Cold seeps are hot spots for life
Thousands of white crabs grazing on an extensive mussel bed: Up to now such high biomasses in the deep sea were only known from hot vents. Now scientists from the MARUM at the University of Bremen have found such scenes at a cold vent off the coast of Pakistan.
Another first was achieved by the videos they took of the cold-vent fluids seeping from the sea floor. Furthermore, the scientists were astonished at the wide variety of seep types. The scientists returned from an expedition with the RV Meteor heavily laden with new data. The expedition investigated the continental margin south of Pakistan from 31 October to 27 November.
Widely accepted doctrine has suggested that life at hot vents, such as black smokers, is much more bountiful than on cold vents. But the images sent up by the deep diving vehicle, Quest, of the Marum_Research Center Ocean Margins in Bremen told a different story: Mussel beds of more than 30 meters in diameter literally crawling with white crabs. “This puts to rest the credo that cold vents generally are less lively than hot vents”, says Gerhard Bohrmann, leader of the expedition. “The organisms seem to have a similar amount of chemical energy – in the form of methane or hydrogen sulfide – available to them as hot-vent organisms. This results in an equally high biomass.”
More surprises were in store for the scientists, like the variety of seep types: “We had a close look at nine separate seeps, and every one was different. The oxygen level in the water, which varies strongly with depth in the research area, is especially influential on the seep communities”, explains Gerhard Bohrmann.
The great differences in seep types are also a product of the geological subsurface. “On satellite images the Pakistani coast north of the area we investigated looks wrinkled. The wrinkles continue under water off the coast. This is because the whole area is being compressed; at a speed of four centimetres a year the Arabian plate is being pushed beneath the Eurasian plate. While diving underneath Pakistan the muddy sea-floor sediments on the Arabian plate are literally being squeezed dry. The water, containing a heavy load of methane, hydrogen sulfide, and a host of other compounds, bubbles out of the sea floor at the so-called cold vents. “Normally such muddy sediments are about two to four kilometres thick, here they reach an astonishing seven kilometers”. A good reason to look for vents in this area. “Where there is a lot of sediment to be squeezed, more fluids can seep out “, reasons Gerhard Bohrmann.
For the first time, the scientists from Bremen were able to observe fluids without associated bubbles seeping from a cold vent directly: “This is due to the extremely high-resolution video images of MARUM's diving vehicle, Quest. This has very likely never been seen before”, enthuses Gerhard Bohrmann. Up to now such seepages have been postulated from measurements, but never directly observed because of technical limitations in the image quality. Cold seeps were either found through gas bubbles escaping with the fluids or because of the associated organisms growing at the seep sites.
“Seepages at the sea floor are of great importance to us, because they link the crust of the Earth and the ocean”, reflects Gerhard Bohrmann about the relevance of the research. “Underwater vents transport enormous amounts of material like methane, sulfides and others as well as heat into the ocean, and therefore into the atmosphere. However, our understanding of these processes and how they shape the Earth are still sketchy.” After all, methane is 30 times stronger as a greenhouse gas than carbon dioxide. “Every expedition teaches us more about how these systems work. On this one, we made a big step towards a better understanding of cold seeps at the ocean floor”, resumes Professor Bohrmann.
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