Time-Series Exploration and Biological, Geological, and Geochemical Characterization of the Rosebud and Calyfield Hydrothermal Vent Fields at 86°W and 89.5°W on the Galapagos Rift

In May-June 2002, the Galapagos Rift axis was explored during a NOAA Ocean Exploration Program expedition that investigated three sites along the Rift between 86°W and 89.5°W. Two major vent fields with markedly different community structures were discovered near 86° 13'W (Rosebud) and 89° 37'W (Calyfield) and were biologically, geologically and geochemically characterized. Alvin dives were initially conducted to extend the long-running biological and geochemical time-series at some of the earliest discovered vent sites (e.g., Rose Garden, 1979; Musselbed, 1977). Alvin dives to the Rose Garden (RG) area (86°W) revealed: 1) a notable absence of the 14 seafloor markers and ~7 stacks of Alvin dive weights that were observed during the last visit to RG in 1990; 2) relatively recent lava flows (as indicated by a 0.6 wt. percent increase in evolved N-MORB MgO in RG [1985] and Rosebud lava flows); and 3) relatively young (small/juvenile) species assemblages. Thus, the well-developed vent faunal communities documented 12 years earlier at RG were apparently buried by eruptive lava flows. Within 200 meters of RG, a new site, Rosebud (2470m), has emerged to support communities presently in the early stages of their development in fluids exhibiting the highest temperatures (23°C) and H₂S concentrations (>0.55 mmol per L) observed on the Galapagos Rift. Photomosaics of the Rosebud vent field (70 x 50 m) reveal this site to consist of 4 major venting areas containing vestimentiferan tubeworms (majority less than 6cm in length), linear rows of bathymodiolid mussels (average ~1cm in length), and adjacent carpets of amphianthid anemones (ca. 50 per square meter). A single assemblage of larger mussels (>10cm) was observed on the margin of the field. Vesicomyid clams (ca. 10 individuals), all less than 3 cm, were observed along cracks in the central sheet flow. Based on extensive time-series analysis of biological community structure on the East Pacific Rise, the age of the Rosebud communities are likely not more than 2.5 years old. The western-most known vent field on the Galapagos Rift, Calyfield, (60m x 60m; 1679m), was discovered at 89° 37'W, and is dominated by the Vesicomyid clam Calyptogena magnifica. This field is similar in appearance to the Clam Acres site at 21°N on East Pacific Rise). Photomosaics constructed via images acquired from Alvin revealed the distribution of large clams (ca. 18-32cm) to predominate in the flow contacts between pillow lavas, along with numerous clumps of bathymodiolid mussels (individuals >10 fold larger than those found at Rosebud), sparse vestimentiferan (Oasisia) tubeworms, numerous amphipod swarms, and the presence of large patches of a grey encrusting sponge on the pillow basalts. This Demospongiae sp. appears endemic to Calyfield. Differences in these communities, their relationship to vent fluid chemistry, and patterns of community succession at eastern Pacific vents will be presented.