Shallow-water redox changes and nitrogen cycles in the early Cambrian Nanhua Basin, South China

Abstract

Widespread oceanic anoxia has been frequently invoked for metazoan extinctions during the Cambrian. However, this hypothesis remains to be debated. In this study, we measured total organic carbon (TOC), total bulk nitrogen (TNbulk), iron speciation, major and trace elements, and isotopic compositions of sedimentary bulk nitrogen (δ15Nbulk) for the Shiyantou Formation in the Meishucun section. Our results suggest ferruginous conditions and high productivity in the lower member (LM: 0–16.0 m), dominantly suboxic conditions and low productivity in the middle member (MM: 16.0–23.7 m), and oxic conditions and low productivity in the upper member (UM: 23.7–28.0 m). Low δ15Nbulk values (average 1.0 ‰) in the LM suggest quantitative assimilation of NH4+ and strong nitrogen fixation as a consequence of high primary productivity and widespread water-column reducing conditions in the Nanhua Basin. δ15Nbulk values close to 0 ‰ in the MM suggest intense nitrogen fixation owing to quantitative denitrification and anammox in an oligotrophic environment. Low δ15Nbulk values (average 1.2 ‰) in the UM are indicative of marine nitrogen limitations in an oligotrophic environment. These findings suggest a similar range of low δ15Nbulk values might result from different environments. In comparison with marine redox conditions and δ15Nbulk data from the other coeval sections in the Nanhua Basin, we propose that widespread anoxic conditions might cause the extinction of small shelly faunas. More importantly, the oligotrophic environment and anoxic conditions might delay the recovery of small shelly faunas in the shallow and deep waters of the Nanhua Basin, respectively.