Click on figures for larger images

Figure 17. Map showing the station locations used to verify the acoustic data with bottom sampling and photography during U.S. Geological Survey cruises 2010-033-FA and 2010-005-FA.

Figure 22. Detailed planar view of the bathymetric data collected during National Oceanic and Atmospheric Administration survey H11922 showing the extent of the scour on the bathymetric high in the northeastern part of the study area and the locations of stations 922-1, 922-2, 922-3, and 922-194.

Figure 23. Detailed planar view of the bathymetric data collected during National Oceanic and Atmospheric Administration survey H11922 showing the extent of the scour in the southeastern part of the study area and the locations of stations 922-7 and 922-8.

Figure 24. Detailed planar view of the bathymetric data collected during National Oceanic and Atmospheric Administration survey H11922 showing the gradual transition from winnowed bouldery sea floor to the surrounding Holocene deposits in the south-central part of the study area and the location of station 922-14.

Figure 28. Map showing the station locations from U.S. Geological Survey cruises 2010-033-FA and 2010-005-FA, that were used to verify the acoustic data, color coded for sediment texture.

Figure 29. Distribution of sedimentary environments based on the digital terrain model from National Oceanic and Atmospheric Administration survey H11922 and the sampling and photography data from U.S. Geological Survey cruises 2010-033-FA and 2010-005-FA that were used to verify the acoustic data.

Figure 30. Bottom photographs from stations 922-3 and 922-7 showing the sessile fauna and flora that commonly cover boulders in the high-energy environments.

Figure 31. Bottom photographs from stations 922-13 and 922-14 showing a view of the gravel sea floor that surrounds the bouldery sea floor in the south-central part of the study area.

Figure 32. Bottom photographs from stations 922-10 and 922-16 showing views of rippled sea floor composed of fine-grained sand.

Figure 33. Bottom photographs from stations 922-12 and 922-17 showing a view of the flat to undulating to faintly rippled sea floor composed of muddy sand typical of the deeper parts of the study area.

Figure 34. Bottom photographs showing a comparison of the undulating muddy sand at station 922-22, typical of the Holocene sediments in Rhode Island Sound, with the gravelly sea floor at station 922-18, located in an adjacent scour depression.

Sediments and Sedimentary Environments

Gravel is the dominant surficial sediment in high-energy environments, such as the floors of scour depressions and on isolated bathymetric highs (figs. 17, 28, 29; for example, stations 922-1, 922-7, and 922-13). These are areas where the Holocene section is thin or absent, indicating that finer grained sediments have been eroded, exposing the coarser lag deposits of Pleistocene drift (figs. 22, 23, 24; O'Hara and Oldale, 1980; McMullen and others, 2009b). While similar to scour depressions reported earlier (Cacchione and others, 1984; Garnaud and others, 2005), floors of the depressions in Rhode Island Sound are apparently not rippled. We suspect that the coarseness of the gravel may be responsible for the absence of bedforms in these scour depressions.

Boulders and cobbles, where present, are typically covered by sessile fauna, including hydrozoans, anemones, sponges, soft corals, barnacles, and hydroids (fig. 30). The attached fauna are ecologically important because they also add to the overall benthic roughness, and indicate that the coarser gravel is immobile even during severe storms. Pea- and pebble-sized gravels and gravelly sediments surround and are present within the bouldery deposits (fig. 31). Episodic mobilization of the finer grained gravel during storm events is suggested by the presence of only sparse sessile biota and thin veneer of fine-grained sediment. Together, the gravels and gravelly sediments delineate areas of the sea floor characterized by sedimentary environments associated with processes dominated by long-term erosion and nondeposition (fig. 29; Knebel and Poppe, 2000).

Sand is the prevalent textural class in the shallower areas across northern and northeastern parts of the study area (fig. 28; for example, stations 922-10, 922-16, and 922-194). The sand there is fine to very fine grained and moderately to poorly sorted. The sea floor in the sandy areas is typically current rippled; small burrows, worm tubes, amphipod communities, and shell hash are variably concentrated in the ripple troughs (fig. 32). Sedimentary environments characterized by processes associated with coarse bedload transport prevail in the areas where small fields of megaripples have developed (figs. 22, 29).

Silty sand is the prevalent textural class in the deeper, predominantly lower energy parts of the study area (figs. 17, 28, 33; for example, stations 922-12, 922-17, and 922-175). The sand is very fine grained, mud averages over 30 percent of the samples, and the sediments are poorly to very poorly sorted. The sea floor in these muddy areas has an undulating to faintly rippled appearance and is heavily bioturbated. Large and small burrows are abundant; animal tracks are common. Sea-floor sedimentary environments in the areas characterized by these muddy sediments are dominated by the processes of sorting and reworking.

The storm-event-driven scour discussed in the bathymetry section of this report results in the juxtaposition of sea-floor areas in eastern Rhode Island Sound with distinct gravel, sand, and muddy sand textures (fig. 34). This textural heterogeneity in turn creates a complex patchwork of contrasting sedimentary environments and, presumably, habitats. Our observations of local variations in community structure suggest that this small-scale textural heterogeneity adds dramatically to the regional soundwide benthic biological diversity.

Sediment Data

The sediment grain-size dataset provided in the Data Catalog section of this report contains information on the collection, location, description, and texture of sediments at 25 stations occupied during the 2010 USGS RV Rafael verification cruise 2010-015-FA and RV Connecticut 2010-005-FA verification cruise (figs. 17, 28). All analyses were conducted in the Sedimentation Laboratory at the USGS Woods Hole Coastal and Marine Science Center. Records without textural data and statistics are based on visual descriptions. The basic structure of the data is a flat-file format, a matrix where records are rows representing individual samples and the columns contain sample- and station-specific information. This matrix consists of 42 fields which are defined in the Data Dictionary below.

The sediment data are provided in three formats: ESRI shapefile, Microsoft Excel worksheet, and delimited ASCII text format. In the delimited ASCII text file, each field or column of data is separated from the next by commas and can be downloaded into many types of software. These files are available through the Data Catalog section of this report.

Data Dictionary

An integral part of any database is the dictionary that explains the structure and content. It contains a list of the fields and the definitions of parameters measured. Data utilization is facilitated by reference to this compilation because it defines abbreviations and lists field names. A null value of -9999 is listed in fields where sediment analysis was not performed.

LABNO - Unique sample identifier assigned in the laboratory

STATIONID - Sample name or number assigned in the field

PROJECT - Project under which samples were taken or data generated

CRUISEID - Name or number of cruise on which sample was collected or station occupied

PRINCIPAL - Name of principal investigator

LATITUDE - Latitude in decimal degrees

LONGITUDE - Longitude in decimal degrees (west longitudes are negative values)

DEPTH_M - Depth of water measured by a hull-mounted fathometer overlying sediment at the time of sampling, not corrected for tides, in meters

T_DEPTH - Top depth of the sample below the sediment-water interface, in centimeters

B_DEPTH - Bottom depth of the sample below the sediment-water interface, in centimeters

DEVICE - Device used to collect the sample

MONTH - Number of calendar month during which the sample was collected

DAY - Calendar day on which the sample was collected

YEAR - Calendar year during which the sample was collected

WEIGHT - Dry weight of sample, in grams

ZGRAVEL - Gravel content in percent dry weight of the sample (particles with nominal diameters greater than 2 millimeters; -1 phi and larger)

ZSAND - Sand content in percent dry weight of the sample (particles with nominal diameters less than 2 millimeters but greater than or equal to 0.0625 millimeter; 0 through 4 phi, inclusive)

ZSILT - Silt content in percent dry weight of the sample (particles with nominal diameters less than 0.0625 millimeter but greater than or equal to 0.004 millimeter; 5 through 8 phi, inclusive)

ZCLAY - Clay content in percent dry weight of the sample (particles with nominal diameters less than 0.004 millimeter; 9 phi and smaller)

SEDCLASS - Sediment description based on a rigorous definition (Shepard, 1954)

MEDIAN - Middle point in the grain-size distribution, in phi units

MEAN - Average value in the grain-size distribution, in phi units

STDDEV - Standard deviation (root mean square of the deviations) of the grain-size distribution, in phi units (that is, sorting)

SKEWNESS - Deviation from symmetrical form of the grain-size distribution

KURTOSIS - Degree of curvature near the mode of the grain-size distribution

PHI _11 - Weight percent of the sample in the 11 phi fraction (nominal diameter of particles greater than or equal to 0.0005 millimeter but less than 0.001 millimeter); fine clay

PHI_10 - Weight percent of the sample in the 10 phi fraction (nominal diameter of particles greater than or equal to 0.001 millimeter but less than 0.002 millimeter); medium clay

PHI_9 - Weight percent of the sample in the 9 phi fraction (nominal diameter of particles greater than or equal to 0.002 millimeter but less than 0.004 millimeter); coarse clay

PHI_8 - Weight percent of the sample in the 8 phi fraction (nominal diameter of particles greater than or equal to 0.004 millimeter but less than 0.008 millimeter); very fine silt

PHI_7 - Weight percent of the sample in the 7 phi fraction (nominal diameter of particles greater than or equal to 0.008 millimeter but less than 0.016 millimeter); fine silt

PHI_6 - Weight percent of the sample in the 6 phi fraction (nominal diameter of particles greater than or equal to 0.016 millimeter but less than 0.031 millimeter); medium silt

PHI_5 - Weight percent of the sample in the 5 phi fraction (nominal diameter of particles greater than or equal to 0.031 millimeter but less than 0.0625 millimeter); coarse silt

PHI_4 - Weight percent of the sample in the 4 phi fraction (nominal diameters of particles greater than or equal to .0625 millimeter but less than 0.125 millimeter); very fine sand

PHI_3 - Weight percent of the sample in the 3 phi fraction (nominal diameter of particles greater than or equal to 0.125 millimeter but less than 0.25 millimeter); fine sand

PHI_2 - Weight percent of the sample in the 2 phi fraction (nominal diameter of particles greater than or equal to 0.25 millimeter but less than 0.5 millimeter); medium sand

PHI_1 - Weight percent of the sample in the 1 phi fraction (nominal diameter of particles greater than or equal to 0.5 millimeter but less than 1 millimeter); coarse sand

PHI_0 - Weight percent of the sample in the 0 phi fraction (nominal diameters of particles greater than or equal to 1 millimeter but less than 2 millimeters); very coarse sand

PHIM1 - Weight percent of the sample in the -1 phi fraction (nominal diameter of particles greater than or equal to 2 millimeters but less than 4 millimeters); very fine pebbles (granules)

PHIM2 - Weight percent of the sample in the -2 phi fraction (nominal diameter of particles greater than or equal to 4 millimeters but less than 8 millimeters); fine pebbles

PHIM3 - Weight percent of the sample in the -3 phi fraction (nominal diameter of particles greater than or equal to 8 millimeters but less than 16 millimeters); medium pebbles

PHIM4 - Weight percent of the sample in the -4 phi fraction (nominal diameter of particles greater than or equal to 16 millimeters but less than 32 millimeters); coarse pebbles

PHIM5 - Weight percent of the sample in the -5 phi fraction (nominal diameter of particles greater than or equal to 32 millimeters); very coarse pebbles to boulders