Peat Thickness Measurements from Grand Bay, Mississippi and Alabama

During the field missions at each site, 50-cm vertical sections of sediment were extracted from the marsh subsurface using a Russian Peat Auger; each section was successively deeper than the previous until the auger met refusal at depth. Each vertical core consisted of half-cylinder sections of marsh subsurface sediment. Each core segment was scaled with a measuring tape, described in the field for basic/visual stratigraphic and sedimentologic characteristics, and photographed in the field. If the peat unit was fully penetrated, the overall thickness of the overlying organic-rich unit was documented and recorded on site. Upon return from the field, photographs of the cores were examined to verify the peat thickness field observation. If the field description and photographed organic-rich unit for each were in agreement, the marsh thickness was included. A total of 57 cores were collected within the Grand Bay study area and are included in this release.

Positional data (location and elevation) and the associated methods used to derive the horizontal and vertical positions have been published and described in Marot and others (2018), Marot and others (2019), Ellis and others (2022), and Smith and others (2020). All USGS collected location data utilized a global positioning system (GPS) base station erected on National Geodetic Survey (NGS) permanent identifier (PID) DO5987 (B166) located at the boat ramp at the end of Bayou Heron Road in Grand Bay, MS. A differential GPS (DGPS) rover receiver, with a known measured antenna height, was used in the field to collect positional data with occupation times of 30 minutes. Base station coordinates were established using the NGS On-Line Position User Service (OPUS, https://geodesy.noaa.gov/OPUS/). Location data was processed with NovAtel's GrafNav. Location data collected by the Grand Bay National Estuarine Research Reserve utilized a Trimble R10 GNSS and TSC3 data collector. Vertical and horizontal errors associated with each dataset can be found in the metadata of its associated data release.

All field-collected location data was brought into ArcGIS Pro (v. 3.1) in its native format (varied by FAN and available in the associated metadata files) and converted/projected to a Universal Transverse Mercator (UTM) planar coordinate system (zone 16) to allow for all measurements to be cartesian lengths. Geospatial variables were extracted from the marsh core locations relative to the arcuate-shaped feature of the estuarine-marsh and upland-marsh boundaries in the study area which reflect the antecedent morphology. Using ArcGIS Pro 3.1 (hereafter AGP) with a UTM projected coordinate system, the upland and river boundary from the 2015 shoreline data set used by Terrano and others (2018) were digitized. Additional smoothed, offset boundaries (buffered) were created from the detailed boundaries to reflect the more general arcuate nature of the Middle and Grand Bays using the Buffer tool in AGP and a 50-meter offset. The shoreline boundary line was converted to points with ten-meter spacing using Terrano and others (2018) transects using the AGP 'Intersect' geoprocessing tool. The 'Near Table' geoprocessing tool was used with Geodesic setting and with the core dataset as the input layer and the shoreline point shapefile as the near layer. Core coordinates (original as measured with GPS), Core Site ID (as a static input), near angle, distance, and targeted shoreline point coordinates were produced as part of the tool s results. A temporary point file shapefile (hereafter the target-shoreline point) was created from the Near Table results (approximately 500 points). The near angle computed as core-to-shoreline was transformed to bearing to reflect shoreline-to-core and added to the target-shoreline point file. The Bearing Distance to Line geoprocessing tool was used to construct a new line shapefile with the target-shoreline point as the origin, transformed bearing, and a set distance of 5000 meters. The point intersection (coordinates) between the generated lines and the upland boundary was added to the target-shoreline point file. The lines were then extended to intersect the buffered boundaries using the Extend Line tool in AGP and the distance of the line updated (i.e., distance between the modified boundaries). This distance was added to target-shoreline-point file. The target-shoreline-point file was reduced to one record per core site using the minimum distance between the modified boundaries per core site. Distance to shoreline, distance to upland, and width were computed from the coordinates of each core and the respective near shoreline point and near upland point. Distance of each new line segment (distance from to the upland boundary, distance to the shoreline, and width) along with bearing of near transect was written to the original core point shapefile s attribute table. The closest distance between a core site and nearby major tidal creeks (Bayou Heron, Middle Bayou, and Bayou Cumbest) and the water s edge (closest shoreline location) were measured. The Near tool in AGP was executed with the core point shapefile as input and line shapefile of the thalweg of each major tidal creek. The shortest distance between each core point and three tidal creeks was added to core point shapefile as distance to Bayou Heron, distance to Middle Bayou, and distance to Bayou Cumbest. Similarly, the Near tool was run between the core site and the shoreline shapefile and added to the core point shapefile as distance to water s edge. The resultant point shapefile was exported as a comma-separated values (.csv) file. The CSV file was modified in Microsoft Excel to include the following attributes: altFAN, SiteID, UTMxNAD83, UTMyNAD83, dGPS elevation, peat thickness, base peat, marsh width, bearing of marsh transect, distance to shoreline, distance to upland, distance to water's edge, distance to Bayou Heron, distance to Middle Bayou, and distance to Bayou Cumbest. All distance metrics are in meters.