Explosive Underwater Volcanoes a Major Feature of Snowball Earth During Cryogenian NeoProterozoic

Around 720-640 million years ago, much of the Earth's surface was covered in ice during a glaciation that lasted millions of years. Explosive underwater volcanoes were a major feature of this 'Snowball Earth', according to new research led by the University of Southampton.

Many aspects of this extreme glaciation remain uncertain, but it is widely thought that the breakup of the supercontinent Rodinia resulted in increased river discharge into the ocean. This changed ocean chemistry and reduced atmospheric CO2 levels, which increased global ice coverage and propelled Earth into severe icehouse conditions.

Because the land surface was then largely covered in ice, continental weathering effectively ceased. This locked the planet into a 'Snowball Earth' state until carbon dioxide released from ongoing volcanic activity warmed the atmosphere sufficiently to rapidly melt the ice cover. This model does not, however, explain one of the most puzzling features of this rapid deglaciation; namely the global formation of hundreds of metres thick deposits known as 'cap carbonates', in warm waters after Snowball Earth events.

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Lead author of the study Dr Tom Gernon, Lecturer in Earth Science at the University of Southampton, said: "When volcanic material is deposited in the oceans it undergoes very rapid and profound chemical alteration that impacts the biogeochemistry of the oceans. We find that many geological and geochemical phenomena associated with Snowball Earth are consistent with extensive submarine volcanism along shallow mid-ocean ridges."

During the breakup of Rodinia, tens of thousands of kilometres of mid-ocean ridge were formed over tens of millions of years. The lava erupted explosively in shallow waters producing large volumes of a glassy pyroclastic rock called hyaloclastite. As these deposits piled up on the sea floor, rapid chemical changes released massive amounts of calcium, magnesium and phosphorus into the ocean.

Dr Gernon explained: "We calculated that, over the course of a Snowball glaciation, this chemical build-up is sufficient to explain the thick cap carbonates formed at the end of the Snowball event.

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Does Undersea Volcanic Activity Affect Climate on 100,000 Year Cycle?

The intensity of volcanic activity at deeply submerged mid-ocean ridges waxes and wanes on a roughly 100,000-year cycle, according to a new study that might help explain poorly understood variations in Earth's climate that occur on approximately the same timetable.

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.

In particular, 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 Earth due to temporarily larger and smaller distances between our planet and the Sun can't fully explain the magnitude of the climatic shifts.

The new research 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 these 100,000-year changes in Earth's orbit and climate. Given that volcanic eruptions release the climate-altering gas carbon-dioxide, significant emissions of the gas might take place during upswings of undersea volcanic activity, potentially affecting the climate at 100,000-year intervals.

"Generally, mid-ocean ridges are thought of as this tiny, not very significant contributor to the carbon cycle and that is true, but that's because they are thought of as a steady-state process. But, if they go through periods of significantly enhanced volcanism and significantly suppressed volcanism, then they may be more important than we thought," said Maya Tolstoy, an associate professor at Lamont-Doherty Earth Observatory at Columbia University in New York and sole author of the new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

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