Surface water productivity and paleoceanographic implications in the Cenozoic Arctic Ocean

Abstract

Study of bulk nitrogen contents and isotopic composition in Arctic Ocean sequences (Integrated Ocean Drilling Program Expedition 302) over the past 60 Ma revealed changes in the export flux and sources of sedimentary nitrogen. The paleoproductivity calculated from the fraction of organic nitrogen to total nitrogen is distinctly lower (<20 g C m−2 a−1) during the ice‐covered Neogene compared to the ice‐free, warm, and biologically active early Paleogene (∼50–100 g C m−2 a−1). Nitrogen isotope measurements from late Paleocene to early Eocene sediments provide evidence for a stepwise stratification and nutrient depletion in surface water masses. Cyanobacterial nitrogen fixation appeared to be the main source of nutrient N in the Arctic Ocean during the early‐middle Eocene characterized by conditions of strong oxygen depletion, high nutrient N losses, and high organic carbon accumulation rates. We speculate that biological CO2 sequestration in the Arctic Ocean and enhanced organic carbon burial rates may have contributed to lower atmospheric CO2 subsequent to the early Eocene climate optimum.