Redox variations and organic matter accumulation on the Yangtze carbonate platform during Late Ediacaran–Early Cambrian: Constraints from petrology and geochemistry
Abstract In order to understand redox variations and organic matter accumulation on the Yangtze carbonate platform during the Late Ediacaran–Early Cambrian, petrological and geochemical studies of several wells were carried out in this work. Our data suggest that depositional environments were dominated by oxidizing bottom water conditions during the late Ediacaran, and evolved to anoxic conditions, triggered by blooms of microbial organisms in surface waters during the earliest Cambrian. Subsequently, massive release of H 2 S derived from both anaerobic recycling of organic matter and, probably, hydrothermal venting promoted a sulfidic ocean. The discovery of a Ni-Mo sulfide ore layer in the basal Cambrian implies that such a sulfidic condition spread onto the Yangtze carbonate platform interior during the late Cambrian Stage 2. Further, transgressive flooding led to widespread black shale deposition and persistently anoxic conditions, as indicated by geochemical proxies. During the late Ediacaran to earliest Cambrian, local and widespread phosphogenesis indicates that organic matter accumulation was intimately associated with microbial (especially cyanobacterial) blooms driven by phosphorus cycling. The organic matter accumulations in early Cambrian black shales, however, were in connection with anoxic bottom water conditions and intermittent replenishment of recycled organic phosphorus to surface waters.