Nitrogen isotopes and geochemistry of the basal Datangpo Formation: Contrasting redox conditions in the upper and lower water columns during the Cryogenian interglaciation period

Abstract

The late Neoproterozoic witnessed series of remarkable geological evolutions, including the Sturtian and Marinoan glaciation events during the Cryogenian. A stratified oceanic redox structure has been proposed for this period, and the dynamics of the deep water redox conditions have been well illustrated by trace metals and their isotopes. However, variations in the redox conditions of the upper water column have received much less attention. Sedimentary nitrogen isotopes (δ15N) are critical for reconstructing the redox conditions in the upper water columns of ancient oceans. Existing research on the N cycle during the Cryogenian period is scarce, and the majority of studied samples were collected from shallow and medium water depth facies. To address this paucity of studies, we conducted a high-resolution N isotope study in combination with Fe speciation and Mo concentration data from two deep-water sections of black shales in the basal Datangpo Formation in the Nanhua Basin, a typical organic-rich stratum deposited during the Cryogenian interglaciation period. While the trace metal proxies suggest a fluctuating euxinic–ferruginous bottom water column for the lower interval and a stable euxinic bottom water column for the upper interval, the redox conditions of the upper water column were relatively stable, as indicated by minimal changes in the recorded δ15N values. Furthermore, the δ15N values of black shales in the basal Datangpo Formation of the Nanhua Basin have a limited range (3.1‰ − 5.8‰, average of 4.3‰, standard deviation (SD) = 0.5‰, n = 104) and show no water-depth gradient, indicating the presence of a relatively large and stable nitrate reservoir and a deep redox chemocline in the Nanhua Basin. The characteristic of the nitrate reservoir reflected by these δ15N signals can be roughly analogized to the global interglacial ocean, although only the counterpart northeast Svalbard can be used for comparison. Therefore, the nitrate availability in the marine water column may have had little effect on animal radiation during the Cryogenian interglacial period. These findings help to clarify the N cycle in the Cryogenian Nanhua Basin and shed light on its significance on bioevolution.