Award: OCE-1558356

Many deep-sea volcanoes occur along island arcs at convergent plate boundaries. Hydrothermal activity associated with these submarine arc volcanoes contribute to global geochemical cycles and are host to diverse ecosystems nurtured by the hydrothermal fluids. Yet, they are still some of the least explored features on the seafloor. Brothers volcano is an arc volcano northeast of New Zealand in the Kermadec that harbors two different styles of hydrothermal vents within its caldera: (1) a seawater-influenced system where high-temperature black smoker-type fluids discharge from Cu-Zn-Au-rich sulfide chimneys (the NW Caldera site); and (2) a magmatically-influenced hydrothermal system characterized by moderate temperature, gassy, acid-sulfate fluids and the formation of native sulfur chimneys and extensive Fe-oxyhydroxide crusts on the seafloor (the Upper and Lower Cone sites). These two fluid types are common to many submarine arc volcanoes and represent different parts of an evolving hydrothermal system. The purpose of this 2018 field expedition with the remotely operated vehicle (ROV) Jason was to address several hypotheses relating to the thermal heat flux and the subsurface geometry of hydrothermal circulation, and the role of low pH, magmatic volatile-rich hydrothermal fluids on the formation of sulfides, rock alteration, microbial diversity, and metabolic pathways in this hydrothermal system. During the research expedition, day-to-day operations were shared with students and the general public on the newly redesigned Dive and Discover website (https://divediscover.whoi.edu/). Six undergraduate and graduate students participated in the cruise and received training in ROV operations and various shipboard analytical techniques. A heat flow survey on the caldera floor and rim revealed that large‐scale subsurface fluid circulation is structurally controlled, with deep (~1‐2 km depth) central recharge through the caldera floor and lateral discharge along the caldera walls and at the summits of the resurgent cones. In addition, there are shallow (~ 0.1‐0.2 km depth), small-scale recharge zones (200-300 m in diameter) located very close to the active vent sites. Hydrothermal fluids from the NW Caldera had temperatures up to 303?C, salinities near seawater, and pH ranging from 7.6 down to 2.8. In contrast, the Upper Cone fluids had maximum temperatures of 160?C, very low pH values (2.0-2.2), and salinities near that of seawater. The Lower Cone samples were very gassy, and are dominated by CO2. This cruise also provided one of the first in-depth microbiological studies of a deep-sea volcano. Over 800 genomes from 18 hydrothermal vents at Brothers volcano were reconstructed and demonstrate that the complex hydrologic evolution and subsurface hydrology of the volcano drives the rich microbial genomic diversity. The multidisciplinary dataset collected on this expedition demonstrates the complex interplay between geology, hydrothermal circulation, and microbial diversity in this arc volcano. The existence of two geochemically distinct hydrothermal systems, most likely driven by heat from one magma chamber, appears to be related to the evolution of the volcano, with faulting of the caldera walls being important in the creation of fluid flow pathways. The complexity of the subsurface hydrologic environment manifests itself as an underlying imprint on differences in the diversity of the microbial community. Last Modified: 04/02/2020 Submitted by: Susan E Humphris