Nitrogen isotope constraints on the early Ediacaran ocean redox structure

Nitrogen (N) is a macronutrient essential to all living organisms and its availability in Precambrian oceans may have played an important role in the early evolution and diversification of eukaryotes. It has been hypothesized that the billion-year evolutionary stasis of eukaryotes during the mid-Proterozoic (ca. 1.8-0.8 Ga) was linked with N limitation. As a corollary, the rapid diversification of eukaryotes during the Ediacaran implies the lift of this barrier. Indeed, recent nitrogen isotope (δ15N) studies suggested a stable oceanic nitrate (NO3-) pool and perhaps oxygenated mid-depth oceans since ca. 750 Ma. This inference, however, contrasts with the iron, sulfur, and trace element geochemical data that suggested much later ocean oxygenation during the Ediacaran or Paleozoic. To better understand the relationship between nitrogen isotope and other redox proxy data, we have conducted a nitrogen isotope study on the organic-rich black shales of the basal Doushantuo Formation (ca. 635-632 Ma) in multiple sections across a shelf-to-basin transect, from which iron, sulfur and trace element geochemical data are available. The results show exclusively positive δ15N values in all sections across the basin, with most values within the range of +3‰ to +12‰ and a modal value of +5 ± 1‰. The positive δ15N values from the basal Doushantuo black shales are comparable with those of the modern ocean (+3‰ to +14‰; modal value of +5‰) and most likely resulted from an aerobic nitrogen cycle with partial water column denitrification in the presence of a stable nitrate pool. In general, it is difficult to maintain a stable oceanic NO3- reservoir in strongly stratified oceans. Therefore, the new δ15N data, in combination with other geochemical data, provide evidence for an oxygenated mid-depth ocean during the early Ediacaran. The co-occurrence of aerobic N isotope signal and euxinia in some samples implies that the early Ediacaran ocean in South China could be periodically stratified well below the photic zone, with a chemocline at least down to the mid-depth (e.g., >1000 m) or close to the water column/sediment interface.