Grain-Size and Data Analysis Results from Sediment Samples Collected at Crocker Reef, Florida, Between 2017 and 2019

Surface sediment samples were collected by USGS SPCMSC Self-Contained Underwater Breathing Apparatus (SCUBA) divers at 7-8 different locations within the Crocker Reef study site during 2017-2018 and 10 locations during 2018-2019. The sediment sampling sites’ water depths ranged from 2 to 16 meters. SCUBA divers wearing gloves dragged open sampling bags along an undisturbed sediment layer atop the seafloor in a single direction to ensure all grain sizes were sampled. Samples were washed with 95% ethanol to remove contaminants and brought back to the SPCMSC Sediment Laboratory to air dry completely in the fume hood.

Particle size analysis was performed on all the sediment samples collected from the Crocker Reef study site. Grain-size analyses on the samples were performed using a Coulter LS13 320 (https://www.beckmancoulter.com/) particle-size analyzer (PSA), that used laser diffraction to measure the size distribution of sediments from 0.4 microns to 2 millimeters (mm), encompassing clay to very coarse-grained sand. To prevent large shell fragments from damaging the instrument, particles greater than 1 mm in diameter were separated from all samples prior to analysis using a number 18 (1 mm) U.S. standard sieve, which meets the American Society for Testing and Materials (ASTM) E11 standard specifications for determining particle size using woven-wire test sieves. The material removed by sieving at 1 mm is recorded and reported as a percent of the total bulk dry weight with the final processed grain-size data. Two subsamples from each sample were processed through the instrument a minimum of four runs each. Once introduced into the Coulter PSA the sediment in the sample well was sonicated with a sonicator wand for 30 seconds to separate any aggregates before starting the first run for both subsamples. The Coulter PSA 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. Light is scattered by the particles into characteristic refraction patterns measured by an array of photodetectors as intensity per unit area and recorded as relative volume for 92 size-related channels (bins).

All raw grain-size data were run through the free software GRADISTAT version 8, (Blott and Pye, 2001; kpal.co.uk/gradistat), which calculates the mean, sorting, skewness and kurtosis of each sample geometrically in metric units and logarithmically in phi units (Φ) (Krumbein, 1934) using the Folk and Ward (1957) scale. 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 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 averaged results for all samples, including the number of averaged runs and the standard deviation of the averaged results were summarized in an Excel workbook with samples from each FAN on its own tab. A comma-separated values (CSV) file containing the tabular data in plain text is included in the download file.

Due to the assumption of grain sphericity of the Fraunhofer optical model used by the Coulter LS13 320, angular particles are measured by their longest axis. When enough thin, angular material is present, the Coulter output files often report a percentage of the grain-size distribution within the 1-2 mm fraction, despite all samples being sieved at 1 mm before analysis. To better assess the impact of the high content of angular shell material present at Crocker Reef on the grain-size distribution reported in the Coulter analysis, a dry sieve analysis was performed at the same major size classes for comparison. A 10–15-gram representative subset of each sample was passed through a stack of sieves with progressively smaller openings using the assistance of a mechanical sieve shaker to determine grain-size distribution. U.S. standard sieves numbers 230 (63 µm), 120 (125 µm), 60 (250 µm), 35 (500 µm), 18 (1 mm), and 10 (2 mm) were used, which meet the ASTM E11 standard specifications for determining particle size using woven-wire test sieves. All dry sieve data was reported as a percent fraction of the total bulk dry weight of each sample subset. The results for all samples were summarized in an Excel workbook with samples from each FAN on its own tab. A CSV data file containing the tabular data in plain text is included in the download file.