Location and radiochemical data from sediment cores collected on Deer Island, Mississippi

At the three marsh sites, four marsh push cores were collected with 10.16-centimeter (cm) diameter polycarbonate barrels. Upon retrieval, the cores were visually inspected for disturbances (for example, slumping, washout, scouring, cracking, bubbling, and/or discontinuities) to ensure the core was intact and representative of the site. If the core appeared disturbed, it was discarded, and a new core was collected and inspected. These methods are similar to those described in Osbourne and DeLaune (2013), with the exception of not adding water for extraction when sediments were already saturated and calculation of compaction due to coring (Morton and White, 1997). Core lengths ranged between 19 and 41 cm. The cores were transported upright to avoid slumping and preserve the natural sediment orientation. Cores were transferred to a contact at the U.S. Army Corps of Engineers (USACE) for sectioning and drying. At each marsh site, Russian peat augers were collected in agreement with the methods described in Osbourne and DeLaune (2013) and manufacturer recommendations. Visual characteristics of the peat augers were described (for example, general color; visual organic matter texture and type such as roots, bivalves, and level of decomposition; and sediment texture such as sandy silt or clayey silt) and thickness of the upper organic-bearing unit (peat) was recorded in field forms, in centimeters. Once described and photographed horizontally with a scale bar and label, peat augers were discarded in the field. The field forms associated with those peat augers include handwritten notes visible on the field forms for each site. Sample identifiers consist of a site-specific identifier (for example, DR021), and appended with an alphabetic identifier to differentiate the sediment collection method (M for marsh push core, R for Russian peat auger). Marsh site coordinates were recorded using a handheld GPS in addition to an the SP80 GNSS receiver’s RTK position (NAD83_2011, NAVD88) in case of RTK failure. Site location information includes site ID, core ID, date collected, easting, northing, latitude, longitude, elevation, core lengths and coring compaction which are reported in an Excel spreadsheet (.xlsx). Comma-separated values (.csv) data files containing the tabular data in plain text are included in the download files.

Dried, ground sediment intervals from the four push cores were received from the USACE and analyzed for the detection of radionuclides by standard alpha and gamma-ray spectrometry in the USGS SPCMSC radioisotope labs. Total lead-210 (Pb-210, half-life = 22.3 years) activity was measured by alpha spectrometry on two-centimeter intervals of four marsh push cores for the entire length of the cores. Polonium-210 (Po-210, half-life = 138 days) is assumed to be in secular equilibrium with its parent Pb-210, allowing for the determination of the total Pb-210 activity in environmental sediments by directly measuring the activity of Po-210 through alpha particle decay. The USGS SPCMSC radioisotope laboratory uses a method that was originally developed by Martin and Rice (1981) to chemically separate Po-210 from sediments. This method utilizes polonium’s affinity to autoplate to silver planchets in order to isolate Po-210 for alpha counting (Flynn, 1968). To separate the Po-210, 5.00 grams (g) of dried, ground sediment sample was leached with 10 milliliters (mL) 16 normality (N) nitric acid (HNO3) and a polonium-209 (Po-209) tracer was added. The solution digested overnight and was then dried on a hotplate. The dried solution was washed with 5 mL of 8 N hydrochloric acid (HCl), followed by three washings with 30% hydrogen peroxide (H2O2) to break down organics. After two additional washings with 5 mL of 8 N hydrochloric acid, 5 mL of 8 N hydrochloric acid was added. Once the sample dissolved completely, the solution was brought to 50 mL by adding deionized water. Three reagents were added prior to the deposition of Po-210 onto the silver planchets: 20% hydroxylamine hydrochloride and 25% sodium citrate were added to reduce the inference of oxidants, such as iron+3 (Fe+3) and chromium+6 (Cr+6) and a 2.2% weight by volume solution of bismuth nitrate was added to help prevent the deposition of bismuth-212 (Martin and Rice, 1981). A calibrated handheld pH meter was used to monitor the pH of the solution. Ammonium hydroxide was added dropwise while mixing to achieve a pH of 1.85 to 1.95. The Po-210 was autoplated onto the 1.9-centimeter diameter sterling silver planchets as the solution was heated and stirred on a hotplate for 2 hours. All planchets had one side covered with tape to ensure Po-210 plating only occurred on one side. The planchets were removed from solution, rinsed with deionized water, and dried. The planchets were counted in low-level alpha spectrometers coupled to a pulse-height analyzer for 24 hours. Samples with low Po-210 activity were counted for 48 hours. The total Pb-210 activity and counting error are reported in the Excel spreadsheet. A .csv data file containing the tabular data in plain text is included in the download file.

Select two-centimeter intervals were analyzed from each core by gamma-ray spectrometry. The sediments were sealed in airtight polypropylene containers with plumbers' tape around the threads for the planar detectors, or polystyrene test tubes for the well detector. Sediments placed in the test tubes were sealed with a layer of epoxy. The sealed samples were stored for a minimum of 3 weeks prior to analysis to allow radium-226 (Ra-226) to come into secular equilibrium with its progeny isotopes, lead-214 (Pb-214) and bismuth-214 (Bi-214). The sealed samples were then counted for 24-95 hours on a 16 x 40-millimeter well, or 50-millimeter diameter planar-style, low energy, high-purity germanium, gamma-ray spectrometer. The suite of naturally occurring and anthropogenic radioisotopes measured along with their corresponding photopeak energies in kiloelectron volts (keV) are lead-210 (Pb-210, 46.5 keV), thorium-234 (Th-234, 63.3 keV), Pb-214 (295.7 and 352.5 keV; proxies for Ra-226), beryllium-7 (477.6 keV), Bi-214 (609.3 keV; proxy for Ra-226), cesium-137 (Cs-137, 661.6 keV), and potassium-40 (K-40, 1640.8 keV). Sample count rates were corrected for detector efficiency determined with International Atomic Energy Agency RGU-1 reference material, standard photopeak intensity, and self-absorption using a uranium-238 (U-238) sealed source (planar detectors only, Cutshall and others, 1983). All activities, with the exception of short-lived Pb-214 and Bi-214, were decay-corrected to the date of field collection. The radioisotopic activities reported in the Excel spreadsheet include the counting error for all samples. A .csv data file containing the tabular data in plain text is also included in the download file.