On Hydrate Ridge

Sat, 26 Jul 2008 11:38 PM PDT

The day began with the tolling of midnight, and while most of the ship slept we remapped Hydrate Ridge for 9 hours using the EM300 system on the Thompson. This fascinating feature was discovered years ago by scientists from Oregon State University and GEOMAR in Germany. There were several reasons for remapping, including our wish to be able to manipulate all of the raw data in new ways with the software tools we now have available.

The Sentry Survey
By 9 am the ship moved to the Sentry deployment site and by 1030 the AUV was in the water to begin its mapping work at very high definition in 1200- to 800-meter water depth. Sentry will be deployed for 17 hours, so as the day ends, it will still be in the water, working away, to be recovered tomorrow at 0330h. It is programmed to survey 5 lines 11-km long oriented north-south in the area where plans call for the Hydrate Ridge secondary node to be located. The lines extend northward to cover the initial experimental site where major seafloor expressions of hydrate deposits are exposed.

Why Study Hydrate Ridge?
Hydrate Ridge has become a major, well known, and well studied example of a major circum-Pacific phenomenon: the accumulation of icy methane and water mixtures that are stable on or below the seafloor in sediments that accumulate over zones where one tectonic plate plunges beneath another. Most developed nations have major research efforts focused on understanding, and learning to use, the energy-rich deposits of what is essentially frozen swamp gas. Our job is to configure an entirely new approach to these studies by providing novel supplies of nearly unlimited power and high-speed computer connectivity, With these capabilities, provided by the regional cabled observing system to be built and supported by the NSF Ocean Sciences Division, we will create an interactive, 25-year, seafloor laboratory that can be visited by anyone in cyberspace and will be used to launch an entirely new suite of in situ studies of the methane ice phenomena.

TowCam Surveys
Nested inside the Sentry survey were two successful TowCam lowerings also in the secondary node area on the south flank of Hydrate Ridge. The goal was to get a sense of what the seafloor was like in this area where the node and experimental sites will be deployed. The bottom sediment as expected turned out to be soft and muddy.  The operators were able to fly the camera at about 4-meter altitude above ocean bottom in a straight 1-mile north-south run covering the key areas of interest.

Working in shifts, various pilots “flew” TowCam for five passes at 4 meters above this exciting region. On the fourth line, TowCam was raised up several meters so that Sentry could continue safely on its preprogrammed survey track underneath.
Careful examination of the EM300 data indicated that the bottom sediments on the rest of the gentle slope that leads up to the summit of Hydrate Ridge were fairly uniform. This guided the decision to bring the camera up and leap frog to a survey location right at the top of the Ridge where there are known methane seeps and carbonate pinnacles located by previous studies.

Signs of Deep-Sea Venting Activity
Researcher Ko-ichi Nakamura from the National Institute of Advanced Industrial Science and Technology of Japan had mounted noel sensors on TowCam. These detectors measure changes in conductivity of the seawater. Such changes can indicate the presence of methane or sulfides in the water, both of which can be associated with deep-sea venting activity. Signals from his oxidation potential detectors are transmitted up the wire to the waiting scientists in the computer lab onboard the Thompson. These impulses indicated strong changes in water properties, so two water samples were taken with the Niskin water sampling bottles (routine equipment on the camera sled)at the precise locations exhibiting anomalously low conductivity.

Microbes that Eat Methane
As the day ends, the Arizona State University scientists will carefully filter the water samples to collect the microbial populations that come streaming out of these seafloor vent sites. Expected results may include microbes that generate methane from other gases (methanogens) or microbes that consume methane (methanotropes). The water will also be analyzed for trace chemical makeup and optical absorption.

The Data Processing Pipeline
The data from the EM300 survey that was finished at 9 am this morning has already gone through multiple processing steps and a 3D version is incorporated into the COVE software and is ready to be used to guide further exploration and fine scale mapping.  The processing pipeline starts when the raw EM300 data is stored directly on the computers operated by the ship’s marine technicians, Bill Martin and Rob Haag.

They then export it to Michael Reed and Andrew Kennedy (College of Charleston) working in the main lab at a dual console. They are equipped with the newest versions of CARIS software, which is designed to allow efficient and precise editing and refinement of the raw data. This seemingly simple processing step requires not only knowledge of the software, but also experience in working with sonar data and the characteristics of the seafloor being surveyed. Some have compared this process to “electronic barbering” where extraneous information and artifacts of overlapping track lines are snipped out to create a clean set of bathymetric data points.

After CARIS processing, Andrew, or Michael, or colleagues Josh Hill and Dax Soule, “ship” the data to Mitchell Elend, 20 feet away, who works his magic using the Fledermaus software to render the data into colorful 3D bathymetric maps.

Next in the data pipeline, Mitch sends his map on to Keith Grochow, the COVE guru, who incorporates the new map into the ever-growing database of information that is being used in real time, accessible to anyone onboard this cruise. These data will also be helpful to inform all future planning for the OOI regional scale nodes program and will be made available to all OOI facilities’ users.

Contributed by Chief Scientist John Delaney