Abstract:

Five subtidal or marsh sediment cores were collected from 2015-12-15 to 2017-05-18 at sites in the vicinity of Cocodrie, LA, and Port Sulphur, LA, and were dated by determining the downcore distribution of 210Pb and 137Cs activity. The three Cocodrie area marsh cores (LC1, LC3, T4) were collected with 10 cm diameter x 50 cm long stainless steel core tubes. The two Port Sulphur area marsh cores (VC3a, PS6a) were collected with vibracores to obtain deeper (older) sediments. Sample weight and % Water are reported for all five cores (LC1, LC3, T4, VC3a, PS5a), cumulative mass for one core (T4) and bulk density for the two vibracores (VC3a, PS6a). Samples were collected in December 2015 (LC1, LC3), May 2016 (T4) and May 2017 (VC3a, PS6a).

Suggested Citation:

Turner, R. Eugene. 2019. Sediment dating in two south Louisiana marsh regions from 2015-12-15 to 2017-05-18. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/n7-hm67-2d60

Purpose:

To date the sediment layers and then run other analyses for constituents with a known age.

Data Parameters and Units:

Worksheet - Sample Locations: Sample Collection Date: Date the core was collected, MM/DD/YY; Core ID: User-defined identifier assigned to each core; Core Description: Describes the two types of cores collected; LAT (dec degrees): Latitude of the core sampling location in decimal degrees; LONG (dec degrees): Longitude of the core sampling location in decimal degrees; Site description: Describes the nature of the area from which the core was collected (subtidal or marsh). The five core data worksheets (LC1, LC3, T4, VC3a, PS6a) all contain the same data variables, except for Cumulative mass (only measured on T4) and bulk density (only measured on VC3a and PS6a). Core Slice ID: User-assigned identifier for each core slice; Mid-depth (cm): the middle of the core segment (layer) down core, in cm; Excess Pb-210 Activity: isotopic activity per weight of sediment (pCi/g); Ln Pb-210 Activity: the natural log of the Pb-210 activity (pCi/g); CS-137 Activity: isotopic activity per weight of sediment (pCi/g); Sample weight (g): weight of the sample analyzed, in grams; % water: the percent of dry weight / wet weight of the sample; Cumulative mass: the cumulative weight of the sediment from the surface downwards; Bulk density: the weight of the sample analyzed per volume (g/cc). Note: “NA” indicates no data.

Methods:

The three sediment samples collected with 10 cm diameter x 50 cm long stainless steel core tubes were kept upright and refrigerated until cutting into 1 cm sections. Half of the sections were dried at 60°C, then ground to a 20-micron mesh sieve size, weighed in sealed 26 cubic cm plastic petri dishes and held for 3 weeks before analysis to allow equilibrium to occur between 222Rn and 210Pb in the petri dish. We used an integrated gamma-spectroscopy system consisting of a Princeton Gamma-Tech 60 mm diameter intrinsic germanium "N" type coaxial detector (40% efficiency) interfaced to an EG&G Ortec 92X Spectrum Master® mutichannel spectrum analyzer. Ortec’s GammaVision® software was used to analyze the data. The system was calibrated using a multinucliide (60-1836 keV) standard solution (Isotope Products #7600) that was mixed with an estuarine soil and then packed in a 26 cm3 plastic petri dish. The calibration of the system was checked quarterly for 210Pb and 137Cs with standard sediment from the International Atomic Energy Agency (IAEA-300). Samples were counted for at least 24 h. The background, with an empty petri dish, was counted every 6 days for 24 h. The excess (unsupported) 210Pb was calculated by subtracting the supported 210Pb activity from the total 210Pb activity. Supported 210Pb was estimated from the activity of 214Pb (295 and 352 keV). Accretion rates were calculated assuming that there is negligible migration of 210Pb and associated radionuclides in sediments, a constant input of 210Pb from the atmosphere, and a constant proportion of 210Pb trapped from the water to the sediments by weight. This model is applied under the assumption that the excess 210Pb in sediments deposited at the surface will be the same regardless of sedimentation rate. The activity of 210Pb in the first few centimeters of the surface can aid in determining how much mixing has occurred.

Error Analysis:

Counting errors were calculated using the counting uncertainties of 210Pb and 214Pb and the formula from Binford (1990). The counting uncertainty ranged from < 10% for surface samples (< 10 cm) to 15% for deeper samples. The sum of the excess 210Pb was estimated for the core using the interpolated values for core segments not counted and the measured sample dry weights.

Provenance and Historical References:

Binford, M. (1990). Calculation and uncertainty analysis of 210Pb dates for PIRLA project lake sediment cores. Journal of Paleolimnology, 3(3). doi:10.1007/bf00219461 Nittrouer, C. A., Sternberg, R. W., Carpenter, R., & Bennett, J. T. (1979). The use of Pb-210 geochronology as a sedimentological tool: Application to the Washington continental shelf. Marine Geology, 31(3-4), 297–316. doi:10.1016/0025-3227(79)90039-2 Robbins, J. A. (1978). Geochemical and geophysical applications of radioactive lead. In: J. O.Nriagu (ed.), Biogeochemistry of Lead in the Environment. Elsevier Scientific, Amsterdam. pp. 285–393.

Individual Files: