Climate: Slower currents during last ice age helped oceans store more carbon

A new study shows how ocean currents play a huge role in the global carbon cycle. @bberwyn photo.
A new study shows how the speed of ocean currents play a huge role in the global carbon cycle. @bberwyn photo.

New study helps explain how carbon flux changes over time

Staff Report

The shells of tiny ocean organisms called foraminifera have once again given climate researchers huge clues about the long-term carbon cycle in the world’s oceans. The information helps show the buildup of heat-trapping greenhouse gases will affect the climate.

A pair of new studies led by University of Cambridge scientists show that cold oceans at the peak of the last ice age about 20,000 years ago, circulated much more slowly, enabling them to store more carbon for longer than modern oceans.

“During the period we’re looking at, large amounts of carbon were likely transported from the surface ocean to the deep ocean by organisms as they died, sunk and dissolved,” said Emma Freeman, the lead author of one of the papers. “This process released the carbon the organisms contained into the deep ocean waters, where it was trapped for thousands of years, due to the very slow circulation.”

Analyzing the foraminifera shells helped the scientists characterize the seawater in the Atlantic Ocean during the last ice age, including its ability to store carbon. During the last ice age, CO2 concentrations in the atmosphere were about 30 percent lower than during the pre-industrial era, before the combustion of fossil fuels started. The researchers wanted to know if the was stored in the deep ocean. Their results are reported in two separate papers in Nature Communications.

Along with forests, land-use and other factors, ocean current regulate atmospheric CO2 levels by conveying heat, nutrients and gases around the globe. Basically, warmer water from near the equator moves north, becoming saltier, colder and denser as they go, causing them to sink to the bottom. These deep waters flow into the ocean basins, eventually ending up in the Southern Ocean or the North Pacific Ocean. A complete loop can take as long as 1,000 years.

The scientists used radiocarbon dating to determine how old the water was in different parts of the ocean, finding that carbon was stored in the slowly-circulating deep ocean. One of the studies, led by Cambridge University’s Jake Howe, analyzed certain isotopes in the shells to reach similar conclusions.

 

“We found that during the peak of the last ice age, the deep Atlantic Ocean was filled not just with southern-sourced waters as previously thought, but with northern-sourced waters as well,” said Howe.

The studies helped tease out nuances in the ocean circulation, showing that a layer of deep water in the Atlantic is a mixture of slowly circulating northern- and southern-sourced waters with a large amount of carbon stored in it.

“Our research looks at a time when the world was much colder than it is now, but it’s still important for understanding the effects of changing ocean circulation,” said Freeman. “We need to understand the dynamics of the ocean in order to know how it can be affected by a changing climate.”

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