Marine redox variation and hydrographic restriction in the early Cambrian Nanhua Basin, South China

Abstract

The early Cambrian Nanhua Basin is an important archive of information related to contemporaneous marine redox evolution and its relationship to the radiation of early animals. However, the nature of watermass conditions in the Nanhua Basin remains uncertain owing to conflicting redox signals. Here, we generated a redox proxy dataset comprising Fe speciation, redox-sensitive element (RSE), pyrite framboid size distributions, and molybdenum isotopic data (δ98Mo), in combination with proxies for hydrographic restriction (i.e., Co × Mn, Cd/Mo, and Mo/TOC), in order to evaluate environmental conditions in the basinal Yuanjia section. We then compared these data with previous results for outer-shelf (Jinsha, Zhongnancun, and Yangjiaping), slope (Daotuo, Songtao, and Longbizui), and basinal facies (Silikou) to develop an integrated picture of environmental (especially redox) changes in the early Cambrian Nanhua Basin. At Yuanjia, the lower member (LM, 0–28 m) is characterized by weakly euxinic conditions and an intense Fe-Mn shuttle, and the upper member (UM, 28–65 m) by moderately euxinic conditions and a weak Fe-Mn shuttle. Our integrated analysis demonstrates co-existence of oxic surface waters, euxinic mid-depth waters, and ferruginous deep waters in the Nanhua Basin, with euxinic conditions prevailing in shelf-slope facies of the Yangtze Block and basinal facies of the Cathaysia Block but declining in intensity from the LM to the UM. The Nanhua Basin changed from being moderately restricted with strong upwelling during the LM to relatively more restricted with weak upwelling during the UM. We infer that redox fluctuations in the early Cambrian Nanhua Basin were linked to hydrographic restriction, although global factors such as low seawater sulfate concentrations may have played a role also. Our study highlights the need for caution in assuming that redox variation in the early Cambrian Nanhua Basin was representative of contemporaneous global-ocean redox states.