Sediment Texture of the Sea Floor offshore of western and southern Martha's Vineyard and north of Nantucket, Massachusetts

The texture and spatial distribution of sea-floor sediment were qualitatively analyzed in ArcGIS using several input data sources (listed in the source contribution), including acoustic backscatter, bathymetry, seismic-reflection profile interpretations, bottom photographs, and sediment samples. The interpretation was initiated by creating a new polygon feature class in an ArcMap 10.5 file geodatabase based on the extent of the regional bathymetric DEM (Andrews and others, 2018, and clipped to the extent of this study area). The polygon was then partitioned into multiple sediment texture polygons using 'cut polygon' and 'auto-complete polygon' in an edit session. In general, polygon editing was done at scales between 1:8,000 and 1:25,000, depending on the size of the traced feature and the resolution of the source data. Some areas interpreted as a single sediment textural unit may contain multiple polygons that indicate different interpretation confidence levels. The following numbered steps outline the workflow of the data interpretation. 1. Backscatter intensity data was the first input. Changes in backscatter were digitized to outline possible changes in sea-floor texture on the basis of acoustic return. Areas of high backscatter (light colors) have strong acoustic reflections and suggest boulders, gravels, and generally coarse sea-floor sediments. Low-backscatter areas (dark colors) have weak acoustic reflections and are generally characterized by finer grained material such as muds and fine sands. 2. The polygons were then refined and edited using gradient, rugosity, and hillshaded relief images derived from interferometric and multibeam swath bathymetry. Areas of rough topography and high rugosity are typically associated with rocky areas, while smooth, lower-relief regions tend to be blanketed by fine-grained sediment. These bathymetric derivatives helped to refine polygon boundaries where changes from primarily rock to primarily gravel may not have been apparent in backscatter data, but could easily be identified in hillshaded relief and slope changes. 3. The third data input (where available) was the stratigraphic interpretation of seismic-reflection profiles, which further constrained the extent and general shape of sea-floor sediment distributions and rocky outcrops, and also provided insight concerning the likely sediment texture of the feature on the basis of glacial or post-glacial origin. Seismic lines and the surficial geologic maps derived from them are used here as input data and were collected at typically 100-meter spacing, with tie-lines generally spaced 1-km apart. 4. After all the sea-floor features were traced from the geophysical data, a spatial join was conducted to provide grain size statistics based on sediment samples. The fields that contain sediment texture statistics or mean water depth information were created and populated using data joins or zonal statistics functions within ArcMap (version 10.5). The fields beginning with "Avg_" and the 'Count_' field were automatically generated by computing a data join where the CZM sample database (vector points) was edited to include only the samples with laboratory sediment analysis and joined to the qualitatively derived polygon file. The fields beginning with "Avg_" and ending in "_1" were automatically generated by computing a data join where the CZM sample database (vector points) included samples with laboratory and visual sediment analysis. Each polygon was given an average of the numeric attributes of the points (with laboratory grain size analysis or with and without laboratory analysis) that fall inside it, and the count field shows how many laboratory analyzed points fall inside each polygon. 153 samples were analyzed in the laboratory. Several fields that were not needed were deleted after the join. A mean water depth (NAVD 88) field was created using ArcMap (version 10.5): ArcToolbox - Spatial Analyst Tools > Zonal > Zonal Statistics as Table, where the mean water depth for each polygon (input zone data using the zone field sed_type) was derived from the regional bathymetric DEM (see Andrews and others, 2018). No data raster values were ignored in determining the output value for each polygon zone. The output was saved to a table, which was joined with the sediment type polygon shapefile. All of the joined fields except MEAN were turned off. 5. A new field was created in the shapefile called 'Barnhardt'. Seafloor composition observations from sediment samples and bottom photographs/video were used to define sediment texture for the polygons using Barnhardt and others (1998) classification. Samples with laboratory grain size analysis were preferred over visual descriptions when defining sediment texture throughout the study area; however within large polygons with a small number of samples (1 or 2), visual samples were considered and used to provide additional insight on the mean sediment texture. Bottom photo stations are typically around 2-km apart, but do not always provide a clear view of the sea floor, and the density of sediment samples varies throughout the study area. Some polygons contained more than one sample with grain-size statistics, while others contained none. For multiple samples within a polygon, the dominant sediment texture was used to classify sediment type (often aided by the 'data join' sediment statistics described in an earlier processing step). In rocky areas, bottom photos were used in the absence of sediment samples to qualitatively define sediment texture. Polygons that lacked sample information were texturally defined through extrapolation from adjacent or proximal polygons of similar acoustic character that did contain sediment samples.

After some additional qualitative polygon editing and reshaping was done in order to create a sediment map that was in the best agreement with all input data: lidar, bathymetry, backscatter, seismic interpretations, bottom photographs, and sediment samples, 4 more fields were added (ArcMap version 9.3.1). The first field, 'simple' is just 3 classes: sand, mud, or hardbottom. A field 'Confidence' was added as a data interpretation confidence, which describes how confident we are in the interpretation on the basis of the number and quality of the input data sources (see the entity and attribute sections for more information on these fields). The last 2 ('Carbonate' and 'Biogenic_C') fields were added and populated manually to incorporate percent carbonate information that accompanied sample analyses in Nantucket Sound. Finally, a second join was conducted similar to the previous processing steps, but allowing all samples with analysis (visual and laboratory) to be incorporated into the statistics.
The polygon feature class containing the sediment texture units was assigned topology rules, (i.e. no gaps and no overlaps). Topology errors were identified and remedied using the topology toolbar in ArcMap (10.5). Finally, the sediment texture was exported from the geodatabase as a shapefile.

Added keywords section with USGS persistent identifier as theme keyword.