Marine biogeochemical cycling during the early Cambrian constrained by a nitrogen and organic carbon isotope study of the Xiaotan section, South China

The Precambrian–Cambrian boundary is claimed as one of the most crucial biological breakthroughs on our planet, when changes in chemical and physical conditions, together with key biological innovations, helped to trigger a biodiversity “explosion”. The Yangtze Platform (South China), mainly characterized by continuous and unaltered successions of this age, is ideally suited to high-resolution, palaeomarine investigations of this event. In this study, δ15N and δ13Corg records from Xiaotan Section (Yunnan, SW China) were investigated in order to provide insight into variations in primary productivity, ecological developments and marine environment. The Ediacaran-early Cambrian Xiaotan section is characterized by relatively high sedimentation rates and variable lithologies (carbonates, cherts, phosphorites, sandstones and siltstones) that alternate through nearly 600 meters thickness. Organic carbon isotope values vary between −36‰ and −21‰, tightly following and helping to complete published δ13Ccarb trends. The base of the Cambrian is characterized in this section by a significant drop from −25‰ to −35‰, mirroring the δ13Ccarb fall observed at this and other sections worldwide. Higher in the section in the Dahai Member, values increase to −20‰, again demonstrating communication between DOC and DIC pools during the early Cambrian on 105–106 year time scales. From the base of the overlying Shiyantou Formation, δ13Corg values remain around −30‰ until the end of the section, testifying to an interval of more modest change in DIC δ13C that would be consistent with biostratigraphic correlations. Nitrogen isotope values vary independently from carbon isotope trends, exhibiting several major δ15N cycles, with values fluctuating between +9‰ and −1‰. Robust trends in δ15N within the Dahai Member testify to changes in the ratio between nitrogen fixation and denitrification, possibly reflecting fluctuations in the water column chemocline as δ13C values reached their global acme. Up section, nitrogen isotope values show dampened cyclicity with values remaining between 0‰ and +5‰, indicating establishment of an equilibrium state in marine biogeochemistry and nutrient cycles. The overall variability in δ15N closely resembles that of recent marine sediments, while the observed cyclicity is defined by several samples in all cases. These observations argue for good preservation of original isotopic signatures which bodes well for future N isotope studies of the Precambrian-Cambrian boundary.