Long-Term Ocean Venting: Seafloor system has been active for ages

Analyses of mineral deposits in and around a unique set of hydrothermal vents beneath the Atlantic Ocean suggest that the site’s tallest towers of minerals have been growing for at least 30,000 years.

The vent system, dubbed the Lost City because its lofty chimneys of carbonate rock can reach the height of an 18-story building, is situated on the side of an undersea mountain about 2,500 kilometers east of Bermuda and 15 km away from the Mid-Atlantic Ridge (SN: 7/14/01, p. 21: New type of hydrothermal vent looms large). The

mineral-rich fluids that spew from the ocean floor are warmed by heat-generating chemical reactions between ocean water and subsurface rocks rather than by volcanic activity, a characteristic that sets this vent site apart from all others yet discovered.

The Lost City’s formations consist primarily of calcium carbonate, says Gretchen L. Früh-Green, a geochemist at the Swiss Federal Institute of Technology in Zurich. She and her colleagues used carbon-dating techniques to determine the age of material gathered from active and extinct vent structures, as well as seafloor sediments nearby.

The oldest chimney material analyzed by the scientists was deposited about 25,000 years ago, well before the peak of the last ice age. Carbonate deposits in cracks near the vents are about 32,000 years old. Although the team has dated only a few mineral samples, the Lost City vents appear to have been continually active for at least 30,000 years.

White, feathery formations that surround the active vents are much younger, deposited only in the past few decades, the researchers report in the July 25 Science.

The hydrothermal activity at the site depends on contact of seawater with freshly exposed rock surfaces, says Früh-Green. Although tectonic activity in the region probably opened the cracks that initiated the heat-generating chemistry beneath the Lost City, the cracking may persist today because those reactions cause the rocks to swell by up to 40 percent. Früh-Green and her colleagues estimate there’s enough virgin rock beneath the vent system to fuel suburban sprawl around the Lost City for hundreds of thousands of years to come.

These findings could be important for several reasons, says Susan E. Humphris of the Woods Hole (Mass.) Oceanographic Institution. Similar systems may be much more common than scientists have suspected because they’ve typically confined their search for hydrothermal activity to mid-ocean ridges and the boundaries of tectonic plates. Also, she notes, the analyses could illuminate the role of nonvolcanic hydrothermal systems in the ocean’s geochemical cycles, many of which are poorly understood.

Perhaps most intriguing is that the Lost City vent system, which hosts a thriving microbial ecosystem, may be the closest analog available to conditions early in Earth’s history. Therefore, Humphris says, it’s probably the best place to study how evolution first unfolded.

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