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U.S. Geological Survey Open-File Report 2009-1031

Vibracore, Radiocarbon, Microfossil, and Grain-Size Data from Apalachicola Bay, Florida


Methods

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Thumbnail image of Figure 6, photo of instrument being deployed from a research vessel, and link to larger figure
Figure 6: Rossfelder electric vibracore system deployed from the R/V Gilbert. U.S. Geological Survey photo.
Thumbnail image of Figure 7, handwritten field notes, and link to larger figure
Figure 7: First page (representing a vertical distance of 1-m) of vibracore field sheet from Core Site App-10.
Thumbnail image of Figure 8, photograph of a vibracore, and link to larger figure
Figure 8: Vibracore photograph from Core Site App-10.
Thumbnail image of Figure 9, seismic-reflection profile, and link to larger figure
Figure 9: Seismic-reflection profile with adjacent vibracore App-24.
Thumbnail image of Figure 10, diagram of vibracore contents, and link to larger figure
Figure 10: Vibracore summary sheet from Core Site App-10.

The following sections provide a basic description of shipboard vibracore collection and post-collection processes and preservation procedures. More processing descriptions and detailed sample information for each data type are provided in the metadata files for each data layer in the Data Catalog. Table 1 provides a quick summary and links to the vibracore imagery and subsample information in this report. The Data Catalog offers comprehensive explanations, download instructions, and metadata for data products associated with this report.

Vibracore Collection

Twenty-four vibracores were collected within Apalachicola Bay, Florida during May 2007. Vibracore sites were chosen based on 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 with a Rossfelder electric percussive (P-3) vibracore system with an aluminum core barrel 7.62 cm in diameter by 6.10 m long with a copper core catcher riveted to the bottom end. The vibracoring rig was mounted, deployed, and recovered on the port side of the R/V Gilbert (fig. 6). The ship was anchored from the bow and stern to maintain 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 through a hole in the aluminum tube immediately above the sediment/water interface, and the pipe was cut at this same interface. Each core was cut into 2-m sections (measure downward from the sediment/water interface) on the boat deck, both ends were capped, and each section was labeled and stored on deck for transport.

Core Processing and Description

Core sections for all 24 vibracores were transported to the U.S. Geological Survey Florida Integrated Science Center (FISC) where the cores were cut into 1-m sections, split in half (lengthwise), described, and photographed in the FISC core laboratory. The archive half of each core section was preserved and sealed in a clear plastic sleeve with the ends taped shut to minimize moisture loss. The working half of the core was used for visual descriptions, photographs, and eventually subsampling. Of the 24 cores, 12 were split in the dark so they could be sampled at a later date for optically stimulated luminescence (OSL) dating. The archive halves of these twelve cores (cores 9, 10, 14, 15, 18B, 20, 21, 22, 23, 24, 26, and 27), once split, were covered with a sheet of aluminum foil and then wrapped in black plastic to shield them from exposure to light. 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 (fig. 7). Each core-description sheet covered 1-m of the core length, so several sheets were needed to describe each core. The field labeled "station" on these sheets was not used for the station number because it was the same as the core number, but instead was used to indicate whether the core was covered for OSL dating. The cores were photographed by aligning all sections of a vibracore abreast with depth indicators between sections (fig. 8). After being photographed, 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.

Sample Collection

Grain-Size Analyses:

Sediment samples within select vibracores were collected at specific depth intervals for grain-size analysis. A total of 117 grain-size analyses were conducted on samples from 13 of the 24 vibracores. Samples approximately 2 by 2 by 2 cm in size were collected away from the edges of the cores to minimize contamination by core disturbance and stored in labeled plastic bags. Grain-size analyses were completed in the Sediment Laboratory at the U.S. Geological Survey Woods Hole Science Center. Grain-size results for the coarse fraction in all the cores were derived from sieving fraction fraction great than 62 microns. The fine fraction (less than 62 microns) was analyzed by Coulter Counter, and one core had to be analyzed with settling tubes (App-19) because the fine fraction was overwhelming the Coulter Counter. Sample information was recorded in a spreadsheet with 44 unique fields. Information on core location, core id, sample id, top of sample, base of sample, bulk weight, percentage of sample in each 1-phi size class from -5 phi to 11 phi, percentage sand, sediment classification, name of analyst, skewness, kurtosis, and other sediment-related statistics were recorded (Poppe and others, 2005).

Radiocarbon Age Dates:

Wood fragments and shells were collected in select vibracores for radiocarbon age analysis. A total of 34 radiocarbon ages were determined 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 14C ages were determined on these targets at the Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory in Livermore, CA. The age results recorded in a spreadsheet include core id, top of sample, base of sample, material dated, δ13C, 14C age, and age error. All dates are 14C dates in uncorrected radiocarbon years before 1950.

Microfossils:

Microfossil samples were collected in select vibracores for foraminiferal analysis. A total of 98 microfossil samples were selected from 8 of the 24 vibracores. Each 1-cm sediment sample collected 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, which contained 250-ml of water and approximately 10-ml of 5-percent Calgon solution, and agitated for less than 60 minutes. Fines were removed by washing the samples over a 63-micron sieve and then placing the residue in a low-temperature oven to dry (cooler than 60° C). The dried fraction was sieved at 125 microns, and split as necessary by using a microsplitter to obtain approximately 100–200 specimens. By using a binocular stereo microscope to view the sample, foraminifers were removed from the sample with a wet brush, placed on a glued 60-mm slide, sorted, and identified by comparison with standard literature. Total foraminiferal counts along with percentage by specie analyses were completed for all 95 samples. Twenty-eight unique species were identified, as well as a few less common species that were lumped into an 'other species' category (table 2 ). All faunal slides will be curated at the Smithsonian Institution National Museum of Natural History at the conclusion of the study.

Seismic-Reflection Profiles and Tracklines

In order to give the vibracores greater stratigraphic context, seismic images and trackline data from the geophysical survey completed in 2006 are included in this report (Twichell and others, 2007). The original seismic-trackline shapefile was edited to include only the tracklines that are adjacent to vibracore sites. The seismic-reflection profiles corresponding to the adjacent tracklines are included as JPEG images with the approximate vibracore locations and lengths shown on the seismic lines (fig. 9). Depths on the seismic profiles are expressed as two-way traveltime, and core lengths were converted to travetime based on an assumed speed of sound in water and sediment of 1500 m/sec.

Core Summary Sheets

Final figures for 23 of the 24 vibracores were produced after the field descriptions, core photographes, and grain-size, microfossil and age-date sampling were complete. A core summary sheet was not created for core App-18A, because core App-18B is a more complete core from the same site. These summary sheets were created with Corel Draw software and are expanded and polished versions of the original core field descriptions. Each core summary sheet is a stratigraphic representation of the vibracore with standardized symbols representing core lithology and grain-size and age data when available (fig 10). The grain-size information is shown to the right of the core log as mean grain size (except for core 19, which shows the percentage of sand), and age dates in uncorrected 14C years are shown to the left of the core log.



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