Paleoceanographic Insights on Recent Oxygen Minimum Zone Expansion: Lessons for Modern Oceanography
New research on sea floor core samples collected from across the planet shows oxygen levels in the world’s oceans plummeted as the last ice age came to an end, a discovery that sheds light on the speed and extent to which modern-day climate change could alter global marine environments with potentially staggering results.
The study, made public this week by UC Davis Bodega Marine Laboratory, suggests expansive low-oxygen zones that characterized world oceans during a period of melting ice sheets 10,000 to 17,000 years ago could be predictive of a trend already underway, potentially leading to mass die-offs of marine species and drastically disrupted food systems.
“The potential for our oceans to look very, very different in 100 to 150 years is real,” lead researcher Sarah Moffitt, a postdoctoral scholar at the marine lab, said in a statement unveiling the work.
The study was based on analyses of 36 sedimentary cores drilled from sites along the continental edges in the Pacific and Indian oceans. The meters-long plugs of sediment serve as records of geochemical and biological changes within ocean layers.
They revealed extreme, rapid oxygen loss in every region, occurring in some cases over 100 years or less but persisting for thousands of years, Moffitt said.
As with the current period of climate change, the deglaciation period was a time of increasing temperatures, surging levels of carbon dioxide in the atmosphere and rising sea levels, Moffitt said.
But “what we’re doing right now is much more extreme and abrupt and bizarre than these past events,” Moffitt said.
In an interview in her lab at the bluff-top research facility near Bodega Head, Moffitt said the study reflects volatility in an ocean environment most people think of as relatively stable and predictable.
The implications are dire for all kinds of sea life, challenging coastal economies and threatening food sources that sustain billions of people on the planet.
Oxygen-depleted areas become “essentially a no-go environment for large swaths of marine biodiversity,” Moffitt said. “None of the shellfish or fisheries that we rely on for our coastal economies can subsist in low-oxygen zones. So when you’re talking about the expansion of low-oxygen zones, you’re also talking about dramatic habitat loss for marine inhabitants … all of our fisheries and shell fisheries, and the charismatic marine mammals that we all are kind of attached to.”
Also striking was the coherency of the data from around the globe, including samples of sea floor stretching from Alaska to Chile, Moffitt said.
“This is a global story,” she said.
Levels of dissolved oxygen — critical to many marine life forms — already are declining in oceans throughout the world, the study said.
Ocean oxygen appears from the research to be highly sensitive — tied to rising temperatures, surging levels of carbon dioxide in the atmosphere and rising sea levels, all modern-day trends that were present during the deglaciation period studied.
Earlier studies have predicted an overall decline of 1 percent to 7 percent in the global stock of ocean oxygen by year 2100. In the same period, the volume of low-oxygen ocean water was predicted to rise by 50 percent, the study said.
Moffitt said a “suite of mechanisms” appear to cause oxygen levels to decline, including complex marine circulation patterns and a food web imbalance nurtured by global warming on the surface that leads to rising oxygen consumption at intermediate ocean layers.
The new research found evidence of oxygen depletion in vertical columns to about 3,000 meters deep, reaching in some areas into more shallow zones just more than 100 meters below the surface, and spreading into the interior ocean, Moffitt said.
Though disheartening, the work is part of a growing body of science that can help inform environmental management and conservation.
“We are not without resources,” Moffitt said.
The study has been published in the journal PLOS ONE and is available on its website.