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Diatoms like this, microscopic plants with silica shells, trapped nitrogen in their shells as they grew. Researchers at Sigman's laboratory at Princeton University were able to extract this tiny amount of nitrogen from countless fossil diatoms and create a model for the Southern Ocean's activity during the summer. Holocene, a period that began about 11,000 years ago. Credit: Anja Studer, Max Planck Institute of Chemistry
The oceans are the planet's largest deposit for atmospheric carbon dioxide at time scales ranging from decades to millennia. But the process of locking greenhouse gases is weakened by the activity of the Southern Ocean, so an increase in its activity could explain the mysterious heat of the past 11,000 years, reports an international team of researchers
. Daniel Sigman, professor of geological and geophysical sciences at Dusenbury, Princeton, says scientists have proposed various hypotheses for this carbon dioxide. increase, but its ultimate cause remained unknown. Now, an international collaboration led by scientists from Princeton and the Max Planck Institute of Chemistry indicates an increase in the upwelling of the Southern Ocean. Their research appears in the current issue of the journal Nature Geoscience .
"We think we may have found the answer," Sigman said. "The increased circulation in the Southern Ocean has allowed carbon dioxide to enter the atmosphere and warm the planet."
Their findings on ocean change may also have implications for predicting how Global warming will affect ocean circulation and the amount of atmospheric carbon dioxide For years, researchers have learned that the growth and sinking of phytoplankton pump carbon dioxide deep into the ocean, a process often called "biological pump ". the low latitude ocean, but is defeated closer to the poles, where carbon dioxide is released into the atmosphere by the rapid exposure of deep water to the surface, Sigman said. The worst offender is the Southern Ocean, which surrounds Antarctica. "We often view the Southern Ocean as a leak in the biological pump," said Sigman.
Sigman and his colleagues discovered that an increase in the upwelling of the Southern Ocean could be responsible for stabilizing the climate of the Holocene. reaching more than 10,000 years before the industrial revolution.
Most scientists agree that the heat of the Holocene was essential to the development of human civilization. The Holocene was an "interglacial period", one of the few warm climate intervals that has occurred during glacial cycles of the last million years. The retreat of glaciers has opened up a wider landscape for humans, and higher concentrations of carbon dioxide in the atmosphere have allowed for more productive agriculture, which has allowed people to reduce their harvesting and building activities. permanent colonies.
Sigman Lab at Princeton University has extracted traces of fossil nitrogen to create a model of the Southern Ocean's activity during the Holocene, a hot period which began about 11,000 years ago, during which agriculture and human civilization flourished. The fossils studied included (left): the planktonic foraminifer Globigerina bulloides, a centered diatom and the deep coral Desmophyllum dianthus. Credit: From left to right: Ralf Schiebel, Max Planck Institute of Chemistry; Anja Studer, Max Planck Institute of Chemistry; Dann Blackwood, United States Geological Survey
The Holocene differed from other interglacial periods in many ways, say the researchers. On the one hand, its climate was unusually stable, with no major cooling trend typical of other interglacials. Second, the concentration of carbon dioxide in the atmosphere increased to about 20 parts per million (ppm), from 260 ppm in the early Holocene to 280 ppm at the end of the year. Holocene, while carbon dioxide was generally stable or declining. ] For comparison, from the beginning of industrialization to the present, the concentration of carbon dioxide in the atmosphere has increased from 280 to over 400 ppm following the burning of fossil fuels .
"In this context, the Holocene may seem small," said Sigman. "However, scientists believe that this small but significant increase has played a key role in preventing the progressive cooling of the body. Holocene, which facilitated the development of complex human civilizations. "
To investigate the potential causes of carbon dioxide Researchers studied three types of fossils from different regions of the Southern Ocean: diatoms and foraminifera, both shell microorganisms found in the oceans, and deep-sea corals.
Isotopic reports of nitrogen in the mineral walls of these fossils, scientists have been able to reconstruct the Evolution of nutrient concentrations in surface waters of the Southern Ocean over the last 10,000 years
"The method used to badyze fossils st unique. study past changes in ocean conditions, "says Anja Studer, first author of the study, who conducted the research while a graduate student Sigman's Lab
The isotope measurements of the # Nitrogen-related fossils indicate that during the Holocene, increasing amounts of water, rich in nutrients and carbon dioxide, rise from the depths to the surface of the sea. Southern Ocean. Although the cause of the increase in upwelling is not yet clear, the most likely process seems to be a change in the "Roaring 40", a belt of winds blowing towards the east. surround Antarctica.
The biological pump was weakened during the Holocene, which allowed a greater amount of carbon dioxide to escape from the oceanic depths into the atmosphere, which perhaps explains the increase of 20 ppm of atmospheric carbon dioxide. invade back into the atmosphere, "Sigman said. "We are basically punch holes in the membrane of the biological pump."
The increase in levels of atmospheric carbon dioxide during the Holocene worked to counter the trend of progressive cooling that dominated most of the previous interglacials. Thus, the new findings 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 slows the increase in atmospheric carbon dioxide The rise of the ocean Southern Ocean still allows some of this carbon dioxide to fall back into the atmosphere. "If the results of the Holocene can be used to predict how the upwelling of the Southern Ocean will change in the atmosphere, in the future, it will improve our ability to predict changes in atmospheric carbon dioxide and therefore in global climate, "said Sigman.
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