| Abstrakt: |
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. Plain Language Summary: A current key element of uncertainty is how the Arctic cryosphere will respond to and drive an increasingly warmer future climate. These questions can be addressed by investigating past climate states, yet paradoxically, the geological archives of the key region, the Arctic Ocean, are underexplored. I use material from a single borehole from the central Arctic Ocean (Lomonossov Ridge) to provide the first insights into Arctic Ocean nutrient utilization over the past 60 million years. The results show the prevalence of oceanic nitrogen fixation as the prime nutrient source to sustain marine productivity in an oxygen‐depleted Arctic Ocean during the early Cenozoic (Paleogene—Middle/Late Eocene) with carbon dioxide concentrations significantly higher than today. Moreover, I provide evidence that the opening of the gateway to the North Atlantic caused major changes in nitrogen cycling expressed by a distinct shift to Atlantic‐sourced nitrate as the prime source of new nitrogen to the surface ocean. Key Points: Reduced mixing in a tectonically closed Arctic Ocean created the conditions for N2 fixation by phytoplankton during the early CenozoicA positive shift of 5‰ in the Arctic Ocean δ15N record implies a biotic response in surface waters to rapidly changing nutrient availabilityThis shift indicates a switch to Atlantic‐sourced nitrate and marks the opening to the North Atlantic during the early Neogene [ABSTRACT FROM AUTHOR] |
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