Cyclical variations in Earth’s tilt and orbit – occurring at 23,000-, 41,000- and 100,000-year intervals – are known to strongly influence our planet’s long-term climate; they are associated with the coming and going of ice ages that also takes place about every 100,000 years. A new study published in the journal Geophysical Research Letters has revealed that the intensity of volcanic activity at deeply submerged mid-ocean ridges waxes and wanes on approximately the same timetable.
Eruption of a deep-sea volcano. Image credit: University of Washington.
Changes in the roundness of Earth’s orbit around the Sun unfold on approximately the same 100,000 year cycle as the planet’s global swings between icy and temperate conditions.
But, the variation in solar radiation reaching our planet due to temporarily larger and smaller distances between Earth and the Sun can’t fully explain the magnitude of the climatic shifts.
The new study finds evidence in the profile of sea-floor elevation that volcanic activity at mid-ocean ridges, where molten rock emerges from Earth’s interior and creates new planetary crust, coincides with periodic changes in Earth’s orbit and climate.
Researchers have already speculated that volcanic cycles on land emitting large amounts of carbon dioxide might influence climate; but up to now there was no evidence from underwater volcanoes.
“People have ignored seafloor volcanoes on the idea that their influence is small – but that’s because they are assumed to be in a steady state, which they’re not,” said Dr Maya Tolstoy of Columbia University’s Lamont-Doherty Earth Observatory, who is the author of the study.
“They respond to both very large forces, and to very small ones, and that tells us that we need to look at them much more closely.”
Volcanically active mid-ocean ridges crisscross Earth’s seafloors like stitching on a baseball, stretching some 60,000 km.
They are the growing edges of giant tectonic plates; as lavas push out, they form new areas of seafloor, which comprise some 80 % of the planet’s crust. Conventional wisdom holds that they erupt at a fairly constant rate – but Dr Tolstoy finds that the ridges are actually now in a languid phase. Even at that, they produce maybe 8 times more lava annually than land volcanoes.
Due to the chemistry of their magmas, the carbon dioxide they are thought to emit is currently about the same as, or perhaps a little less than, from land volcanoes – about 88 million metric tons a year.
“But were the undersea chains to stir even a little bit more, their carbon dioxide output would shoot up,” Dr Tolstoy said.
Some scientists think volcanoes may act in concert with Milankovitch cycles – repeating changes in the shape of Earth’s solar orbit, and the tilt and direction of its axis – to produce suddenly seesawing hot and cold periods.
The major one is a 100,000-year cycle in which the planet’s orbit around the Sun changes from more or less an annual circle into an ellipse that annually brings it closer or farther from the Sun.
Recent ice ages seem to build up through most of the cycle; but then things suddenly warm back up near the orbit’s peak eccentricity. The causes are not clear.
Researchers have suggested that as icecaps build on land, pressure on underlying volcanoes also builds, and eruptions are suppressed. But when warming somehow starts and the ice begins melting, pressure lets up, and eruptions surge.
They belch carbon dioxide that produces more warming, which melts more ice, which creates a self-feeding effect that tips the planet suddenly into a warm period.
A recent study has found that land volcanoes worldwide indeed surged 6 to 8 times over background levels during the most recent deglaciation, 12,000 to 7,000 years ago.
The corollary would be that undersea volcanoes do the opposite: as Earth cools, sea levels may drop 100 meters, because so much water gets locked into ice. This relieves pressure on submarine volcanoes, and they erupt more. At some point, could the increased carbon dioxide from undersea eruptions start the warming that melts the ice covering volcanoes on land? That has been a mystery, partly because undersea eruptions are almost impossible to observe.
In the new study, Dr Tolstoy studied 10 present-day eruptions at mid-ocean ridges and the topography around one mid-ocean ridge where ancient volcanic eruptions had left traces.
Because newly formed crust pushes outward from those ridges at a regular pace as it forms, the elevation of the sea floor, representing the buildup of new rock, provides a temporal record of how much outpouring of magma took place when.
From the recent eruptions and seismic studies of them, Dr Tolstoy finds that volcanic activity intensified when crust-compressing gravitational forces due to the Moon and Sun were low, underscoring that eruption rates are sensitive to forces of this type, which are also at play as the shape of Earth’s orbit gradually changes.
As for the ancient eruptions and the topographic pattern they left behind, she finds that there were fewer or smaller eruptions coinciding with periods when there was less carbon dioxide in the atmosphere and when Earth’s orbit was closer to being a perfect circle. Also, there were more eruptions during periods of more carbon dioxide in the atmosphere and Earth’s orbit being more out-of-round.
Dr Tolstoy’s analysis also shows a correspondence between periods of low sea level and high volcanism and the other way around.
The findings suggest that changes in both sea level and the shape of Earth’s orbit may influence the rate of eruption at these mid-ocean ridges. These changes in seafloor volcanic activity may, in turn, feed back into climate cycles, possibly contributing to glacial cycles, the abrupt end of ice ages and the dominance of the 100,000-year climate cycle.
This study does not put a number on how much carbon dioxide could be released by these undersea eruptions.
However, the study suggests that these volcanic eruptions could be acting as a climatic valve that causes the flow of greenhouse gases to fluctuate, and that models of the Earth’s climate system might be rendered more accurate by including them.
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Maya Tolstoy. Mid-ocean ridge eruptions as a climate valve. Geophysical Research Letters, published online February 05, 2015; doi: 10.1002/2014GL063015
