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Sampling the Gulf of Maine Seabed from the Ocean Survey Vessel Bold
The geology of the ocean floor of the Gulf of Maine is spatially complex at many scales. Regionally there are areas of shallow rocky ledges adjacent to deeper muddy valleys and basins. Some basins tend to be isolated while other deep areas can be connected around shoals or underwater ridges. The complex spatial relationships between bottom relief (water depths) and sediment types may be important in understanding a variety of marine habitats and ecosystems in the Gulf of Maine (Figure 1; Dickson, 2004). From July 3 through 9, 2012 the U.S. Environmental Protection Agency Ocean Survey Vessel (OSV) Bold (Figure 2) worked offshore of mid-coast Maine in an area of limited prior research, including depth measurements (Figure 3). This web site describes the geological aspect of a collaborative investigation conducted in the summer of 2012 to map an area about 12 nautical miles south of Boothbay Harbor and Pemaquid Point (Figure 3).
Maine's inner continental shelf is primarily composed of rock and mud environments. Together these two geological categories make up about 80% of the seafloor (Figure 4; Kelley and others, 1998). The remainder of the sea bed is composed of sand or gravel. In the mid-coast region, prior investigations (Kelley and others, 1987; see also other Maine Geological Survey Open-File Reports) determined that sediments are often a legacy of the last Ice Age. Deposition of sediment such as sand, gravel, and boulders in the form of glacial till occurred either under ice or at the ice margin (Figure 5) as the glacier retreated northward (Kelley and others, 1987).
Following the Ice Age, fine silty and muddy sediments have been deposited in the open Gulf of Maine and often bury the glacial sediments. In some areas these post-glacial or "modern" mud deposits can be thick while in other areas they can be so thin that glacial boulders are not yet buried. Geologists can investigate these same relationships with remote sensing and sampling on land in areas that were once submerged by marine waters (Thompson and Tolman, 2011). Maine's Ice Age Trail (Thompson and Borns, 2007) as well as MGS Surficial Geology Field Localities provide examples and land locations to visit in Maine.
The Maine Geological Survey and University of Maine have collaborated to map the Surficial Geology of the Maine Inner Continental Shelf (Barnhardt and others, 1996) to reveal the complexity of the seafloor using a combination of acoustic surveying using side-scan sonar and seismic reflection profiling and bottom sampling with grab samples and cores (Kelley and others, 1987, 1998). Sediment samples from the Gulf of Maine have been compiled by the U.S. Geological Survey (Poppe and others, 2005; Reid and others, 2005).
Depth measurements in some areas are sparse and lead to low-resolution bathymetric maps that only represent broad characteristics of the bottom over wide areas (Figure 3) so interpretation of sediment characteristics with low-resolution maps can be challenging. On a small spatial scale of a nautical mile or less there can be dramatic changes in water depth, slopes, and geology.
Maine Seafloor Mapping Survey
From July 3 through 9, 2012 the U.S. Environmental Protection Agency Ocean Survey Vessel (OSV) Bold conducted a benthic mapping research cruise under the direction of EPA Chief Scientist Matt Liebman and Principal Investigators Matthew Nixon of the Maine Coastal Program, Stephen Dickson of the Department of Conservation, Maine Geological Survey, and Carl Wilson of the Maine Department of Marine Resources. Additional scientists from the Maine Geological Survey, University of Maine, University of New Hampshire, and the Biodiversity Research Institute of Gorham, Maine collaborated in a multidisciplinary effort that included multibeam bathymetric mapping as well as seabird and marine mammal observations.
Mapping with Side-scan Sonar
One of the best ways to map the ocean floor is with side-scan sonar. Using sound sent out from a "towfish" sound waves are bounced off the bottom, received back at the tow-fish, and processed into images based on the strength of the returning sound energy. Sonar imagery is the equivalent of aerial photographs that are used by geologists on land to study landforms and sediments. The OSV Bold uses a Klein side-scan sonar (Figure 6) that sends signals into the sonar lab (Figure 7). Screen images of the sea floor are selected and stored by scientists and technicians on board the ship in real time (Figure 8). These images are about 650 feet (200 meters) in width and have the ship track in the center. This collection of sonar images (Figure 9) is then analyzed and prioritized to select images representative of various bottom types and broad areas of the sea floor.
The description provided below is on the geological sampling that was done in order to "ground truth" side-scan sonar images and document sediment types offshore of mid-coast Maine.
On the ship, 12 samples were collected with a grab sampler (Figure 10) by Kara Jacobacci, Robert Johnston, Patrick Ryan, and Robin Arnold with the technical assistance of the crew of the OSV Bold. All of the samples were taken from water depths - not corrected for tidal stage - between 440 and 525 feet (134 to 160 m) with the exception of one sample (MSMS_0135L) that was from a shallower depth of 390 feet (118 m). The color of all the samples was dark yellowish brown (Munsell color 10 YR 4/2; GSA, 1994). Table 1 summarizes the location of the samples and what was noted in the sonar images. Figure 11 shows sample locations on a map.
Grain size analysis was completed at the University of Maine by Jacobacci (2012) with the methods of Folk (1980) using settling tubes for the silt and clay fractions. Results are shown in Figure 12, Figure 13, and Figure 14. Most of the samples can be classified as clay or sandy clay with very little silt. Two samples were coarser and contained gravel. The shallowest sample (MSMS_0135L) was gravelly muddy sand but a gravelly mud was also recovered from a water depth of 485 feet (148 m; MSMS_192L; Table 2).
In addition to grain size, both the water content and organic content of the sediments was measured. Water content averaged 57% with a range of 45% to 62% (Table 3). Organic matter averaged 9% by weight within a range of 5 to 16% (Table 4). Marine worms or worm casts were common in all samples and a sea mouse was recovered at station MSMS_0135.
The area investigated with grab samples represents conditions over about 9 square nautical miles across a section of the Gulf of Maine sea floor that is about 500 feet deep. The seafloor is predominantly sandy mud containing nearly 60% of its weight in water and 10% of its weight in organic matter. Side-scan sonar images show boulders ranging in size from a few feet to tens of feet in diameter are scattered across this muddy bottom. Sonar images also detected bottom trawl marks, some new and some perhaps old, from fishing activity. These data, combined with processed side-scan sonar mosaic maps, will be critical to geological mapping a new area of Maine’s inner continental shelf.
References and Related Web Sites
Barnhardt, W. A., Belknap, D. F., Kelley, A. R., Kelley, J. T., and Dickson, S. M., 1996, Surficial Geology of the Maine Inner Continental Shelf, a series of 7 maps covering coastal Maine, scale 1:100,000: Maine Geological Survey, Augusta, Maine.
Dickson, S. M., 2004, An Underwater View of the Gulf of Maine Sea Floor, Maine Geological Survey, May 2004 Site of the Month.
Folk, Robert L., 1980, Petrology of Sedimentary Rocks, Hemphill Publishing Company, Austin, Texas, 184 p.
GSA, 1984, Rock-Color Chart, prepared by the Rock-Color Chart Committee, E. N. Goddard, Chairman, Geological Society of America, Boulder, Colorado, 16 p.
Jacobacci, K., 2012, OSV Bold Grab Sample Report, July 2012 Maine Seafloor Mapping Survey, prepared for the Maine Geological Survey and Maine Coastal Program, Maine Department of Agriculture, Conservation and Forestry, September 25, 2012.
Kelley, J. T., Barnhardt, W. A., Belknap, D. F., Dickson, S. M. and Kelley, A. R., 1998, The Seafloor Revealed - The Geology of the Northwestern Gulf of Maine Inner Continental Shelf: Maine Geological Survey, Open-File Report 96-6, 55 p.
Kelley, J. T., Belknap, D. F., and Shipp, R. C., 1987, Geomorphology and Sedimentary Framework of the Inner Continental Shelf of South Central Maine: Maine Geological Survey, Open-File Report 87-19, 76 p.
Maine Geological Survey Open-File Reports on the Geomorphology and Sedimentary Framework of the Maine Inner Continental Shelf.
Poppe, L. J., Eliason, A. E., Fredericks, J. J., Rendigs, R. R., Blackwood, D. S. and Paskevich, V. F., 2005, USGS East-Coast Sediment Analysis: Procedures, Database, and GIS Data, U.S. Geological Survey Open-File Report 2005-1001.
Reid, J. M., Reid, J. A., Jenkins, C. J., Hastings, M. E., Williams, S. J., and Poppe, L. J, 2005, usSEABED: Atlantic coast offshore surficial sediment data release: U.S. Geological Survey Data Series 118, version 1.0.
Thompson, W. B. and Borns, H. W., Jr., 2007, Maine's Ice Age Trail, Maine Geological Survey, July 2007 Site of the Month.
Thompson, W. and Tolman, S., 2011, Lidar Imagery Reveals Maine's Land Surface in Unprecedented Detail, Maine Geological Survey, December 2011 Site of the Month.
U.S. Environmental Protection Agency, 2012, Ocean Survey Vessel (OSV) Bold.
Through the advocacy of Maine's Senators Olympia Snowe and Susan Collins, the U.S. Environmental Protection Agency, under the leadership of Lisa Jackson, provided ship time in August. Chief Scientist Matthew Liebman from EPA provided great knowledge and leadership on board at all hours of the day and night. Principal Investigator Matthew Nixon of the Maine Coastal Program was visionary in arranging a large collaborative effort that integrated several marine sciences and maximized ship time.
We are very thankful for the exceptional skills and cooperation provided by Captain Jerel Chamberlain, mates, technicians, and crew of the OSV Bold provided through Seaward Services, Inc. of Dania Beach, Florida. With their strong commitment to safety and success, the research cruise and data quality are sound and will advance our scientific understanding and maximize use of our ocean resources.
Text by Stephen M. Dickson. Laboratory analysis by Kara Jacobacci.
Originally published on the web as the October 2012 Site of the Month.
Last updated on October 29, 2012
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