CORVALLIS, Ore. – Every year, blooms of the plankton that feed the marine food chain in the Pacific Ocean off the Oregon coast draw enormous quantities of carbon dioxide out of the atmosphere, annually negating the effects of approximately 100 million tanks of gasoline feeding greenhouse gas discharges into the air.
But when these carbon-fed marine organisms die and sink to the bottom, it isn’t clear whether their remains are transported out to the deep ocean, or if they decompose locally, consuming oxygen and releasing carbon dioxide into coastal waters, where it may soon return to the atmosphere.
The National Science Foundation would like to find out, and just awarded a $2.8 million grant to Oregon State University to lead a multi-institution research effort to track that carbon.
Discovering what happens to the huge amounts of carbon – an estimated 2 megatons a year off the Oregon coast alone – is critical to understanding the interface between the atmosphere and the open ocean that influences marine dead zones, atmospheric pollution and ultimately climate change.
“If the dead organisms are transported to the deep ocean, which we think they are, the carbon dioxide that is released as they decompose will stay there for as long as a thousand years,” said Burke Hales, an associate professor of oceanic and atmospheric sciences at OSU and principal investigator on the NSF study. “If they get decomposed on the shelf, and the carbon dioxide just returns to the atmosphere, we don’t have the annual net ‘carbon sink’ that we think we do.
“It’s a difference in time scale,” Hales added. “Eventually, carbon works its way back to the atmosphere but if it is cycled back immediately, the biological pump in the ocean has to work a lot harder.”
In their three-year study, five OSU researchers will team with colleagues from the University of Chicago, the University of Washington and Columbia University to trace carbon generated from decomposing marine life and follow it either to the deep ocean, or back to the atmosphere. The technology needed to trace microscopic dead plankton in the Pacific Ocean, Hales says, is daunting.
The researchers must inject tracers – an inert gas and dye – into the water containing the dead organisms and use sensors and pumps towed behind a ship to get detailed measurements of the tracers, carbon and other important dissolved gases and nutrients. The dye tracer will help track the movement of the water; the gas tracer will help determine if the carbon dioxide has escaped back into the atmosphere.
Most of their work will be off the central Oregon coast, between Cascade Head and Cape Perpetua, because of the researchers’ knowledge of ocean currents and undersea terrain.
The OSU researchers’ ability to even attempt such a project is possible because of their recently completed study of ties between net oxygen production and net carbon production in coastal waters. These two production rates are tied together because during photosynthesis, phytoplankton produce oxygen as they convert carbon dioxide and upwelled nutrients into organic matter.
Led by Hales, the study was the first comprehensive analysis of the coast-wide net exchange between the huge oxygen production off the Oregon coast during the summer upwelling and subsequent phytoplankton blooms, and the amount of organic carbon produced by the growth of those plants.
Past studies that looked at oxygen-carbon ties looked only at ocean productivity at the surface waters, but in a multi-year effort, OSU researchers examined 95 percent of the water column in a series of measurements ranging from the near-shore to the edge of the continental shelf.
“We were able to account for all of the oxygen in the system, but when we measured for organic carbon, there just wasn’t as much there as there should have been,” Hales said. “Our carbon measurements were the same in August as they were in May, and with the extreme upwelling and high oxygen production, they should have been ten-fold higher. So the question is, where did the carbon go?
“It wasn’t respired, or we would have measured that,” he pointed out. “Other measurements of burial in the sediment show it can’t have gone there. It didn’t move up and down the coastline because we measured the movement of the oxygen and organic carbon in the currents off the coast.
“So it either went back into the atmosphere after the summer upwelling season, or it was transported off the shelf and into the deep ocean, which drops off to depths of thousands of meters off the Oregon coast,” Hales said. “Our money is on the deep ocean.”
Hales said he thinks the process that transports the carbon to the deep ocean occasionally shuts down, causing marine die-offs, or dead zones, which have plagued the central Oregon coast for five consecutive summers. Summer upwelling is caused by northern winds that cause deep, nutrient-rich water to move up from below and mix into the surface waters.
Occasional “relaxation” events, during which summertime upwelling-favorable north winds quiet – or even reverse direction – help keep the system from becoming choked, he added.
“It’s like garbage-collection day,” Hales said. “The dead phytoplankton carbon piles up in the near-bottom waters for several days, and then once a week or so, it gets taken out from the near-shore to the edge of the continental shelf and dumped into the deep ocean.”
Those relaxation events didn’t take place with their normal frequency in 2002 and 2006, when the hypoxia was at its worst, Hales pointed out. And though these “dead zones” have received a great deal of attention, they would be much worse without some kind of a mechanism for taking carbon out of the near-shore system.
“If the carbon were allowed to sink to the seafloor and accumulate on the shelf, it would literally suck the oxygen out of the water across the entire shelf,” Hales said. “You wouldn’t see dead and dying fish, because they’d never come into those waters to start with. The whole system would be oxygen-starved below the surface every summer.”