Deep within the eastern North Pacific Ocean lies the “shadow zone,” a massive reservoir of nearly stagnant seawater trapped between the ocean floor and shallower currents. Oxygen levels in this zone are vanishingly low—less than 5 millimoles per cubic meter, compared to more than 200 millimoles per cubic meter at the ocean surface—and the only organisms that thrive there are anaerobic microbes. A new study traces the sources of oxygen pulses that periodically refresh the shadow zone, offering findings that could help scientists predict how climate change will affect oxygen-depleted seas.
Oxygen deficient zones (ODZs) like the North Pacific shadow zone occupy only 0.35% of the ocean’s volume worldwide. Yet they are growing as ocean oxygen levels drop—a trend linked to global warming that threatens marine ecosystems—and play an important role in global biogeochemical cycles: The anaerobic microbes that live in these zones, for example, use up roughly half the ocean’s nitrogen, producing the greenhouse gas nitrous oxide. In addition, ODZs affect concentrations of trace metals such as dissolved iron in seawater, which phytoplankton like diatoms require to grow. Phytoplankton are the foundation of marine food webs and remove vast quantities of carbon dioxide from the atmosphere.
To study how oxygen-rich water enters the North Pacific shadow zone, Margolskee et al. examined oxygen measurements from Argo floats, a vast array of autonomous sensors that drift the world’s oceans, and from the World Ocean Database. They also used a computational technique called Lagrangian particle tracking to trace specific parcels of oxygen-rich water back to their origins.
The shadow zone receives infusions of oxygen more frequently than previously thought, the team found, with oxygen arriving by several routes. For example, oxygenated water is delivered via eddies spinning off the eastward flowing North Equatorial Countercurrent along the zone’s southern boundary and via deep, narrow flows called the North Equatorial Undercurrent jets along the northwestern boundary.
The computer models used today to simulate global ocean circulation and climate don’t accurately reproduce the fine-scale dynamics of such currents or the frequent intrusions of oxygen observed to occur within ODZs. Nor do they reliably predict how changes in ODZs will alter nitrogen and carbon cycles. By incorporating the sort of high-resolution modeling and particle tracking used in the new study into these models, scientists may be able to more accurately forecast how the North Pacific shadow zone and other ODZs will respond to climate change. (Global Biogeochemical Cycles, https://doi.org/10.1029/2018GB006149, 2019)
—Emily Underwood, Freelance Writer