New insights into nitrogen cycle dynamics from phase-specific nitrogen isotope data during the terminal Ediacaran period

Abstract

The terminal Ediacaran period marked the decline of the Ediacara biota, a transition potentially linked to marine redox changes. Nitrogen cycling, a key biogeochemical process tied to marine redox conditions, offers critical insights into paleoenvironmental dynamics. However, interpreting bulk nitrogen isotopes (δ15Nbulk) alone may introduce uncertainties due to their non-unidirectional response to redox shifts. To better constrain nitrogen cycling and assess the role of anoxia in Ediacaran biotic extinction, we present new nitrogen isotope data from kerogen-bound nitrogen (δ15Nkerogen) and silicate-bound nitrogen (δ15Nsilicate), as well as organic carbon isotope (δ13Corg) from a drill core spanning the Ediacaran-Cambrian successions in South China. The integration of consistently high phase-specific nitrogen isotope data with complementary redox proxies reveals a stratified water column, with well‑oxygenated surface waters and anoxic to euxinic deep waters as the dominant redox structure during the deposition of the Doushantuo Formation. In contrast, the extremely low δ15Nsilicate values (reaching as low as −23.3 ‰) observed during the deposition of the lower Shibantan Member coincide with a slight negative excursion in δ15Nkerogen and a rapid positive excursion in δ13Corg. This pattern most probably reflects intensified organic carbon sequestration, which triggered the expansion of deep-water anoxia and stimulated anaerobic nitrogen cycle processes associated with the DNRA process (dissimilatory nitrate reduction to ammonium). Critically, while δ15Nkerogen alone may obscure redox signals due to isotopic buffering by DNRA, the integration of δ15Nsilicate enables more robust reconstruction of nitrogen cycling dynamics. This advancement offers substantial improvements in interpreting redox conditions, particularly in anoxic and nitrogen-limited marine environments. Furthermore, our study establishes a mechanistic link between nitrogen cycle disruptions and the evolution of Ediacaran biota, demonstrating that the expansion of marine anoxia and subsequent nitrogen limitation may have constituted critical environmental stressors during the terminal Ediacaran period.