The Earth’s oceans lock away atmospheric carbon dioxide, but a ‘leak’ in the Southern Ocean, which surrounds Antarctica, brings the greenhouse gas back into the atmosphere. A team of scientists led by Princeton University and the Max Planck Institute for Chemistry looked at concentrations of nitrogen embedded in deep-sea corals and shelled microorganisms called diatoms and foraminifers to identify an increase in Southern Ocean upwelling during the past 11,000 years, which could explain the mysterious warmth of the Holocene period that allowed human populations to flourish.
Antarctica. Image credit: Robyn M.
“We think we may have found the answer. Increased circulation in the Southern Ocean allowed carbon dioxide to leak into the atmosphere, working to warm the planet,” said Princeton University’s Professor Daniel Sigman, co-author of the study.
For years, researchers have known that growth and sinking of phytoplankton pumps carbon dioxide deep into the ocean, a process often referred to as the ‘biological pump.’
“The biological pump is driven mostly by the low latitude ocean but is undone closer to the poles, where carbon dioxide is vented back to the atmosphere by the rapid exposure of deep waters to the surface. The worst offender is the Southern Ocean,” Professor Sigman said.
Most scientists agree that the Holocene’s warmth was critical to the development of human civilization.
The Holocene was an ‘interglacial period,’ one of the rare intervals of warm climate that have occurred over the ice age cycles of the last million years. The retreat of the glaciers opened a more expansive landscape for humans, and the higher concentrations of carbon dioxide in the atmosphere made for more productive agriculture, which allowed people to reduce their hunter-gathering activities and build permanent settlements.
A map of Antarctica and the surrounding oceans. Image credit: Hogweard / CC BY-SA 3.0.
In order to study the potential causes of the Holocene carbon dioxide rise, Professor Sigman and co-authors investigated three types of fossils from several different areas of the Southern Ocean: diatoms, foraminifers and deep-sea corals.
From the nitrogen isotope ratios of the trace organic matter trapped in the mineral walls of these fossils, the scientists were able to reconstruct the evolution of nutrient concentrations in Southern Ocean surface waters over the past 10,000 years.
“The method we used to analyze the fossils is unique and provides a new way to study past changes in ocean conditions,” said study first author Dr. Anja Studer, a postdoctoral researcher at the Max Planck Institute for Chemistry.
The fossil-bound nitrogen isotope measurements indicate that during the Holocene, increasing amounts of water, rich in nutrients and carbon dioxide, welled up from the deep ocean to the surface of the Southern Ocean.
While the cause for the increased upwelling is not yet clear, the most likely process appears to be a change in the ‘Roaring 40s,’ a belt of eastward-blowing winds that encircle Antarctica.
Because of the enhanced Southern Ocean upwelling, the biological pump weakened over the Holocene, allowing more carbon dioxide to leak from the deep ocean into the atmosphere and thus possibly explaining the rise in atmospheric carbon dioxide.
“This process is allowing some of that deeply stored carbon dioxide to invade back to the atmosphere. We’re essentially punching holes in the membrane of the biological pump,” Professor Sigman said.
The increase in atmospheric carbon dioxide levels over the Holocene worked to counter the tendency for gradual cooling that dominated most previous interglacials.
Thus, the new results suggest that the ocean may have been responsible for the ‘special stability’ of the Holocene climate.
The same processes are at work today: the absorption of carbon by the ocean is slowing the rise in atmospheric carbon dioxide produced by fossil fuel burning, and the upwelling of the Southern Ocean is still allowing some of that carbon dioxide to vent back into the atmosphere.
“If the findings from the Holocene can be used to predict how Southern Ocean upwelling will change in the future, it will improve our ability to forecast changes in atmospheric carbon dioxide and thus in global climate,” Professor Sigman said.
The study was published online this week in the journal Nature Geoscience.
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Anja S. Studer et al. Increased nutrient supply to the Southern Ocean during the Holocene and its implications for the pre-industrial atmospheric CO2 rise. Nature Geoscience, published online July 30, 2018; doi: 10.1038/s41561-018-0191-8
