Controls on organic matter accumulation on the early-Cambrian western Yangtze Platform, South China

The origin of widespread organic-rich shales on the early Cambrian Yangtze Platform of Nanhua Basin (South China) has been attributed to high primary productivity. To test this hypothesis, we generated multiple paleoproductivity (OCAR and PAR), paleoredox (C-org/P ratios), upwelling/restriction (Co-EF x Mn-EF, Co (ppm) x Mn (%), Mo/TOC, and Cd/Mo ratios), and terrigenous flux proxies (Al and unreactive Fe (Fe-U)) for the lower Cambrian Niutitang Formation in the outer-shelf Jinsha section, which records a typical transition from the organic-rich black shales to overlying organic-lean mudstones or siltstones. These new data were integrated with published paleoredox proxies (iron speciation and redox-sensitive trace metals) for the same section. Our results suggest that the organic-rich (TOC = 4.6 +/- 1.8%) lower member (LM; 0-37 m) of the Niutitang Formation records ferruginous conditions, high productivity levels, and low siliciclastic fluxes, and that the organic-poor (TOC = 0.1 +/- 0.1%) upper member (UM; 37-329 m) was characterized by dominantly oxic conditions, declining productivity levels, and high siliciclastic fluxes. Our results suggest a redox shift from ferruginous to oxic conditions and enhanced terrigenous fluxes in response to falling sea level from the LM to the UM. Secular variations in primary productivity and redox conditions are strongly correlated, which is consistent with a positive feedback: high primary productivity led to increasing water-column anoxia, which enhanced organic P recycling, stimulating further productivity. Variations in primary productivity may reflect variable upwelling intensity in a semi-restricted shelf setting linked to sea-level changes. Taken together, these findings suggest that a combination of primary productivity, redox conditions, and terrigenous fluxes driven by sea-level changes played a key role in organic matter accumulation on the early-Cambrian western Yangtze Platform.