Laboratory Methods And Analysis

Physical Sediment Parameters

In the laboratory, samples were homogenized. Each 1-cm interval was processed for basic sediment characteristics including: dry bulk density, water content, porosity, and radionuclides. Dry bulk density, water content, and porosity were calculated by determining water mass lost during drying. Thirty milliliters (mL) of each wet subsample were packed into a graduated syringe with 0.5 mL 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). Water content (θ) was determined as the mass of water (lost when dried) relative to the initial wet sediment mass. 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), which is within the range of average particle density for mineral soils. Salt-mass contributions were removed under the assumption that pore water salinity was equivalent to an average salinity of 25.

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 to a fine powder with a porcelain mortar and pestle. Approximately 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 450 °C over 30 minutes and kept at 450 °C for 6 hours. The furnace temperature was then lowered to 60 °C, at which point the sediments could be reweighed. The latter step prevents the absorption of moisture, which can affect the measurement. Samples were reweighed on the analytical balance with a precision of 0.01 g. The mass lost during the 6-hour baking period relative to the initial dry mass is used as a metric of organic matter content. The physical parameters for each core interval are included in this report's data downloads page.

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 microns [µm]) to very course grained sand (2 millimeters [mm]). A total of 164 samples from 8 sediment 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 (1000 µm) 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 of sodium hexametaphosphate solution to act as a deflocculant. The sediment slurry was sonicated with a wand sonicator for 1 minute 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 apiece. 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 each sample 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 highlight the 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 this report's data downloads page.

Radiochemistry

Subsamples of dry, homogenized sediment prepared during the physical parameter analyses were used for the detection of radionuclides by standard gamma-ray spectrometry (Cutshall and Larsen, 1986) at the USGS SPCMSC radioisotope lab. Dried ground sediments (15–30 g) were sealed in airtight polypropylene containers. The sample weights and counting container geometries were matched to pre-determined calibration standards. The sealed samples sat for a minimum of 3 weeks 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 48–72 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 lab 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 International Atomic Energy Agency (IAEA) RGU-1 reference material, standard photopeak intensity, and self-absorption using a U-238 sealed source (Cutshall and others, 1983). Data from gamma spectroscopy counts are available in this report's data downloads page.