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1 Institute of Oceanographic Sciences, Wormley, Surrey GU8 5UB, UK
2 Woods Hole Oceanographic Institute, MA 02543, USA
3 Marine Biological Association, Citadel Hill, Plymouth PL1 2PB, UK
Deep-towed sidescan sonar and manned submersible studies have shown that hydrothermal activity within the Broken Spur vent field, located at 29°10'N on the Mid-Atlantic Ridge, is controlled by a combination of recent volcanic and tectonic activity. Three sulphide mounds, with high-temperature fluid vents, and two weathered sulphide mounds, with low-temperature fluid seeps, are aligned across an axial summit graben that lies along the crest of a neovolcanic ridge within the axial valley floor. The largest high-temperature venting sulphide mound, which is up to 40 m high, lies in the centre of the graben. Two further, but smaller, high-temperature sulphide mounds are located to the east and west of the larger mound. All three high-temperature venting mounds lie on an axis that strikes 115°, orthogonal to the trend of the axial summit graben and neovolcanic ridge. These directions are similar to the strike of two sets of faults that locally intersect at the vent site and which probably control the location of hydrothermal emission. The fauna colonizing the vents are distinct, at least at a species level, from those found at other hydrothermal sites on the Mid-Atlantic Ridge. New species of bresiliid shrimp, and a new genus of brittle star have been found along with other fauna in an ecosystem that is otherwise similar to those found at high-temperature hydrothermal sites elsewhere on the Mid-Atlantic Ridge. However, the populations of the bresiliid shrimp at Broken Spur are significantly lower in abundance than those of the same genus found elsewhere. The size, shape and state of alteration of the sulphide mounds and the extent of their oxyhydroxide sediments and weathered sulphide talus aprons, suggests that hydrothermal activity at the Broken Spur vent field has been long-lived, probably for several thousand years. This is supported by the unique speciation of fauna present, which also suggests that the Broken Spur vent field is isolated (in terms of faunal accessibility) from other vent sites on the Mid-Atlantic Ridge. The low population of the shrimp, in an otherwise active and long-established hydrothermal habitat, suggests that the hydrothermal activity at Broken Spur is in a state of change and may have been recently rejuvenated.
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