Reduced N2 fixation in the Atlantic Ocean during the Warm Late Pliocene

Recent studies show that water column denitrification, the main oceanic nitrogen sink, decreased under warmer than present conditions, but the response N ₂ fixation, the main oceanic nitrogen source, remains unresolved. Here we address this gap using novel foraminifera bound δ¹⁵N records from the Western and Eastern tropical Atlantic spanning the warm Pliocene, which is considered a geological analog for predicted warming. Our records reveal that N ₂ fixation was substantially reduced during the Pliocene, likely due to decreased excess phosphorous supply, driven by lower global water column denitrification. After ~ 2.8 Ma, as glaciations intensified, obliquity paced variability in N ₂ fixation emerged, reflecting sea level driven changes in continental shelf area and benthic denitrification. These findings unveil N ₂ fixation’s sensitivity to both long term as well as glacial interglacial scale changes, which is critical because reduce N ₂ fixation under warmer conditions could influence the oceanic nitrogen balance and global ocean productivity.