ABSTRACT

The regional distribution of hydrothermal vent activity in the northeastern (NE) Lau Basin was recently reported by the Ridge 2000 program; however, active venting sources have yet to be located. Here, we report geological and hydrological evidence that indicates the presence of three active hydrothermal venting sources in the NE Lau Basin: the Fonualei Rift and Spreading Center (FRSC), the Northeast Lau Spreading Center (NELSC), and an off-axis caldera (MTJ-1). These examples of hydrothermal activity were recognized by the appearance of hydrothermal plume signals in the water column, including anomalies in light-transmission, methane, adenosine 5'-triphosphate (ATP), and trace metal concentrations (TDMn and TDFe). Three active venting sources were identified by the observation of possible buoyant plumes during conductivity-temperature-depth (CTD) tow-yo surveys and by the recovery of hydrothermal precipitates (chimneys and altered rocks). The strongest light-transmission anomaly, an order of magnitude greater than those at other sites, was observed at the central cone of the MTJ-1 caldera. The recovery of eruption debris at a central volcanic cone, including molten sulfur, volcanic ash, and lapilli, indicates an active volcanic eruption and hydrothermal venting at the MTJ-1 caldera. Our results suggest extensive and various hydrothermal activity in the NE Lau Basin, thereby providing valuable insight into hydrothermal and volcanic processes in back-arc environments.