Over geologic time, Earth’s atmosphere has changed. The carbon dioxide concentration of the atmosphere has cycled through periods of higher and lower concentration, which is correlated with variations in Earth’s climate. This variation, occurring over multi-million year timescales, is partially the result of changes in volcanic activity, with more or less CO2 transferred between Earth’s deep mantle and the surface carbon system.
As the largest reservoir of carbon in Earth (the solid Earth contains about 10,000 times more carbon than the atmosphere and oceans), deep carbon is a key factor in controlling this long-term climate change. But what causes volcanic degassing to increase or decrease?
A new paper from Jonathan Burley and Richard Katz at the University of Oxford, UK, outlines a new model for carbon flux at mid-ocean ridges, and implicates changing sea level as an important driver of volcanic CO2 degassing .
As Earth’s glaciers melt, they return water to the ocean, raising sea level. This increase in ocean volume increases pressure on Earth’s crust at mid-ocean ridges. Beneath the ridge, mantle melting is determined by pressure. If the pressure applied by the ocean increases, the depth at which the mantle begins melting becomes shallower. This impacts mantle melt chemistry by changing the way certain elements are distributed in solid and liquid phases. Ultimately, this results in changes in volatile degassing during ridge formation.
Burley and Katz constructed a model for CO2 degassing at mid-ocean ridges based on the principles governing silicate mantle melting and melt transport. They subjected their model to various hypothetical and historical situations to ask how changes in sea level might alter atmospheric CO2 concentrations, and over what time scales this might occur.
They calculated that after a sudden drop in sea level, mid-ocean ridge CO2 emissions remain constant for 60,000-120,000 years, but this is followed by a sharp rise in CO2 emissions, with emissions then remaining elevated for tens of thousands of years. Using a time-series of reconstructed global sea level over the last million years they again saw a lag on the order of 100,000 years between sea level decreases and increased CO2 emissions, with global mid-ocean ridge CO2 emissions varying by up to 12%.
“This is the first work to quantify glacial cycles’ effect on carbon fluxes from the mantle into the surface carbon system,” said Burley. “We calculate that these global variations in mid-ocean ridge carbon flux are up to ~12% and last for tens of thousands of years.”
Future work will elucidate how these changes in mid-ocean ridge degassing, along with degassing at arc volcanoes, translate to climate changes on Earth over millions of years.
This work was funded by the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013) / ERC grant agreement number 279925.
Image: Lava from Kilauea volcano, Hawaii, flowing into the ocean. Credit: Alexandre Socci/Green Pixel