Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: Evidence from nitrogen and organic carbon isotopes

Abstract

The early Cambrian is a crucial interval, during which the marine environment encountered significant perturbations alongside key biological innovations. The nitrogen isotopic composition of ancient sediments has the potential to record co-variation between the marine ecosystem and environmental conditions. Here we report bulk nitrogen and organic carbon isotopic compositions accompanied by Mo concentration data (a proxy for redox conditions) for a shelf marginal drill core section (Sancha) and a deep basinal outcrop section (Yuanjia) in Hunan province, South China.For the Cambrian Fortunian Stage, variable δ15Nbulk values (−4 to +2‰) and low δ13Corg values (−35‰ to −33‰) from the Yuanjia section indicate that anoxic waters shoaled into the photic zone, with ammonia assimilation competing with anammox in the basinal area. In the late Cambrian Stage 2, low δ15Nbulk and δ13Corg values, together with high Mobulk concentrations (δ15Nbulk=−5 to 0‰; δ13Corg=−33.5 to −31.0‰; >100ppm Mobulk in Sancha section and δ15Nbulk=∼0‰; δ13Corg=∼−33‰ >100ppm Mobulk in Yuanjia section), reflect intense N2-fixation accompanied by intermittent ammonia assimilation under expanded euxinic condition with shoaling chemocline possibly into the photic zone. In the early Cambrian Stage 3, relatively high δ15Nbulk values, alongside positive δ13Corg excursions, at both sections indicate an increasing contribution from 15N-enriched nitrate assimilation and hence more oxic conditions. Shelf margin sediments from the middle and late parts of Cambrian Stage 3 are characterized by low δ15Nbulk values (∼0‰), being lowest in the basinal area (−3 to 0‰). In combination with high Mobulk concentration (>100ppm) and high Mo/TOCbulk ratios (averaging ∼18) in deep basinal sediments, these data suggest that the photic zone became anoxic again, but that sulfidic conditions occurred in the basinal area only. Compared with Stage 2, euxinia was less extensive during this interval, with photosynthetic biomass dominating over chemoautotrophic biomass at both localities.Based on a combination of δ15N data from this study and published data from studies on contemporaneous sections in Yunnan Province and the Three Gorges area, it is possible to reconstruct the spatial variability and temporal evolution of redox conditions in the various marine environments along the southern margin of the Yangtze platform. The basinal area appears to have been dominated by anoxia and intermittent euxinia in the photic zone during most of the early Cambrian (Fortunian Stage to Stage 3) with the exception of more oxic conditions during early Stage 3. On the platform, oxygenated shallow water prevailed with the exception of the notable Cambrian Stage 2 anoxic event. The outer shelf area experienced anoxia from Stage 2 to Stage 3 of the early Cambrian, although of possibly limited areal extent. The high Mobulk concentrations and Mo/TOCbulk ratios suggest that the South China basin was well connected with the considerably oxygenated open ocean and that the mean low δ15Nbulk values (∼+1.6‰) from the basin may reflect generally lower δ15N composition for seawater nitrate in the early Cambrian compared with modern value.