Laboratory methods and analysis
Vibracores | Grain-size samples | Radiocarbon Dating | Optically Stimulated Luminescence (OSL) Dating
Vibracores
In the core-analysis laboratory at the USGS SPCMSC, each marine vibracore was cut into 1-m sections and split lengthwise. One-half of each core was described using standard sediment-logging methods, photographed, and wrapped in plastic sleeves for archive storage. The other core half was sampled at the top, middle, and bottom of each described sediment unit for grain-size analysis. For terrestrial core methods, see Optically Stimulated Luminescence (OSL) Dating section below.
Grain-size samples
Samples for grain-size analysis were collected from the top, middle, and bottom of each lithologic unit in approximately 2-3 cm intervals. Grain-size analyses on the core sediment samples were performed using a Coulter LS 200 (https://www.beckmancoulter.com/) particle-size analyzer, which uses laser diffraction to measure the size distribution of sediments ranging in size from 0.4 microns (µm) to 2 millimeters (mm) (clay to very coarse-grained sand). A total of 367 core samples were analyzed (Data Products and Downloads).
In order to prevent shell fragments from damaging the LS 200, particles greater than 1 mm in diameter were separated from all samples prior to analysis using a number 18 (1,000 µm) 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. Two subsamples from each sample were processed through the LS 200 a minimum of three runs each. The LS 200 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. Light is scattered by the particles into characteristic refraction patterns measured by an array of photodetectors as intensity per unit area and recorded as relative volume for 92 size-related channels (bins). The size-classification boundaries for each bin were specified based on the ASTM E11 standard.
The raw grain-size data were then run through the free, widely available program GRADISTAT (Blott and Pye, 2001; http://www.kpal.co.uk/gradistat.html), which calculates the geometric (in metric units) and logarithmic (in phi units, Φ; Krumbein, 1934) mean, mode, sorting, and skewness of each sample using the Folk and Ward (1957) method as well as the cumulative particle-size distribution. 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 developed by the USGS was applied to calculate the average and standard deviation of each sample set (six runs per sample) and to calculate highlight runs that varied from the set average by more than plus or minus (±) 1.5 standard deviations. Those runs were removed from the results, and the sample average was recalculated using the remaining runs.
Radiocarbon Dating
Two shells were prepared to send to the Woods Hole Oceanographic Institution (WHOI) National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS): an intact, articulated bivalve shell in core 10BIM06-28 from a depth of 3.75 m and an intact small bivalve shell in core 10BIM06-14B from a depth of 2.92 m. The shells were broken into smaller pieces, weighed, and soaked in vials overnight in Ultrapure (Type 1) filtered water (Millipore Milli-Q Advantage A10 water system fed by a Millipore Elix 10). The water was pipetted out of the vials, replaced with new water, and sonicated for 15 minutes (in the vial). The process was repeated three times. The samples were air dried and weighed prior to sending to NOSAMS. Care was taken to clean instruments with alcohol, to use gloves while handling the samples, and to use new pipettes for all rinsing procedures. For specific sampling and processing procedures please refer to the NOSAMS Web site (http://www.whoi.edu/nosams/), and for results from NOSAMS included in this report see Data Products and Downloads.
Optically Stimulated Luminescence (OSL) Dating
Sediment samples from all 11 terrestrial vibracores were collected for optically stimulated luminescence (OSL) dating. Care was taken not to expose the sediment to light, and all sampling was conducted in the dark with only a photographer's red lamp; however, six of the cores were split into 1-m lengths before sampling. Therefore, in all the cores, sampling locations were only selected from core sections greater than (>) 10 cm from the cut ends of the cores. Cores were split and the sample half was wrapped in lightproof material and aluminum foil, while the archive half was photographed, described, and sealed in plastic wrap. Core descriptions were used to select samples from sandy sections of the sample half of the core for OSL dating. Samples were collected in 2-cm intervals, placed in lightproof film canisters, labeled, and sent to the USGS Luminescence Dating Laboratory for analysis. For detailed information on the analysis processes, refer to the USGS Luminescence Dating Laboratory Website (http://crustal.usgs.gov/laboratories/luminescence_dating/). For results from OSL analyses included in this report see Data Products and Downloads.
