Nitrogen Isotope Evidence for Changing Arctic Ocean Ventilation Regimes During the Cenozoic

Abstract

Abstract In this work, I report on the coupling of dinitrogen (N2) fixation and denitrification in oxygen‐deficient waters of the Arctic Ocean during the Paleogene. This coupling fertilized marine phytoplankton growth and favored organic carbon burial. Reduced vertical mixing due to salinity stratification in a tectonically closed oceanic basin created conditions favorable for N2‐fixation by phytoplankton harboring diazotrophic bacterial symbionts. A positive shift of 5‰ in the δ15N record indicates a change in the main source of biologically available nitrogen due to rapidly changing nutrient availability. I interpret this shift as a switch to Atlantic‐sourced nitrate as the main nitrogen source owing to the opening of the Arctic‐Atlantic gateway to the northern North Atlantic. While the timing of the opening is still disputed among the available Arctic records, I use evidence from the northern North Atlantic to argue that the Arctic Ocean has been fully ventilated since the early Neogene.

Keywords

• Arctic Ocean
• Cenozoic
• nitrogen isotopes
• climate