Oceans may be acidifying faster today than in the last 300 million years. Credit: NOAA

The
oceans may be acidifying faster today than they did in the last 300
million years, according to scientists publishing a paper recently in
the journal Science.

“What
we’re doing today really stands out in the geologic record,” says lead
author Bärbel Hönisch, a paleoceanographer at Columbia University’s
Lamont-Doherty Earth Observatory.

 

“We
know that life during past ocean acidification events was not wiped
out—new species evolved to replace those that died off. But if
industrial carbon emissions continue at the current pace, we may lose
organisms we care about—coral reefs, oysters, salmon.”

The oceans act like a sponge to draw down excess carbon dioxide from the air.

The gas reacts with seawater to form carbonic acid, which over time is neutralized by fossil carbonate shells on the seafloor.

If
too much carbon dioxide enters the ocean too quickly, it can deplete
the carbonate ions that corals, mollusks and some plankton need for reef
and shell-building.

In
a review of hundreds of paleoceanographic studies, the researchers
found evidence for only one period in the last 300 million years when
the oceans changed as fast as today: the Paleocene-Eocene Thermal
Maximum, or PETM.

In ocean sediment cores, the PETM appears as a brown mud layer flanked by thick deposits of white plankton fossils.

About 56 million years ago, a mysterious surge of carbon into the atmosphere warmed the planet and turned the oceans corrosive.

In
about 5,000 years, atmospheric carbon doubled to 1,800 parts per
million (ppm), and average global temperatures rose by about 6 C.

The
carbonate plankton shells littering the seafloor dissolved, leaving the
brown clay layer that scientists see in sediment cores today.

As
many as half of all species of benthic foraminifera, a group of
one-celled organisms that live at the ocean bottom, went extinct,
suggesting that deep-sea organisms higher on the food chain may have
also disappeared, said paper co-author Ellen Thomas, a paleoceanographer
at Yale University.

“It’s really unusual that you lose more than 5 to 10% of species,” she said.

Scientists
estimate that ocean acidity—its pH—may have fallen as much as 0.45
units as the planet vented stores of carbon into the air.

“These
scientists have synthesized and evaluated evidence far back in Earth’s
history,” said Candace Major, program officer in the National Science
Foundation’s (NSF) Division of Ocean Sciences, which funded the
research.

“The
ocean acidification we’re seeing today is unprecedented,” said Major,
“even when viewed through the lens of the past 300 million years, a
result of the very fast rates at which we’re changing the chemistry of
the atmosphere and oceans.”

In
the last hundred years, rising carbon dioxide from human activities has
lowered ocean pH by 0.1 unit, an acidification rate at least 10 times
faster than 56 million years ago, says Hönisch.

The
Intergovernmental Panel on Climate Change (IPCC) predicts that pH will
fall another 0.2 units by 2100, raising the possibility that we may soon
see ocean changes similar to those observed during the PETM.

More catastrophic events have happened on Earth before, but perhaps not as quickly.

The
study finds two other analogs for modern day ocean acidification—the
extinctions triggered by massive volcanism at the end of the Permian and
Triassic eras, about 252 million and 201 million years ago,
respectively.

But
the authors caution that because ocean sediments older than 180 million
years have been recycled back into the deep Earth, scientists have
fewer records to work with.

During the “Great Dying” at the end of the Permian, about 252 million years ago, about 96 percent of life disappeared.

Massive
eruptions from what is known as the Siberian Traps in present-day
Russia are thought to have triggered earth’s biggest extinction.

Over 20,000 years or more, carbon in the atmosphere rose dramatically.

Scientists
have found evidence for ocean dead zones, and preferential survival of
organisms predisposed to carbonate-poor seawater and high blood-carbon
levels, but so far they have been unable to reconstruct changes in ocean
pH or carbonate.

At
the end of the Triassic, about 201 million years ago, a second burst of
mass volcanism associated with the break-up of the supercontinent
Pangaea doubled atmospheric carbon and touched off another wave of
die-offs.

Coral reefs collapsed and an entire class of sea creatures, the eel-like conodonts, vanished.

On land, large plant-eating animals gave rise to meat-eating dinosaurs like Tyrannosaurus rex as the Jurassic era began.

A
greater extinction of tropical species has led some scientists to
question whether global warming rather than ocean acidification was the
main killer at this time.

This
study finds that the most notorious of all extinctions, the one that
ended the Age of Dinosaurs with a falling asteroid 65 million years ago,
may not have been associated with ocean acidification.

The
asteroid impact in present-day Mexico 65 million years ago released
toxic gases and possibly set off fires that sent surges of carbon into
the air.

Though many species of plankton went extinct, coral reefs and benthic foraminifera survived.

In
lab experiments, scientists have tried to simulate modern ocean
acidification, but the number of variables currently at play—high carbon
dioxide and warmer temperatures, and reduced ocean pH and dissolved
oxygen levels—make predictions difficult.

An
alternative to investigating the paleo-record has been to study natural
carbon seeps from offshore volcanoes that are producing the
acidification levels expected by the year 2100.

In
a recent study of coral reefs off Papua New Guinea, scientists found
that during long-term exposure to high carbon dioxide and pH 0.2 units
lower than today—at a pH of 7.8 (the IPCC projection for 2100)—reef
biodiversity and regeneration suffered.

SOURCE