Web Extra Friday, January 24

Arctic bounty of underwater plumes

The Arctic's Gakkel Ridge has recently surprised oceanographers with signs of abundant hydrothermal venting. Geological conditions and the heat or energy of a ridge determine how much water can circulate through the crust and vent its way back to the ocean. Generally, the slower the spreading ridge the cooler it is and the less it vents. The Gakkel Ridge is the slowest of all, but its heat may provide it an extra circulation boost. A reconnaissance study has identified plumes emanating from at least nine to 12 discrete vents, more than twice as many as predicted for this section of the Arctic Mid-Ocean Ridge.

"It's amazing," says chemical oceanographer Hedy Edmonds of the University of Texas at Austin and lead author of the report about the find, which appeared in the Jan. 16, Nature. "I've dreamed of getting to be the person to look for hydrothermal activity on the Gakkel Ridge, and even to be asked to go on the cruise was exciting," she says. "That there are so many sites and so much evidence for venting is beyond my wildest expectations."

Map of the Arctic ocean floor, indicating the Gakkel Ridge. The black dotted line is the cruise track of the Healy. Yellow stars indicate the nine vent sites located during the AMORE expedition. Credit: Hedy Edmonds.

The Gakkel Ridge lies on the seafloor near the North Pole and is a difficult locality to explore, as only icebreakers or submarines are equipped to handle the Arctic conditions at the surface while simultaneously investigating the topography below.

Peter Michael of the University of Tulsa spent almost a decade planning the 2001 Arctic Mid-Ocean Ridge Expedition (AMORE) cruise, funded in part by the National Science Foundation and the National Oceanic and Atmospheric Administration (NOAA) Vents Program. The cruise sampled and surveyed more than a thousand kilometers of the Gakkel Ridge using the U.S. Coast Guard Cutter Healy and German PFS Polarstern icebreakers. Michael asked Edmonds to accompany him on the cruise in an effort to learn what hydrothermal activity might be occurring in the area.

Every time a dredge to collect rock samples went overboard, a miniature device went with it attached to the cables, recording the temperature of the seawater and how much light could penetrate the water at the various locations. Areas where the light could not penetrate easily indicated suspended material in the water column, one sign of a hydrothermal plume. When those areas also showed signs of warm water, the plume was near to the vent. These clues allowed the oceanographers to map the trail of plumes leading to the vent sites.

A Miniature Autonomous Plume Recorder is deployed on a dredge wire. Below sunset on the Arctic. Photos by Hedy Edmonds.

In several locations conductivity, temperature and depth profiles, along with measurements of manganese, a mineral linked with hydrothermal circulation, helped Edmonds and her colleagues to confirm the location of nine vents along the ridge. In some parts of the ridge the dredges themselves brought up hydrothermally altered rocks.

The discovery is a strong reminder that the Arctic seafloor is still a region full of mystery and that the rules on plate tectonics regarding hydrothermal venting and spreading ridges are still being written. Says Julie Brigham-Grette of the University of Massachusetts-Amherst: "Because practical logistics and political access has limited research here, the unexpected is still what we should expect from the arctic regions. It remains an exciting frontier in the natural sciences."

Christina Reed


Arctic Mid-Ocean Ridge Expedition cruise

January Geotimes feature article: Investigaing an Arctic Gateway



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