New insights into controlling factors of organic matter enrichment in Carboniferous−Permian shales based on machine learning

Shale deposits constitute Earth’s largest and most stable organic carbon reservoir. Organic matter enrichment of fine-grained sediments is paramount for carbon sink research and oil and gas exploration. The Carboniferous to Permian interval, an especially critical period in Earth’s ecosystem evolution that encompassed the late Paleozoic ice age, witnessed widespread accumulation of organic-rich shale. However, the mechanisms driving organic matter enrichment and the role of organic carbon burial as a carbon sink during this period remain contentious. Our study focuses on upper Carboniferous and lower Permian shale deposited on the North China Block (NCB). A total of 370 elemental geochemical datasets were obtained to elucidate paleoenvironmental conditions associated with these deposits. We employed random forest (RF) and artificial neural network (ANN) algorithms to elucidate the controlling mechanisms of organic matter enrichment during this time period and to offer a quantitative assessment of the magnitude of organic carbon burial. Our results suggest that upper Carboniferous and lower Permian shale accumulated contemporaneous with a dominantly warm, humid climate that experienced episodes of cooler, drier conditions. The water column exhibited low primary surface productivity, oxic-suboxic, saline water conditions, and elevated terrigenous input and sedimentation rates. RF and ANN analysis reveals that paleoclimate was the dominant factor influencing organic matter enrichment of fine-grained sediments during this time. The inferred warm, humid climate not only promoted enhanced organic carbon production but also favored increased delivery of organic matter as a result of increased chemical weathering and associated terrestrial input to the basin. Concurrently, elevated sedimentation rates and the establishment of saline water conditions facilitated enhanced preservation of deposited organic matter. Organic carbon burial associated with accumulation of the upper Carboniferous and lower Permian shale succession of the NCB reached 95.5 × 10³ PgC. These deposits appear to have served as a significant carbon sink based on an estimated organic carbon burial rate of shale containing total organic carbon >6% of as great as 65.6 gC/m²/yr. Results of the present study enhance our understanding of the carbon cycle during the Carboniferous to Permian interval and provide guidance for shale gas exploration and development of Carboniferous and Permian shale deposits.