Gorda Ridge Cruise
August 5 - 21, 2002

August 16, 2000: Day #12

A deep fissure in the sediments in the northern Escanaba Trough.

Dave Clague writes: Dive 197 returned to the active hydrothermal vents at the northern Escanaba Trough where we collected four more fluid samples (since most of the samples collected during earlier dives were mainly seawater with only a small hydrothermal fluid component). Preliminary analyses indicate that the latest samples are also mainly seawater, so we did not acquire high-quality samples of the hydrothermal fluids from this site. The difficulty in collecting good samples reflects the low flow rates and diffuse nature of the fluid flow at the present time.

After completing the water sampling, we explored the region north of the known active vents. Several dredges recovered fresh high-grade sulfide samples from the region in the 1980s. The northern part of the uplifted sediment hill has many inactive sulfide mounds, which are easy to find using the sonar on Tiburon. We collected some samples from these mounds to compare their mineralogy with that of the active mounds. North of the sediment hill, the sediments are complexly fractured and fissured. A lava flow drapes the bottom in places, but is broken up by the faulting. Hydrothermal sulfide also is abundant through the area, but we located no active vents.

An exciting discovery on the dive was the occurrence of tar hydrocarbon deposits on the seafloor. These formed from petroleum produced by the hydrothermal heat from organic carbon in the sediments. We recovered a large mound of the tar and a core collected beneath it smelled strongly of hydrocarbons. We looped through the chaotically fractured area and returned to the sediment hill, looking for additional sulfide deposits. The dive ended at the edge of a large young lava flow that will be the region explored on tomorrow's dive.

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