Organic matter enrichment in a terrestrial-marine transitional environment driven by global/regional climate recorded in the Upper Permian succession from the Qiangtang Basin, northern Tibet

Compared to organic matter (OM) accumulation in marine environments, OM enrichment mechanism in terrestrial-marine transitional environments remain unclear. The Late Permian was an important coal-forming period in China. Late Permian coal-bearing mudstones are important petroleum source rocks in the Qiangtang Basin. In this study, the coal-bearing rocks in the Late Permian Nayixiong Formation from the Qiangtang Basin are selected to address OM accumulation process in transitional environments. Our work demonstrates three stage climatic changes during the deposition of coal-bearing sedimentary rocks: stages I, II and III, corresponding to a semi-warm and semi-humid climate, warm and humid climate, and cold and arid climate, respectively. Based on degree of pyritization (DOP), enrichment factor (EF) of elements, the Late Permian coal bearing rocks were deposited under oxic-dysoxic environments. Low excessive barium (Ba-xs) and biogenic silica (Si-bio) contents in the coal-bearing rocks indicates a low primary productivity. We suggest that warm and humid conditions, freshwater and terrestrial plants input are the primary factors controlling OM accumulation during the Late Permian coal-bearing deposition. We summarized a global/regional climate-driven model for OM accumulation in transitional environments. The warming climate created a habitable environment for both terrestrial and marine organisms. Subsequent freshwater input brought abundant nutrients and terrestrial OM to the aqueous system to build high OM accumulation.