Reduced N-fixation in the Low Latitude Atlantic during the Warmer Pliocene

Date:
23
Sunday
March
2025
Lecture / Seminar
Time: 11:00-12:00
Location: Sussman Family Building for Environmental Sciences
Lecturer: Maayan Yehudai
Abstract: N2 fixation is the primary pathway by which bioavailable nitrogen is added to th ... Read more N2 fixation is the primary pathway by which bioavailable nitrogen is added to theoceans. However, the drivers of N2 fixation on orbital timescales are uncertain. Wepresent high-resolution foraminifera-bound (FB) δ15N records from the Westernand Eastern tropical Atlantic Ocean (WTA and ETA respectively) throughout thelate Pliocene (~3.60 to ~1.97 Ma), where WTA ODP Site 999 represents N2fixation changes and EEA ODP Site 662 represents changes in pycnocline δ15N.Our results show that, compared to the past 160 ka, N2 fixation in the WTA wassignificantly lower throughout the late Pliocene as reflected by an average of ~2 ‰higher FB-δ15N values. A possible explanation to the higher Pliocene FB-δ15N inthe WTA could be lower rates of global denitrification that were balanced by lowerglobal N2 fixation levels. We suggest that this reduced N2 fixation was due todecreased excess P in the pycnocline/subsurface ocean, driven by lower globalwater column denitrification. This finding implies a coupling between decreasedwater column denitrification and reduced level N2 fixation rates under warmerclimates.On orbital timescales, our N2 fixation record display obliquity-paced cycles thatprogressively intensified after the Northern Hemisphere glaciation intensification ~2.8 Ma, and the onset of equatorial upwelling pulses documented during glacialperiods in the EEA (ODP Site 662; [1]). The observed changes in N2 fixation of thelast 160 ka were previously explained by precession-paced upwelling in the EEAthat imported excess P into the oligotrophic WTA [2]. However, precessionalcyclicity is not dominant in the Pliocene FB- δ15N, which calls for other candidatesto explain the variations after 2.8 Ma. The best explanation is a response to sealevelpaced sedimentary denitrification. Glacial lower sea levels exposedcontinental shelves, reducing regional benthic denitrification and inhibiting thesupply of excess P, thereby limiting N2 fixation in the WTA, whereas interglacialsubmerged shelves increased excess P availability.
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