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Eight large (up to 200 m across) and many small massive sulfide deposits were photographed and sampled at the sediment-covered Escanaba Trough, southern Gorda Ridge, during S.P. Lee cruises L1-86-NC and L2-86-NC in 1986. The deposits are associated with two volcanic edifices within the sedimentary fill along the axis of Escanaba Trough. The sulfide and other hydrothermal minerals are deposited on the seafloor as well as within the sedimentary section. High concentrations of Pb and As in some of the sulfide samples indicates significant interaction between hydrothermal fluids and sediment at depth. INTRODUCTION Gorda Ridge is located in the northeast Pacific Ocean 200 to 300 km west of northern California and southern Oregon, within the U.S. Exclusive Economic Zone (EEZ). Escanaba Trough comprises the southernmost segment of Gorda Ridge, extending 130 km northward from the intersection of the ridge with the Mendocino Fracture Zone (40°25'N latitude, 127°30'W longitude) to a rightlateral offset of the ridge axis near 41°35'N latitude, 127°25'W longitude (Figure 1). The total spreading rate along Escanaba Trough segment is about 2.3 cm/yr (Atwater and Mudie, 1973; Riddihough, 1980), which is characteristic of a slow-rate spreading center. Along the southernmost 90 km of Escanaba Trough, the spreading axis is covered by as much as 500 m of sediment. DSDP site 35 (Figure 1) located at 40°40.4'N latitude, 127°28.5'W longitude penetrated 390 m of primarily Pleistocene turbidites without reaching basement (McManus et al., 1970). Reconnaissance seismic-reflection surveys suggested that igneous rocks intrude the sedimentary fill at several localities near the axis of Escanaba Trough (Moore and Sharman, 1970; Clague and Holmes, 1987). A detailed seismic-reflection survey conducted in 1985 aboard the U.S. Geological Survey (USGS) research vessel S.P. Lee revealed that the igneous bodies are discrete edifices with both intrusive and extrusive components and are spaced at approximately 15 km intervals along the axis (Holmes and Morton, 1986; Morton et al., 1987). The sedimentary fill of the trough in the vicinity of some intrusions has been uplifted as much as 100 m. In places fresh basalt flows are exposed on the sea floor. The areas of the volcanic edifices appear as nearly circular, highly reflective zones within the less reflective axialvalley sediments on GLORIA side-scan records (EEZ-SCAN 84 scientific staff, 1986). Reconnaissance station work conducted on the 1985 S.P. Lee cruise suggested that the volcanic centers are sites of active or recent hydrothermal activity. Heat-flow values in excess of 1200 mW/m were measured adjacent to two of the volcanic centers (Abbott et al., 1986). Pyrrhotite-rich massivesulfide samples were dredged from the flanks of the volcanic center near 40°45'N latitude, 127°30'W longitude (Holmes and Morton, 1986; Koski and Kvenvolden, 1986; Morton et al., 1987). These sulfide deposits were the first ones identified on a mid-ocean ridge spreading axis within the U.S. EEZ. One of the sulfide samples contained non-biodegraded asphaltic petroleum of hydrothemal origin (Kvenvolden et al., 1986). In 1986 we chose the volcanic edifices at 40°45'N latitude, 127°30'W longitude (SESCA) and 41°00'N latitude, 127°30'W longitude (NESCA) as sites to study the processes and products of hydrothermal circulation at a sedimentcovered spreading center. In June, a detailed bathymetric survey of a 12 x 12 km area at the SESCA site was conducted aboard U.S.N.S. Narragansett. Extensive sampling work and limited geophysical surveying were conducted aboard S.P. Lee on cruises L1-86-NC (June 30 July 14, 1986) and L2-86-NC (July 17-28, 1986). The U.S. Navy submersible Sea Cliff accompanied S.P. Lee during cruise L2-86-NC. During this operation and a subsequent leg in August, Sea Cliff completed a total of eight dives at the SESCA and NESCA sites. The submersible program was coordinated by the state/federal Gorda Ridge Technical Task Force and the results of the dive program will be reported elsewhere. In this report we summarize the station work conducted aboard S.P. Lee during cruises L1-86-NC and L2-86-NC. METHODS An acoustic-transponder positioning system provided a common base for all sample stations and geophysical lines as well as the positions for the Sea Cliff dives. We used five transponders at SESCA and four transponders at NESCA attached to tethers 184 m above the sea floor. An integrated satellite (NNSS) doppler sonar-Loran C navigation system and a satellite Global Positioning System (GPS) provided supplemental navigation during deployment and calibration of the transponder net and near the fringes of the transponder net. GPS coverage was available about 14 hours per day. We deployed the seafloor transponders during GPS coverage, thus greatly facilitating an accurate and rapid calibration of the transponder locations. During station work, a relay transponder was attached to the wire above the camera or sampling device to provide direct position information for the samples. When travel times to three or more sea-floor transponders were obtained, position errors were generally less than 3 to 5 meters, sufficiently accurate for re-occupation of station sites. In general, navigation at the SESCA site was excellent. At NESCA, the northwestern transponder lost flotation and was frequently shadowed from the ship or relay transponder. Consequently, positions for some stations in the northern part of the NESCA area were calculated using only two transponder ranges and the position errors for three-and-four-range fixes are somewhat greater than at SESCA. Underway geophysical equipment consisted of a 12-kHz echosounder and a 3.5-kHz sub-bottom profiler, both hull mounted. In addition, 160 km of single-channel reflection data were collected at SESCA and NESCA during cruise L1-86-NC utilizing an 80-in water gun as an energy source. A bathymetric survey of the SESCA site with tracklines spaced at 350 m intervals was conducted during the Narragansett cruise and of the NESCA site with tracklines spaced at approximately 300 m intervals during S.P. Lee cruises L1-86-NC and L2-86-NC. The 12 kHz echo-sounding profiles were digitized on board the ship at five minute intervals and at all inflection points. Sound velocity control for the bathymetric mapping and the transponder navigation was based on a conductivity-temperature-depth profile obtained in 1985 at 41°N latitude, 127°30'W longitude by NOAA (E. Baker, oral communication). The primary activities during both S.P. Lee cruises were sea-floor photography, dredging, and gravity coring. In addition, we conducted two hydrocast stations during cruise L1-86-NC and two fish trap deployments during cruise L2-86-NC. In addition to a relay transponder, we used a 12-kHz pinger to help position sampling devices and the deep-sea camera sled. A total of 13 camera tows were made in the NESCA and SESCA areas (Table 1). The photographic system consists of both a 35-mm still camera and a video system mounted in a rugged steel frame (Chezar and Lee, 1985). The sled was generally towed 3 to 5 m above the sea floor which yielded images about 3 to 4 meters wide. The still camera obtained one exposure every 14 seconds on Ektachrome color film. Test strips of film were developed aboard ship to check performance of the system; the remainder of the film was developed post cruise. The video system is self-contained on the camera sled. It operated simultaneously with the still camera and recorded up to 4-1/2 hours of color video during each station. Following recovery of the camera sled, the video tape was reviewed on board ship. Output from a sled-mounted temperature sensor was acoustically telemetered to the ship during camera stations providing an indication of any increases in bottom water temperature. Dredge samples (Table 2) were obtained with a round-frame chain-bag dredge. A small cloth-lined pipe dredge was mounted inside the mouth of the main dredge and was useful for collecting soft, friable material. Sediment cores (Table 3) were obtained by gravity coring using a 3-m long, 9-cm diameter barrel and a 500-kg weight stand. After recovery, cores were cut into 1 to 1.5 m sections and subsamples were taken for light and heavy hydrocarbon measurements. During cruise L2-86-NC, pore-water subsamples (Table 4) were also taken from some cores. The cores were then capped and refrigerated. Following the cruise, the cores were split, photographed on color-film with a continuous-feed camera, and X-radiographed. The subsamples for pore-water analysis were both stored and squeezed under refrigeration The two hydrocast stations utilized six 30-1 Niskin bottles attached at 50-m intervals to the main dredge wire. The two stations were conducted at approximately the same location because the lowermost two bottles did not close during the first attempt. During the second hydrocast, all except one bottle worked properly. Subsamples were drawn into acidified bottles for post-cruise determination of total dissolvable manganese concentration. The remainder of the water was filtered to determine the concentration of particulate trace metals. PRELIMINARY RESULTS The bathymetry data from the SESCA and NESCA sites were contoured at 10-m intervals and are shown in Figures 2 and 3, respectively. The SESCA site is characterized by a broad, low uplift over the entire volcanic edifice. Three prominent hills, less than 1 km in diameter and up to 100 m in elevation, are located in the northeast quadrant of the SESCA site. Seismic reflection and 3.5 kHz profiling and photographic work indicate that these hills are composed largely of sediment and have steep slopes. At the NESCA site, the axial valley floor is flanked on either side by broad, sedimented terraces that have been uplifted 100 to 150 m above the valley floor (Morton et al., 1987). The bathymetric map of the NESCA site (Figure 3) shows the floor of the axial val

Preliminary report, cruises L1-86-NC and L2-86-NC, Escanaba Trough, Gorda Ridge