Hydrography and Geochemistry of Northern Gorda Ridge

Abstract

The physics and chemistry of the near-bottom water column above Gorda Ridge were studied for indications of hydrothermal venting. Based on initial sea-floor surveys, three field programs focused on Northern Gorda Ridge, using state-of-the-art shipboard and in situ determinations of physical, optical, and chemical properties useful in “prospecting” for active venting. The results of these investigations demonstrate the presence of active venting in at least two regions of the northernmost ridge segment—the “Narrowgate” section (GR14), and the east slope of the axial valley near its intersection with Blanco Fracture Zone (GR15). Distinct temperature anomalies (≈30 millidegree C) and suspended particle “plumes” were found at both locations during two separate expeditions in 1985. These plumes had stabilized at depths between 2450 and 2900 m and were separated from the bottom by at least 200 m. Detailed sections through the plume at GR14 demonstrated dimensions of at least 2 km (east-west) and 2 to 4 km (north-south), centered over the east wall of the axial valley. The plumes at GR14 and GR15 were not physically connected along the ridge axis. Chemical anomalies in the plumes included high concentrations of dissolved manganese, helium-3, radon-222, and iron-rich hydrothermal precipitates. The composition and hydrography of the plumes suggest that they originated from high-temperature vents. A brief reoccupation of the GR14 site in 1986 failed to relocate these strong plume features.