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AGU Chapman: Could seafloor vents control atmospheric CO2?

050104114942.jpgAs the Earth has alternated between glacial and inter-glacial periods, the steep climatic ups and downs have gone hand in hand with changes in atmospheric carbon dioxide levels. But where was the CO2 going to and coming from? Scientists have pointed to the ocean – currently a vast sponge for the greenhouse gas.

But a talk at the AGU Chapman Conference today by palaeoclimatologist Lowell Stott of the University of Southern California suggests a radically different reservoir: pools of liquid carbon dioxide trapped in seafloor hydrothermal vents.

These pools were spotted in the mid 2000s unleashing bubbles of liquid CO2 from the Okinawa trough in the Pacific Ocean (see the video here).

The CO2 pools form when one oceanic plate buckles under another and carbonates in the sediment break down under the intense heat. Perforations around underwater volcanic vents can allow CO2 droplets to escape and bubble up to the surface, but where the seawater is cold enough it effectively freezes the CO2 into a solid, or hydrate, form that acts as a lid. NOAA has a further explanation and diagram.

Stott points out that the carbon isotope signatures in some mid-latitude ocean sediment don’t tally with the conventional view of carbon entering the ocean system via photosynthesizing algae. The chemistry of the sea-vent carbon is a much better match, he argues. What’s more, unpublished work by Stott and colleagues shows that past changes in deep sea temperatures around the vents would have been sufficient to destabilize hydrate caps and thus modulate the vents’ release of CO2 in time with the rising and falling atmospheric CO2 concentrations.

The idea is speculative at this stage. For one thing, the researchers still need to find out where all the vents are and how much CO2 they are releasing. But if the amount proves reasonable, underwater volcanoes could be a dark-horse candidate for a controlling influence on past atmospheric CO2 levels.

If you’re wondering whether carbon dioxide hydrates could be as treacherous as methane hydrates, I should pass on Stott’s warning that the mechanisms involved are different. The workings of carbon dioxide vents are still largely unknown, but the type of deep-sea warming that could destabilize CO2 hydrates isn’t what most experts think will happen with future climate change, he says.

Any extra carbon that does get vented as a result of climate change, he adds, would be dwarfed by anthropogenic emissions. Whatever their future influence, though, it would be remarkable if these newly discovered gas sources turned out to be a major player in climatic history.

Anna Barnett

Image: NOAA


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