Plio-Pleistocene denitrification in the eastern tropical North Pacific; intensification at 2.1 Ma

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doi: 10.1029/2008GC002044
Author(s): Liu, Zhonghui; Altabet, Mark A.; Herbert, Timothy D.
Author Affiliation(s): Primary:
Brown University, Department of Geological Sciences, Providence, RI, United States
University of Massachusetts at Dartmouth, United States
Volume Title: Geochemistry, Geophysics, Geosystems - G<sup>3</sup>
Source: Geochemistry, Geophysics, Geosystems - G>3`, 9(11). Publisher: American Geophysical Union and The Geochemical Society, United States. ISSN: 1525-2027
Note: In English. Includes appendices. 57 refs.; illus.
Summary: 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 δ 15N 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 δ 15N 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 per mil in δ 15N) 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.
Year of Publication: 2008
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Biogenic processes; California; Cenozoic; Climate change; Continental margin; Denitrification; Dissolved oxygen; East Pacific; Geochemistry; Glaciation; Global change; Hydrochemistry; Isotope ratios; Isotopes; Leg 167; N-15/N-14; Neogene; Nitrogen; North Pacific; Northeast Pacific; ODP Site 1012; Ocean Drilling Program; Ocean circulation; Oxygen; Pacific Ocean; Paleo-oceanography; Paleoclimatology; Pleistocene; Pliocene; Quaternary; Sea water; Solutes; Stable isotopes; Tertiary; Tracers; United States
Coordinates: N321658 N321658 W1182302 W1182302
Record ID: 2009080455
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