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Plio‐Pleistocene denitrification in the eastern tropical North Pacific: Intensification at 2.1 Ma

Global climate has changed substantially over the past ∼4 million years (Ma) toward, on average, colder conditions and higher amplitude oscillation between glacial and interglacial periods. Corresponding changes in marine biogeochemistry could have linked oceanic circulation changes to the global carbon cycle, but remain poorly understood. We report a high‐resolution, 4‐Ma‐long δ15N record from Ocean Drilling Program Site 1012 on the California margin, which monitors denitrification intensity in one of the principal Oxygen Minimum Zones (OMZs) of the open ocean. The δ15N record demonstrates that eastern tropical North Pacific (ETNP) denitrification was weak for much of the late Pliocene. Though there was no apparent change associated with Northern Hemisphere glaciation (NHG) at ∼2.7 Ma, denitrification strengthened substantially (by ∼2‰ in δ15N) at ∼2.1 (Ma). Since 2.1 Ma, intense denitrification in the ETNP occurred during interglacial periods and was generally very climate‐sensitive. We infer that two oceanic changes, the shoaling of the thermocline in the eastern Pacific (and thus the strengthened Walker circulation) and the development of a modern‐like frontal system in high‐latitude southern ocean around ∼2.1 Ma provided favorable conditions for intensification of ETNP denitrification that was also likely mirrored south of the equator. Intermediate water circulation changes likely created the poorly ventilated zones susceptible to the suboxic condition required by denitrification. 2.1 Ma thus marks a critical point for the reorganization of the ocean's nitrogen cycle in which open ocean denitrification became a significant loss term during interglacial phases of climate. Given linkage between the ocean's N and C cycle, ETNP denitrification changes probably contributed to Plio‐Pleistocene climatic changes.