Controls on the organic carbon content of the lower Cambrian black shale in the southeastern margin of Upper Yangtze

Abstract

Control of various factors, including mineral components, primary productivity and redox level, on the total organic carbon (TOC) in the lower Cambrian black shale from southeastern margin of Upper Yangtze (Taozichong, Longbizui and Yanbei areas) is discussed in detail in this article. Mineral components in the study strata are dominated by quartz and clay minerals. Quartz in the Niutitang Formation is mainly of biogenic origin, and the content is in positive correlation with TOC, while the content of clay minerals is negatively correlated with TOC. Primary productivity, represented by the content of Mobio (biogenic molybdenum), Babio (biogenic barium) and phosphorus, is positively correlated with TOC. The main alkanes in studied samples are nC18–nC25, and odd–even priority values are closed to 1 (0.73–1.13), which suggest the organic matter source was marine plankton. Element content ratios of U/Th and Ni/Co and compound ratio Pr/Ph indicate dysoxic–anoxic bottom water, with weak positive relative with TOC. In total, three main points can be drawn to explain the relationship between data and the factors affecting organic accumulation: (1) quartz-rich and clay-mineral-poor deep shelf–slope–basin environment was favorable for living organisms; (2) high productivity provided the material foundation for organic generation; (3) the redox conditions impact slightly on the content of organic matter under high productivity and dysoxic–anoxic condition.