in the catchment sediments of China but regional-scale natural sources (e.g. lithologies) predominate. Sedimentary sandstones and carbonate rocks, alluvium in plain terrain and aeolian materials (e.g. loess) account for the extensive low concentrations ( 1.2 μg/g) in the Yangtze and Cathaysia blocks. Simultaneously, some local mineralized areas can be also identified at this scale because the sampled sites just happened to fall on sediments derived from mineralized rocks or anthropogenic addition by mining activities. Furthermore, the TOS share the similar patterns with the BOS indicating relatively stable inheritance and no apparent extensive secondary enrichment or depletion in the catchment sediments of China. The CGB demonstrates that a series of baseline levels of Mo are needed at different regions and scales and ultra-low density mapping can still deliver robust geochemical patterns.