The first large-scale bioavailable Sr isotope map of China and its implication for provenance studies

Strontium isotope ratio (Sr-87/Sr-86) has been widely used to trace human migrations across geologically distinct landscapes in archaeological and forensic sciences. Sr-87/Sr-86-based provenance studies require a detailed strontium reference map to make valid interpretations of mobility patterns for a given region. In this study, we first review the previous development for Sr-87/Sr-86 isoscapes relevant to research on migration and then present the first map of bioavailable Sr-87/Sr-86 ratios for China, based on 1935 measurements on biosphere samples collated from published research and supplemented by our targeted collection strategy. The results show that Sr-87/Sr-86 ratios in China span a large range and exhibit regional patterns largely dependent on the divergent geological conditions of each of China's tectonic blocks. We further combine river water Sr-87/Sr-86 ratios with lithology, integrated elemental data, and chemical weathering rates, to determine which dominant weathering patterns (e.g. silicate, carbonate or evaporite) have the most significant influence on Sr-87/Sr-86 ratios. The low ranges of Sr-87/Sr-86 ratios (< 0.711) are predominately present in the Xing'an-Mongolian orogenic belt, which is composed of mafic-ultramafic silicate rocks, as well as in the Yangtze Block and the Junggar terrane-Tian Shan-northern Tarim Block, which are both influenced by the weathering of carbonate rocks. The intermediate ranges of Sr isotopes (0.711-0.7125) are mainly found in the North China Block, the northern Tibetan Plateau, and the southern Tarim Block, which may be because of evaporite dissolution and silicate/carbonate weathering processes. The high ranges of Sr isotope ratios (> 0.7125) appear in the Cathaysia Block as well as some orogenic belts and the southeastern margin of the Tibetan Plateau, likely due to the influence of the metamorphic rocks and granitoid sources. Additionally, topography, differential weathering among lithologies, and eolian deposits (e.g. loess) are also significant factors that influence Sr-87/Sr-86 ratios in China. Our study reveals that the large variation of Sr-87/Sr-86 ratios across China is predominately driven by geological conditions, revealing that future Sr isotope provenance studies can shed on ancient and historical migrations in China.