Nitrogen isotope evidence for stratified ocean redox structure during late Ediacaran to Cambrian Age 3 in the Yangtze Block of South China

Abstract

The Ediacaran–Cambrian (E–C) transition witnessed profound biological and oceanic changes, among which the appearance of explosive radiation of skeletonized animals is considered as the most remarkable one. Although the prominent rise of oxygen concentration in atmosphere and ocean is considered to be the major trigger of the “Cambrian Explosion”, the links between the oxygenated process of deep ocean and the diversification of Cambrian metazoans are still under debate. In this study, δ15N and δ13Corg values, TOC and TN contents, as well as trace elements concentrations of fresh shale core samples were analyzed to explore the redox condition and nitrogen cycle of deep oceans and their relationships with biological evolution. The Mo concentrations, Mo–U covariations and Th/U ratios of our samples indicate suboxic/anoxic waters in the mid-depth ocean during the late Ediacaran to Cambrian Fortunian age, widespread anoxic/euxinic waters with temporary sulfidic waters during Cambrian Age 2 to early Age 3, and suboxic waters again during the middle to late Age 3. The positive δ15N values from late Ediacaran to Cambrian Age 2 indicate a large NO3− reservoir with a deep chemocline. In contrast, the low δ15N values (<0‰) in the deepwater slope regions suggest a dominant biological N2-fixation, and/or partial NH4+-assimilation in the photic zone during Cambrian Age 3. Integrated δ15N values of this study and early publications suggest a continuous redox stratified paleo-ocean with anoxic/suboxic deep waters from late Ediacaran to Cambrian Age 3. It is also worthy to note that although the deep ocean may not experience substantial oxygenation during the main phase of Cambrian diversification, the expanding oxic surface waters and supply of bioavailable fixed nitrogen may satisfy complex high O2-demanding animals, and thus at least have promoted the Cambrian biodiversity in shallow waters.