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Data Series 999

Sedimentologic Characteristics of Recent Washover Deposits From Assateague Island, Maryland

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Laboratory Methods and Analyses

Navigation Post Processing

Base station data were post processed through the National Geodetic Survey On-Line Positioning User Service (OPUS), and time-weighted average positions were calculated following SPCMSC standard procedures. The time-weighted average positions were compared with the NPS network control numbers for each base station; the time-weighted elevations for each base station occupation were within 2 standard deviations of the NPS control elevations. The NPS control coordinates were used for subsequent post processing.

The NPS control coordinates of the GPS base stations were imported into GrafNav, version 8.4 (spring data) or 8.5 (fall data) (NovAtel Waypoint Product Group), and the data from the rover GPS (the GPS unit used to collect data at each sample site) were post processed to the concurrent GPS session data at the nearest base station. The final sample locations, including elevation, are the post-processed DGPS coordinates based on processing to the nearest base station; baseline distances for all sand auger sites were less than about 6 km. These data are included in the site locations files on the Data Downloads page.

Core Processing

At the SPCMSC sediment laboratory, the cores were imaged using an Ecotron EPX-F2800 x-ray unit at 90 kilovolts for 20 milliampere-seconds from a height of 79 cm. The x-radiograph was captured on an 11 by 14-inch phosphor cassette, which was scanned on an iCRco, Inc., iCR3600+ scanner at 254 pixels per inch and exported as a 16-bit Tagged Image File Format (TIFF) image. The raw x-radiographs show a slight anode heel effect, which is a variation in x-ray intensity along the anode-cathode axis that results in non-uniform pixel intensity across the image. This effect was corrected by subtracting a background pixel intensity template from each raw image (Bernier and others, 2014; Buster and others, 2014), a technique similar to methods used in coral densification studies (Chalker and others, 1985; Carricart-Ganivet and Barnes, 2007; Carilli and others, 2010). The anode heel effect has mostly been removed from the filtered images, and variations in the down-core pixel intensity correspond to variations in lamination densities and can be compared against other cores. Images were then edited in Adobe Photoshop Creative Cloud (CC) 2014 by applying a grayscale color inversion; as a result, organic-rich sediments in the cores show up white on the processed images. Cores longer than approximately 35 cm, that either did not fit onto the phosphor cassette or extended into the area at the bottom of the x-radiograph where the filter could not completely remove the heel effect, were x-rayed in "top" and "bottom" segments and then merged in Adobe Illustrator CC 2014.

Each core was split lengthwise, photographed, described macroscopically using standard sediment-logging methods, and subsampled at 1.5- to 2-cm intervals for grain-size analysis. Sample frequencies varied between cores depending on observed sedimentologic changes. Some cores were sampled at high frequencies to characterize the washover deposits, whereas in other cores the sampling strategy was more targeted. In addition to whole-core photographs, the cores were photographed in approximately 5-cm, overlapping segments with a Nikon D5200 digital camera using a macro zoom lens from a fixed height. The raw images were "stitched" together using The Panorama Factory version 5.3 software, providing seamless high-resolution whole-core images. The core descriptions, x-radiographs, and photographs can be downloaded from the Data Downloads page. Physical descriptions, including textural characteristics, for the core logs are based on macroscopic observations; the quantitative grain-size data are represented by down-core plots on the core logs (fig. 6).

Example core log 14CTB-328W.
Figure 6. Example core log 14CTB-328W. The physical description, including textural characteristics, is based on macroscopic observations; the quantitative grain-size data are represented by the down-core plots. Sediment color is based on the Munsell® soil color system. [Click image to enlarge]

Grain-Size Analysis

Grain-size analyses were performed using a Coulter LS 13 320 particle-size analyzer, which uses laser diffraction to measure the size distribution of sediments ranging from 0.4 microns (µm) to 2 millimeters (mm) (clay to very coarse-grained sand). A total of 400 samples (including 39 replicates) from 46 core sections were analyzed. Samples consisted of a 1.5- to 2-cm section of the core based on the minimum amount of material needed for analysis.

In order to prevent shell fragments from damaging the LS 13 320, particles greater than 1 mm in diameter were separated from all samples prior to analysis using a number 18 (1,000 µm, 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. Prior to sieving, each down-core sample was dried at 60 degrees Celsius for 24 hours, and the fraction of sediment greater than 1 mm was recorded as a percentage of the bulk sample dry weight. For samples (N = 60) that were visually identified as organic rich, organic material was chemically removed using 30 percent hydrogen peroxide (H202). Wet sediment was dissolved in H202 overnight. The H202 was then evaporated through slow heating on a hot plate, and the sediment was washed and centrifuged twice with deionized water. The samples were stored in the centrifuge tubes with several milliliters of deionized water until instrument analysis. Due to a limited quantity of sediment available, quantitative measurements of organic content were not practical.

Each sample was processed through the LS 13 320 a minimum of six runs. The LS 13 320 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 run through the free, widely available program GRADISTAT (Blott and Pye, 2001), which calculates the geometric (in metric units) and logarithmic (in phi units, Φ; Krumbein, 1934) mean, sorting, skewness, and kurtosis of each sample using the Folk and Ward (1957) method as well as the cumulative particle-size distribution. 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 developed by the USGS was applied to calculate the average and standard deviation of each sample (six runs per sample) and highlight runs that varied from the set average by more than plus or minus (±) 1.5 standard deviations. Excessive deviations from the mean are likely the result of equipment error or extraneous material in the sample and are not considered representative of the sample. Those runs were removed from the results and the sample average was recalculated using the remaining runs. The grain-size data are included as down-core plots with the core logs; the individual run statistics as well as the averaged run statistics and graphical class-size distributions are also available from the Data Downloads page.

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