N_² fixation is the primary pathway by which bioavailable nitrogen is added to the oceans. However, the climatic and oceanographic drivers of N₂ fixation on orbital timescales are uncertain. I this talk I will discuss the evolution of N₂ fixation throughout the Pliocene-Pleistocene (PP) transition (~3.60 to ~1.97 Ma), which marked the intensification of Northern Hemispheric glaciations and the onset of glacial-interglacial cycles, with a new N₂ fixation record reconstructed from the analysis of foraminifera-bound δ¹⁵N in the tropical North Atlantic (ODP Site 999). 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 (Straub et al., 2013). 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 variations in δ¹⁵N, including changes in glacial-interglacial sea level changes and global orbitally-paced changes in denitrification.

 

References: