Mendocino Fracture Zone Cruise - 2001
August 20 - 30, 2001
Over 300 km off the California-Oregon Coast

August 26, 2001: Day #7

Fluidization structures found at 1575 meters during the dive today. This is an indication of methane release in the past which had caused some sediment instability.

Log Entry: Tiburon was lowered into the water at 0627 hours local time. Our aim was to follow several bathymetric features where methane might be expected to seep out from the sediments according to the seismic section and EM300 multibeam data. We started at a depth of 2,155 meters and Tiburon moved up an erosional channel on the eastern part of the Gorda Escarpment. At 1,575 meters we encountered fluidization structures (see picture above), which was an indication to us that methane release had caused instability in the sediments. The seafloor was covered in a thin layer of pelagic sediment, with parallel coarse-grained channels running downhill. These channels are around 0.5 m wide and do not show significant topography. Locally manganese coated carbonate blocks were lying chaotically near the channel. We followed one of the channels to its’ source where it disappeared into a pavement of continuous carbonate. It was clear that the hard substrate underneath the pelagic sediment was carbonate. It was disappointing that there was no evidence for recent venting.

We continued to see fluidization structures at frequent intervals. But no sign of clams or recent fluid flow. Eventually we came across a clamshell, and our hopes for finding some (we need 30, the magic number for population studies) clams alive were high. We did indeed find a clam field (see picture). The clams populated a radial structure in sulfide rich mud, however there was little evidence for fluid flow. We know that methane must be seeping out from the sediment to support this chemosynthetic community. The radial pattern suggests that the methane seepage is controlled by fractures and cracks in the underlying rock. The sulfide-rich muds were surrounded by blocks of carbonate which were collected for further study.

We also collected several pushcores to study the microbial population in the sediments (see picture) both near and away from the clams.


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