Mining the ocean floor

But with each new type came the discovery that it, too, displayed significant hydrothermal activity. While vents on fast-spreading ridges were fueled by heat from plumes of magma pushing up close to the surface of the crust, the ultraslow ridges displayed a different type of heat source. Water heats as it seeps into deep faults that reach the bottom of the crust. It returns through fault networks to the ridge surface, picking up dissolved minerals and metals along the way.

From the perspective of mining exploration, sulfide deposits along ultraslow ridges have the potential for some of the highest yields. Exposed deposits can build over millenniums. On volcanically active ridges, fresh deposits often get buried by fresh lava flows or undersea landslides.

Yet even here, researchers say they have found significant sulfide deposits (often under sediment) away from ridge centers, where most of the action takes place. That difference between fast and ultraslow has a bearing on the marine ecosystems each type contains and how they might respond to mining disturbances.

On fast-spreading ridges, organisms may have adapted to sudden changes brought about by quakes and eruptions. On the ultraslow spreaders, one could imagine organisms there as the “old-growth forests” of the mid-ocean ridges – evolving over time under relatively stable conditions.

It might seem that after closely studying 200 to 300 active and inactive vent systems worldwide, researchers would have many of the answers they need to begin crafting conservation strategies. Yet, based on estimates of the number of vent fields along the ridges, this represents only about 3 percent or less of the world’s estimated vent fields. Researchers have barely scratched the surface as they try to gauge the biodiversity there.

Yet, ecologists may have enough experience with land-based and shallower marine ecosystems to offer a couple of plausible ways vent communities recover from a catastrophe, suggests Kim Juniper, a marine biologist at the University of Victoria in British Columbia.

Organisms may be hard-wired to recover in place. Or the devastated spot could get reseeded with larvae drifting in from elsewhere, leading to a far different mix of organisms from the ones that were there originally.

Scientists got a chance to check on the incoming-larvae idea following a series of eruptions on the East Pacific Rise in 2005 and 2006, he says. All the organisms in several vent communities were destroyed, although the vents continued to function.

“The really shocking thing about what happened here,” he says, “is that the organisms that repopulated the site came from 180 miles to the north. No one had seen them here before.”

That gives some sense of how far larvae travel and lends weight to the idea that a disturbed site may not host the same critters it did originally.

He and his team are now trying to monitor the area to see if the changes are permanent.

The implications of this for mining, he adds, are that when someone asks for a prediction of its effects, the answer is: It depends – on the distance to an alternative source of larvae, on the timing of organisms’ reproductive cycles, and on the changes mining can bring to the seafloor topography.

Such information is crucial if scientists and regulators try to design marine- protected areas when any mining operations are approved.

Craig Smith, a University of Hawaii biological oceanographer who has played a key role in designing marine reserves and protected areas, notes that reserves must be built in networks of protected areas large enough to contain a representative sample of ecosystems and close enough to allow for larvae to travel from one to the other quickly enough to reduce the chance of being eaten by predators. Ideally, reserves would protect 30 to 50 percent of each habitat type in a region.

“We should be thinking about these steps for hydrothermal vents and massive sulfide deposits,” he says. “Ocean ecosystems, including vents, are increasingly being impacted by human activity and urgently need protection.”

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