AT26-09

Ship

Vehicles

Cruise Party

Departure: Dec 7, 2013

Arrival: Dec 23, 2013

Operations Area

Science Objectives

Intellectual Merit: Scientific Motivation and Proposed Program

            Most of the advective heat loss from the oceanic lithosphere occurs on "ridge flanks”, areas far from the magmatic influence of seafloor spreading. Global hydrothermal fluid flows to and from the seafloor on ridge flanks, driven by lithospheric cooling and influenced by basement relief and variations in sediment thickness, are of the same magnitude as the flows from all of Earth's rivers and streams to the oceans. Yet, pristine fluids from a typical ridge-flank hydrothermal system have never been sampled, mainly because it has not been possible to locate a site of focused discharge where representative samples could be collected. We have located a small basement feature, Dorado outcrop, on 23 m.y.-old seafloor on the eastern flank of the East Pacific Rise, that is hypothesized to discharge 10³–10⁴ L/s of cool (10–20°C) hydrothermal fluid that carries 200-350 MW of lithospheric heat, equivalent to the heat output of a black-smoker vent field.

            We propose to sample this fluid to determine its composition, and to assess the rate of discharge from Dorado outcrop, so that we can quantify the chemical impact of this hydrothermal system. The physical and chemical state of this fluid is typical of fluids that extract heat from the crust and exchange solutes with the overlying ocean across much of the seafloor. Collection of uncontaminated samples from this feature will provide the most accurate estimates of the global impact of ridge-flank hydrothermal circulation. We anticipate that this fluid will be modestly different in composition from bottom seawater, which is why collection of pristine samples is so important, because of the relatively low temperatures of reaction and short residence time of the fluid within basaltic basement. Despite being slightly altered, fluid of this kind can have a large influence on the geochemical budgets of important solutes in the ocean because the flows are so large (from both Dorado outcrop and on a global basis).

            We propose an 18-day expedition that combines the surveying capabilities of the AUV Sentry (bathymetric, sub-bottom sonar, photo mosaics, water column anomalies) and an ocean-class vessel capable of collecting high-quality multi-beam data and CTD samples, and supporting the survey and sampling capabilities of the ROV Jason II for collection of spring and plume fluids, heat flow data, sediment push cores, and still and video photography. These data and samples will be combined to generate the first well-constrained estimates of hydrothermal flows from Dorado outcrop. This single expedition will result in the collection of samples and data from a “fire hose” of ridge-flank, hydrothermal system, challenging the commonly held view that discharge from ridge flank hydrothermal systems occurs primarily from diffuse seeps.

Broader Impacts

            Undergraduate and graduate students and a junior faculty member (Hulme) will gain experience by participating in this program and through the development of ROV manipulated samplers and sensors. Results of this work will be broadcast in publications, theses, presentations (to scientific and lay audiences), and in K-16 curriculum development. The P.I.s have strong histories of involvement in educational and outreach programs at multiple levels. Wheat and Hulme are developing hands-on science-technology packages to engage K-8 students during week-long learning opportunities that include three technology-based activities per grade level and meet national science, mathematics, and language arts standards. Fisher will involve university students through USCS’s Institute for Scientist and Engineering Educators and Cal Teach programs. Wheat and Fisher are involved in the NSF-funded Science Technology Center for Dark Energy Biosphere Investigations (C-DEBI). Materials from this expedition will be incorporated in C-DEBI education and outreach activities, reaching a diverse audience that includes K-8 students in the Los Angeles area.

Science Activities

WHOI support personnel estimate 8 hours of bottom time for a complete bathymetry survey of Dorado outcrop (216,000 m²/hr), yielding a processed map with 20-30 cm resolution. Subbottom penetrating sonar and water column thermal and eH surveys will be completed simultaneously and continuously during bathymetric mapping. The initial bathymetric map will guide additional Sentry dives to collect additional bathymetric and sediment thickness data on and surrounding the outcrop; should it be necessary, additional higher altitude (50 m) mapping of the outcrop perimeter will be conducted to ensure complete bathymetric and subbottom coverage of the outcrop and adjacent sediments. Sentry also will be used to conduct photographic surveys 5 meters above the seafloor (5400 m²/hr) during which we will monitor for thermal and chemical anomalies in the water column. WHOI personnel expect to mount a new strobe system in late 2011 or early 2012 on Sentry, and have indicated that the entire outcrop can be visually surveyed in about seven days of operation (includes battery recharging, etc) at an elevation of 5 m with this new strobe system. It may not be necessary to image the entire outcrop, but we will focus photo-mosaic operations on sites of interest for closer evaluation, and run repeat surveys at multiple altitudes to define better the distribution and size of water column anomalies.

            Jason II deployments will be guided using the initial bathymetric map and data collected with Sentry to locate and sample springs and hydrothermal plumes at/near the seafloor, and to conduct heat flow and sediment push core surveys on sedimented areas on Dorado outcrop and along the outcrop edge. Sentry and Jason II can work concurrently provided they are 300 m apart. Thus, after the initial day of mapping with Sentry, 6 days of bottom time (8 operating days) with Jason II are required to conduct six radial heat flow surveys along transects defined by the sub-bottom sonar and bathymetric data (each measurement takes 0.5 hours with 1.5 hours to take a push core, transit and set-up for the next measurement. Twelve measurements will be made on each survey, with transects oriented radially away from the edge of the outcrop). The combination of sediment pore water and heat flow data will allow quantification of hydrothermal fluxes, as described above. Four additional days of bottom time (6 operating days) are needed to sample springs using inverted funnels, barrels, or short “pipes” [e.g., Johnson et al., 2003] that minimize entrainment of bottom seawater during sampling.

            In aggregate, we can complete outcrop-wide mapping, water column surveys, sonar data acquisition, heat flow measurements, and seafloor sampling in 18 days on site, including one day for the initial map, eight days to conduct heat flow transects, six days to sample springs, one day for CTD operations to collect additional fluid samples near the seafloor, one day for vehicle timing (coordinating vehicles in/out of water) and maintenance, and one day for weather/contingency.

In summary

Sentry - will map Dorado Outcrop and possibly another nearby oucrop (Bathymetry, eH, temperature anomaly, chirp sonar, and likely photo mosaic

Jason - will conduct heat flow and push core surveys. When springs are found we will collect spring fluids, rocks, mats, clams (if any).  The springs should be 10 to 20 C.

Additional Info

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Supporting documentation:

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