Some time between March 26 and April 4, 1991, on the crest of the East Pacific Rise between nine and ten degrees north of the equator, the ocean floor erupted. By the morning of April 4, the research vessel Atlantis II and the deep submersible Alvin were above the site now known as Nine North to conduct the second part of a site survey for the Ocean Drilling Program. Scientists on board Atlantis II could not have known that 2,550 meters below their bunks, they would soon witness the very processes that drive this fast-spreading mid-ocean ridge and thus create ocean crust. The eruption on the East Pacific Rise in the spring of 1991 gave geologists, geochemists and biologists a singular opportunity to study volcanic, hydrothermal and biological processes that accompany seafloor spreading. This article describes the original destruction and subsequent recovery of the vent communities in the years following the 1991 event at Nine North. The eruption was not the first time that chance played a significant part in biological studies of the ocean floor. When hydrothermal venting from mid-ocean ridges was initially suggested and then confirmed in the late 1970's no one expected to find rich communities of animals around the deep vents. But one of the great de-lights of research is surprise, and in 1977, surprised and delighted scientists found a self-contained yet diverse ecological systm around vents along the Galápagos Rift, about a thousand kilo-meters southeast of Nine North. This chance discovery gave impetus to what would become entirely new areas of biological research, and our understanding of vent communities grew steadily throughout the '80s and '90s- accompanied by expressions of aston-ishment from biologists. The communities around hydrother-mal vents function without light and in an environment adjacent to water that would seem to be too hot and toxic to support life. Yet life there is fecund. Tubeworms, clams, mussels, gastro-pods,octopods, fish, grazing poly-chaete worms, scavenging crabs and other crustaceans inhabit there sites. To date more than 500 new species, more than 150 new genera-including vestimentiferans, the tubeworms-have been described from vents throughout the world's oceans. This abundance of life depends entirely on geothermal energy captured by chemosynthetic microbes,which use this energy to synthesize organic compounds through various chemical reactions. (See "Hydrothermal Vent Communities of the Deep Sea," July-August 1992.) The microbial base of the food chain in vent communities provides the ultimate support for the entire population, including uniquely adapted invertebrates- tuberworms, clams, mussels and others-living symbiotically with the microbes.
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