USGS CMG W-4-09-WO Metadata

Gas hydrate investigation

Age and characterization of methane mounds

Our AUV mapping and ROV diving operations associated with MBARI’s northern 
expedition this summer are focused on investigatingknown gas vents, 
chemosynthetic biological communities (CBC), and gas hydrate bearing sites along 
the North American margin. The plans for the expedition involve collecting a 
series of AUV mapping surveys from the R/V Zephyr at four sites. This data will 
then be used to direct Doc Ricketts dives to ground truth the mapping surveys 
and to help direct the design of other experiments and installations. Among 
sites include the two summits of Hydrate Ridge
(Oregon), Bullseye Vent (Canada), and Barkley Canyon (Canada). These sites 
continue to be targets for exploration and experimentation by researchers at 
MBARI and many other institutions.
            The installation of a seafloor observatory to monitor natural variations 
      in gas and gas hydrate dynamics and to conduct perturbation experiments remains 
      one of the most supported concepts within the various Ocean Observing Initiative 
      efforts. The planned sites for the installation of gas hydrate observatories, as 
      part of NEPTUNE Canada (including future capacity to include an IODP CORK), are 
     Bullseye vent and Barkley
Canyon. The other candidate for a cable-connected system is Hydrate Ridge. While 
an initial deployment of instruments is planned for NEPTUNE Canada in 2009, 
detailed high-resolution mapping is still lacking in parts of these areas and is 
necessary for the long-term planning for expansion of hydrate observatory 
infrastructure. This can be best (and perhaps only) provided with a system like 
the MBARI mapping AUV.
            A particular focus of this expedition is the nature and origin of the 
      micro-topography associated with these gas-rich seafloor environments. Visual 
      observations in many such environments have already shown that the seafloor 
      where CBCs are on the surface and gas hydrate are believed to be in the near 
      subsurface is commonly associated with seafloor blisters or mounds of various 
      sizes, shallow depressions (e.g., ~3 m wide and ~1 m deep), as well as an 
      occasional small up-turned ridge of truncated strata. The origin of this 
      topography remains unexplained, yet is potentially critical for assessing the 
      extent to which gas hydrates are geo-hazards and to understand how gas hydrates 
      may shape the seafloor. Moreover, up-to-date maps that document these features 
      do not exist, largely because of the limitations of previously available 
      surveying tools. We are interested in documenting these features and evaluating 
     two possible processes that may play a role in creating these features:
(a) The area around these vents may be experiencing seafloor modification 
because
of gas hydrate growth. Subsurface gas hydrate formation may inflate the seafloor 
producing pingo-like features that range from 1 to 10 m in height and width. 
When sections of seafloor contain >92% gas hydrate by volume they are buoyant in 
seawater.
Thus, gas hydrate-bearing sediment masses may break loose from the seafloor and 
float away, leaving depressions on the seafloor (Paull et al., 1996). Repeat 
visits with an ROV or HOV to vent sites in the Gulf of Mexico (Roberts et al., 
2001) and Barkley Canyon
(Ross Chapman, personal communication) have shown that gas hydrate mounds 
observed in previous years have disappeared, apparently leaving holes. 
Presently, it is unknown whether these changes are of a magnitude that will be 
within the resolution of the
mapping tools on the AUV.
(b) We will also focus on documenting the extent and importance of seafloor 
erosion on the flanks of these structures. Occurrences of carbonate-cemented 
nodules and concretions exposed on the seafloor that contain cements with light 
carbon isotopes, indicating a contribution of methane-derived carbon, are 
commonly interpreted to be indicators of seafloor fluid venting and occur around 
all these vent systems. However, the fine-grained clastic sediments that 
typically host these carbonates indicate that many, if not most, of these 
carbonates were formed within the subsurface, presumably associated with 
anaerobic oxidation of methane at the sulfate-methane transition zone. We 
hypothesize that the widespread occurrence of these carbonates on the seafloor 
may result from persistent seafloor erosion, which preferentially removes the 
uncemented sediments and leaves the more resistant, cemented sediments standing 
in relief. Combined multibeam bathymetry, sub-bottom profiling, and side-scan 
sonar data obtained by the mapping AUV will provide a basic context to 
understand the nature of these systems and the broad area around them. ROV 
ground truth sampling also will be required to evaluate these questions.
      An MBARI mapping AUV survey was conducted at Barkley Canyon in 2007.
However, we are interested in returning, in part, to assess whether any changes 
in the morphology can be quantified and to refine and enlarge the existing 
survey coverage.
The existing schedule allows for up to 7 18-hour long AUV mapping survey 
missions. We have scheduled this as 2 at Hydrate Ridge (44° 34’N 125° 09’W); 2 
at Bullseye vent (48° 41’ N 126° 51’W); 2 at Barkley Canyon (48° 19’N, 126° 04 
W); and one as a contingency. These would be followed by a 10-day Western Flyer 
expedition out of Newport, Oregon, to ground truth these survey areas. We intend 
to work with Ross Chapman (University of Victoria) on the Barkley Canyon surveys 
and Michael Riedel (McGill University) on the Bullseye vent surveys, both as a 
scientific collaboration and to assist in plans for the installation of Neptune 
observatories.
            The day-by-day plan for this trip will be highly dependant on the weather 
      and the results of the AUV survey’s. However, it is likely that we will dive on 
      Hydrate Ridge the first day, as it will make logistical sense. On this dive we 
      will focus on sampling up the side of the feature called the Pinnacle which has 
      that has been described as an upward growing chemoherm (e.g., Teicher et al., 
     2005), but we hypothesis it is an erosional feature (Paull and Ussler, 2008).
At Barkley canyon we anticipate picking up some gear of Ross Chapman’s. At 
Bulleye Vent we expect to deploy an Osmo-sampler probe that is to be stuck into 
the sediment. This probe is designed by Laura Lapham. We believe that this is of 
a size that will allow it to be carried on the Doc Ricketts.

____, 2009, 2009 Pacific Northwest expedition leg 3
Monterey Bay Aquarium Research Institute, accessed 22 October 2009, URL:
http://www.mbari.org/expeditions/Northern09/L3/index_L3.htm