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

Water column denitrification, the main oceanic sink of nitrogen (N), has been shown to weaken as Earth warms. However, the impact of climate on N2 fixation, the main oceanic source of N, remains poorly constrained. For the Pliocene epoch, a potential geological analogue for predicted warming over the next century, we present high-resolution foraminifera-bound (FB-) δ15N records from the Western and Eastern equatorial Atlantic Ocean (WTA and EEA, respectively). Our records reveal little difference in FB-δ15N between the WTA and EEA during the Late Pliocene, contrasting with the large EEA-WTA difference of the Late Pleistocene. The data indicate reduced Atlantic N2 fixation rates during the Late Pliocene which, given reconstructions of reduced water column denitrification in the Pacific oxygen-deficient zones, appear to be linked to less excess phosphorus (relative to N) in the Pliocene pycnocline/subsurface ocean. Our records indicate that obliquity-paced cycles in FB-δ15N progressively strengthened as Northern Hemisphere glaciations intensified after ~ 2.8 million years ago, approaching the amplitude of orbital FB-δ15N change observed during the Late Pleistocene. The inferred orbital cycles in N2 fixation are best explained as a response to sea-level modulation of sedimentary denitrification: Lower sea level during glaciations exposed continental shelves, reducing regional benthic denitrification and inhibiting the supply of excess phosphorus to the surface ocean, thereby limiting N2 fixation in the WTA. Thus, since the warm Pliocene, Atlantic N2 fixation has at least partially compensated for oceanic N loss at both global and regional scales.