Nitrogen cycle perturbations linked to metazoan diversification during the early Cambrian

Abstract The rapid diversification of metazoans during the late Ediacaran-early Cambrian is one of the most remarkable evolutionary events in Earth history. However, the triggering mechanism remains highly contentious, particularly concerning the relationship of this biological event to contemporaneous changes in ocean redox and nutrient cycling. In this study, we report new bulk nitrogen isotope (δ15N) and organic carbon isotope (δ13Corg) data from the lower Cambrian Niutitang Formation in two deep-water sections in South China to provide a better understanding of the relationship of the nitrogen cycle, ocean redox, and biological evolution during this key period. The δ15N data from two study sections are characterized by consistently low values (–1‰ to +2‰) throughout the Niutitang Formation despite large variations in δ13Corg. We interpret these low δ15N values to have resulted from assimilation of recycled ammonium (NH4+) and N2 fixation. Compilation of available δ15N data reveals large temporal and spatial δ15N variations from –1‰ to +7‰ in time-equivalent strata in South China. Such δ15N variations imply the co-existence of aerobic and anaerobic N cycles and highly heterogeneous ocean redox conditions during the early Cambrian in South China, which is consistent with inferences from other geochemical proxies. Temporal fluctuations and spatial heterogeneity in ocean redox conditions may have controlled the distribution and evolution of metazoans during the early Cambrian.