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

Sediment push cores were collected at 14 estuarine sites across Grand Bay, Alabama/Mississippi. The push cores were collected with 10.2-cm diameter polycarbonate barrels, driven into the sediment until refusal. Upon retrieval, the push cores were capped, labelled, and inspected for integrity. Push core recovered lengths ranged between 22 and 88 cm. Two push cores were collected at site GB232, labeled as (A) and (B). At each push core site, bottom 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 slowly removed, and the uppermost one centimeter of sediment was sampled for sediment characterization. A replicate surface sample was collected at push sites GB230, GB237, and GB240 and labeled as site GB242, GB241, and GB243, respectively, to investigate homogeneity of the sediments. Water quality properties at each core site were measured with an YSI Professional Plus multi-sensor meter. Water column profiles of temperature, conductivity, and pressure were recorded with a SonTek Castaway-CTD sensor at each core site. Core and surface sample identifiers consist of the USGS project ID (16CCT03) and a site-specific identifier (for example, GB230). An alphabetic identifier was appended to each site identifier to differentiate the sediment collection method (M for push core and G for ponar grab sample). Site positions and elevations were determined using an Ashtech DGPS receiver with a 15-minute occupation time. DGPS locations were not collected at sites GB233, GB235-GB238, and GB240. All site positions were also recorded with a vessel-mounted Garmin echoMAP 50s chartplotter. Site locations, elevations, date of collection, YSI measurements, and CTD water column profiles are reported in Excel spreadsheets. Comma-separated values data files containing the tabular data in plain text are included in the download files.

Vibracores were collected at 17 estuarine sites across Grand Bay, Alabama/Mississippi. The vibracores were obtained by using the assemblage of an 8-horse power Briggs and Stratton motor attached to a Dreyer 5.4-cm concrete vibrator head with a 4.9-m flexible shaft. Bolted to the vibrator head is a galvanized steel clamp with a quick release system for connecting the clamp to 7.6-cm diameter, 9.1-m long aluminum core barrels. The aluminum barrel is vibrated into the subsurface until refusal, and then cut to discard any unused portion of the barrel. Measurements were taken on the inside and outside of the barrel to determine compaction or core shortening values. The empty void within the barrel was filled with water and an expansion plug was inserted onto the top of the barrel to ensure a solid vacuum. The full core barrel was extracted from the subsurface using an aluminum tripod and a 2-ton come-along winch. Once the barrel was completely removed from the subsurface, core caps were affixed to the openings on both ends of the barrel and reinforced with duct tape. Vibracore recovered lengths ranged between 0.79 and 3.98 m. At the vibracore sites, bottom sediments were collected with a petite ponar grab sampler following the same procedure as outlined above with the exception of sites GB255, GB257, GB259, GB261, and GB266. Water quality properties at each core site were measured with an YSI 556 MPS multi-sensor meter. Water column profiles of temperature, conductivity, and pressure were recorded with a SonTek Castaway-CTD sensor. CTD profiles are not collected at sites GB255, GB259, GB261, GB262, and GB266. Core and surface sample identifiers consist of the USGS project ID (16CCT03) and a site-specific identifier (for example, GB230). An alphabetic identifier was appended to each site identifier to differentiate the sediment collection method (V for vibracore and G for ponar grab sample). Site positions and elevations were determined using an Ashtech DGPS receiver with an 8 to 10-minute occupation time. DPGS data were not collected at GB255, GB257, GB259, GB261, and GB266. All site positions were also recorded with a hand-held Garmin GPSMAP 78sc receiver. Site locations, elevations, date of collection, YSI measurements, CTD water column profiles are reported in Excel spreadsheets. Comma-separated values data files containing the tabular data in plain text are included in the download files.

One marsh push core was collected near the North Rigolets channel along the northern edge of Point Aux Chenes Bay. The push core completed a transect of cores collected in April 2014 (project ID 14CCT01). The push core was collected with a 10.2-cm diameter polycarbonate barrels, driven into the sediment until refusal. Measurements were taken on the inside and outside of the barrels to determine compaction or core shortening. Upon retrieval, the push core was capped, labeled, and inspected for integrity. The uppermost 1 cm of surficial sediment was collected for sediment characterization. Surficial sediments were also collected along two marsh transects near Bayou Heron and Bayou Cumbest. Samples collected along the transects followed an elevation transition from the tidal channel to upland. Microfossil assemblages for the surface sediments and cores are available in Haller and others, 2018. Surficial water quality properties at each site were measured with an YSI Professional Plus multi-sensor meter with the exception of sites GB280, GB281, and GB282 where there was no standing water. Core and surface sample identifiers consist of the USGS project ID (16CCT03) and a site-specific identifier (for example, GB280). An alphabetic identifier was appended to each site identifier to differentiate the sediment collection method (M for push core and S for surficial sediment sample). Site positions and elevations were determined using an Ashtech DGPS receiver with a 30-minute occupation time (site GB285 occupation time was 15 minutes only). All site positions were also recorded with a hand-held Garmin GPSMAP 62stc receiver. Site locations, elevations, date of collection, and YSI measurements are reported in an Excel spreadsheet. A comma-separated values data file containing the tabular data in plain text is included in the download files.

DGPS Post-Processing: The base station coordinates were established using the NGS OPUS. Every base station session was processed using OPUS and a solution in IGS08 was received and logged into a spreadsheet. The position from each OPUS solution was then converted from the International GNSS Service reference frame of 2008 (IGS08) into NAD83 (2011). The weighted average of the base station positions was calculated and any elevations greater than 3 standard deviations were excluded from the average. Benchmark B166 has been occupied multiple times for SSIEES project surveys. For improved accuracy in comparison of the positional measurements for SSIEES surveys, the OPUS solution from USGS FAN 2015-315-FA (https://cmgds.marine.usgs.gov/fan_info.php?fan=2015-315-FA) is used as the default for all subsequent data processing if the current OPUS solution is within 3 standard deviations of the 2015 values. All DGPS rover sessions were processed in GrafNav version 8.7. For all DGPS sessions, the antenna height, antenna model and DGPS session type (rover or base) were accounted for and then processed. GrafNav version 8.7 reports the processed marsh core and the surface sediment grab sample coordinates in the NAD83 (2011) datum for the horizontal component and the North American Vertical Datum of 1988 referenced to GEOID12A for the vertical component. A vertical cumulative error is calculated that combines the errors associated with data processing, instrument measurement, and the baseline distance between the rover and base station. The horizontal coordinates, elevation, and errors are reported in an Excel spreadsheet. A comma-separated values data file containing the tabular data in plain text is included in the download files.

At the SPCMSC, the 16 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 core-analysis laboratory at the USGS SPCMSC, each estuarine vibracore was cut into 1-m sections and split lengthwise. The core was photographed, described, and sealed in plastic wrap. The core was described using standard sediment-logging methods. The reported core lengths on the core logs are the final laboratory measurements. Core elevations reported for sites GB255, GB257, GB259, GB261, and GB266 are estimated from the digital elevation model in DeWitt and others (2017). The elevations were converted from mean lower low water (MLLW) to NAVD88. The core photographs and descriptions are available as Portable Document Format (PDF) files. Samples were collected from 5 vibracores (GB250V, GB255V, GB256V, GB260V, and GB265V) at various depth intervals for radiocarbon dating. All utensils and containers were cleaned by ashing in an oven at 550 degrees Celsius (°C) for 1 hour prior to use. Nitrile gloves were worn while sampling, ashed tweezers or razor blades were used to remove the samples from the core. Samples were washed with Milli-Q water to remove all the sediment from the organic or carbonate sample. One shell cast sample was soaked in a 30% hydrogen peroxide (H2O2) solution to remove surface mold and then thoroughly rinsed with Milli-Q water. Once cleaned the samples were dried in small aluminum dishes and weighed, the weight was recorded. Samples were then placed in glass vials and shipped to the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) in Woods Hole, Massachusetts. Select depth intervals from 2 vibracores were also sampled for sediment physical properties analysis (GB250V and GB250V) and grain size analysis (GB255V only).

In the SPCMSC laboratory, a subsample from each 1-cm interval of the 16 push cores, the surficial sediment samples (G & S samples), and select intervals from two vibracores (GB250V and GB255V) 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, 8–120 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 °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 salinity measured at the time of sample collection. 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 2-6 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 each 1-cm depth interval for 9 estuarine push cores, one vibracore (GB255V), and the surficial sediment (G & S) samples. All intervals from the push cores were analyzed to a depth of 40 cm in the core. Prior to particle size analysis, organic material was chemically removed for the samples using 30% 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 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 uppermost three 1-cm depth intervals from the 15 estuarine push cores, all depth intervals from the one marsh push core (GB34M), and the marsh surface grab sample were analyzed for the detection of radionuclides by standard gamma-ray spectrometry (Cutshall and Larsen, 1986) at the USGS SPCMSC radioisotope lab. The sediments (5-50 g) were sealed in airtight polypropylene containers. The sample weights and counting container geometries were matched to pre-determined calibration standards. The sealed samples were stored for a minimum of three 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 24-72 hours on a 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 Pb-210 (46.5 keV), Th-234 (63.3 keV), Pb-214 (295.7 and 352.5 keV; proxies for Ra-226), Be-7 (477.6), 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 (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.

Total Pb-210 activity was measured by alpha spectroscopy for 15 estuarine push cores. The Pb-210 (half-life = 22.3 years) activity is determined by directly measuring the activity of its granddaughter Po-210 (half-life = 138 days) via alpha particle decay. Po-210 is assumed to be in secular equilibrium with its parent. The analytical method exploits the ability of polonium to self­plate onto silver planchets, which facilitates the alpha counting (Flynn, 1968). The laboratory method used at the USGS SPSMSC for chemical separation of Po-210 from sediments was developed by Martin and Rice (1981). Po-210 was acid leached from 5 grams of dried, ground sediment with concentrated nitric acid and a known activity of the tracer Po-209 was added to the solution. The solution digested overnight and then was dried on a hotplate, followed by several washings with 30% hydrogen peroxide and 8N hydrochloric acid. The final acidic solution was brought to 70 ml with deionized water. Several buffering solutions were added to reduce interference from other cations and oxidants present during the plating process. The pH was adjusted to between 1.8-1.9 with ammonium hydroxide. The Po-210 was autoplated onto 1.9-cm diameter sterling silver plancets while stirring and heating the solution. After 2 hours, the planchets were removed from solution, rinsed and dried. The planchets were counted for 24 hours in low-level alpha spectrometers coupled to a pulse-height analyzer. The radioisotopic activities reported in the Excel spreadsheet include the counting error for all samples. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Samples were collected from 5 vibracores (GB250V, GB255V, GB256V, GB260V, and GB265V) at various depth intervals for radiocarbon dating. This material was sent to the National Ocean Sciences accelerator mass spectrometry (NOSAMS) laboratory at Woods Hole Oceanographic Institution (Woods Hole, MA, USA), where accelerator mass spectrometry (AMS) radiocarbon dating was performed by means of a 500 kilovolt compact pelletron accelerator. The NOSAMS facility is supported by National Science Foundation Cooperative Agreement number, OCE-1239667. Calibrated radiocarbon ages were calculated using the Bchron package for R statistic software. A marine reservoir correction was applied to the calibrated age for the carbonate shell sample. Values were obtained from the Marine Reservoir Correction database (http://calib.org/marine/). The nearest 9 available Delta R (ΔR) estimates from the northwest coast of Florida (Hadden and Cherkinsky, 2015) were used to calculate a weighted mean ΔR of 164 and standard deviation (square root of variance) of 267 using the equations presented in Bevington (1969). The radiocarbon analytical results and calibrated ages 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.

Added keywords section with USGS persistent identifier as theme keyword.