Sedimentary Data From Grand Bay, Alabama/Mississippi, 2014-2016 (ver. 1.1, April 2020)

Two transects of four cores each were collected at two salt marsh sites along Grand Bay, Alabama/Mississippi. At each site, the 4 cores were collected at distances of 5, 15, 25, and 50 meters landward of the shoreline. Push cores were collected with 10.2-centimenter (cm) diameter polycarbonate barrels, driven into the sediment until refusal. Measurements were taken on the inside and outside of the barrel to determine compaction or core shortening values. Upon retrieval, the push cores were capped, labeled, and inspected for integrity. Push core recovered lengths ranged between 42 and 54 cm. Core identifiers consist of the USGS project ID (16CCT07) and a site-specific identifier (for example, GB301). An alphabetic identifier was appended to each site identifier to differentiate the collection method (M for push core). Site positioning and elevations for cores GB301M-GB304M were determined using an Ashtech differential GPS receiver. GPS locations were not recorded at the core sites GB305M-GB308M, the site location was approximated from a reference map. Elevations at core sites GB305M-GB308M were determined using a total station during site reoccupation in January 2017. Site locations, elevations, date of collection, distance from the shoreline, core lengths, and core compaction are reported in an Excel spreadsheet. Comma-separated values data files containing the tabular data in plain text are included in the download files.

DGPS Acquisition: DGPS base stations were erected on two NGS benchmarks located within the Grand Bay National Estuarine Research Reserve, B166 (PID DO5987) and 189A (PID DO5977). At each base station, an Ashtech Z-Xtreme DGPS receiver recorded the 12-channel full-carrier-phase positioning signals (L1/L2) from satellites via a Thales choke-ring antenna. A similar instrument combination (Ashtech Z-Xtreme receiver and Ashtech geodetic antenna) was used for the rover GPS systems. The base receiver and the rover receiver record their positions concurrently at 1 second (s) recording intervals throughout the survey. A stop-and-go rapid-static survey technique was used, with static occupation durations of either 300 or 30 s, depending on sample site.

GPS Post-Processing: The final, time weighted coordinates from 16CCT07 for the GPS base stations were imported into GrafNav, version 8.7 (Novatel Waypoint Product Group) and the data from the rover GPS were post-processed to the concurrent GPS session data from the nearest base station; baseline distances for all sample sites were less than about 8 km dependent upon site location. The GPS data were acquired in the World Geodetic System of 1984 (WGS84, (G1150)) geodetic datum, processed and exported in the North American Datum of 1983 (NAD83) geocentric datum. The exported file from GrafNav was converted using the National Oceanic and Atmospheric Association (NOAA) VDatum software conversion tool version 3.6 (http://vdatum.noaa.gov/). The sample locations were transformed from the GPS acquisition datum (WGS84) horizontal and vertical, to NAD 83, Universal Transverse Mercator (UTM) Zone 16 north (16N) horizontal reference frame and the North American Vertical Datum of 1988 (NAVD 88) orthometric elevation using the NGS geoid model of 2012A (GEOID 12A). The site information data files provided in the data release are in the Geographic Coordinate System (latitude, longitude, decimal degrees) NAD83.

At the SPCMSC, the eight push cores were vertically extruded and sectioned into 1-cm intervals. The outer circumference of each interval was removed to avoid use of sediment that was in contact with the polycarbonate barrel. Each sediment interval was bagged and homogenized. The bagged intervals were refrigerated until processing.

In the SPCMSC laboratory, a subsample of each 1-cm interval from the 8 push cores was 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, 10–60 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 an estimation of salinity to be 25. 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 2-3 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 of loss on ignition for a representative subset of the core intervals 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.

Down-core particle size analysis was performed on 1-cm depth interval for the eight push cores. Select intervals throughout the entire length of each core were chosen for analysis. A total of 96 samples were analyzed for particle size. 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 (8 runs per sample), and highlighted 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 with each core on its own tab. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Dried, ground sediment from the 1-cm depth intervals of the 8 push cores were analyzed for the detection of radionuclides by standard gamma-ray spectrometry (Cutshall and Larsen, 1986) at the USGS SPCMSC radioisotope lab. Intervals from the uppermost 30 cm were analyzed from each core with alternating intervals analyzed from lower depths in the cores. A total of 306 depth intervals were analyzed for radioisotopic activities. The sediments (3.3-30 g) were sealed in airtight polypropylene containers or polystyrene test tubes. Sediments placed in the test tubes were sealed with a layer of epoxy. 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 Ra-226 to come into secular equilibrium with its daughter isotopes Pb-214 and Bi-214. The sealed samples were then counted for 48-72 hours on a 16 x 40-mm well or 50-mm 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 Pb-210 (46.5 keV), 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), Cs-137 (661.6 keV), and 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 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. The critical level is reported for each core. 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.