Detection of hydrothermal plumes along the Southeast Indian Ridge near the Amsterdam‐St. Paul Plateau

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A new self‐contained profiling instrument, attached to 91 lowerings of waxcores and dredges along a 1600 km stretch of the Southeast Indian Ridge, found six sites having optical back‐scatter anomalies indicative of hydrothermal plumes from high‐temperature seafloor venting. At five other sites, back‐scatter anomalies were less distinct, and at the remaining sites, anomalies were absent. These are the first documented hydrothermal sites along this intermediate‐rate spreading center and are among the very few yet discovered in the Indian Ocean basin. Some profiles exhibit complex back‐scatter anomalies suggestive of multiple sources of seafloor discharge. At one site, near‐bottom temperature anomalies of ∼0.1°C and the recovery of a vent‐specific barnacle provide the first precise seafloor location of active venting in the deep Indian Ocean. Although this study is only reconnaissance in nature, venting along this portion of the SEIR appears to be significantly less than predicted by global correlations of plume incidence as a function of spreading rate. Nevertheless, these sites may be important as biological connections between distinct faunal assemblages of Atlantic and Pacific vent fields.