Reduced N₂ fixation in the tropical Atlantic Ocean during the warm late Pliocene.

Abstract

Water column denitrification, a major oceanic nitrogen (N) sink, decreased under past warmer-than-present climates, but the response of N₂ fixation, the main oceanic N source, remains unresolved. Here we address this gap using foraminifera-bound N isotope records from the tropical Atlantic spanning the warm late Pliocene, a geological analog for projected 21^st century warming. Our records show that tropical North Atlantic N₂ fixation was reduced during the late Pliocene and had increased markedly by the late Pleistocene, likely due to an increase in water column denitrification, a process that globally engenders N-poor, phosphorus (P)-bearing surface waters. After ~ 2.8 million years ago, as glaciations intensified, our records show that obliquity-paced variations in the intensity of N₂ fixation emerged, likely reflecting sea-level controlled changes in continental shelf area and benthic denitrification in the western tropical Atlantic, again through the generation of N-poor, P-bearing surface waters. These findings show that N₂ fixation responds to the ocean's N loss processes both over millions of years and on orbital timescales, suggesting that the global marine fixed N budget may rebalance under warmer climates by reducing both water column denitrification and N₂ fixation.