Grain size and charcoal abundance in sediment samples from Los Padres reservoir, Carmel River watershed, California

GRAIN-SIZE ANALYSIS: Grain-size analysis was conducted by the USGS Pacific Coastal and Marine Science Center (PCMSC) sediment lab 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 pcSDSZ program (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).

INTERPOLATED SIZES: Grain sizes at select cumulative percentage values (D) were calculated by the PCMSC Sediment Lab 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.

CHARCOAL ANALYSIS: Charcoal abundance was analyzed in 1.23-milliliter sediment samples that had been sub-sampled from dry sediment samples. The 1.23-ml sub-samples were treated with 6 percent sodium hypochlorite solution (bleach), slowly agitating the samples and bleach together in a 50-ml test tube for two hours. Samples were then wet-sieved at 125 microns, and the fraction coarser than 125 microns was washed into disposable petri dishes and oven-dried at 50 degrees C. Once dried, the samples were examined under a binocular microscope at 15x magnification using a gridded background to facilitate thorough, accurate counting coverage. Charcoal particles were counted by one technician to ensure consistency. These methods are based on those of Anderson and others, 2022.