Paleo-marine environments of the Early Cambrian Yangtze Platform
Zhang et al
Previous studies have generated conflicting interpretations of the nature of oceanographic conditions on the Early Cambrian Yangtze Platform. We investigated paleo-environmental conditions there through an analysis of major elements and redox-sensitive trace elements in the lower Cambrian Niutitang Formation from two new drillcores (RY1 and RY2) in Guizhou Province. High TOC and excess Ba concentrations record elevated productivity in the study area. Redox variation was documented based on enrichment factors (EFs) for redox-sensitive trace elements (Mo, U, V, Cr, Ni, and Co) and Corg:P ratios. These proxies indicate that the Lower Unit of the Niutitang Formation was deposited under fluctuating but mostly anoxic conditions, whereas the Upper Unit was deposited under predominantly suboxic conditions. Mo/TOC ratios suggest persistently moderate to strong basinal restriction, although Mo–U covariation patterns show no evidence for operation of a particulate shuttle despite the semi-restricted, anoxic nature of the deep watermass. Episodic deep water oxygenation events occurred within the Yangtze Platform sea, and longer-term secular variation may record eustatic influences. Additionally, patterns of trace-metal enrichment record evidence of submarine hydrothermal inputs. We propose an integrated paleoenvironmental model to account for the observed productivity, redox, and hydrographic patterns in the Early Cambrian Yangtze Platform. On the basis of paleo-marine environmental analysis, we ascribe the scarcity of trace fossils in the bottom to oxygen deficiency, and deny considering the Songlin Biota as the onset of the intense biogenic evolution after global atmospheric and oceanic oxygenation.