Ancient marine fossils, like the tiny shells of foraminifera, helped scientists trace shifting CO2 and pH levels in Earth's oceans. Photo by Erik Thomsen/Tine Rasmussen/CAGE

March 27 (UPI) -- Today, Earth's oceans absorb excess carbon dioxide from the atmosphere. Research shows, in the past, the opposite was true.

In recent decades, scientists estimate the Arctic and Nordic seas absorbed as much as 40 percent of CO2 emissions. But the ocean hasn't always been a carbon repository. At times, it has been primary provider.


Analysis of sediment cores suggests Earth's northern seas pumped significant amounts of CO2 into the atmosphere in the past.

"Our research shows that areas in Norwegian Sea have changed their function on several occasions through the past 135,000 years," Mohamed Ezat, a geoscientist at the Arctic University of Norway, said in a news release. "Instead of absorbing CO2 from the air, they released more of the greenhouse gas into it."

Researchers were able to track changes in ocean pH and CO2 emission rates by analyzing the ratios of boron isotopes in the fossil shells of foraminifera, a single-celled marine organism. The foraminifera fossils were sourced from sediment cores representing the Norwegian Sea's biochemical history spanning the last 135,000 years.

"We saw that at the end of several of the severe cooling periods in the region, so-called Heinrich events, the ocean became more acidic and later released CO2 into the atmosphere," Ezat explained. "These episodes of CO2 pumping from the Nordic Seas coincide with times of increase in atmospheric CO2."

Analysis suggests an influx of terrestrial organic matter, as well as shifts in marine biology and ocean currents, influenced fluctuations in CO2 and pH levels near the ocean's surface. The findings were published in the journal Nature Communications.

The research also showed modern changes in CO2 and pH levels are unnatural.

"Results of our study actually show that the sea surface pH throughout the last 135,000 years has never been as low as today in our study area," Ezat said. "This is not an unexpected result. It is similar to previous studies conducted in other ocean areas. It does however add a body of evidence to the hypothesis that human activity is profoundly affecting the chemistry of our oceans."