Laboratory Methods And Analysis
Physical Sediment Parameters
In the laboratory, samples were homogenized. The sample bag and a subsample of each 1- or 2-cm interval was processed for basic sediment characteristics: dry bulk density and porosity; and radionuclides. Water content, porosity and dry bulk density 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 °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)/ρs]
where ρs is grain density assumed to be 2.5 grams per cubic centimeter (g cm-3). Salt-mass contributions were removed under the assumption that pore water salinity was equivalent to average salinity of 25. Dry bulk density (ρb; g cm-3) was determined as
ρb = ρs(1 – φ).
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 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.
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 284 samples from 49 cores were analyzed.
Prior to particle size analysis, organic material was chemically removed from the marsh core samples using 30 percent hydrogen peroxide (H2O2). The organic matter content in the back-barrier core samples was estimated from the LOI values to be ‹ 3 percent and therefore would not interfere with the Coulter LS200 measurements. 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), 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, 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 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.
Radiochemistry
Additional subsamples of wet sediment were used for the detection of radionuclides by standard gamma-ray spectrometry (Cutshall and Larsen, 1986) at the USGS SPCMSC radioisotope lab. Each subsample (approximately 80 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 (15–30 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 IAEA RGU-1 reference material, standard photopeak intensity, and self-absorption using a U-238 sealed source (Cutshall and others, 1983). Samples collected in March and July 2012 (FAN 12BIM01 and 12BIM02) were kept in the sealed counting containers for a minimum of 5 months following the initial count to allow for the decay of unsupported Th-234. The aged samples were recounted. Data from the initial and aged gamma spectroscopy counts are available in this report's data downloads.
Foraminiferal Analysis
Using a spatula, replicate samples (A and B) of surface sediment (0–2 cm) were collected at each sampling location and sealed in 50-mL centrifuge tubes. A solution of 2 g of Rose Bengal per liter of ethanol was added to each centrifuge tube to stain living protoplasm. Stained samples were transferred to the USGS SPCMSC foraminiferal lab. To ensure accurate staining, samples were inverted daily for 2 weeks. Prior to washing, the relative volume (mL) of sediment was recorded using the volume gradation on the tube (±1 mL). Each sample was washed over a 63 micron (µm) sieve under warm water; in some cases, an additional wash was needed to remove clay particulates. The remaining sample (> 63 µm size fraction) was oven-dried at ≤ 60 °C to remove moisture and then reweighed to assess the weight of the sand fraction. Then the sample was dry sieved to separate the 63–125 µm, 125–250 µm, and > 250 µm sub-fractions.
If more than 300 specimens were encountered in a sample, the sample was split with a micro-splitter. In most cases, the entire > 125-µm fraction was spread across a 45-square, hole-punched tray, one or more times, for examination under a microscope. When a stained specimen, which was probably alive at collection, was encountered, it was picked up with a wet brush and dropped through a hole in the tray onto a stationary, 60-square micropaleontological slide for later sorting and identification. Both A and B samples were picked, sorted, and counted individually, and then combined to provide a complete representation of the living foraminiferal community. Unstained benthic foraminifera were picked onto the same micropaleontological slide and processed in the same manner as living specimens. The foraminferal assemblages are included in this report's data downloads.
