Open-File Report 2015-1143
Laboratory Methods And Analysis
Physical Sediment Parameters
A split of each subsample was processed for basic sediment characteristics (dry bulk density and porosity) and bulk organic matter content (data downloads). Porosity and dry bulk density were determined from approximately 50 cubic centimeters (cm3) of wet sediment packed into a syringe. Water content was determined as the fraction of mass loss of known volume relative to the initial mass after drying at 60 degrees Celsius (°C) for 48 hours; porosity (cm3voids/cm3wet) was estimated from water content and assumed density of water was 1.00 grams per cubic centimeter (g cm-3). Dry bulk density (ρb g cm-3) was determined as the dry mass normalized to the initial wet measured volume. Bulk organic matter content was determined by loss-on-ignition on dried, homogenized samples. A small aliquot of each sample (3–5 g) was weighed in porcelain crucibles, dried at 110 °C for 2 hours, re-weighed, and then combusted at 550 °C in a muffle furnace for 6 hours. Samples were held at 60 °C in the furnace until post-combustions weights could be recorded. The loss-on-ignition for organic matter content were estimated by the ratio of mass-loss during combustion at 550 °C to mass after drying at 110 °C. Approximately 10 percent of the samples were replicated. The physical parameters for each core interval are included in this report's data downloads.
Additional subsamples of wet sediment were used to detect radionuclides with standard gamma-ray spectroscopy (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). Data from the initial and aged gamma spectroscopy counts are available in this report's data downloads.
Per standard protocol, 15 cubic centimeter (cm3) subsamples were taken from core samples using a 0.5 cm3 graduated syringe. When a sample contained less than 15 cm3, the entire sample was processed while leaving approximately 5 cm3 of sediment to archive. Sediment samples for faunal analyses were wet sieved by soaking in water with a small amount of 10 percent sodium hexametaphosphate solution, slowly agitated for up to 1 hour to aid disaggregation, and then washed over a stainless-steel, 63 micron (μm) sieve. The >63 µm fractions were oven dried at 60 °C and then dry sieved at 125 µm. Percent mud was determined by the mass of the >63 µm and the initial sample mass minus water content.
Surface samples were collected to analyze the variability of living foraminifera assemblages. Surface sediments were collected with a Ponar grab sampler containing 27 to 40 cm3 with a spatula and immediately placed in two graduated centrifuge tubes (labeled A and B) containing a solution of ethanol and Rose Bengal. The sample tubes were shaken daily for two weeks before processing the sediment in the laboratory to ensure equal staining of the protoplasm. The volume of the sediment was recorded using the volume gradation of the tube. The stained, wet sediment was washed over a 63 µm sieve. After the washing, the >63µm fraction was oven dried at 60°C and then dry-sieved at 125 µm.
The foraminifers were picked, counted, and identified from the >125 µm fraction (data downloads). Processed core samples contained abundant benthic foraminifers in the top half of the core, but decreased in abundance in the bottom one-third of the core. A representative subsample of approximately 200-300 specimens was obtained for faunal analysis using a microsplitter. When needed, the entire sample was used for faunal analysis, with several samples containing less than 100 specimens. The split-size fraction was spread across a 45-square, hole-punched tray, and each specimen was dropped through a hole and onto a stationary 60-square micropaleontology slide to be later identified and counted. The stained surface samples were also picked, counted and identified from the >125-µm fraction using the same processing procedures (data downloads). Because the dead (non-stained) surface foraminifers were more abundant, the microsplitter was used to obtain the correct sample size for identification and counting. Data from the initial foraminiferal counts are available in this report's data downloads.