Upwelling controls organic matter enrichment and organic carbon cycle in the shelf area of the Doushantuo Formation

Abstract

As the oldest shale in South China with potential for exploration, the black shale of the second member of the Doushantuo Formation holds significant research importance and economic value because it records the organic carbon cycle during that period. Previous studies have suggested that the formation of the organic-rich black shale in the second member of the Doushantuo Formation was primarily controlled by an anoxic depositional environment. However, investigations of samples from shelf lagoons have revealed otherwise. In this research, microstructural observations, organic carbon content, elemental geochemistry and C_org-N isotopes of the black shale at the Qinglinkou section and well YD in the middle Yangtze were conducted. The results indicate that the formation mechanisms and paleoenvironment of organic-rich shale in the lower and upper halves of the second member are distinct. The presence of apatite and the characteristics of upwelling proxies suggest the occurrence of upwelling during the sedimentation period of the upper half of the second member. The research results indicate that upwelling directly enhances primary productivity levels during sedimentation in the upper half, bolstering the exchange of nutrients and elements within the water column, which facilitates organic matter enrichment and exacerbates the anoxia of deeper waters. In contrast, the organic matter enrichment in the lower half of the Doushantuo Formation's second member is conventional and is governed primarily by anoxic depositional environments; however, it lacks high productivity and is worse overall than the upper half. Moreover, because the water column is restricted, the organic carbon cycle in the lower half is slow and limited. The organic carbon cycle in the upper half benefits from the nutrients brought by upwelling, which results in better formation and preservation. The role of upwelling in altering the deep-water environment of shelf lagoons and in organic matter accumulation and the organic carbon cycle was emphasized in this study, providing guidance for the study of shale deposition globally during the same period.