Sediment Channels Discovered

Thu, 24 Jul 2008 11:06 PM PDT

At 0330h the Thompson arrived near the survey site for Node 1, Hydrate Ridge. The speed of the ship was slowed from 11 to 9 knots and the next 19 hours were spent using the EM300 multibeam sonar system, mounted on the hull of the ship, to map an area 67- by 20-km in size. The slower speed helps ensure higher quality bathymetric maps.

Main Cruise Goal

Our main goal for this cruise is to ascertain the optimal location for two of the primary nodes on the Regional Scale Nodes. These nodes will be large, heavy, and delicate power and communication stations designed to last 25 years on the seafloor. Putting them in protected locations is very important. Of course finding a protected location at the edge of a large tectonic plate that is plunging under North America is a bit of a challenge.

Choosing the Location for Node 1

The surveyed area had not been previously mapped with an up-to-date multibeam system. Our results provided crucial input to the immediate decision we had to make about where Node 1 should be located.  A number of months ago a team of scientists and engineers at the University of Washington, lead by Dr. Deborah Kelley, had chosen a site based on the available but somewhat limited data. Our cruise is configured with two types of multibeam systems that will add to the data available for final evaluation of the node site near Hydrate Ridge.

The purpose of the EM300 survey was to zero in on the best location for Node 1 before the Sentry AUV, which “swims” just above the bottom and is equipped with a very high resolution multibeam mapping system, is sent down to conduct a much higher resolution survey of the selected area.

Unexpected Features

The EM300 maps revealed unexpected features on the seafloor in the area under consideration: several north-south trending, well-developed channels in the sediments in the flat-lying edge of the abyssal plain at a depth of ~2000m below sea level.

These channels lie generally parallel to the steep scarp that forms the edge of the continental slope. The geologists onboard are speculating about how these channels form and how frequently they host energetic sediment “flooding.” A land-based analogue might be streams on the high plains east of the Rocky Mountain front near Denver, Colorado. It is possible that these undersea channels could pose hazards to a nearby node.

Analysis of the new maps led to the decision to keep the initial location that had been earlier indentified by the team led by Deb Kelley as a likely site for Node 1. In fact, one of the channels, estimated to be about 7 meters deep, is positioned between the scarp and the node and may protect the node from the sediments and rock debris that continually cascade down the steep face of the continental slope toward the edge of the abyssal plain.

Finalizing the Night's Schedule

As the evening closes, the night’s schedule is finalized. Once we reach the selected site, four hours will be spent deploying the transponders that Sentry uses for navigation. Nested surveys will then begin, with Sentry mapping out in great detail a 3- by 2-km box on the eastern edge of the abyssal plain, and the TowCam simultaneously photographing the same seafloor. This Sentry/TowCam “minuet” is a dance of the two platforms on the deep seafloor that has been done several times before; it requires skill and a fine sense of timing on the part of the two operators (Dana Yoerger for Sentry and Dan Fornari for TowCam). TowCam’s photography of the seafloor will help characterize sediment types and give the geologists and engineers a sense of the details 2000m deep. With the blessing and innovation of Dan Fornari, we also intend to use TowCam in a new mode to collect a sediment core from the prospective node site.

Stay tuned…..

Contributed by Chief Scientist John Delaney