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

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.