At the end of the last Ice Age, atmospheric carbon dioxide
levels rose rapidly as the planet warmed; scientists have long hypothesized
that the source was carbon dioxide released from the deep ocean.
But a new study using detailed radiocarbon dating of
foraminifera found in a sediment core from the Gorda Ridge off Oregon reveals that the
Northeast Pacific was not an important reservoir of carbon during glacial
times. The finding may send scientists back to the proverbial drawing board
looking for other potential sources of carbon dioxide during glacial periods.
The study, which was supported by the National Science
Foundation and the University
of Michigan, was published
online in Nature Geoscience.
“Frankly, we’re kind of baffled by the whole thing,” says Alan
Mix, a professor of oceanography at Oregon
and an author on the study. “The deep North Pacific was such an obvious source
for the carbon, but it just doesn’t match up. At least we’ve shown where the
carbon wasn’t; now we just have to find out where it was.”
During times of glaciation, global climate was cooler and
atmospheric carbon dioxide was lower. Humans didn’t cause that carbon dioxide
change, so it implies that the carbon was absorbed by another reservoir. One
obvious place to look for the missing carbon is the ocean, where more than 90%
of the Earth’s readily exchangeable carbon is stored.
The Pacific Ocean is the
largest ocean by volume. The deep water mass longest isolated from the
atmosphere and most enriched in carbon is found today in the Northeast Pacific,
so the researchers focused their efforts there. They hypothesized that the
ventilation age in this basin—or the amount of time since deep water was last in
contact with the atmosphere—would be older during glacial times, allowing
carbon dioxide to accumulate in the abyss.
“We were surprised to find that during the last ice age, the
deep Northeast Pacific had a similar ventilation age to today, indicating it
was an unlikely place to hide the missing carbon,” says David Lund, a
paleoceanographer at the University of
Michigan, formerly at Oregon State,
and lead author on the Nature Geosciences paper.
“This indicates that the deep Pacific was not an important
sink of carbon during glacial times,” Lund
adds. “Even more intriguing is that we found the ventilation age increased
during the deglaciation, at the exact time that atmospheric carbon dioxide
levels were rising.”
The researchers reconstructed the ventilation history of the
deep North Pacific, examining the sediments at a site about 75 miles off the
coast of southwestern Oregon.
There the water is more than a mile-and-a-half deep and is known as the oldest
water mass in the modern oceans, Mix says. By radiocarbon dating both the
planktonic, or surface-dwelling, and benthic (seafloor-dwelling) foraminifera,
the scientists can determine whether the isotopic signatures of the
foraminifera match “values predicted by the assumption of oceanic control of
The organisms that lived on the seafloor have older “apparent” radiocarbon ages than the organisms that lived at the sea surface,
Mix says, even though both come from the same sediment sample and are of the
same true age. The radiocarbon dating was performed using an advance particle
accelerator by the authors’ colleague, John Southon of the University of California
“Different sources of carbon dioxide have different apparent
ages, depending on how long they have been isolated from the atmosphere,” Mix
says. “We use these dates as kind of a ‘return address label’ rather than to
establish precise ages of the events. The bottom line is that the deep North
Pacific wasn’t the source of rising carbon dioxide at the end of the last ice
The study is important not just in tracing climatic history,
scientists say, but in forecasting how the Earth may respond to future climate
change. The Earth “breathes carbon in and out,” Mix says, inhaling carbon into
sediment and soils, while exhaling it via volcanism and a slow exchange between
the oceans, soils, and plant life with the atmosphere.
When everything is in balance, the Earth is said to be in a “steady state.” But on numerous occasions in the past, the carbon balance has
shifted out of whack.
“Because the ocean is such a huge repository of carbon, a
relatively small change in the oceans can have a major impact,” Mix says. “We
know ocean circulation changed during the ice ages and that is why many
scientists assumed the deep Pacific Ocean was
the source for rising carbon dioxide levels during the last deglaciation.”
says it “is conceivable that we are misunderstanding the radiocarbon signal by
assuming it is controlled by ocean mixing.”
“These are volcanically active regions, so the input of
carbon from volcanoes, which lacks radiocarbon because of its great age, needs
to be looked at,” Lund
points out. “But it is premature to draw any conclusions.”
The researchers’ next step will be to look for chemical
traces of volcanic influence.
Another source of carbon could be from land, though the
authors say it would be difficult to account for the magnitude of atmospheric
carbon increase and the apparent radiocarbon age of released carbon by
pre-industrial terrestrial sources alone.
“If we can better understand how carbon has moved through
the Earth’s systems in the past, and how this relates to climate change, we
will better predict how the carbon we are now adding to the atmosphere will
move in the future,” Mix says.