Expansion of denitrification and anoxia in the eastern tropical North Pacific from 1972 to 2012

Rachel E.A. Horak, Wendi Ruef, Bettie Ward, Allan H. Devol

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32 Scopus citations


The eastern tropical North Pacific (ETNP) is a large region of anoxic water that hosts widespread water column N loss (denitrification). There is some disagreement about the long-term trends of denitrification and anoxia and long-term studies of water column denitrification within the anoxic zone are lacking. In this study, we compared ETNP water column nitrite, N*, and O2 data along the same transect for four studies ranging from 1972 to 2012. Anoxic water volume increased, and low-oxygen conditions expanded into shallower isopycnals from 1972 to 2012. A geochemical marker for cumulative N loss indicates that denitrification was highest in 2012 and the upper oxygen-deficient zone (ODZ) experienced the most change. Oxygen and N loss changes in the world's largest ODZ for 2012 could not be explained by the Pacific Decadal Oscillation, and decreased O2 in supply currents and increased wind-driven upwelling are likely mechanisms contributing to increased N loss and anoxia.

Original languageEnglish (US)
Pages (from-to)5252-5260
Number of pages9
JournalGeophysical Research Letters
Issue number10
StatePublished - 2016

All Science Journal Classification (ASJC) codes

  • Geophysics
  • General Earth and Planetary Sciences


  • anoxia
  • denitrification
  • deoxygenation
  • nitrogen cycle
  • oxygen-deficient zone


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