High-resolution hydrothermal mapping of Brothers caldera, Kermadec arc

Submarine edifices with caldera summits are common along volcanic arcs and much more likely than simple cones to host hydrothermal venting. Compared with cones, however, locating all vent field locations on a caldera's complex bathymetry is a daunting logistical challenge. Here we describe the first use of an autonomous underwater vehicle, ABE, to fully map the distribution of near-bottom hydrothermal tracers over the caldera walls and cone complex of Brothers Volcano, the most active hydrothermal source on the southern Kermadec arc. Sensors on ABE simultaneously measured hydrothermal plume anomalies in temperature, light backscattering (particle concentration), and oxidation-reduction potential (dissolved reduced species) every 2 to 3 m along track. Local maxima in these tracers confirmed known sites on the northwestern wall and the cone summits, and more precisely mapped their extents. We discovered evidence for new sites throughout the entire northwestern half of the caldera wall, at the base of the southeastern wall, and on the flanks of both cones. Systematic variations in the backscattering/temperature ratio identified different types of fluid discharge. Plumes from high-temperature, metal-rich sources dominated the caldera wall, while on top of the caldera wall and on the cones we found only plumes from low-temperature diffuse flow, including occasional S-rich plumes. Source distribution on the walls appears fault controlled, but we detected no sources along the deepest fault defining the rim of the downdropped caldera floor. Advanced deep-sea survey techniques using autonomous vehicles is an indispensable and cost-effective tool for acquiring a comprehensive understanding of the relationship between hydrothermal discharge and geology on individual volcanoes.