Award: OCE-1130074

Dorado outcrop, located ~100 km west of the Middle America Trench, lies on 20 MA ocean crust in 3000m of water and is approximately 2km by 500m in length and width and 200m above the surrounding seafloor. Despite its small size, regional heat flow surveys have determined that Dorado plays a major role in the regional hydrogeologic system by siphoning significant volumes of warm water from the buried basement aquifer. Until this project, little was known about Dorado, other than the aforementioned secondary evidence of fluid circulation. This project funded a seagoing expedition aboard the R/V Atlantis that hosed two deep-sea vehicles, the AUV Sentry and the ROV Jason-Medea, in an effort to confirm the hypothesis that Dorado was a site of basaltic aquifer discharge and quantify the fluxes resulting from this fluid circulation. A follow-up expedition was also conducted aboard the R/V Atlantis with the manned submersible Alvin, in which experiments deployed on the first expedition were recovered one year later, in addition to continued exploration and characterization of Dorado Outcrop by mapping and an additional gravity-coring program. During the course of these investigations, a regional multibeam survey was conducted using the hull-mounted multibeam system during periods of opportunity between dive operations. This survey both improved the resolution and expanded the coverage of existing seafloor maps in the region. The process of mapping Dorado Outcrop started with making an initial map of the outcrop with the hull-mounted multibeam in order to plan AUV Sentry flight paths over the outcrop. This immediately increased the resolution of the seafloor map of Dorado from 100m to 10m. Using this initial data, Sentry bathymetric surveys were conducted, and a 1m resolution map was produced, that could be used for ROV Jason navigation around the outcrop. Real-time support for the ROV team was provided by supplying geo referenced maps of the outcrop for display on their navigation software. This allowed the pilots and scientists to conduct a comprehensive survey of the outcrop over the course of the expedition. In addition to bathymetric mapping, the AUV Sentry was used to detect thermal anomalies in the water column, that ultimately led to the discovery of the region of most active venting on the outcrop. Sentry was also used to conduct sub-bottom profiler transects across the outcrop and adjacent area. This information was used to guide heat flow surveys, which could combine sediment thickness information from the sub-bottom profiles with the heat flow results to model the fluxes of warm fluids beneath the outcrop. Sentry was also used as a visual-surveying tool by photo-mosaicing areas of the outcrop that could not be surveyed by Jason in the given amount of time. ROV Jason operations on the outcrop consisted largely of probing the sediments and rocky substrate with a temperature wand to determine the presence of fluid upwelling. Because of the low-temperature nature of fluid venting there, it was very difficult to visually determine the presence of upwelling fluids. By conducting over 200 discrete temperature measurements across the outcrop, it was possible to not only determine the best locations for sampling, but also to identify areas of diffuse fluid upwelling and the distribution of fluid venting across the outcrop. Once the best areas of fluid venting were identified, discrete water samples were taken using ROV-deployed samplers. These samples were processed for both chemical and biological properties. Rock samples were also taken from across the outcrop in order to map the geologic properties of the outcrop in combination with visual survey data from ROV Jason and AUV Sentry. Sediment cores were taken in areas of thin sediment drape in order to assess the physical and microbial properties of sediment in relation to upwelling fluids from the ocean crust. After sampling the outcrop, experiments were deployed at vent sites for...