Radiochemistry 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. In the field, the cores were sectioned at 2-cm intervals to a depth of 10–16 cm, placed in individual storage bags, and stored on ice. The only exception was the back-barrier marsh cores collected in March 2012 (12BIM01-M) that were subsampled at 1 cm intervals. The section core samples were transported to the USGS SPCMSC.

Each subsample (approximately 80 g of wet sediment) was oven-dried at 60 degrees Celsius (°C) for 48 hours. The dried sediment was homogenized to a fine powder using 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-made calibration standards. The sealed samples were allowed to sit 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. Sample count rates were corrected for detector efficiency (referenced to the International Atomic Energy Agency RGU-1 standard), standard photopeak intensity, and self-absorption corrections using a U-238 sealed source. Samples collected in March and July 2012 (12BIM01, 12BIM02) were kept in their sealed counting containers for a minimum of five months following their initial count to allow for the decay of unsupported Th-234. The aged samples were recounted.

Added keywords section with USGS persistent identifier as theme keyword.