Wetland Paleoecological Study of Coastal Louisiana: Surface Sediment and Diatom Calibration Dataset

Sediment sample field collection: Surficial sediment samples were collected from a network of permanent wetland monitoring stations called the Coastwide Reference Monitoring System (CRMS). All sampled stations were located in the Chenier Plain in southwest Louisiana and were sampled to include representative salinity regimes for the region. Samples were collected by pushing a 10-cm diameter polyvinyl carbonate cylinder into the sediment and removing the surface 0-2 cm surface by extrusion. This procedure was selected because it replicated the sampling procedure interval of the sediment cores. Samples were labeled using CRMS site identification numbers and environmental data (salinity, water level, elevation, sediment accretion, and vegetation) for each site was obtained from CRMS monitoring data. All samples were stored in leak-proof containers, treated with isopropyl alcohol and placed on ice immediately to preserve the sample and reduce the chance of bacteria growth during transport. The samples remained refrigerated until processed, where they were homogenized and divided into sub-samples for laboratory processing.

Laboratory procedure: All surface sediment samples were processed in the laboratory for bulk density, organic carbon (by loss-on-ignition), and grain size. A 30-mL sediment sample was weighed wet, dried in an oven at 60 °C, and weighed again to calculate bulk density (dry mass divided by volume). Loss-on-ignition (LOI) was determined by grinding the dried sample to a fine powder and combusting a subsample at 650 °C for four hours. Approximately 50 percent of the samples were measured in triplicate to calculate sample error. Grain size was measured by laser diffraction using a Coulter LS 200 particle-size analyzer after organic digestion. For organic matter digestion, approximately 5g of sediment was placed in a 50-mL test tube, 15-mL of sodium hypochlorite was added, and placed in a warm water bath (80 °C) for 15 minutes. The sample was then centrifuged and supernatant was carefully removed using a pipette as to not remove sediments. The sodium hypochlorite digestion was conducted at least three times to remove all organic matter. After digestion, 7 percent sodium hexametaphosphate solution was added to deflocculate clays. At minimum, three aliquots were analyzed three times to generate a total of nine grain-size distributions for each sample. Estimates of percent sand, silt, clay and mean phi were calculated from grain size distributions for each aliquot using GRADISTAT version 8. The nine datasets were checked for any unusual variations (for example, exceptionally high or low values which can be caused by partially undigested organics or debris) and data were removed if values deviated significantly from the majority of aliquots (1 standard deviation from the mean). The remaining data were averaged to create a single estimate of grain size (percent sand, silt, and clay, and mean sediment size) for each sample. Data was copied into Microsoft Excel, double-checked for errors, and exported to a comma-delimited text file.

Microfossil analyses: Approximately 0.5 mL of sediment was digested in a 100-mL beaker using hydrogen peroxide and potassium dichromate. Samples were rinsed twice with deionized (DI) water (the beaker was filled with DI water; sample was allowed to settle overnight, decanted to 500-mL mark, and refilled, etc.). Since samples contained a significant fraction of fine-grained sediment (silt and clay), a 7 percent sodium hexametaphosphate solution was added to deflocculate fine-grained sediments. Each sample was continually rinsed until decant was clear. A known volume (5 or 10 mL depending on the density of diatoms and sand grains) of diatom solution was added to settling trays containing four cover slips and allowed to dry (Battarbee, 1973). Cover slips were sealed to microscope slides using Zrax diatom mountant (refractive index ~ 1.7+). A minimum of 300 diatoms were identified to species and counted using a Nikon Optiphot light microscope at 1000x magnification (Plan 100x / 1.25 oil DIC objective). The identification of species and autecology, including general salinity preferences, was determined from taxonomic literature. Data was copied into Microsoft Excel, double-checked for errors, and exported to a comma-delimited text file.

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