Sedimentary Data From Grand Bay, Alabama/Mississippi, 2014-2016

At 21 sites along the western margin of the Grand Bay estuary, bottom surface sediments were collected with a petite ponar grab sampler. The sediment recovered in the grab sampler was inspected for an undisturbed sediment-water interface. If the sediment was disturbed, the sediment was discarded and a new grab sample was collected. If the sediment surface was intact, the overlying water was gently removed, and the uppermost one centimeter of sediment was sampled for sediment characterization. The grab sample identifiers consist of the USGS project ID (16CCT04) and a site-specific identifier (for example, GB300). An alphabetic identifier was appended to each site identifier to denote the collection method (G for bottom sediment grab samples). The site position was recorded with a vessel-mounted Ashtech Proflex DGPS GNSS receiver and GNSS antenna. Site locations and date/time of collection are reported in an Excel spreadsheet. A comma-separated values data file containing the tabular data in plain text are included in the download files.

In the SPCMSC laboratory, the surficial sediment grab samples were processed for basic sediment characteristics (dry bulk density and porosity). Water content, porosity and dry bulk density were determined using water mass lost during drying. For each 1-cm interval, 48–60 milliliters (mL) of each wet subsample was packed into a graduated syringe with 0.5 mL resolution. The wet sediment was then extracted into a pre-weighed aluminum tray and the weight of the wet sediment and the volume was recorded. The wet sediment and tray were placed in a drying oven for a minimum of 48 hours at 60 degrees Celsius (°C). Water content (θ) was determined as the mass of water (mass lost when dried) relative to the initial wet sediment mass. Dry bulk density was determined by ratio of dry sediment to the known volume of sediment packed into the syringe. Porosity (φ) was calculated from the equation φ = θ / [θ+(1-θ)/ρs] where ρs is grain density assumed to be 2.5 grams per cubic centimeter (g/cm^3). Salt-mass contributions were removed based on the average salinity (18.5) measured in the Grand Bay estuary in May 2016 (FAN_2016-331-FA, project ID 16CCT03). Water content, porosity and dry bulk density are reported in the Excel spreadsheet. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Organic matter content was determined with a mass loss technique, referred to as loss on ignition (LOI). The dry sediment from the previous process was homogenized with a porcelain mortar and pestle. Approximately 5 grams (g) of the dry sediment was placed into a pre-weighed porcelain crucible. The mass of the dried sediment was recorded. The sample was then placed inside a laboratory muffle furnace with stabilizing temperature control. The furnace was heated to 110 °C for a minimum of 6 hours to remove hygroscopic water absorbed onto the sediment particles. The furnace temperature was then lowered to 60 °C, at which point the sediments could be reweighed. The dried sediment was returned to the muffle furnace. The furnace was heated to 550 °C over 30 minutes and kept at 550 °C for 6 hours. The furnace temperature was then lowered to 60 °C and held at this temperature until the sediments could be reweighed. The latter step prevents the absorption of moisture, which can affect the measurement. The mass lost during the 6-hour baking period relative to the 110 °C-dried mass is used as a metric of organic matter content. Data are reported as a ratio of mass (g) of organic matter to mass (g) of dry sediment (post-110 °C drying). Replicate analyses were performed on approximately 20 percent of the samples as a representative subset for quality assurance. Loss on ignition measurements, including replicate analyses, are reported in the Excel spreadsheet. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Particle size analysis was performed on the 37 surficial sediment (G & S) samples. Prior to particle size analysis, organic material was chemically removed from the samples using 30% hydrogen peroxide (H2O2). Wet sediment was dissolved in H2O2 overnight. The H2O2 was then evaporated by gentle heating and the sediment washed and centrifuged twice with deionized water. Grain size analyses on the sediment cores were performed using a Coulter LS 13 320 (https://www.beckmancoulter.com/) particle-size analyzer (PSA), 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). To prevent shell fragments from damaging the Coulter instrument, 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 depth interval were processed through the instrument a minimum of three runs each. The sediment slurry made from the digested sample and deionized water was sonicated with a wand sonicator for 1 minute before being introduced into the Coulter PSA to breakdown aggregated particles. 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). 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. GRADISTAT also calculates the fraction of sediment from each sample by size category (for example, clay, coarse silt, fine sand). 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 (6-8 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. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Added keywords section with USGS persistent identifier as theme keyword.