Image: UNIVERSITY OF WASHINGTON/WOODS HOLE OCEANOGRAPHIC INSTITUTION
Scientists have found a new type of hydrothermal vent at the bottom of the Mid-Atlantic Ocean that formed very differently from those previously known. An analysis of the discovery, published in today's issue of Nature, indicates that hydrothermal vents may be far more abundant than scientists thought.
Using the submersible craft Alvin, an international team of researchers discovered the field of vents this past December on an underwater mountain, the Atlantis Massif. The chimney-like structures reach a height of up to 180 feet¿100 feet higher than most other vents¿and their composition is quite different from that of the well-known black smokers. The new vents consist almost entirely of carbonate and appear gray or even white in color (see image).
The most important difference is their location. Black smokers form near mid-ocean ridges¿areas where two tectonic plates drift apart and magma bubbles up from the earth's core, forming new crust. Heat from that magma powers a cycle in which water seeps into the crust and then washes dissolved metals and other chemicals back up to the ocean floor, creating the vents. But this new site¿christened the Lost City¿is situated on an older part of the ocean floor, 15 kilometers away from the ridge. "We did not realize that hydrothermal activity of this sort could be taking place on seafloor generated millions of years ago," says Margaret Leinen, Assistant Director for Geosciences at the National Science Foundation.
According to the new study, hydrothermal circulation at the Lost City site works as follows: water permeates into the ground, where it turns olivine in the rock to serpentine. This process, called serpentinization, probably drives the hydrothermal circulation and produces fluids that contain large amounts of methane and hydrogen. In the early 1990s researchers found indication that these two chemicals were common over sites tectonically similar to the Lost City, and proposed venting as the cause. Until now, though, no one had discovered vents at such sites.
The tectonic conditions of the Lost City are prevalent on the sea floor and the study's lead author, Deborah Kelley of the University of Washington, suggests that there may be a large number of similar places along the Mid-Atlantic Ridge. "If these systems prove to be common and operate for long periods on old ocean crust," she says, "these environments may host a significant and important amount of microbial life."