Textural description of surface sediment samples collected in August 2015 from Dauphin Island and the surrounding areas

303 surface sediment samples were collected on and around Dauphin Island by hand, PONAR grab, or a VanVeen sediment sampler attached to a miniSEABOSS. Samples were transported to the USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) for analysis. Of the 303 surface samples collected, seven were not analyzed due to an abundance of oyster shells. Prior to particle size analysis, sediment samples which appeared to consist of greater than 5 percent organic material had the organic matter chemically removed from the samples using approximately 8 mL of 30 percent hydrogen peroxide (H2O2) overnight. The H2O2 was then evaporated through slow heating on a hot plate, and the sediment was washed and centrifuged twice with deionized water. Samples which appeared to consist of less than 5 percent organic material were dried at 60 degrees C. To prevent shell fragments from damaging the Coulter LS200, particles greater than 1 mm in diameter were separated from all samples prior to analysis with a number 18 (1000 µm) U.S. standard sieve, which meets the American Society for Testing and Materials (ASTM) E11 standard specifications for determining particle size with woven-wire test sieves. The samples were washed through the sieve with filtered municipal water and a few milliliters of sodium hexametaphosphate solution to act as a deflocculant. The sediment slurry was sonicated with a wand sonicator for 30–60 seconds before being introduced into the Coulter LS200 to break down aggregated particles. Two subsamples from each sample were processed through the Coulter LS200 with a minimum of four runs apiece. The Coulter LS200 measures the particle-size distribution of each sample by passing sediment suspended in solution between two narrow panes of glass in front of a laser. The particles scatter light into characteristic refraction patterns that are measured by an array of photodetectors as intensity per unit area and recorded as relative volume for 92 size-classification channels, or bins. The size-classification boundaries for each bin were based on the ATSM E11 standard specification for wire cloth and sieves for testing purposes.

The raw grain-size data were then run through the free software program, GRADISTAT version 8, (Blott and Pye, 2001; kpal.co.uk/gradistat),which calculates the mean, median, sorting, skewness, and kurtosis of each sample geometrically in metric units and logarithmically in phi units (Krumbein, 1934) using the Folk and Ward (1957) method. GRADISTAT also calculates the fraction of sediment from each sample by size category (for example, clay, coarse silt, fine sand) based on a modified Wentworth (1922) size scale. A macro function in Microsoft Excel, developed by the USGS SPCMSC, was applied to the data to calculate the average and standard deviation for each sample set (8 runs per sample), and highlight runs that varied from the set average by more than ± 1.5 standard deviations. Excessive deviations from the mean are likely the result of equipment error or extraneous organic material in the sample and are not considered representative of the sample. The highlighted runs were removed from the results, and the sample average was recalculated using the remaining runs. The raw, unaltered, individual run statistics and class size distribution as produced by GRADISTAT version 8 (Blott and Pye, 2001), as well as the averaged run statistics and class size distributions, are available upon request. The averaged results for all samples, including the number of runs included and the standard deviation of the averaged results were summarized in an Excel workbook.

Added keywords section with USGS persistent identifier as theme keyword.