Sediment and Radiochemical Characteristics from Shore-Perpendicular Estuarine and Marsh Transects in the Grand Bay National Estuarine Research Reserve, Mississippi

In April 2017, two, 35-meter shore-perpendicular transects were sampled in the Grand Bay National Estuarine Research Reserve, Mississippi. Each transect extended from the shoreline (identified as 0), 10 meters into the estuary (identified with negative values) and 25 meters into the marsh (identified with positive values). Surface samples were collected every 2.5 meters by hand using a spoon and/or scoopula from approximately the top 1-centimeter (cm) of the marsh or estuarine surface and transferred and stored in a labeled and sealed baggie on ice for sedimentological analyses (for example, organic matter content, grain-size) for surface sediment characterization. Sample identifications all begin with altFAN, 17CCT02, contain GB, indicating Grand Bay, followed by a three-digit number starting with either 1 (indicating a site 1 sample), or 3 (indicating a site 3 sample), and are denoted with a S, for surface sample (for example, GB181S-GB195S and GB381S-GB395S). Following collection, samples were stored on ice until they reached the St. Petersburg Coastal and Marine Science Center where they were then stored refrigerated. Site location information includes distance from shoreline, date collected, latitude, longitude, and elevation which are reported in an Excel spreadsheet. Comma-separated values data files containing the tabular data in plain text are included in the download files.

In the sedimentology laboratory, surface sediment samples were homogenized in the sample bag. The wet sediment sample was placed in an aluminum tray/weigh boat, and the wet sediment sample and tray were placed in a drying oven for 48 hours at 60° Celsius (C) to remove water content; no measurements were made for determination of dry bulk density (a mass/volume measurement). Organic matter (OM) content was determined with a mass loss technique referred to as loss on ignition (LOI). The dry sediment 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 with a precision of 0.01 g on an analytical balance. 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 adsorbed onto the sediment particles. The furnace temperature was then lowered to 60 °C, at which point the sediments could be reweighed safely (modified from Dean, 1974 who heated the furnace to 100 °C for 1 hour). The dried sediment was returned to the muffle furnace and heated to 550 °C over a period of 30 minutes and kept at 550 °C (Galle and Runnels, 1960) for 6 hours (optimal exposure times for complete combustion of organic carbon are reported ranging between 1–12 hours; Dean, 1974; Wang and others, 2011; Heiri and others, 2001; Santisteban and others, 2004). Following the 6-hour burn time for removal of organic carbon, the furnace temperature was lowered to 60 °C, at which point the sediments could be reweighed safely while preventing 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 OM content (Dean, 1974). Twenty percent of the field samples were run in triplicate for LOI to assess precision. Data are reported as a ratio of mass (g) of organic matter to mass (g) of dry sediment (post-110 °C drying). Triplicate analyses of loss on ignition are reported for quality assurance 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 all surface transect samples. Prior to analyses, sediment samples were digested with 8 milliliters (mL) of 30 percent hydrogen peroxide (H2O2) overnight to remove excess organics. The H2O2 was then evaporated slowly on a hot plate, and the sediment was washed and centrifuged twice with deionized water. Grain-size analyses 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 (sets) from each interval were processed through the PSA (4 runs per set). 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 bin boundaries are determined by the manufacturer and GRADISTAT groups the data from the bins into sediment-size classifications. Individual run statistics for all sample are available upon request for detailed grain-size distribution analyses.

The raw grain-size data were processed with the free software program, GRADISTAT version 8, (Blott and Pye, 2001), which calculates the mean, median, sorting, skewness, and kurtosis of each sample geometrically in metric units and logarithmically in phi units (Φ) (Krumbien, 1934) using a modified 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 Friedman and Saunders (1978), a modified Wentworth (1922) size scale. A macro function in Microsoft Excel, developed by the USGS SPCMSC, was applied to the data to calculate the average and standard deviation for each sample set (4 runs per 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 of Excel workbook. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Dried, ground sediment from the surface samples were analyzed for the detection of radionuclides by standard gamma-ray spectrometry (Cutshall and Larsen, 1986) at the USGS SPCMSC radioisotope lab. The sediments were sealed in airtight polypropylene containers with plumbers' tape around the threads for the planar detectors. The sample weights and counting container geometries were matched to pre-determined calibration standards. The sealed samples were stored for a minimum of 3 weeks prior to analysis to allow radium-226 (Ra-226) to come into secular equilibrium with its progeny isotopes lead-214 (Pb-214) and bismuth-214 (Bi-214). The sealed samples were then counted for 24-200 hours on a 16 x 40-millimeter well or 50-millimeter diameter planar-style, low energy, high-purity germanium, gamma-ray spectrometer. The suite of naturally occurring and anthropogenic radioisotopes measured along with their corresponding photopeak energies in kiloelectron volts (keV) are lead-210 (Pb-210; 46.5 keV), thorium-234 (Th-234; 63.3 keV), Pb-214 (295.7 and 352.5 keV; proxies for Ra-226), Bi-214 (609.3 keV; proxy for Ra-226), cesium-137 (Cs-137; 661.6 keV), and potassium-40 (K-40; 1640.8 keV). Sample count rates were corrected for detector efficiency determined with International Atomic Energy Agency RGU-1 reference material, standard photopeak intensity, and self-absorption using a uranium-238 (U-238) sealed source (planar detectors only, Cutshall and others, 1983). All activities, with the exception of short-lived Pb-214 and Bi-214, were decay-corrected to the date of field collection. The radioisotopic activities reported in the Excel spreadsheet include the counting error for all samples; results from each core are on its own tab. Throughout the data, ND stands for Not Detectable, indicating the radioisotope activity for that interval was lower than the critical level. The critical level (Lc) is reported for each core. A comma-separated values data file containing the tabular data in plain text is included in the download file.