Grain size, bulk density, and organic carbon of sediment cores from San Pablo Bay and Grizzly Bay, California, 2020

Subsamples were collected from the top 5 centimeters (cm) of push cores collected in San Pablo Bay and Grizzly Bay, California. The top two subsamples were each 0.5 cm thick, and all following subsamples were each 1 cm thick. Subsamples were placed into pre-weighed Whirl-pak bags.

Wet samples weights were measured, then samples were placed in a 60 degrees C oven until fully dried. Once samples were fully dried, they were placed in a desiccator for an hour, then dry weights were measured. Samples were then placed back in the oven for another 4 hours at 60 degrees C, moved to the desiccator for 1 hour, and reweighed. Samples were considered dry when their weights were equal to or less than 0.004g of their previously recorded dry weight. The bulk density reported here was calculated with the formula: bulk density = (water density * sediment density) / (water density + (sediment density - water density) * water content), where water density is 1 g/cm3 and sediment density is 2.65 g/cm3. Measurements were made in the USGS Pacific Coastal and Marine Science Center sediment lab.

The grain-size distributions of samples were determined in the USGS Pacific Coastal and Marine Science Center sediment lab. Approximately 20 g of sediment was subsampled and 10 mL of 30 percent hydrogen peroxide was added to remove organic material in the sample. The sample was heated at 250 degrees C to remove any remaining hydrogen peroxide and was placed in an ultrasonic bath to liberate the fine fraction. Subsequently, the sample was centrifuged for 1.5 hours to remove soluble salts. Samples were funneled through a 1-mm and a 63-um sieve to separate the gravel (greater than 1 mm), sand (63 um to 1 mm), and mud (less than 63 um). The sand and gravel fractions were dried in an 80-degree C oven and weighed. Five mL of sodium hexametaphosphate was added to the mud fraction and a 20 mL aliquot was taken from a 1 L graduated cylinder to determine the weight of the mud. The particle-size distribution of the mud and sand fractions was analyzed separately using a Beckman Coulter LS 13 320, and the gravel, sand, and mud results were integrated into a single grain-size distribution for each sample. Where used, phi sizes are equal to the negative log base 1 of millimeter size.

Percent carbon measurements were made in the USGS Pacific Coastal and Marine Science Center sediment lab. To determine percent carbon in sediment samples, approximately 5g of sample sediment was sub-sampled and dried in an 80 degrees C oven overnight. Samples were then pulverized into a fine powder. To obtain percent total carbon (TC), approximately 20mg of the pulverized sample was weighed and wrapped in an aluminum tin. A UIC CM 5200 furnace attached to a CM 5015 coulometer was used to incinerate the samples at 950 degree C which converted the produced CO2 into a value to obtain total carbon as percent dry weight. For percent total inorganic carbon (TIC), approximately 20mg of sample was weighed into and placed in glass sample tubes which were loaded into a UIC CM 5230 acid digester attached to a CM 5015 coulometer. The samples were digested with 2mL perchloric acid and the produced CO2 was measured to obtain a total inorganic carbon value as percent dry weight. To determine percent calcium carbonate of a sample, the percent TIC value was multiplied by 8.333. The percent total organic carbon (TOC) value was determined by subtracting the percent TIC value from the percent TC value. Two analyses were run per system and the results were averaged.