Sedimentologic Data from Point aux Chenes Marsh and Estuary, Mississippi (18CCT09)

Surface grab samples were taken from estuarine sites (GB542G-GB563G) using a petite PONAR grab sampler, deployed according to manufacturer specifications. The sediment recovered in the grab sampler was inspected for an undisturbed (i.e., free of slumping, washout, scouring, cracking, and/or other disturbance features) sediment-water interface to ensure the bulk sample collected was representative of the surface and material was not lost. If the sediment was disturbed, the sediment was discarded, and a new grab sample was collected and assessed. If the sediment surface was intact, any overlying water and the mesh screens were slowly removed in accordance with the product manual, and the uppermost one centimeter of sediment was subsampled with a spoon or scoopula for sediment characterization as described in Osbourne and DeLaune (2013) and for consistency with related products (Ellis and Smith, 2021; Haller and others, 2019) and as is standard and recommended for recent surficial sediment and microfossil analyses (Schonfeld and others, 2012). Water quality properties at each estuarine site were measured with a YSI Pro-DSS and values were recorded. At ten sites on the Point aux Chenes marsh, push cores (GB530M-GB535M; GB538M-GB541M) were collected with 10.16-cm diameter polycarbonate barrels. Upon retrieval, similar to methods described in Osbourne and DeLaune (2013; with the exception of not adding water for extraction when not necessary due to the sediments being saturated) and calculation of compaction due to coring, the cores were visually inspected for disturbances (i.e., slumping, washout, scouring, cracking, bubbling, and/or discontinuities) to ensure the core was intact and representative of the site. If the core appeared disturbed, it was discarded, and a new core was collected and inspected. Core lengths ranged between 36 and 65 cm. The cores were transported upright, in order to avoid slumping and preserve the natural sediment orientation, to the SPCMSC laboratory for sectioning. At each marsh site, Russian peat augers were collected in agreement with the methods described in Osbourne and DeLaune (2013) and manufacturer recommendations. Visual characteristics of the peat augers were described (i.e., general color, visual organic matter texture and type such as roots, bivalves, and level of decomposition, and sediment texture such as sandy silt or clayey silt) and thickness of the upper organic-bearing unit (peat) was recorded in field logs, in centimeters. Once described and photographed horizontally with a scale bar and label, peat augers were discarded in the field. The field logs associated with those peat augers are available upon request. Sample identifiers consist of the USGS alternate FAN or project ID (18CCT09), a site-specific identifier (for example, GB542), and appended with an alphabetic identifier to differentiate the sediment collection method (S for marsh surface sample, M for marsh push core, G for estuarine grab sample, R for Russian peat auger). Estuarine site coordinates and water depth were recorded with a Garmin EchoMapPlus 63v boat GPS and depth sounder; push core locations were initially recorded at the time of collection using a Garmin GPSMAP 76S handheld GPS receiver. Marsh site coordinates were also recorded using a GPS antenna with base station set up. Site location information includes sample type, date collected, location relative to the shoreline, latitude, longitude, elevation, core lengths, and YSI measurements, which are reported in an Excel spreadsheet (.xlsx). Comma-separated values (.csv) data files containing the tabular data in plain text are included in the download files.

DGPS Acquisition: A DGPS base station was erected on a NGS benchmark located within the Grand Bay National Estuarine Research Reserve, B166 (PID DO5987). At the 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 30 minutes per sample site, which provide more accurate horizontal and vertical control than handheld GPS units.

GPS Post-Processing: The final, time weighted coordinates 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. 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 NAD83, Universal Transverse Mercator (UTM) Zone 16 North (16N) horizontal reference frame and the North American Vertical Datum of 1988 (NAVD88) orthometric elevation using the NGS geoid model of 2012A (GEOID12A). The site information data files provided in the data release are in the Geographic Coordinate System (latitude, longitude, decimal degrees) NAD83.

Upon return from the field, all marsh push cores were X-rayed vertically using a stand, to avoid sediment slumping, onto an iCRco 11 x 14-inch cassette using an Ecotron EPX-F2800 X-ray unit at a distance of 79 cm for a 1:1.015 ratio. The cassette was inserted into and scanned using an iCR3600+ cassette scanner and processed using iCRco QPC XSCAN32 version 2.10. Images were then exported as Tag Image File Format (.tiff) files and edited in Adobe Photoshop, using grayscale color inversion, and inserting a reference scale bar. X-ray images were used as visual aids prior to extraction and subsampling to assess peat thickness in comparison with peat augers, presence of macrofossils which may impede sectioning, and to ensure cores are intact for the length of the core.

Following x-raying, all cores were vertically extruded and sectioned into 1-cm intervals using a serrated knife, pre-measured polycarbonate ring, and extruder (in accordance with methods described in Osbourne and DeLaune, 2013 and in 1-cm intervals as is standard for sediment and radiochemical analyses; Nittrouer and others, 1979) at the USGS SPCMSC sediment core laboratory. The outer circumference of each sample interval was removed to avoid use of sediment that was in contact with the polycarbonate barrel which could result in contamination by sediment from other depths due to movement within the barrel both during collection and extruding; each sediment interval was bagged in a zipped baggie, homogenized, and refrigerated (Osbourne and DeLaune, 2013).

In the laboratory, marsh core samples were homogenized in the sample bag to ensure a representative subsample from the 1-cm interval, and the subsample for sediment parameters was extracted and processed for basic sediment characteristics: dry bulk density and porosity. Water content, porosity, and dry bulk density were calculated by determining water mass lost during drying. To calculate, known volumes of each wet subsample, usually 30-60 milliliters (mL), were packed into a graduated syringe with 0.5 cubic centimeter (cm^3) resolution. The wet sediment sample was then extruded into a pre-weighed aluminum tray/weigh boat, and the weight was recorded. The wet sediment sample and tray were placed in a drying oven for 48 hours at 60 °C (Celsius) to remove water content. A 1 mL split from the original wet sample was also taken for diatom analysis and was dried using the same procedure. Water content (θ) was determined as the mass of water (lost when dried) relative to the initial wet sediment mass. Porosity (φ) was estimated 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), representative of a silty quartz sand. Salt-mass contributions were removed based on the salinity measured at the time of sample collection. If salinity was not measured in the field, pore water salinity was estimated to be 25. Dry bulk density (g/cm^3) was determined by the ratio of dry sediment to the known volume of sediment packed into the syringe. Water content, porosity and dry bulk density are reported in the Excel spreadsheet (.xlsx). A comma-separated values (.csv) data file containing the tabular data in plain text is included in the download file. Some depth intervals in the .csv file display as dates due to csv formatting.

Organic matter (OM) content was determined with a mass loss technique referred to as loss on ignition (LOI). The dry sediment subsample from the previous process step, measuring dry bulk density, was homogenized with a porcelain mortar and pestle. Approximately 2 to 6 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). In error, marsh push core GB541M was heated to 800 °C instead of 550 °C for 6 hours, which may result in an overestimate of organic matter content due to potential degradation of inorganic carbon at the higher temperature, which is considered to peak at 950 °C (Galle and Runnels, 1960). Following the 6-hour burn time for removal of organic carbon, the furnace's 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); OM content for core GB541M may be overestimated due to the increased furnace temperature. Approximately 13 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). Replicate analyses of loss on ignition are reported for quality assurance in the Excel spreadsheet (.xlsx). A comma-separated values (.csv) data file containing the tabular data in plain text is included in the download file. Some depth intervals in the .csv file display as dates due to csv formatting.

Particle size analysis was performed on selected 1-cm depth intervals for the 10 marsh push cores, based on dry bulk density variations down-core, and on all 23 surficial sediment samples. Prior to analyses, sediment samples were digested with 8 mL of 30 percent hydrogen peroxide (H2O2) overnight to remove excess organics (Poppe and others, 2000). The H2O2 was then evaporated slowly on a hot plate, and the sediment was 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 processed with the free software program, GRADISTAT version 8, (Blott and Pye, 2001; kpal.co.uk/gradistat), which calculates the mean, median, sorting, skewness, and kurtosis of each sample geometrically in metric units and logarithmically in phi units (Φ) (Krumbein, 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 (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 (.xlsx) with each core on its own tab. A comma-separated values (.csv) data file containing the tabular data in plain text is included in the download file. The individual run statistics or all sites, prior to averaging, is available upon request. Some depth intervals in the .csv file display as dates due to csv formatting.

Dried, ground sediment from the 1-cm depth intervals of the 10 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. The sediments were sealed in airtight polypropylene containers for planar detectors or polystyrene test tubes for the well detector. 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 radium-226 (Ra-226) to come into secular equilibrium with its daughter isotopes lead-214 (Pb-214) and bismuth-214 (Bi-214). The sealed samples were then counted for 48-72 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 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 (.xlsx) 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 (.csv) data file containing the tabular data in plain text is included in the download file. Some depth intervals in the .csv file display as dates due to csv formatting.

Version 2.0: This data release was versioned on August 23, 2023, to correct an error in the 18CCT09 site information dataset. Sites 18CCT09-GB539 through GB541 were collected at site 9 rather than site 8. The site number in the "Transect Site ID" column was updated to correct this error. Also added data for an additional field activity (alternate FAN 21CCT02): sediment physical properties, site information, gamma spectroscopy, x-ray images, and field logs. Appended alternate FAN to zip filenames (18CCT09_21CCT02), and added a new metadata file, 21CCT02-GB_metadata (.txt and .xml). Please see the version_history.txt file for more information.