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

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 21st 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.