Laboratory Methods and Analysis

Vibracores

All 65 cores collected in the field were transported to the USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) core-analysis laboratory located in St. Petersburg, Fla., where each vibracore was cut into 1-m sections and split in half lengthwise. One-half of each core was described using standard sediment-logging methods, photographed, and archived. The other half was sampled within each described sediment unit and analyzed for grain size. To retain sedimentological properties as long as possible, the core halves were sealed and stored within the USGS SPCMSC core repository.

Grain-size samples

Samples for grain-size analysis were collected from lithologic units of interest in approximately 2- to 3-cm intervals. 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 micron (µm) to 2 millimeters (mm) (clay to very coarse-grained sand). A total of 312 core samples were analyzed. The data can be acquired from the data products and downloads page.

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 (1,000 µm) 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. To provide results suitable for reproducibility and statistical error analysis, 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 ATSM E11 standard.

The raw grain-size data were then run through the free, widely available program GRADISTAT (Blott and Pye, 2001; http://www.kpal.co.uk/gradistat.html), which calculates the geometric (in metric units) and logarithmic (in phi units, Φ; Krumbein, 1934) mean, mode, sorting, and skewness of each sample 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, which was developed by the USGS, created in the computer program Excel Visual Basic, was applied to calculate the average and standard deviation of each sample set (6 runs per sample), and highlight runs that varied from the set average by more than ±1.5 standard deviations. Those runs were removed from the results and the sample average was recalculated using the valid five runs.