Led by: Geoff Wheat, University of Alaska, Fairbanks
The Dorado Outcrop program builds on decades of research that observed differences between measured and theoretical heat flow values in the ocean crust. Lister (1972) postulated that these differences resulted from the cooling of the crust by the circulation of seawater. The magnitude of this circulation is large, redistributing 10 TW (one fourth) of the Earth’s heat loss. Then, in 1977, the first seafloor hydrothermal system was discovered; however, such systems, driven by the intrusion of magma, account for 20% of the total convective heat loss from oceanic crust. The remaining heat loss is transported on the ridge flanks at much cooler temperatures, yet with a net fluid flux that is commensurate with that of rivers. Given this magnitude of fluid flow, it has been postulated that even a minimal (1-5%) loss or gain from seawater-crustal exchange could impact global geochemical budgets in the ocean; however, until recently such a representative system had yet to be sampled.
Off the west coast of Costa Rica on 23 Ma-old crust lies the Dorado Outcrop (DO), a small (0.5 km wide by 2 km long and 150 m high) basaltic outcrop that that trends southeast-northwest and is characteristic of typical ridge flank hydrothermal systems. In 2013 we surveyed DO using the autonomous underwater vehicle Sentry and the remotely operated vehicle Jason II (AT26-09) (cruise report). Surveys generated a bathymetric map from which visual and thermal surveys located sites where crustal fluids vented and placed them within a geologic context. The outcrop consists of basaltic pillows and sheet flows with linear faults that are tens to hundreds of meters long and in the general trend of the outcrop. Surprisingly hydrothermal fluids do not currently vent from these linear features.
Instead, diffuse hydrothermal activity was identified from thermal anomalies determined from temperature probes mounted on the ROV and AUV. Such anomalies guided closer inspection with a manipulator-held device. Over one hundred thermal measurements were made with the warmest recorded temperature of 12.3°C, well above the bottom water temperature (1.85°C). Most of the hydrothermal flow occurred ubiquitously along the southwestern portion of the outcrop with the most vigorous site that spans hundreds of square meters (Marker K). In 2013, spring samples were collected and continuous fluid samplers (OsmoSamplers), and temperature loggers were deployed. Because of the diffuse nature of the springs, we developed a novel syringe-based fluid sampler that we used in 2014 with the submersible Alvin during which we recovered samplers and sensors, collected additional fluids, and measured dissolved oxygen in situ (cruise report).
Dorado Outcrop Site Maps. Top left: The Tico Flux area is located off the coast of Coast Rica on 18-24 Ma seafloor of the Cocos Plate, comprising lithosphere produced at the Cocos-Nazca Spreading Center and the East Pacific Rise. Top right: Multi-beam data overlain on gravitational bathymetry illustrate seafloor relief (modified from Hutnak et al., 2008). Dorado Outcrop and Tengosed Seamount are located within a 14,500 km2 area of cool seafloor, where the heat flux is generally 10-40% of lithospheric predictions. Bottom left: Fourteen gravity cores were taken on and near Dorado Outcrop providing a measure of the fluid composition in upper basaltic basement (from Wheat and Fisher 2008).
Recent Operational Accomplishments
During the last expedition (AT26-24) we completed ten Alvin dives, collected thirteen gravity cores, conducted one CTD hydrocast, and completed four surveys using the hull-mounted bathymetric mapping system. Through these activities we collected hundreds of samples for hydrologic, biogeochemical, and microbial studies, all of which have been the focus of shore-based analysis undergoing a spectrum of analytical and modeling efforts. Samples and data collected include:
- Kilometers of high resolution bathymetric with the hull-mounted system,
- Video and still images of Dorado Outcrop,
- Discrete samples of spring fluids (19 microbial and 35 chemistry),
- Measurements of heat flow (4),
- Measurements of temperature (31),
- Sediment push cores (50),
- Rocks (30),
- Bottom water via Niskin bottles (8) on Alvin and one CTD,
- Discrete dissolved oxygen measurements (26),
- A three-day and a four day record of dissolved oxygen and temperature at two (2) sites,
- Year-long record of temperature at five (5) sites with two sensors at each site,
- Continuous fluid sampling for one year at four (4) sites,
- Year-long enrichment experiments at four (4) sites.
These accomplishments were greatly aided with the help of the Alvin team. They seamlessly rig the oxygen optode, heat flow probe, and thermistor for real-time measurement within the sphere and provided quality navigation that minimized transit times on the seafloor.
Recent Scientific Accomplishments
Much of the focus for the Alvin dives in 2014 were on the collection and sensing of pristine spring fluids for biogeochemical and microbial analysis and the recovery of sensors, samplers, and experiments to constrain biogeochemical pathways and activity. Based on visual observations, discrete measurements of dissolved oxygen, dye experiments, three-day and four-day continuous records of dissolved oxygen and temperature, and year-long temperature and chemical records from probes and OsmoSamplers, the rate of flow from these springs varies on time scales of hours to days to years.
Data and samples from the two DO expeditions (AT26-24 and AT26-09) form the foundations for a series of manuscripts. Critical findings in three manuscripts include:
- The presence of dissolved oxygen in the chemically altered (relative to bottom seawater) spring fluids that, when extrapolated globally, have an impact on oceanic geochemical budgets (Wheat et al.)
- A survey of 16S rRNA gene sequence diversity of communities on seafloor rocks reveal a community that is dominated by Gamma-, Alpha-, and Deltaproteobacteria, and Thaumarchaeota with much greater richness and diversity than in the surrounding seawater and when these results are integrated with existing datasets that provide a global distribution of seafloor basalt experiments the results suggests that substrate age correlates with community structure (Lee et al.);
- Simulations of outcrop-to-outcrop crustal hydrology that match regionally expected fluid fluxes and new findings that local convection in and out of individual, large outcrops also removes a significant fraction of lithospheric heat (Lauer et al.)
Technical accomplishments for Expedition AT26-24 included (1) submersible integrations of an Aanderaa Optode for discrete measurements of dissolved oxygen, (2) development of a dye releasing manifold for assessing fluid flux, (3) the redeployment of RBR oxygen and temperature probes, which were developed for NP and are now commercially sold by RBR, Inc., and (4) and hydrologic models for elucidating subsurface flow patterns and magnitude.
DORADO OUTCROP EXPEDITION HISTORY
-- UNOLS Expedition, December 7- December 23, 2013
AT26-09 Discovery, sampling, and quantification of flows from cool yet massive ridge flank hydrothermal springs on Dorado Outcrop, eastern Pacific Ocean
Chief scientist: C. Geoffrey Wheat
-- UNOLS EXPEDITION, November 30 - December 12, 2014
R/V Atlantis Expedition AT26-24 with the Submersible Alvin
Chief scientist: C. Geoffrey Wheat
> See the Dorado Outcrop NSF Proposal [PDF]
> Dorado Outcrop cruise prospectus [PDF]
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