Marine C, S and N biogeochemical processes in the redox-stratified early Cambrian Yangtze ocean

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Abstract
The temporal change of redox conditions of the Yangtze ocean has been revealed by investigating the Ediacaran–Cambrian transition section at Zhalagou, South China. During the earliest Cambrian, cherts and shales were deposited under an anoxic and ferruginous bottom water with significantly increasing total organic carbon and P contents, and negative shift in kerogen δ 13 C values in the lowest part of the section. Euxinic bottom water conditions occurred during the earliest Cambrian Stage 2, with the surface water dominated by N 2 utilization by cyanobacteria or sulphur bacteria leading to negative kerogen δ 15 N values. During Stage 3, dissolved oxygen and sulphate concentrations were significantly increased, and thus the oxidized surface water and the redox transition zone overlying a euxinic bottom water may have been expanded, resulting in an increase in kerogen δ 15 N increasing to 2–4‰, a decrease in pyrite δ 34 S decreasing to as low as –24.6‰ and differences in δ 34 S values between kerogen and pyrite as high as 37‰. This period coincided with the abrupt appearance of large-body metazoans. Thus, the expanding oxic surface water may have reinforced the evolution of animals or vice versa. Interestingly, kerogen δ 34 S values show negative relationships to FePy/FeHR ratios and pyrite sulphur contents, indicating that they can be used to reflect redox conditions, with the lightest values being obtained from euxinic environments.

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