Regulation of carbon–nitrogen cycle by chemoautotrophic bacteria during the early Cambrian: Evidence from the silicate-bound nitrogen in the Yangtze Block, South China

Abstract

The Ediacaran-Cambrian (E–C) transition was a critical period in Earth’s history marked by environmental changes and an unprecedented biological evolution event. During the early Cambrian, carbon isotopes (δ¹³C_carb and δ¹³C_org) exhibited multiple large negative excursions. The first and second excursions, known as the Basal Cambrian Carbon Isotope Excursion (BACE) and the Shiyantou Carbon Isotope Excursion (SHICE), are closely associated with the extinction of the Ediacaran biota and the small shelly biota, respectively. While it is hypothesized that intensified anoxic conditions contributed to the perturbations in carbon cycling and life evolution during the early Cambrian, the specific details of the mechanisms behind the negative carbon isotope excursions still require further investigation. Fixed ammonium is incorporated within the crystal structure of silicate minerals (mostly K-bearing phyllosilicates and feldspars) and is also referred to as silicate-bound nitrogen. It is the primary form of inorganic nitrogen in rocks and is increasingly being recognized for its role in reconstructing nitrogen cycles in paleo-oceans. In this study, we conducted detailed geochemical analyses of kerogen-bound nitrogen and silicate-bound nitrogen found in marine sequence from the lower Cambrian strata within the South China Craton. The isotopic composition of kerogen-bound nitrogen overall exhibits a negative excursion, indicating an expansion of the anoxic zone in seawater and a concomitant contraction of the nitrate reservoir during deposition of the Shuijingtuo Formation. The ¹⁵N-depleted signal of kerogen (<0‰) is concurrent with the notably elevated δ¹⁵N_silicate values (up to 9.5 ‰) in the Shuijingtuo Member I, displaying offsets of up to 10.0 ‰. We attribute the abnormally high δ¹⁵N_silicate values to intensified anammox associated with expansive anoxic bottom waters. Our research reveals a strong coupling between marine carbon and nitrogen cycles during the early Cambrian. Insofar as the biomass formed by carbon fixation coupled to ammonium oxidation is significantly depleted in ¹³C relative to CO₂ in the atmosphere due to respiratory processes, the SHICE may be attributed to progressively greater inputs from these chemoautotrophic sources. This new explanation broadens our understanding of the mechanisms controlling the negative excursions of carbon isotopes during the early Cambrian. Furthermore, our results also suggest that applying the δ¹⁵N_kerogen and the δ¹⁵N_silicate proxies, as observed in the lower Cambrian Shuijingtuo Formation, enables speculation on the origin of changes with analogous δ¹⁵N distributions associated with intensified anammox over other time periods.