Laboratory Methods and Analysis
Sediment from the push cores and surface samples was analyzed for bulk density, organic matter content, and detailed grain size to evaluate potential changes in sediment texture or provenance and chronology. Initially, cores were x-rayed to image the intact core. Cores were subsequently sectioned at 1-cm intervals over the entire length of the core, and major lithologic changes were noted during sectioning. Samples were refrigerated at approximately 3.3 degrees Celsius (°C) until further processing. Sediment from the auger cores was photographed in the field, logged in the laboratory, and used as visual reference.
X-Radiographs
The 10.2-cm diameter cores were x-rayed vertically using an Ecotron EPX-F2800 at a distance of 79 cm for a 1:1.015 ratio onto an iCRco 11” x 14” 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 using grayscale color inversion and inserting a reference scale. The x-ray images are included in the data downloads section.
Physical Sediment Parameters
In the laboratory, samples were homogenized in the sample bag and a subsample of each 1-cm interval was processed for basic sediment characteristics (dry bulk density and porosity). Water content, porosity, and dry bulk density were determined using water mass lost during drying. Approximately 20–30 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 volume and weight of the wet sediment was recorded. The wet sediment and tray were placed in a drying oven for approximately 48 hours (h) at 60 degrees Celsius (°C). Dry bulk density was determined by 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 (in other words, mass lost when dried) relative to the initial wet mass. Salt-mass contributions were removed based on the salinity measured at the time of sample collection. Porosity was calculated 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).
Figure 3. Graph showing the strong correlation among replicated loss-on-ignition (a metric for organic matter) measurements for surface sediments. This technique aids in identifying potential outliers in the dataset with samples that fall outside the 95 percent prediction bands. [Click image to enlarge].
Organic matter (OM) content was determined using a similar mass loss technique, commonly referred to as loss on ignition (LOI). The dry sediment from the processing described above was homogenized using a porcelain mortar and pestle. Approximately 1–5 g (average weight used for all core 4.39 ± 1.85 g; n = 380) of dry sediment was placed into a pre-weighed porcelain crucible. The mass of the dried sediment was recorded with an analytical balance to a precision of 0.01 g. The samples were then placed into a laboratory muffle furnace with stabilizing temperature control. The furnace temperature was increased to 110 °C over a 30 minute interval and then held at 110 °C for 6 h. Samples were removed and weighed to determine excess moisture loss. Samples were then placed back in the furnace. The furnace temperature was increased to 550 °C over a 30 minute interval and then held at 550°C for 6 h. The furnace temperature was then lowered to 110 °C and held at this temperature until the sediments could be reweighed. The latter step prevents the absorption of moisture, which can affect the measurement. Samples were reweighed using the same balance and to a precision of 0.01 g. The mass lost during the 6 h baking period relative to the initial dry mass is used as a metric of OM content (modified from Dean, 1974). Data are reported as a ratio of mass (gOM) of organic matter to mass (gDRY) of dry sediment (post-110 °C drying) The physical parameters for each core interval are included in this report's data downloads.
Four cores (13M, 14M, 25M(A), and 25M(B)) contained an abundance of fibrous organic material. As such, these cores were sieved at 1.7 millimeters (mm) to remove these large organics, which can be fire hazards in open vessel burns. The large mass OM contribution from the coarse fraction was estimated by correcting for residual water and assuming the material was 100 percent burnable organics. The fraction of OM was added proportionally back to greater than 1.7 mm LOI measured as described above.
To assess reproducibility of the LOI method and assess potential sample heterogeneity, approximately 16 percent of the samples were measured in duplicates (fig. 3). Regression analysis from the set of duplicates (n = 58) showed that the two separate runs had a significant correlation (r = 0.995, p < 0.001) and that the average uncertainty based on the slope was approximately 1.5 percent. The raw, absolute, and relative difference between runs did not show a quantitative relation with average OM content. Thus, much of the error observed would suggest it is random and the result of small-scale heterogeneity in the sediment.
Grain Size Analysis
Prior to particle size analysis of all marsh surface sediment and marsh core samples, organic material was chemically removed using 30 percent hydrogen peroxide (H2O2). Wet sediment was 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. The samples were stored in the centrifuge tubes with several milliliters of dionized water until instrument analysis.
For surface samples (S) taken on the washover fans, the low to negligible organic matter content made chemical pretreatment unnecessary. In general these samples contained numerous large-sized particles that needed to be removed prior to instrument analysis. The samples were dried for 24 h in a drying oven at 60 °C. The dried samples were weighed and then dry-sieved through a number 18 (1,000 microns [μm] 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. The two size fractions were weighed and bagged. The weight percentage of the coarse fraction to the total sample weight is reported in the grain-size workbooks (data downloads).
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 from 0.4 μm to 2 mm (clay to very coarse-grained sand). In order to prevent shell fragments from damaging the Coulter LS200, particles greater than 1 mm in diameter were separated from all sediments prior to analysis using a number 18 U.S. standard sieve. If there was sediment greater than 1 mm, the material was dried and the dry weight was recorded. After the samples were washed through the sieve with filtered tap water, a few milliliters of sodium hexametaphosphate solution were added to act as a deflocculant. The sediment slurry was sonicated with a wand sonicator for 30–60 seconds before being introduced into the Coulter LS200 to break down aggregated particles. The pre-sieved <1 mm dried washover fan (S) fraction was introduced directly into the Coulter LS200.
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 ASTM E11 standard.
The raw grain size data were then run through the 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, fine sand) on the basis of 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 (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 section.
Alpha Spectroscopy
To aid in identifying event sedimentation, subsamples from each section of the core were processed for the determination of total lead-210 (Pb-210) by standard alpha spectrometry (Marot and Smith, 2012; a modification of Robbins and Edgington, 1975). Total Pb-210 (by way of granddaughter polonium-210; [Po-210]) were determined by alpha spectroscopy following the procedure of Marot and Smith (2012). Briefly, approximately 1–5 g of sediment was spiked with 0.5 mL of Po-209 of known activity (12 disintegrations per minute per milliliter [dpm mL-1]). Sediments were leached with a combination of concentrated nitric and hydrochloric acid with the addition of 30 percent H2O2 to remove organics. Sediments were taken to near dryness and re-saturated three times with hydrochloric acid until all nitric acid was removed. Po-209 and Po-210 were electroplated onto silver planchets and counted on an alpha spectrometer. Count rate efficiency for Po-209 was determined and applied to Po-210 counts. Total Pb-210 was then assumed to be in secular equilibrium with Po-210 in the down-core sediment. Radiochemistry data from alpha spectroscopy are available in the data downloads section.
