Applying water masses as a framework to analyze the distribution of redox active compounds in the Pacific Oxygen Deficient Zones and the subsequent implications for low oxygen concentrations across the eastern Pacific Ocean

Zachary Evans, University of Southern California, Los Angeles, United States, Elisabeth Boles, Stanford University, Civil and Environmental Engineering, Stanford, CA, United States, Jarek V Kwiecinski, Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Science, Cambridge, MA, United States, Susan Mullen, University of California Berkeley, Department of Earth and Planetary Science, Berkeley, CA, United States, Martin Johann Johann Wolf, Massachusetts Institute of Technology, Cambridge, MA, United States, Allan Devol, University of Washington, School of Oceanography, Seattle, WA, United States, Rintaro Moriyasu, University of Southern California, Jülich, United States, SungHyun Nam, Seoul National University, School of Earth and Environmental Sciences, Seoul, South Korea, Andrew R Babbin, Massachusetts Institute of Technology, EAPS, Cambridge, United States and James W Moffett, University of Southern California, Los Angeles, CA, United States
Many studies have estimated the geographic drivers and resulting size of the Pacific Oxygen Deficient Zones (ODZs), but the vertical fine structure responsible for the oxygen deficient layers has not been thoroughly investigated. Boundaries such as the “top of the ODZ” and the “core of the ODZ” often rely on indicator compounds, such as oxygen, which is below detection limits, and nitrite, which has other complicating factors besides oxygen. This study applied Optimum Multiparameter Analysis to several transects in the Eastern Tropical North and South Pacific ODZs, and these water mass analyses reveal coherent regions that are consistent between cruises and redox-active compounds. This framework can be applied to oxygen, nitrite, nitrous oxide, iodide, iodate, ferrous iron, sulfide, and methane data to generate insight into the location of the oxygen deficient layer, the fidelity of these redox-active compounds to this layer, and the importance of subsurface mesoscale eddies on the distribution of these compounds.

Results from this study indicate similar and shared water masses between the two ODZs as well. One significant water mass, the 13 ºC water mass, links low oxygen waters across the entire Eastern Pacific Ocean from the Equatorial Undercurrent through the ODZs poleward to Alaska and Chile via their respective undercurrents. This conclusion is consistent with independent water mass analysis in the California Current System (CCS) and Peru-Chile Current System (PCCS) and modeling studies on the eastern Equatorial Pacific. The relative impact of changing the percent of ODZ source waters exported to these ecosystems will be compared against other climate-changed stressors, such as temperature-driven deoxygenation, to generate a more holistic picture of the consequences of deoxygenation on the eastern Pacific Ocean.