13 January 2016
by Alison F. Takemura
For thousands of years during the last glacial period, Earth’s land and sea stored carbon as both dissolved carbon dioxide and biomass. But as the ice receded, water warmed and organisms decayed, that carbon surged into the atmosphere. Most of the released gas came from the atmosphere originally, but in a new study, a data anomaly hints that a small percentage of it came from volcanoes erupting on the ocean floor.
“We were stunned,” said Thomas A. Ronge, a marine geologist at the Alfred-Wegener-Institute Helmholtz Centre for Polar and Marine Research in Bremerhaven, Germany, lead author of the study. “Many of us never thought that plate tectonics might react to glacial cycles.”
When the glaciers retreated, about 18,000 to 14,000 years ago, carbon dioxide concentrations in the atmosphere rose 37 percent, according to previous research. Scientists weren’t certain how the carbon had been distributed between marine and terrestrial ecosystems during the glacial period, but suspected that the ocean had sequestered a considerable amount, according to Ronge.
The deep ocean can hold vast amounts of carbon dioxide, Ronge explained: up to 60 times more than the atmosphere. Water at the ocean’s surface soaks in the gas directly, while marine organisms also incorporate it into their biomass and shells. Over thousands of years, the surface waters cool and sink, storing dissolved gas in depths below. As organisms die, a fraction drift downward and bring the carbon to deeper waters. But eventually, water masses resurface, releasing the carbon dioxide they’ve held.
To study how carbon dioxide stored in the ocean might have contributed to the sudden carbon surge during deglaciation, Ronge and his colleagues sailed to the South Pacific Ocean, a site scientists believe has significant sway over climate on deglacial time scales.
At the New Zealand Margin and the East Pacific Rise, the team used radiocarbon dating to trace how often water masses released stored carbon dioxide for the last 30,000 years. The team dated carbon in the ocean by analyzing carbon-14 isotopes in shells of long-dead single-celled organisms called foraminifera. Carbon-14 is created when cosmic rays interact with nitrogen in the upper atmosphere. A fraction of the carbon dioxide that dissolves in the ocean, called inorganic carbon, will carry the carbon-14 label.
When the foraminifera were alive, they incorporated labelled and unlabelled inorganic carbon into their shells, Ronge said, so the shells show how much carbon-14 was in the water at the time they were formed. Measuring how much carbon-14 in the shell has radioactively decayed to nitrogen-14 allows scientists to estimate the time since the gas came from the atmosphere: the less shell carbon-14, the longer its time in the ocean.
By calculating the difference in carbon-14 concentrations between two species of foraminifera— one that floats near the surface and another that lives on the ocean floor sediment — Ronge and his team could determine the “ventilation age” of the water, or how long ago the deep water was in contact with the atmosphere. The less carbon-14 in the sediment foraminifera relative to surface foraminifera, then the longer the deep water had remained submerged.
The researchers found that deep water appeared to have gone 8,000 years without contact with the atmosphere. But water that old is anoxic — depleted of dissolved oxygen. These foraminifera can’t survive in these conditions, but the presence of their shells showed they had thrived. On its face, the science contradicted itself, Ronge said.
According to the researchers, an alternative process must have created the conditions needed for the foraminifera to grow that would also account for the unlikely old age of the deep waters. Evidence of volcanic activity at the East Pacific Rise led them to a likely explanation: eruptions of underwater volcanoes.
During glacial periods, ice sheets sitting on land trap water, causing global sea levels to fall. Less water reduces pressure on the earth’s crust, which can cause underwater volcanoes to erupt, according to previous unaffiliated work, Ronge said. These underwater volcanoes spew carbon dioxide free of the carbon-14 isotope into the water. This process would have diluted the concentration of carbon-14 and artificially inflated the water’s age, he explained.
Taking potential volcanic activity into account corrected the water’s age to a more reasonable 2,700 years or younger, according to Ronge. That finding suggests underwater volcanism might have contributed to the deglacial ballooning in carbon dioxide, he said, presenting the group’s work at the 2015 American Geophysical Union Fall Meeting.
But he notes that his group only sampled a small part of the South Pacific, so they can’t yet say how much the results apply to the history of the ocean globally. Wary of misinterpretation by climate change deniers, Ronge and his colleagues are also careful to point out that neither oceanic outgassing nor volcanoes caused the current rise in carbon dioxide.
The team will be embarking in February to collect more data from the Drake Passage to the Antarctic Peninsula. Ronge also hopes to explore the Indian, Atlantic, and northern Pacific oceans to trace their carbon histories.
– Alison F. Takemura is a science communication graduate student at UC Santa Cruz. You can follow her on twitter @AlisonTakemura.