| Summary: | N2 fixation is the primary pathway by which bioavailable nitrogen is added to the oceans. However, the climate and oceanographic drivers of N2 fixation on orbital timescales are uncertain. Here we investigate the evolution of N2 fixation throughout the Pliocene-Pleistocene (PP) transition (~3.60 to ~1.97 Ma) with a new N2 fixation record reconstructed from the analysis of foraminifera-bound δ15N in the tropical North Atlantic (ODP Site 999). Our results show that, compared to interglacials of the past 160 ka, N2 fixation was significantly lower throughout the PP transition as reflected by an average of ~2.5 ‰ higher δ15N values. A possible explanation to the observed higher Plio-Pleistocene δ15N values could be lower levels of global denitrification that were balanced by lower global N2 fixation levels. In addition, we observe an increase in the dominance of obliquity and eccentricity frequencies in δ15N after ~2.7 Ma, coinciding with the intensification of the Northern Hemispheric glaciation and the onset of equatorial upwelling pulses documented during Pleistocene glacial periods in the Eastern Equatorial Atlantic (ODP Site 662; Lawrence et al., 2013) Observed changes in N2 fixation derived from foraminifera bound δ15N of the last 160 Ka were explained in previous studies by precession-paced upwelling in the eastern equatorial Atlantic that imported excess P into the oligotrophic Western Tropical Atlantic (2). However, precessional cyclicity is not dominant in the new Plio-Pleistocene δ15N data, which calls for other candidates to explain the δ15N variations of the early Pleistocene. We propose several mechanisms to explain the early Pleistocene orbitally paced variations in δ15N, including changes in nutrient utilization in the Southern and Equatorial Atlantic, glacial-interglacial sea level changes and global orbitally-paced changes in denitrification. K. T. Lawrence et al., Time-transgressive North Atlantic productivity changes upon Northern Hemisphere glaciation. Paleoceanography. 28, 740–751 (2013). M. ...
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