Grain size analysis and total carbon content of sediment subsamples from gravity cores and push cores from the Escanaba Trough (Gorda Ridge)

Push core and gravity core samples were collected on the R/V Thomas G. Thompson between May 24, 2022, and June 13, 2022, during the TN403 Escanaba Trough expedition, which was led and staffed by USGS employees. Push cores were pushed into sediment at 37 locations by ROV Jason’s manipulator arm in water depths between 3201 and 3301 meters. Gravity cores were collected using a gravity corer at 7 locations in water depths between 3209 and 3304 meters. Gravity cores greater than 60 cm in length were cut into sections onboard the ship and labeled ‘X’ (0-60 cm), ‘Y’ (60-120 cm), and ‘Z’ (>120 cm). All cores were stored upright and refrigerated until return to shore.

In the USGS Pacific Coastal and Marine Science Center (PCMSC) Core Preparation and Analysis Laboratory, subsamples were collected from relevant stratigraphic regions of the core. Pelagic clay and organic-rich sediments were air-dried and sulfidic sediments were dried in a glove bag purged with nitrogen gas.

Grain-size analysis was conducted by the USGS Pacific Coastal and Marine Science Center (PCMSC) Sediment Lab Suite and Carbon Analysis Laboratory using a Beckman Coulter LS 13 320 particle size analyzer with an aqueous liquid module attachment. PCMSC procedures are modified from those described in Carver (1971) and Folk (1974, 1980). 5-50 grams of sediment was sub-sampled and at least 10 milliliters of 30 percent hydrogen peroxide was added to oxidize overnight to remove organics. The samples were then heated to 250 degrees Celsius to drive off excess hydrogen peroxide. Next, samples were placed in an ultrasonic bath to help disperse silt and clay (mud) from sand and larger-sized particles. Samples were then rinsed twice in deionized water with centrifugation to remove soluble ions. Subsequently, the samples were wet sieved with 1 millimeter and 0.063 millimeter sieves to segregate the combined gravel and coarse sand fraction (greater than 1 millimeter), sand (1 millimeter to 0.063 millimeter), and mud (less than 0.063 millimeter). The coarse sand + gravel fractions were then dried, dry sieved at 1/4phi intervals and weighed (Carver, 1971). The sand fraction was dried and weighed. The mud fraction was collected in a 1 liter graduated cylinder, 5 milliliters of at least 10 percent sodium hexametaphosphate (dispersant) was added, and deionized water was added to bring the total the volume to 1 liter. The mud + dispersant solution sat overnight to ensure complete dispersion and to observe if flocculation occurred. Subsequently, a 20 milliliter aliquot was extracted from the graduated cylinder and dried to obtain the mud fraction weight normalized to a 1-liter basis. The mud and sand fractions were analyzed separately via the Beckman Coulter LS 13 320 with the aqueous liquid module attachment. All size fraction data were compiled and converted by the PCMSC Sediment Lab Suite and Carbon Analysis Laboratory pcSDSZ program version 1.71 (Penscil, 2015) to a grain-size distribution at 1/4phi size intervals on a weight percent basis. pcSDSZ also calculated interpolated sizes and graphical statistics on a percent coarser basis (Folk and Ward, 1957; Folk, 1966; Folk, 1974, 1980; Inman, 1952; Trask, 1930, 1932) and moment measures (Collias and others, 1963; Jaquet and Vernet, 1976; Folk 1980).

Grain sizes at select cumulative percentage values (D) were calculated by the USGS Pacific Coastal and Marine Science Center (PCMSC) Sediment Lab Suite and Carbon Analysis Laboratory software pcSDSZ version 1.71 on a percent coarser basis, which summed from coarse to fine bins. D values were interpolated between phi bins bracketing the specified D value. Grain sizes were converted into units of millimeters (mm) using the formula mm=reciprocal of 2(exponent)phi or (phi=-log2(mm)). Interpolated grain sizes at D5, D10, D16, D25, D50, D75, D84, D90, and D95 are reported in phi and millimeter units.

Total carbon and total inorganic carbon of bulk sediment were determined coulometrically (UIC, Inc.) at the USGS Pacific Coastal and Marine Science Center (PCMSC) Sediment Lab Suite and Carbon Analysis Laboratory. Samples were run in duplicate. Coulometer performance was verified relative to a calcium carbonate reference material. Total organic carbon content (TOC) was calculated as the difference between total carbon and total inorganic carbon. Calcium carbonate was calculated as percent TIC * 8.333.

Results from all analyses and core location information (TN403_CoreLocations) were combined into a single comma-delimited text value document (TN403_GrainSize_TotalCarbon.csv).