<p>N2 fixation is the primary pathway by which bioavailable nitrogen is added to the</p><p>oceans. However, the drivers of N2 fixation on orbital timescales are uncertain. We</p><p>present high-resolution foraminifera-bound (FB) δ15N records from the Western</p><p>and Eastern tropical Atlantic Ocean (WTA and ETA respectively) throughout the</p><p>late Pliocene (~3.60 to ~1.97 Ma), where WTA ODP Site 999 represents N2</p><p>fixation changes and EEA ODP Site 662 represents changes in pycnocline δ15N.</p><p>Our results show that, compared to the past 160 ka, N2 fixation in the WTA was</p><p>significantly lower throughout the late Pliocene as reflected by an average of ~2 ‰</p><p>higher FB-δ15N values. A possible explanation to the higher Pliocene FB-δ15N in</p><p>the WTA could be lower rates of global denitrification that were balanced by lower</p><p>global N2 fixation levels. We suggest that this reduced N2 fixation was due to</p><p>decreased excess P in the pycnocline/subsurface ocean, driven by lower global</p><p>water column denitrification. This finding implies a coupling between decreased</p><p>water column denitrification and reduced level N2 fixation rates under warmer</p><p>climates.</p><p>On orbital timescales, our N2 fixation record display obliquity-paced cycles that</p><p>progressively intensified after the Northern Hemisphere glaciation intensification ~</p><p>2.8 Ma, and the onset of equatorial upwelling pulses documented during glacial</p><p>periods in the EEA (ODP Site 662; [1]). The observed changes in N2 fixation of the</p><p>last 160 ka were previously explained by precession-paced upwelling in the EEA</p><p>that imported excess P into the oligotrophic WTA [2]. However, precessional</p><p>cyclicity is not dominant in the Pliocene FB- δ15N, which calls for other candidates</p><p>to explain the variations after 2.8 Ma. The best explanation is a response to sealevel</p><p>paced sedimentary denitrification. Glacial lower sea levels exposed</p><p>continental shelves, reducing regional benthic denitrification and inhibiting the</p><p>supply of excess P, thereby limiting N2 fixation in the WTA, whereas interglacial</p><p>submerged shelves increased excess P availability.</p>
