Sedimentologic Data from Point aux Chenes Marsh and Estuary, Mississippi (21CCT02)

At 2 sites on the Point aux Chenes marsh, push cores (GB537M, GB572M) 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. 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 (21CCT02), a site-specific identifier (for example, GB572), and appended with an alphabetic identifier to differentiate the sediment collection method (M for marsh push core, R for Russian peat auger). Push core locations were initially recorded at the time of collection using a Garmin GPSMAP 76S or Garmin 62stc 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.

A Trimble R10 GNSS and TSC3 data collector were used to collect global positioning system (GPS) coordinates and elevations. Both the R10 and TSC3 were mounted onto a 2-m graphite rod with a mounted foot to keep the rod on the surface of the marsh. A projected coordinate system (NAD_1983_StatePlane_Mississippi_East_FIPS_2301) was used for data collection. At marsh core sites, the "Measure Points" tool was used. Data were exported from the TSC3 as a shapefile (.shp). The .shp was pulled from the export folder and brought into ArcGIS software where the projection was defined as NAD_1983_StatePlane_Mississippi_East_FIPS_2301 (ESRI:102694). Elevation data were sent to the USGS SPCMSC as ArcGIS shapefiles. Within ArcMap, the data were reprojected from NAD_1983_StatePlane_Mississippi_East_FIPS_2301 into NAD_1983_UTM_Zone_16N (North American Datum of 1983 Universal Transverse Mercator Zone 16 North), using the "reproject" geoprocessing tool. The final shapefile was exported into tabular digital data format (Comma Separated Values [CSV]).

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 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 extrusion 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 (Reddy and others, 2013), 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 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 (Sharma and others, 1987; Gardner and Wilson, 2006). 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 (Celsius) 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'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). Approximately 12.3 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.

Dried, ground sediment from the 1-cm depth intervals of 2 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 40 cm were analyzed from each core. 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.

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 four 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 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 (8 runs per sample), and highlight runs that varied from the set average by more than ± 1.5 standard deviations. 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 for all sites, prior to averaging, is available upon request. Some depth intervals in the .csv file display as dates due to csv formatting.

Version 3.0: This data release was versioned in June 2025, to add new co-authors, grain-size information (21CCT02 only), and other required versioning changes to the metadata record. Metadata contact information with Alisha Ellis as the primary contact was changed to Christopher Smith.