The Sedimentological Characteristics and Radiochemistry Data for the Marshes on Dauphin Island, Alabama (U.S. Geological Survey Field Activity Number 2015-322-FA)

At ten sites on Dauphin Island, marsh push cores were collected with 11.4-cm diameter polycarbonate barrels. Upon retrieval, the cores were visually inspected for an undisturbed surface. A replicate core was collected at site DA31. Core lengths ranged between 24 and 59 cm. The cores were transported upright to the SPCMSC laboratory for sectioning. At select sites, Russian peat augers were collected and occasionally sampled in the field for micropaleontology. The data associated with those peat cores will be made available at another time. Water quality properties were measured with an YSI Professional Plus multi-sensor meter in the standing water at each site where there was standing water. Shear vane measurements were recorded within a couple of meters of each marsh push core site using an AMS field shear vane (https://www.ams-samplers.com/pdfs/ams-Field-Vane-Shear-Tester-Kit.pdf). At each site, four measurements were collected, one in the center of each quadrant of a 1 square meter grid, in order to account for variation in vegetation, sediment, and users. Site coordinates were recorded using a GPS antenna in the NAD83 datum with base station set up. Site location information including latitude, longitude, elevation, environment type, vegetation, and YSI measurements are reported in Excel spreadsheets. Comma-separated values data files containing the tabular data in plain text are included in the download files.

Upon return from the field, all marsh push cores were x-rayed vertically 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 .tiff files and edited in Adobe Photoshop, using grayscale color inversion and inserting a reference scale bar.

Following x-raying, all cores were vertically extruded and sectioned into 1-cm intervals at the USGS SPCMSC sediment core laboratory. 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.

In the laboratory, marsh core samples were homogenized in the sample bag 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. 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, and the weight was recorded. The wet sediment sample and tray were placed in a drying oven for 48 hours at 60 °C. Water content (?) was determined as the mass of water (lost when dried) relative to the initial wet sediment mass. Porosity (?) was estimated from the equation ? = ? / [?+(1-?)/?s] where ?s 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. 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. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Organic matter (OM) content was determined with a mass loss technique referred to as loss on ignition (LOI). The dry sediment sample from the previous process was homogenized with a porcelain mortar and pestle. Approximately 2.5 or 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. The dried sediment was returned to the muffle furnace. The furnace was heated to 550 °C over a period of 30 minutes and kept at 550 °C for 6 hours. The furnace temperature was then lowered to 60 °C, at which point the sediments could be reweighed safely. The mass lost during the 6-hour baking period relative to the 110 °C-dried mass is used as a metric of OM content (modified from Dean, 1974). Approximately 19 percent of the field samples were run in duplicate 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. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Prior to analyses, sediment samples were digested with 8 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. To prevent damage to the Coulter LS 13 320, particles greater than 1 millimeter (mm) in diameter were removed with a 1 mm sieve. The samples were washed through the sieve with a five percent solution of sodium hexametaphosphate with filtered municipal water to act as a deflocculant. The sediment slurry was sonicated with a wand sonicator for 30–60 seconds before being introduced into the particle size analyzer. Two subsamples from each sample were processed with a minimum of four runs apiece. The data for each sample was exported as relative volumes for 92 size-classification channels, or bins, which correspond with the size-classification boundaries for American Society for Testing and Materials (ASTM) E11 standard specification for wire cloth and sieves for testing purposes.

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 (?) (Krumbein, 1934) using the Folk and Ward (1957) method. 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. 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 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.

A subsample of sediment from each centimeter of each core was processed by standard alpha spectrometry procedures using polonium-210 (210Po) (Marot and Smith, 2012; a modification of Robbins and Edgington, 1975) to determine total lead-210 (210Pb) down core. In the laboratory, approximately 3.0 g of dried and homogenized sediment was spiked with 0.5 mL of polonium-209 (209Po), with an activity of 12 disintegrations per minute per milliliter (dpm/mL). The sediment was then leached by the addition and subsequent evaporation to near dryness with a combination of concentrated nitric acid, hydrochloric acid, and 30 percent H2O2. Polonium-210 (210Po) and 209Po were electroplated onto silver planchets and counted twice, in two different chambers, for 24 hours on an ORTEC Alpha Ensemble multi-chamber spectrometer. Count rate efficiency was determined for 209Po and applied to the 210Po counts. The total 210Pb was assumed to be in secular equilibrium with its granddaughter, 210Po in the down-core sediment. 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.

Added keywords section with USGS persistent identifier as theme keyword.