Award: OCE-1131210

This project explored a rocky outcrop of basalt in the middle of the eastern Pacific Ocean. We hypothesized that this small, extinct volcano (a "seamount" known as Dorado Outcrop), was a location where there was massive flow of cool (10 to 20 degC) hydrothermal water existing the ocean crust and flowing into the overlying ocean, on the order of 1,000 to 10,000 L/s. This hypothesis was made based on consideration of measurements made of the surrounding seafloor, at a water depth of 3 km, which showed that most of the heat rising from deep within the Earth below this site was missing. One explanation for the missing heat was that it was carried by seawater that flowed into the ocean crust through one seamount, then flowed for 20 km or more until it encountered another seamount that would allow the fluid to flow back out to the ocean. Other evidence supporting this hypothesis included samples of sediment and rock, collected with a surface ship and conventional coring system that suggested that Dorado Outcrop was a site of rapid hydrothermal discharge. This fluid flow is important because it occurs across large parts of the seafloor, and may have a big influence on the chemistry and microbiology of the ocean and crust, but no one has been able to find a site where it discharges from the crust to the ocean. We returned to this site 12 years after discovering it, with a remotely-operated vehicle (ROV) and an autonomous underwater vehicle (AUV), both of which could be used to collect data and samples of the outcrop and help us to find locations where water might flow rapidly out of the seafloor. After about a week of systematic mapping and exploration, we found the first vent site, where shimmering water that was 10-15 degC warmer than bottom water came out of the seafloor. This and other hydrothermal "springs" were often rich in vent animals, including octopus, shrimp, fish, crabs, and small worms. The area with the greatest density of springs was along the southeastern end of the outcrop, near the base, where Dorado Outcrop rises steeply from surrounding sediments ("Marker K," Fig. 1). We collected the first pristine samples of this fluid, and also made measurements of heat flowing from the seafloor over and around the outcrop. We have also completed three dimensional numerical models of this fluid and heat flow system. Our computer models show how hydrothermal circulation between basement outcrops can mine the Earth's heat very efficiently across a large region. These models suggest that in order to explain the how so much heat is extracted from the seafloor around Dorado Outcrop, the permeability (ease of flow of water) in the ocean crust must 100x or 1000x higher than measured in other parts of the seafloor. New models also show that discharge in these systems is favored through smaller outcrops, consistent with field observations around Dorado Outcrop. Finally, models show that the fluid discharge rate through Dorado Outcrop is about 1000 to 3000 L/s, in the range previously hypothesized. Last Modified: 12/01/2015 Submitted by: Andrew T Fisher