Sediment Grain-Size Data from Sediment Cores Collected in March 2012 Along the Northern Chandeleur Islands, Louisiana (U.S. Geological Survey Field Activity Numbers 12BIM01)

At each of the seven sample sites, a 10.2-centimeter (cm)-diameter push core was collected. Eight sediment push cores were collected from the northern Chandeleur Islands in March 2012 (USGS Field Activity Numbers 12BIM01); core lengths varied between 45.5 and 70 cm. Accurate position and elevation data were obtained at all sampling locations with an Ashtech Differential Global Positioning System (DGPS) receiver and antenna with occupation times of 30 minutes. The accuracy of both horizontal position and elevation were improved by post-processing the DGPS data using a local benchmark, which was occupied throughout the course of each field activity by a similar DGPS system. Samples were transported to the USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) for analysis.

Prior to particle-size analysis, organic material was chemically removed for the marsh core samples using 30 percent hydrogen peroxide (H2O2). Wet sediment from the marsh samples were dissolved in H2O2 overnight. The H2O2 was then evaporated and the sediment washed and centrifuged twice with deionized water.

Grain-size analyses were performed using a Coulter LS 200 (https://www.beckmancoulter.com/) particle-size analyzer, which uses laser diffraction to measure the size distribution of sediments ranging in size from 0.4 microns to 2 millimeters (mm) (clay to very coarse grained sand). In order to prevent shell fragments from damaging the LS 200, particles greater than 1 mm in diameter were separated from all samples prior to analysis using a number 18 (1000 microns or 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. Two subsamples from each sample were processed through the LS 200 a minimum of three runs each. The LS 200 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). The size-classification boundaries for each bin were specified based on the ASTM E11 standard.

The raw grain-size data were then run through the free software program GRADISTAT (Blott and Pye, 2001; http://www.kpal.co.uk/gradistat), which calculates the mean, sorting, skewness, and kurtosis of each sample geometrically in metric units and logarithmically in phi units (F) using the 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 size scale. A macro function in Microsoft Excel, developed by the USGS SPCMSC, was applied to the data tocalculate average and standard deviation for each sample set (6 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 runs included and the standard deviation of the averaged results were summarized in an of Excel workbook with results from each core on its own tab.

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