Monterey Bay Aquarium Research Institute

Gorda Ridge Cruise
August 5 - 21, 2002

August 19, 2000: Day #15

Dave Clague writes: Our final dive on Gorda Ridge was the milestone 200th dive for Tiburon. We planned a dive similar to Dive 199 that would cross over several sills and end at an uplifted sediment hill where we expected to locate sulfide deposits.

The dive began in the deep on the west side of the largest sill and traversed up the slope to and across the top. We encountered a deeply weathered sulfide deposit near the top of the slope, and collected several samples. The traverse across the top was entirely sediment and the ubiquitous sea cucumber. The slope down off the sill on the east side also encountered sulfide deposits. As we saw yesterday, the rim of the sill and slope are characterized by faults that are slope-parallel. Unlike yesterdays dive, the faults were entirely buried beneath sediment, and no basalt is exposed. We continued to the uplifted sediment hill and located a large region of sulfide deposits. We collected additional samples and the 200th dive was over.

The thick sediment cover and highly oxidized nature of the sulfide deposits indicate that this region is the oldest of the three that we studied. The northern Escanaba site is the youngest, although the active vents represent only a tiny fraction of past hydrothermal activity, suggesting that even the northern site is a dying system. All the sulfide deposits at each region had similar degrees of weathering and oxidation, indicating that at each region the hydrothermal activity was related to a single period of shallow magma intrusion into the sediments that filled the trough.

We are steaming to the Eel River tonight and will do a short dive there to recover the clams in a corral Jim Barry deployed in 1998 before going to port in Eureka.

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