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

August 24, 2001: Day #5

Shana Goffredi directing the dissection of seep clams, assisted by Susan Potter and Betsy Berg.

Log Entry: The aim of the dive today was to return to the cold seep site discovered last year to see if the vent community was still there. We also wanted to see if we could find more seeps. By 1400 hours, we had come across four new seep communities.

Seismic studies carried out in 1999 revealed that a rocky basement ridge, normally draped by sediment, was exposed at the seafloor at this location. A strong negative polarity reflection (BSR) in the region suggested the presence of free methane gas adjacent to the south face of the basement ridge. Isotopic studies from Ocean Drilling Program hole 1022C sediments suggest extensive methanogenesis, and the methane is apparently channeled laterally and up section to the BSR. Last years’ dive targeted a small section of this exposed basement. The presence of methane supports chemosynthetic communities via the coupled microbial reactions of methane oxidation and sulfate reduction. The reduced sulfide is the primary nutrient for the chemosynthetic fauna diagnostic of such benthic communities.

The cold seep we found last year is unique because it is the first vent community found on an active transform fault and associated with gas hydrate. The community is also unusual in that the animals appear to live both on sediment and tucked in and around basaltic boulders. The basement rocks provide a permeable pathway for the methane to migrate to the surface. This is why we wanted to go back and study this cold seep further. We want to carry out population studies on the chemosynthetic seep animals, and study the microbial activity within the cold seep area. 

Tiburon landed near last years’ seep location at 0730 local time this morning. But there was no sign of last years’ sulfide rich mud populated by chemosynthetic clams. Instead, the area was now populated by big, well fed octopuses. We came to the conclusion that the octopus may just have a taste for hydrogen sulfide. However, we knew were somewhere within 50 meters of last years’ seep location, and may have missed it. 

Tiburon continued its journey on the sediment covered seafloor. We saw octopuses, brittle stars, sea stars, anemones…and eventually we came across a clam. This clam was our guide to the first cold seep location of the morning. The seep was small but we got a good selection of tubeworms and clams. The water temperature at the seep was 2.5 degrees C so we really are talking about a COLD seep here. The clams populated areas of sulfide rich mud (see picture), and tube worms were found in cracks in basaltic bedrock.

We followed the rocky outcrops on the seafloor, and at 1100 hours we found the second seep of the day. This seep was very much like the first one but much larger and extended 200 meters across a region of angular basaltic blocks of bedrock.

The third and fourth community we observed had very little sulfide rich mud to live on and appeared to be located on almost bare bed rock. 

We also came across several benthic communities, which were clearly concentrated within regions of increased organic content, but no chemosynthetic fauna was present. We did follow the exposed ridge most of the dive, and would have hoped to see more chemosynthetic clams and tubeworms but this did not happen. This suggests that the flow of methane is discontinuous, and constrained by the permeability of the overlying rocks.

Tiburon was back on board by 2030 hours local time . The night was spent dissecting the clams and tubeworms collected for preservation and further study back at MBARI (see picture above of Goffredi, Potter, and Berg).

This gave us a chance to use the water sampler to study the microbial content of the cold seep fluid

We discovered a carbonate chimney with some evidence for fluid flow. This carbonate chimney is likely to be the product of authigenic carbonate precipitation.


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