Laboratory Methods and Analysis

X-Radiographs

The 7.6-cm diameter cores were X-rayed vertically using an Ecotron EPX-F2800 unit at a distance of 79 cm for a 1:1.015 ratio onto an iCRco 11- x 14-inch cassette. The cassette was scanned using an iCR3600+ cassette scanner and processed using iCRco QPC XSCAN32 version 2.10. Images were then edited in Adobe Photoshop by grayscale color inversion and inserting a reference scale. The X-ray images are included in the data downloads page of this report.

Physical Sediment Properties

In the laboratory, the wet samples were homogenized to ensure that all samples were well mixed prior to subsampling. A subsample of each 1-cm interval was processed for basic sediment characteristics: dry bulk density, water content, porosity, and radionuclides. Dry bulk density, water content, and porosity were calculated by determining water mass lost during drying. A known volume, generally 30 cubic centimeters (cm³), of each wet subsample was packed into a graduated syringe with 0.5 cm³ resolution. The wet sediment was then extracted into a pre-weighed aluminum tray and the wet sediment's weight was recorded. The wet sediment and tray were placed in a drying oven for 48 hours at 60 degrees Celsius (°C). Dry bulk density was determined by the ratio of dry sediment to the known volume of wet sediment packed into the syringe. Water content (θ) was determined as the mass of water (lost when dried) relative to the initial wet sediment mass. Salt-mass contributions were removed based on the salinity measured at the time of sample collection. Porosity (φ) was estimated with the following equation

φ = θ / [θ+(1-θ)/ρ_s]

where ρ_s is grain density assumed to be 2.5 grams per cubic centimeter (g/cm³).

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 5-7 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 absorbed onto the sediment particles. The furnace temperature was then lowered to 60 °C, at which point the sediment was 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, at which point the sediment was again reweighed. 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. The physical properties for each core interval are included in the data downloads page of this report.

Grain-Size Analysis

Grain-size analyses on the sediment cores were performed using a Coulter LS200 (https://www.beckmancoulter.com/) particle-size analyzer, which uses laser diffraction to measure the size distribution of sediments ranging in size from clay (0.4 micron [µm]) to very course grained sand (2 millimeters [mm]). Thirty surficial sediment samples and a total of 193 samples from the 8 box cores were analyzed.

Prior to particle-size analysis, organic material was chemically removed from the marsh core samples using 30 percent hydrogen peroxide (H₂O₂). Wet sediment from the marsh samples were dissolved in H₂O₂ overnight. The H₂O₂ was then evaporated through slow heating on a hot plate, and the sediment was washed and centrifuged twice with deionized water.

To prevent shell fragments from damaging the Coulter LS200, particles greater than 1 mm in diameter were separated from all samples prior to analysis with a number 18 (1 mm) U.S. standard sieve, which meets the American Society for Testing and Materials (ASTM) E11 standard specifications for determining particle size with woven-wire test sieves. The samples were washed through the sieve with deionized water and a few milliliters (mL) of dilute sodium hexametaphosphate solution to act as a deflocculant. Any material >1 mm remaining on the sieve was dried and weighed. The weights are reported in grain-size data Excel worksheets located in the data downloads page. The contribution from this size fraction, if present, is generally minor and not included in the class-size distribution percentages.

The sediment slurry was sonicated with a wand sonicator for 30 seconds before being introduced into the Coulter LS200 to breakdown aggregated particles. Two subsamples from each sample were processed through the Coulter LS200 a minimum of three runs per subsample. The Coulter LS200 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. The particles scatter light into characteristic refraction patterns that are measured by an array of photodetectors as intensity per unit area and recorded as relative volume for 92 size-classification channels, or bins. The size-classification boundaries for each bin were based on the ATSM 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.html), which calculates the mean, sorting, skewness, and kurtosis of the particle-size distribution of each sample which are expressed geometrically in metric units and logarithmically in phi units (Φ) (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, and fine sand) based on a modified Wentworth (1922) size scale. 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 runs per sample), and to 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 individual run statistics and class-size distribution, as well as the averaged run statistics and class-size distributions, are included in the data downloads page of this report.

Gamma Spectroscopy

Additional subsamples of wet sediment from the uppermost five, 1-cm intervals of each box core were used for the detection of radionuclides by standard gamma-ray spectrometry (Cutshall and Larsen, 1986) at the USGS SPCMSC radioisotope laboratory. Each subsample (approximately 20–30 g of wet sediment) was oven-dried at 60 °C for 48 hours. The dried sediment was homogenized to a fine powder with a porcelain mortar and pestle. Dried ground sediments (15 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 then counted for 36 to 72 hours on a planar-style, low energy, high-purity germanium, gamma-ray spectrometer (Canberra Industries, Inc., http://www.canberra.com/). The standard suite of naturally occurring and anthropogenic radioisotopes measured at the SPCMSC radioisotope laboratory 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 keV), 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 IAEA (International Atomic Energy Agency) RGU-1 reference material, standard photopeak intensity, and self-absorption using a U-238 sealed source (Cutshall and others, 1983). Radiochemistry data from gamma spectroscopy are available in the data downloads page of this report.

Alpha Spectroscopy

Total Pb-210 activity was measured by standard alpha spectroscopy for each box core (Robbins and Edgington, 1975). The Pb-210 (half-life = 22.3 years) alpha method is based on determining the activity of Po-210 (half-life = 138 days), which is assumed to be in secular equilibrium with its parent isotope Pb-210. The analytical method exploits the ability of polonium (Po) to self-plate onto silver planchets, which facilitates the alpha counting (Flynn, 1968). The method used at the USGS SPSMSC for chemical separation of Po-210 for sediments was developed by Martin and Rice (1981). Prior to alpha spectroscopy analysis, the sediment underwent LOI analysis to remove organic material. Five grams of the fired sediment was transferred to a glass beaker, Po-210 was acid leached from the 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 percent hydrogen peroxide and 8 N hydrochloric acid. The volume of the final acidic solution was raised to 70 mL with deionized water. Several buffering solutions were added to reduce interference from other cations and oxidants present during the plating process. Ammonium hydroxide was added to adjust the pH of the final solution to between 1.8 and 1.9. The Po-210 was autoplated onto 1.9-cm diameter sterling silver planchets 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. Select intervals from each core were analyzed in duplicate for quality assurance. Radiochemistry data from alpha spectroscopy are available in the data downloads page of this report.

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