China geochemical baselines: Sampling methodology

Abstract The China Geochemical Baselines Project (CGB) was launched in 2008, and sampling was completed in 2012. Its purpose is to document the abundance and spatial distribution of chemical elements covering all of China. The database and accompanying element distribution maps represent a geochemical baseline against which future human-induced or natural chemical changes can be quantified. The sampling methodology was updated or developed for China's diverse landscape terrains of mountains, hills, plains, desert, grassland, loess and karst in order to obtain nationwide high-resolution and harmonious baseline data. Floodplain sediment or alluvial soil was used as the sample medium in plain and hilly landscape terrains of exorheic river systems in eastern China. Overbank sediment was adopted as the sampling medium in mountainous terrains of exorheic river systems in south-western China. Methods of collecting catchment basin and lake sediments were developed in desert and semi-desert terrains, respectively, in endorheic drainage systems in northern and north-western China. Two sampling sites were allocated to each CGB grid cell of 1° (long.) × 40′ (lat.), approximately equal to 80 × 80 km in size. At each site, two samples were taken; one from a depth of 0–25 cm and a second, deeper sample from a depth greater than 100 cm or the deepest part of horizon C as possible as we can take. A total of 6617 samples from 3382 sites have been collected at 1500 CGB grid cells across the whole of China (9.6 million km 2 ), corresponding to a density of approximately one sample site per 3000 km 2 . In addition, 11,943 rock samples have also been collected to aid in the interpretation of geogenic sources of elements. Before chemical analysis, the soil and sediment samples were sieved to 2 + , organic C, CO 2 , H 2 O + and pH were determined under strict laboratory analytical quality control. An Internet-based software named Digital Geochemical Earth was developed for managing the database and maps. Initial results show excellent correlations of element distribution with lithology, mineral resources and mining activities, industry and urban activities, agriculture, and climate.