Keck Expedition 2004
August 9, 2004 Day 11
Update for Monday August 9, 2004 (by Debra Stakes)
Today we deployed our last seismometer—a shortperiod sensor package in a cement seismonument to be place adjacent to a new cold seep site discovered near the intersection of the Nootka fault and the Cascadia subduction zone. The boxes and counters sit empty as the instruments are now on the seafloor. Electronic testing equipment is being packed away. The ocean is placid for the 6:30 am launch that carried down the instrument, the datalogger in a yellow hardhat and the pink flamingo mascot with a Canadian hockey puck on its nose. This was a site that none of us had visited before. All we have to go on is a description sent by email and a faxed map. I sit in the chief scientist chair all day as the only field geologist on board. The plan is to deploy the last seismonument right in the new seep site. The description was “fields of clams” in basins behind a ridge. Right after we land on the bottom, we ask the ROV to struggle to carry around the seismometer with its cement block and large railroad-tie anchor while we looked for the clams and a flat spot. All we found initially were empty clamshells on rugged terrain littered with fault blocks from above. So the seismometer was taken to a nice flat high point where it could record the movement of the adjacent faults without being placed at risk by a landslide. Finally, the last seismometer until next year is disappearing from view and the hard work of the cruise is over. Now there was exploration to do!
A cold seep is the type of vent associated with fluid flow in low temperature environments such as transform zones and subduction complexes. The characteristic seep animals are chemosynthetic tubeworms and clams dependent upon hydrogen sulfide (H2S), similar to communities at hot vents. What I think is very peculiar is that for cold seeps, hydrogen sulfide is not the primary gas coming from the subsurface, as it is in high temperature mid-ocean ridges. Rather, the cold seeps are venting methane (CH3), a gas produced by subsurface microbial breakdown of carbon-rich organic material that has been buried in marine sediments. But none of the animals in cold seeps can directly use the methane for food! Rather, near surface microbial processing oxidizes the methane to dissolved bicarbonate ion combined with the reduction of seawater sulfate to H2S. Microbes called Archea are thought to do the first part of the processing and bacteria accomplish the sulfate reduction. The combination of this microbial processing is to provide H2S to nourish the chemosynthetic biological communities in cold seep environments. A second byproduct is chemical precipitation of calcium carbonate from the sedimentary pore fluids that get saturated with the bicarbonate. So, in environments such as Nootka, chemosynthetic clams and tubeworms along with carbonate precipitates are fingerprints of the up-flow of fluids.
The edge of the subduction zone is defined by a series of major faults that parallel the coastline. This dive is searching for cold seep communities and carbonates where one of the most western of these faults crosses the Nootka Transform Zone. Much of the fault zone includes dramatically fractured horizons composed of sediments such as mud and sand compressed into rock. Layers of such sedimentary rocks are uplifted and broken into ridges that parallel the subduction zone, where the downgoing plate is first compressed We notice an unusually large number of animals such as octopus, skates, tunicates, crabs, snails, and medusa etc. The abundance of non-seep animals around the seep communities is very reminiscent of my dives on the Mendocino Transform Zone. Since then I have wondered whether the seeps actually support a broader range of benthic life than just the scattered tubeworms and clams. Finally we find the clam fields, nestled in a muddy pond behind the faulted ridge . An expedition earlier in the summer left the flag to mark the site. We have found the needle in the haystack!
As the ROV moves northward to follow the westward facing wall of the fault, we are amazed to find expanses of tubeworms clinging to the fault face. These look for all in the world like twisted patches of tumbleweeds attached to their plates of carbonate layers. Obviously there are methane-rich pore fluids oozing from the cracks within this escarpment. Our seismometers are in the perfect configuration to monitor the relationship between movement on this series of faults and changes in the size and biological diversity of these communities.
The biologists will study the communities themselves to understand their relationship to others in the area. The geologists and seismologists, in contrast, use the biology as a way to map the distribution and intensity of the underlying fluid flow. After the vehicle has started its trip back to the surface the scientists discuss why the animals were so sparsely and discontinuously distributed. Why is it that there were no chemosynthetic communities right at the intersection of the transform and the subduction zone. Why was one site only clams and the other only tubeworms? There are still so many specific questions to answer before we can address the interrelationships of the larger processes.