Gorda Ridge Revisited: the R/V Okeanos Explorer Conducts its First Hydrothermal Plume Survey

Abstract

The first systematic survey of the water column over the Gorda Ridge since 1985 was conducted during a June, 2009, field trial cruise aboard the R/V Okeanos Explorer, “America’s Ship for Ocean Exploration.” The purpose of the cruise was fourfold: (a) give the ship’s officers and crew experience with exploration methods for hydrothermal plumes, (b) conduct CTD vertical casts and tows to determine the current state of hydrothermal activity along the ridge, (c) assess the EM302 system water column data capabilities for detecting and mapping hydrothermal plumes, and (d) map the neovolcanic zone within the axial valley with the ship’s EM302 multibeam system. A total of 15 vertical casts and 2 tows were completed within the axial valley approximately every 20 km between 40.75oN and 43.0oN. A Seapoint Turbidity sensor and PMEL oxidation-reduction potential sensor were added to the ship’s Seabird CTD for the detection of suspended particulates and dissolved hydrothermal species. Water samples were collected for analysis of 3He and total dissolvable metals (Fe and Mn). The Gorda Ridge is a medium to slow spreading rate spreading center with a 600-1400-m-deep axial valley. Previously known high-temperature vents are located at ~41.0oN on the Escanaba segment (NESCA site) and at ~42.75oN on the North Gorda segment (Sea Cliff site). A volcanic eruption in 1996 extruded 18 x 106 m3 of new lava onto the seafloor at the south end of the axial high on the North Gorda segment, south of the Sea Cliff area, and generated event plumes and short-lived chronic plumes that died out within 3 months after the eruption. Response cruises to a seismic swarm located on the Jackson segment in 2001 did not find evidence of either event plumes or new lava on the seafloor. During the 2009 Okeanos Explorer cruise, the Sea Cliff site was confirmed as still active, generating a distinct plume with maximum particulate (dNTU~0.1), ORP (~80 mv), and temperature (0.04oC) anomalies below 2250 m depth. The plume was present nearly all the way to the seafloor (2610 m), and density inversions below 2500 m suggest we were sampling within the buoyant plume. A signal was also detected at the axial high along the Jackson segment (bottom depth ~3100 m). While this plume had no discernable particle signal above the regional NTU increase from ~2000 m (the approximate depth of the axial valley walls) to the seafloor, a very slight (1 mv) ORP anomaly occurred about 200 meters above the seafloor. There was also a density inversion in the bottom 50 m (indicating rising fluids) formed by lower-salinity, slightly warmer water. There were no apparent particle, temperature, or ORP anomalies elsewhere along the Gorda Ridge, although pending analyses of the 3He and total dissolvable trace metal samples might identify additional areas where hydrothermal venting may be active. The EM302 multibeam data does not appear to show a water column signature for the plume over the Sea Cliff site.

Department

Center for Coastal and Ocean Mapping

Publication Date

2-2010

Volume

91, Issue 26

Journal Title

Ocean Sciences Meeting

Conference Date

Feb 22 - Feb 26, 2010

Publisher Place

Portland, OR, USA

Publisher

American Geophysical Union Publications

Document Type

Conference Proceeding

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