Monterey Bay Aquarium Research Institute

Easter Microplate Expedition March 12-April 6, 2005

Please visit the ChEss website for additional information and translations in Español, Português, and Français.

 
This expedition has been made possible by National Science Foundation grants to Dr. Robert Vrijenhoek (NSF OCE-0241613) and Dr. Cindy Van Dover (NSF OCE-0350554)

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Easter Microplate Expedition

Cruise dates: March 12, 2005 to April 6, 2005
Cruise location: East Pacific Rise and Pacific-Antarctic Ridge between 23°S and 38°S
Chief scientist: Robert Vrijenhoek, MBARI, Molecular Ecology Lab 
Ship: R/V Atlantis
Vehicle: DSV Alvin
(image on right: by Mark Spear, Alvin tech.)

Purpose: 
The purpose of the expedition is to locate barriers to gene flow among species of hydrothermal vent organisms in the vicinity of the Easter and Juan Fernandez Microplates. Animals collected elsewhere along the East Pacific and Pacific-Antarctic Rise have shown genetic differences that suggest barriers to larval dispersal exist in this region. The microplates are surrounded on the north and south by transform faults and on the east and west by chains of seamounts and cross-axis currents occur in this region, which may prevent the successful transfer of larvae from vent site to vent site. Known vent sites not previously sampled for genetic studies along the southern extent of the East Pacific Rise will be targeted on this expedition. 

Plan: 
Sixteen dives are planned with the DSV Alvin from 23°S to 38°S along the East Pacific Rise and Pacific-Antarctic Ridge, weather permitting. The ship R/V Atlantis will depart from Tahiti on March 12, 2005 and transit for 9 days to the first site, at 38°S. After several dives, the ship will progress northward to sites at 26°S and 23.5°S along the east and west flanks, respectively, of the Easter Microplate. If time permits, dives will also be made on the enormous "dueling propagator" rifts south of the Easter Microplate, the ridges around the Juan Fernandez Microplate, and at the Pito Seamount, an active volcanic seamount at 23°S. The ship will arrive at Easter Island on April 6.

EasterCruise.jpg (299674 bytes)
Map of the dive targets from 23°S to 38°S along the East Pacific Rise, and showing our ports at Tahiti and Easter Island. Easter Island is one of the most remote places on Earth. The nearest habitable island is Pitcairn, 1,400 miles away and even smaller than Easter, and the nearest continent, South America, is more than 2,000 miles away. Larger image

For more information please visit: DSV Alvin and other equipment 



Our destination: the spreading ridges of the East Pacific Rise and Pacific-Antarctic Ridge (box), near tiny Easter Island, which is not even visible in this view of the Earth. We plan to start near the southern end of the box at 38º South latitude and work our way north as far as 23º South. Image by Jenny Paduan, MBARI, and NGDC/NOAA.


Cruise History and Background  

Biology - Previous molecular analyses showed that mussels and two species of tubeworm from the Galapagos Rift, the East Pacific Rise from 13°N to 17°S, and the Pacific-Antarctic Ridge (31-32°S) have divergent populations (Won et al, 2003, Hurtado et al., 2004). Based on mitochondrial DNA sequences and allozyme variation, the populations to the north of the Easter Microplate region were homogeneous genetically, despite the respective vent fields being separated by hundreds of kilometers. The species to the south of the Easter Microplate, however, were divergent from those to the north. Their genetics indicate that the mussel populations became isolated about 4.4 million years ago, which is roughly when the Easter Microplate formed. However, one tubeworm species had restricted gene flow and two species of polychaetes showed no divergence at all (Hurtado et al., 2004), indicating successful gene flow for these species along the ridge. 

The investigators hypothesized that strong cross-axis currents associated with the topographic highs in the Easter Microplate region washed planktotrophic larvae away from the ridge system, and that more negatively buoyant larvae would be less subject to this loss. The adults of many vent animals are sessile or relatively immobile, and long-distance dispersal takes place mainly through their larvae. Species with positively buoyant larvae can disperse in hydrothermal plumes that rise several hundred meters above the seafloor and be carried by ocean currents as plankton. Negatively buoyant larvae are expected to move primarily with along-axis currents constrained to the axial valley of the ridge system. The 4900 km range of the above studies crossed a series of topographic features in the vicinity of the Easter Microplate, and there are known to be strong cross-axis currents. 

Geology - The segments of the ridge system near the Easter Microplate exhibit the highest spreading rates, ~150mm/year, of all the segments of the global mid-ocean ridge system (in contrast to the moderate ~60mm/year spreading at the Juan de Fuca Ridge, or the slow ~25mm/year spreading at the Mid-Atlantic Ridge). In addition, the ridge is quite complex: rifting zones and transform faults surround the Easter and Juan Fernandez Microplates, which may be slowly rotating in response (Hey et al., 2004). There are two "dueling propagators," where the ridge system is extending to the north and south but not meeting in the middle, and perhaps creating a new microplate in the process. There are also several chains of seamounts that intersect the ridge in this region. The high rates of spreading and excessive volcanism that generated the seamounts causes the crust to be hotter and less dense, so there is virtually no axial valley and the ridge generally stands higher in this region. These factors may contribute to providing topographical barriers to larval transport. 

Hydrothermal venting has been discovered at numerous locations along this part of the East Pacific Rise using towed sensors to detect plumes and multibeam sonar to map the seafloor (Hey et al., 2004). The venting appears to be most active where abundant magma is supplied and voluminous, young lava flows smooth over the fractures found elsewhere along the ridge: at the inflated ends of ridge segments and where seamount chains intersect the ridge axis. 

References* 

Hurtado, L.A., R.A. Lutz, and R.C. Vrijenhoek (2004) Distinct patterns of genetic differentiation among annelids of eastern Pacific hydrothermal vents, Molecular Ecology, 13: 2603-2615. [Abstract] [Article]

Won, Y., C.R. Young, R.A. Lutz, and R.C. Vrijenhoek (2003) Dispersal barriers and isolation among deep-sea mussel populations (Mytilidae: Bathymodiolus) from eastern Pacific hydrothermal vents, Molecular Ecology, 12: 169-184. [Abstract] [Article]

Hey, R., E. Baker, D.W. Bohnenstiehl, G. Massoth, M. Kleinrock, F. Martinez, D. Naar, D. Pardee, J. Lupton, R. Feely, J. Gharib, J. Resing, C. Rodrigo, F. Sansone, and S. Walker (2004) Tectonic/volcanic segmentation and controls on hydrothermal venting along the Earth's fastest seafloor spreading system, EPR 27°-32°S, Geochemistry, Geophysics, Geosystems, 5(12): 10.1029/2004GC000764. [Abstract] [Article]

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