Leg 4 

Leg Summary:  

Leg 4 will use the ROV to compare the ecology and evolution of hydrothermal vent communities at three main localities along a line of transition between the sediment covered vents in the Guaymas Basin to the basaltic vent field on the East Pacific Rise at 21º N. Vent communities will be surveyed for comparison at each site using the ROV’s video camera. Biological samples will be collected with the ROV manipulator arm, push cores, water sampling bottles, and a suction sampler. This material will be used in studies of ecology, physiology, genetics, and species diversity. This research should reveal some of the fundamental relationships between vent fauna, vent site characteristics, and cold seep communities. The leg 4 coordinator is Dr. Robert Vrijenhoek from MBARI, and the Mexican collaborator is Luis Soto from Universidad Nacional Autonoma de Mexico. 

History & Purpose: 

A multidisciplinary team of biologists, chemists, and geologists will characterize ecological changes between a basaltic hydrothermal field on the East Pacific Rise (EPR, 21°N) and the massively sedimented hydrothermal field in Guaymas Basin (27°N). We will also explore the Tamayo Fracture Zone, a potential transition zone between these areas.

The biota at 21°N is typical of vent-endemic fauna along the EPR. The suite of invertebrate species that graze on chemoautotrophic bacteria or employ sulfur-oxidizing bacteria as symbionts dominates hydrothermal fields along this rapidly spreading ridge axis. Common EPR organisms (e.g., vestimentiferan tubeworms, vesicomyid clams, bathymodiline mussels, archaeogastropod limpets, brachyuran crabs, bresiliid shrimp, etc.) show little change in morphology or genetic structure, a feature that is considered evidence for high rates of organismic dispersal and gene flow across thousands of kilometers.  

In contrast, Guaymas Basin hydrothermal vents differ markedly from EPR vents to the south. Only five vent-endemic species are shared between Guaymas hot vents and 21°N, which are only 500 km apart. Species that are shared between these disparate sites (Riftia pachyptila, Paralvinella grasslei, Provanna laevis, and Amphisamytha galapagensis) appear to comprise a complex associated with sulfide edifices and focused hydrothermal flow.  

Many of the non-overlapping species, like the tubeworm Escarpia spicata and several vesicomyid clams, found in Guaymas Basin also occur at cold-seeps in the Gulf of California and along the continental margin from Costa Rica to Oregon. The obvious difference between Guaymas Basin and 21°N on the EPR is the presence of 200–600 m of sediment covering active spreading centers in the Gulf of California. Hydrothermal fluids passing through this sediment carry the chemical signatures of mantle volatiles from below. However, the sediments also carry a complex mixture of biogenic hydrocarbons, which in turn support a complex microbial community. In contrast, hydrothermal communities at basaltic sites along the East Pacific Rise are supported primarily by volcanically produced sulfides.   

Click the movie icon for a short video of a smoking hydrothermal vent.

Participants from MBARI, the Childress lab (UC-Santa Barbara), and Mexican collaborators plan to gather benthic samples for geological, chemical, physiological, genetic, and species diversity studies from four localities.