Sediment Physical Properties Data from Sediment Cores Collected in March 2012 Along the Northern Chandeleur Islands, Louisiana (U.S. Geological Survey Field Activity Numbers 12BIM01)

At each of the seven sample sites, a 10.2-centimeter (cm) diameter push core was collected. Eight sediment push cores were collected, in total, with core lengths varying between 45.5 and 70 cm. Accurate position and elevation data were obtained at all sampling locations with an Ashtech Differential Global Positioning System (DGPS) receiver and antenna with occupation times of 30 minutes. The accuracy of both horizontal position and elevation were improved by post-processing the DGPS data using a local benchmark, which was occupied throughout the course of each field activity by a similar DGPS system. At a local field station, the cores were sectioned at 1-cm intervals to a depth of 50 cm, below 50 cm the core was sectioned at 2-cm intervals to the base of the core. The subsections were placed in individual storage bags, and stored on ice. The sectioned core samples were transported to the USGS SPCMSC.

In the laboratory, samples were homogenized in the sample bag and a subsample of each 1 or 2 cm interval was processed for basic sediment characteristics (dry bulk density and porosity). Water content, porosity and dry bulk density were determined using water mass lost during drying. Thirty ml of each wet subsample was packed into a graduated syringe with 0.5 ml resolution. The wet sediment was then extracted into a pre-weighed aluminum tray and the weight of the wet sediment was recorded. The wet sediment and tray were placed in a drying oven for approximately 48 hours at 60 degrees Celsius (°C). Water content was determined as the mass of water (in other words, mass lost when dried) relative to the initial wet mass. Salt-mass contributions were removed from the porosity values under the assumption that pore water salinity was equivalent to an average salinity of 25. Porosity and dry bulk density were calculated from the reported equations in the document.

Organic matter content was determined using a similar mass loss technique, commonly referred to as loss on ignition. The dry sediment from the processing described above was homogenized using a porcelain mortar and pestle. Approximately 5 g of the dry sediment was placed into a pre-weighed porcelain crucible. The mass of the dried sediment was recorded with an analytical balance to a precision of 0.01 g. The samples were then placed into a laboratory muffle furnace with stabilizing temperature control. The furnace was ramped to 450 °C over a 30 minute interval and then held at 450 °C for 6 hours. The furnace temperature was then lowered to 60 °C and held at this temperature until the sediments could be reweighed. The latter step prevents the absorption of moisture, which can affect the measurement. Samples were reweighed using the same balance and to a precision of 0.01 g. The mass lost during the 6-hour baking period relative to the initial dry mass is used as a metric of organic matter content.

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