Type_of_Source_Media: paper
Source_Contribution:
Twenty-four vibracores were collected within Apalachicola Bay, Florida during May 2007. Vibracore sites were selected based on the identification of target areas. These target areas were chosen from the analysis of geophysical data collected during 2005 and 2006 in collaboration with the National Oceanic and Atmospheric Administration’s (NOAA) Coastal Services Center (CSC) and the Apalachicola Bay National Estuarine Research Reserve. The vibracores were collected using a Rossfelder electric percussive (P-3) vibracore system. The cores were collected in a 7.62 cm (3 in) diameter by 6.10 m (20 ft) long aluminum tube with a copper core catcher riveted to the bottom. The vibracoring rig was mounted, deployed, and recovered on the port side of the R/V Gilbert. The ship was anchored from the bow and stern to maintain ship position as a hydraulic crane deployed and recovered the coring rig. Following recovery, the bottom of the core was capped with a plastic cap, water was drained from the top of the core barrel by drilling a hole in the aluminum tube immediately above the sediment/water interface, and the pipe was cut at the sediment/water interface. Each core was cut into one-meter sections (measuring down from the sediment/water interface) on the boat deck, the ends were capped, each section labeled, and stored on deck for transport.
Core sections for all 24 vibracores were transported to the U.S. Geological Survey Florida Integrated Science Center (FISC) where the 1-meter core sections were split in half (lengthwise), described, and photographed in the FISC core laboratory. One half of each core section was preserved and archived in a clear plastic sleeve with the ends taped shut to minimize moisture loss. The other half, the working half, of the core was used for visual descriptions, photographs, and eventually sub-sampling. Visual core descriptions were recorded on field sheets that contained information regarding: cruise, core number, describer, general station location, lithologic units, sedimentary structures, biota, and a depiction of the stratigraphic column. The cores were photographed by aligning all sections of a vibracore abreast with depth indicators located between sections. After photographing, all core sections were wrapped in plastic sleeves with the ends taped shut to minimize moisture loss, and stored in labeled cardboard boxes in the FISC core repository.
Sediment samples within select vibracores were collected at depth intervals for grain-size analysis. In total 117 grain-size analyses were conducted on samples from 13 of the 24 vibracores. Samples, approximately 2x2x2 cm in size, were collected away from the edges of the cores to minimize core disturbance and stored in labeled plastic bags. Grain-size analyses were completed in the Sediment Laboratory as the U.S. Geological Survey Woods Hole Science Center (WHSC). Grain-size results for the coarse fraction in all the cores was derived from sieving the >62 micron fraction. The fine-fraction (< 62 microns) was analysed by coulter counter, and in one case (core App07-19) a settling tube. Sample information was recorded in 44 fields in a spreadsheet. Information recorded includes: core location, core id, sample id, top of sample, base of sample, bulk weight, percent of sample in each 1-phi size, percent sand, sediment classification, analyst, skewness, kurtosis, and other sediment-related statistics (Poppe and others, 2005).
Wood fragments and shells were collected within select vibracores for Radiocarbon age analysis. In total 34 radiocarbon ages were produced from wood fragments and shells in 10 of the 24 vibracores. Wood and shell samples were converted to graphite targets at a U.S. Geological Survey laboratory in Reston, VA, and the AMS 14C ages were determined on these targets at the Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory in Livermore, CA. The age results were recorded in a spreadsheet including: core id, top of sample, base of sample, material dated, delta 13C, 14C age, and age error. All dates were 14C dates and had no corrections applied to them.
Microfossil samples were collected within select vibracores for foraminiferal analysis. In total of 95 microfossil samples were selected from 8 of the 24 vibracores. Each ~1-cm sediment sample for faunal analyses was processed at the USGS Foraminiferal Research Laboratory in St. Petersburg, FL, without additional drying. The sediment was placed in a 500 ml beaker containing 250 ml water and ~10 ml of 5 percent Calgon solution and agitated for less than 60 minutes. Fines were removed by washing the samples over a 63µm sieve, and then placing the residue in a low-temperature oven to dry (< 60° C). The dried fraction was sieved at 125µm, and split as necessary using a microsplitter to obtain approximately 100-200 specimens. Using a binocular stereo microscope to view the sample, foraminifers were removed from the sample using a wet brush, placed on a glued 60 square slide, sorted and identified using standard literature.
In order to give the vibracores greater stratigraphic context, seismic images and trackline data from the geophysical surveys completed in 2005 and 2006 are included in this report (Twichell and others 2006). The original seismic trackline shapefile was edited to include only the tracklines that were adjacent to vibracore sites. The seismic-reflection profiles corresponding to the adjacent tracklines were included as JPEG images with the approximate vibracore locations and lengths shown on the seismic lines. Depths on the seismic profiles are shown in two-way travel time, and core lengths were converted to travel time assuming a speed of sound in water and sediment of 1500 m/sec.
Final figures for each of the 24 vibracores were produced after the field descriptions, core photographing, and grain-size, microfossil, and age date sampling were complete. These summary sheets are expanded and polished versions of the original core field descriptions. Each core summary sheet is a stratigraphic representation of the vibracore with standardized legends representing core lithology and grain-size and age-data when available. The grain-size information is shown to the right of the core log as mean grain size (except for core 19 which shows percent sand) and age dates in uncorrected 14C years are shown to the left of the core log.
