Three-Week Expedition Images Sediments Beneath the Gulf of Alaska
In June 2011, scientists aboard the research vessel Marcus G. Langseth acquired detailed images of sediment layers and volcanic bedrock beneath the Gulf of Alaska. This research cruise was the first opportunity for the U.S. Geological Survey (USGS) to conduct scientific investigations of the central Gulf of Alaska in more than 20 years, since the GLORIA mapping program in 1989.
The primary objective was to determine the thickness of sediment along the outer parts of the Surveyor and Baranof submarine fans—delta-like piles of sediment deposited by underwater avalanches called turbidites. The scientists sought to determine whether the outer fans have sufficient sediment thickness to satisfy the criteria for U.S. "extended continental shelf"—where the nation can exercise sovereign rights over resources on and beneath the seabed according to the international Convention on the Law of the Sea. (For more information, see http://continentalshelf.gov/.)
Additional objectives were to improve understanding of the geology of this remote and little-imaged part of the ocean, to explore the processes and structures that control the distribution and movement of seafloor sediment in this region, to understand the interaction between sediment pathways and basement topography (the shape of the bedrock surface buried beneath the sediment), and to help refine the history of climatic and tectonic influences on the development of these deep-water sediment fans. Preliminary observations indicate that ridges, valleys, and other elements of the bedrock surface long buried beneath the sediment continue to control the locations of channels and other features on the seafloor.
Leading the cruise were chief scientist Ginger Barth, a USGS geophysicist, and co-chief scientist Sean Gulick, a research scientist from the Institute for Geophysics (UTIG) in the Jackson School of Geosciences, University of Texas at Austin. Also onboard for this science mission were USGS geophysicists Jonathan Childs, Patrick Hart, and Ray Sliter; USGS sound-source mechanic Jenny White; UTIG geophysicists Bobby Reece, Ryan Lester, Maureen LeVoir, and Kevin Johnson; and University of Wyoming geophysicist Erik Everson. Aboard the support vessel Norseman II, USGS scientists George Tate, Dennis Mann, and Diane Minasian, along with Woods Hole Oceanographic Institution seismic-instrument technicians David DuBois and Timothy Kane, completed the science party.
A multichannel seismic-reflection system, commonly abbreviated as a "multichannel seismic" or "MCS" system, was the main tool used to collect data during the cruise. Similar to an echosounder, a seismic-reflection system makes use of sound energy, which travels easily through water and rock and reflects off boundaries between materials with differing acoustic properties. Examples are the boundary between water and sediment (the seafloor), the boundaries between sediment layers of different materials (for example, sand versus mud), and the boundary between sediment and hard volcanic rock (the geologic "basement"). Seismic-reflection systems use sound energy at frequencies that enable the sound to penetrate deep (as much as several kilometers) beneath the seafloor and bounce off boundaries between sub-seafloor layers. The resulting "profile" is a cross-sectional view of the geologic layers beneath the seafloor, similar to what you might see in a canyon wall. The MCS data-acquisition system towed behind the Langseth during the June 2011 cruise consisted of an array of 36 pneumatic sound sources, or "air guns," to generate the sound energy and an 8-km-long streamer containing 636 groups, or "channels," of hydrophones (underwater microphones) to receive the echoes.
During the cruise, the scientists collected 3,321 km of seismic-reflection profiles along the ship's track. Fortuitously, the hydrophones also recorded seismic waves from a magnitude 7.2 earthquake that struck the Fox Islands in Alaska's Aleutian Island chain on June 24, 2011. In addition to seismic-reflection data, the scientists collected approximately 3,800 km of trackline gravity and magnetic data, multibeam bathymetric data, and chirp subbottom profiles. Like MCS reflection profiles, chirp subbottom profiles are cross-sectional views of sedimentary layers. Produced by higher-frequency sound energy, chirp subbottom profiles provide greater detail than MCS reflection profiles but do not extend nearly as deep beneath the seafloor.
To correctly interpret seismic-reflection data, scientists need to know the speed of sound in the sub-seafloor layers, which varies with such factors as porosity, grain size, and rock type. Variation in the speed of sound causes sound waves to bend, or "refract," as they move through different layers. Ocean-bottom seismometers (OBS) helped the scientists pick up refracted sound energy. (Learn more about ocean-bottom seismometers at http://woodshole.er.usgs.gov/operations/obs/whatobs.html.) Thirteen days into the cruise, the research vessel Norseman II rendezvoused with Langseth and deployed 15 ocean-bottom seismometers along two lines, each approximately 200 km long. Data recorded by the ocean-bottom seismometers will be used to calculate the speed of sound in the sub-seafloor layers, which will allow the scientists to calibrate their interpretations of the seismic-reflection images and to develop hypotheses about the nature of the sediment in the various layers and the underlying volcanic rocks of the oceanic crust.
The June cruise yielded a valuable dataset that will allow scientists to reconstruct many aspects of sediment routing and climate and tectonic history in the Gulf of Alaska region. Two major scientific results already evident from this study involve the significance of basement topography—the shape of the bedrock beneath the sediment layers. First, the Kodiak-Bowie seamount province is considerably broader and more voluminous than is evident from a bathymetric map. Sediments are surprisingly thin landward of the Kodiak-Bowies, as the basement itself is shallower than expected, and many significant seamounts (with original elevations greater than 1 km) are buried just beneath the sediment surface. Second, sediment routing throughout the vast Gulf of Alaska appears to be profoundly shaped by basement structure, even long after the primary basement topography has been completely buried. This relation holds true for features of the Surveyor and Baranof fans on a regional scale and over the course of many millions of years, as evidenced by stacked channel systems whose placement was originally controlled by fracture-zone ridges and troughs and by irregular barriers within now-buried groups of seamounts.
The data will also support further development of an Integrated Ocean Drilling Program (IODP) proposal to drill a research hole in the Gulf of Alaska, add to a growing number of seismic oceanography datasets, and contribute to the understanding of earthquake and tsunami processes on the Alaska margin.
in this issue:
Expedition Images Gulf of Alaska Sediments