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Dorado Outcrop

Led by: Geoff Wheat, University of Alaska, Fairbanks

Background
To date, ridge flank hydrothermal system (RFHS) fluids have only been collected from two springs—Baby Bare and Zona Bare along the Juan de Fuca ridge. The Dorado outcrop, which overlies 23 Ma seafloor east of the East Pacific Rise and west of Costa Rica, represents a new and very different sampling opportunity. On the basis of swath bathymetry, seismic and heat flow data, and systematic pore water chemical profiles from nearby gravity cores, Dorado outcrop is a site of focused low temperature (10-20 °C) ridge flank venting. RFHSs transport a significant (35%) portion of the Earth’s heat loss. Globally this discharge through the basaltic portion of the oceanic crust is about equal to the input of water from rivers to the ocean. Locally, the Dorado outcrop discharges 2-3 orders of magnitude more fluid than Baby Bare (103-104 L/s versus 10 L/s) although both outcrops are similar in size. We hypothesized that the Dorado outcrop:

  • Produces 200-350 MW of lithospheric heat (equivalent to a black smoker vent field),
  • Discharges fluids that are chemically different from bottom seawater, resulting from seawater-basalt reactions within the crust and through diffusive exchange with sediment pore waters overlying basaltic crust as the basement (formation) fluid flows from recharge to discharge,
  • Generates fluid, thermal, and chemical fluxes that have a regional influence and are globally representative of RFHS in general, thus allowing an accurate assessment of geochemical budgets from RFHS.

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).

Additional hypotheses state that the hydrothermal system at Dorado outcrop:

  • Harbors microbial communities that resemble those observed in seawater with an overprinting of microbial species observed on cool, seafloor exposed basalts and in warmer environments,
  • Has hydrothermal fluids that contain higher cell densities and more active microbial communities than ambient seawater, owing to more diverse fluid chemistry to support chemolithotrophic reactions in basaltic basement and in the overlying sediment pore waters,
  • Removes dissolved inorganic and organic carbon, affecting the isotopic composition of the discharging fluids and affecting the local C cycle,
  • Results in sedimentary reactions for several solutes that are greater than those produced by seawater basalt reactions.

An intial cruise took place on December 7-23, 2013 (cruise report now available), embarking from and returning to Puntarenas, Costa Rica.  In early February 2014, samples were returned to the United States where the shore-based analytical program will began. A follow-up expedition with the submersible Alvin is scheduled for November 2014 (cruise prospectus now available) to sample spring fluids and recover year-long experiments.

C-DEBI 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
[Cruise Report]


NEXT EXPEDITION: UNOLS EXPEDITION, November 30 - December 12, 2014
R/V Atlantis Expedition AT26-24 with the Submersible Alvin
Chief scientist: C. Geoffrey Wheat
[Cruise Prospectus]

 

Associated C-DEBI Funded Projects


> See the Dorado Outcrop NSF Proposal [PDF]
> Dorado Outcrop cruise prospectus [PDF]
> About our major programs
> Our expedition schedule
 

 

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