Collection, analysis, and age-dating of sediment cores from mangrove and salt marsh ecosystems in Tampa Bay, Florida, 2015

Sites around Tampa Bay were visited on October 19, 20 and 21, 2015 and sediment cores were collected. Three main vegetation types were targeted: salt marsh, dominated by Juncus and Spartina alternaflora; mangrove, including Rhizophora mangle, Laguncularia racemosa and/or Avicennia germinans; and young mangrove, where wetlands were created within the last three decades. E.G. Simmons park was restored in 1990 (Osland and others, 2012). An additional surface sediment sample was collected from a salt barren, as this site was not conducive to coring. USGS core collections were co-located with biomass and soil surveys conducted by study collaborators.
Sediment cores were taken by pounding 4 inch diameter PVC pipe 20 to 40 cm into the ground. Compaction was monitored by measuring the sediment surface inside and outside the core tube. If compaction was greater than 1 cm (2-3% of total core length), the core was discarded and not used. After the core barrel was inserted into the sediment, a rubber gasket and handle were placed over the top and then the core was dug out of the sediment. The gasket allowed the core to be removed without further disturbance. Caps were placed on the top and bottom of each core immediately after collection. The cores were transported upright to the laboratory for further processing. At the salt barren site it was not possible to take a sediment core, so a section of sediment was removed with a shovel, sectioned and bagged in the field. Note at the two restored mangrove sites (E.G. Simmons and Fort de Soto) cores were collected of the organic peat above a compact sand layer, which was not conducive to coring. Thus these cores are only 20-24 cm deep, compared to other cores that were 30-40 cm deep.
Latitude, longitude, and elevation were measured, by study collaborators, with a differential global positioning system (DGPS). Refer to the horizontal and vertical accuracy report sections of the metadata record for more information regarding this measurement. The process date reflects the last occurrence of the work.
Osland, M.J., A.C. Spivak, J.A. Nestlerode, J.M. Lessmann, A.E. Almario, P.T. Heitmuller, M.J. Russell, K.W. Krauss, F. Alvarez, D.D. Dantin, J.E. Harvey, A.S. From, N. Cormier, C.L. Stagg. 2012. Ecosystem Development after mangrove wetland creation: Plant-soil change across a 20-year chronosequence: Ecosystems 15: 848-866, https://doi.org/10.1007/s10021-012-9551-1.

All cores were transported to the U.S. Geological Survey in St. Petersburg, FL, the day of collection and immediately sectioned. Cores were sectioned on a spinning wheel, which was calibrated to push 1 cm of sediment out of the core top. This interval was removed, placed in a labeled whir-pak bag, weighed (wet weight) and frozen. Samples were shipped frozen overnight to the U.S. Geological Survey in Woods Hole, MA, where all further laboratory analysis was done. All sediment samples were kept frozen (-40 degrees Celsius) until freeze dried. Samples were kept in original sample bags and placed in a freeze dryer for 1 week until constant weight was achieved. Samples were immediately weighed (dry weight). Dry bulk density was determined as the dry weight of a known volume of sample.
Approximately 5 g of dried sediment sample was blended and homogenized prior to sealing in a jar for a minimum of three weeks and then placed on a planar-type gamma counter for 24 to 48 hours to measure 7Be, 137Cs, 210Pb, and 226Ra at 477, 662, 46.5 and 352 kiloelectronvolts (KeV) energies respectively (Canberra Inc., USA). Detector efficiency was determined from EPA standard pitchblende ore in the same geometry as the samples. Activities of 7Be, 137Cs, and 210Pb were decay corrected to time of collection. Suppression of low energy peaks by self-absorption was corrected for according to Cutshall and others, 1983. Peak detection, with respect to background activity, is calculated for each radioisotope in the APTEC software during sample analysis. Generally, measured radioisotope activity greater than or equal to 0.30 (210Pb), 0.20 (226Ra), 0.71 (7Be), and 0.10 (137Cs) dpm/g were accepted as above detection limit for this dataset; values below are reported as 0. Sediment ages and accretion rates were calculated with the continuous rate of supply 210Pb age model, a variant on the advection-decay equation (Appleby and Oldfield, 1978; Goldberg, 1963). This model assumes that 210Pb supply to the sediment surface is constant through time, but allows for changing sedimentation rates, in addition to decay, to control the down-core activity of 210Pb. The common form of the CRS (constant rate of 210Pb supply) model as derived by Appleby and Oldfield (1978) solves for age based on the distribution of 210Pb in the sediment record. Prior to application of the age model, 210Pb profiles were evaluated to ensure they were sufficiently resolved to apply the CRS model without bias towards ages that are too old or accretion rates that are too low at depth (Binford, 1990). All gamma analyses were ongoing from 2015 and completed in 2016.
Appleby, P.G., and Oldfield, F., 1978, The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediment: Catena, v. 5, issue 1, p. 1–8, https://doi.org/10.1016/S0341-8162(78)80002-2.
Binford, M.W., 1990, Calculation and uncertainty analysis of 210 Pb dates for PIRLA project lake sediment cores: Journal of Paleolimnology, v. 3, issue 3, p. 253-267, https://doi.org/10.1007/BF00219461.
Cutshall, N.H., Larsen, I.L., and Olsen, C.R., 1983, Direct analysis of 210 Pb in sediment samples—Self-absorption corrections: Nuclear Instruments and Methods in Physics Research, v. 206, issues 1–2, p. 309–312, https://doi.org/10.1016/0167-5087(83)91273-5.
Goldberg, E.D, 1963, Geochronology with 210 Pb, in Miller, J.A., convener, Radioactive dating: International Atomic Energy Agency Symposium on Radioactive Dating, Athens, Greece, November 19-23, 1962, [Proceedings], p. 121-131.

A subsample of freeze-dried sediment (0.5 g) was ball-milled to a fine powder and packaged for carbon and nitrogen analysis at the USGS WHCMSC. Sample preparation involved weighing 10-30 micrograms of ball-milled sediment into a silver capsule, sediment was moistened and placed in a fuming hydrochloric acid desiccator overnight. The sample was subsequently dried at 60 degrees Celsius and then encapsulated for carbon and nitrogen analysis via a Perkin Elmer 2400 Series II CHNS/O analyzer. Standards, blanks and reference sediment with a known carbon and nitrogen content were run to verify results (see attribute accuracy report for further details). Analysis of fumed samples yields organic carbon and nitrogen. These analyses occurred concurrently with analyses in the previous process step; after radioisotope analysis was completed on a core, we would begin the total carbon and nitrogen analysis (as other cores were still in the process of being analyzed for radioisotopes).

Samples were run for loss on ignition (%LOI) to determine the operationally-defined organic matter content. %LOI is commonly used as a proxy for organic carbon content, since %LOI is an easy and inexpensive approach. Thus, these samples will be used to determine a relationship between %LOI and %C. A fraction of sediment (1 to 10 grams) was weighed, then burned in a muffle furnace for 4 hours at 450 degrees Celsius and reweighed to determine percent loss on ignition (%LOI). LOI% = (pre weight-post weight)/(pre weight)×100%

Raw data was entered into an EXCEL spreadsheet where all calculations were completed. The Excel spreadsheet was exported as a CSV file from Excel for Mac version 15.33. The CSV file was processed in Matlab to round calculated values to appropriate place values and again exported as a comma separated text file. Note that all calculations were performed prior to rounding and truncating values reported in this data release.