Abstract

Previous studies on early-Cambrian nitrogen (N) cycle suggested a modern-type nitrogen reservoir with elevated nitrate (NO3−) availability in a globally oxygenated ocean for the Cambrian explosion. However, recent marine redox studies on the Chengjiang Biota revealed a stratified ocean with anoxic deeper waters for the peak Cambrian explosion, asking for further works on the N cycling during this critical period. Chengjiang and Qingjiang biotas are the two top-rank Burgess Shale-type fossil Lagerstätten that recorded peaked metazoan diversification of the Cambrian explosion at different paleoenvironmental regimes during the Cambrian Age 3. In this study, we conducted a detailed geochemical investigation of the early-Cambrian Shuijingtuo Formation from two drill cores containing the Qingjiang Biota in Hubei Province, South China. The results suggest progressive oxygenation of the depositional environment from euxinic to fluctuating, suboxic conditions with a low chemocline in the water column, accompanied by an increase of δ15N values from <−4‰ to between −2‰ and +1‰. Especially, both event and background beds of the Qingjiang Biota are characterized by δ15N values ranging from −1‰ to +1‰. Comparative analysis reveals consistent variations of δ15N signatures and marine redox states at the Qingjiang and Chengjiang areas during the Cambrian Age 3, with the latter exhibiting systematically higher δ15N values ranging mainly from +1‰ to +2‰ at the biota interval. Taken together, the δ15N signatures of the Qingjiang and Chengjiang biotas can be most parsimoniously explained by prevailed aerobic marine nitrogen cycling featured by active biological N2 fixation and a limited nitrate reservoir in the shallow ocean. The nitrogen isotope gradient between the Qingjiang and Chengjiang areas may be reflective of varied NO3− availability in seawater. These findings contribute to a clearer biogeochemical context that supported prosperous marine ecosystems during the peak Cambrian explosion, and provide new insights on spatiotemporal evolution of the marine nitrogen cycle during this critical period.

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