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During the pre-site survey in July 1999 surficial sediments from the site(s) will be characterized from the standpoint of their likelihood of producing oxygen supersaturation (hence bubbles of significance to the acoustic experiments) due to benthic phytoplankton productivity. This represents a change from the original proposed goal of measuring gases produced by heterotrophic activity (carbon dioxide and methane) because the initial reports of the nature of the sediments at the likely sites (relatively coarse sand) indicate that supersaturation with respect to methane is unlikely. On the other hand, given the moderate depth of the site, there may be sufficient light penetration to allow significant benthic productivity and thus possible oxygen supersaturation at, or just below, the sediment-water interface.
Oxygen concentrations in surficial sediments will be determined by microelectrode profiling. Commercially available microelectrodes rugged enough to be used in sand will be positioned and moved via a micromanipulator under a binocular microscope. The probe will be calibrated with samples also determined by standard Winkler oxygen titrations. During the cruise, cores 10-15 cm in length will be retrieved by divers and oxygen profiles will be determined immediately. During the measurements, the photosynthetically active radiation (PAR) regime will be kept as close as possible to that on the bottom, via the use of a tent of multiple layers of plastic screening. The PAR regime will also be varied by addition or removal of layers to determine the response of the benthic photosynthetic community to differing light levels via changes in the oxygen profiles. Differences between light and dark profiles will be used to determine the depth distribution of photosynthetic acitivity in the sediment for later modeling of photosynthetic oxygen production as a check on the feasibility of producing oxygen bubbles in this way.
In addition to the oxygen profiling, samples for methane analysis via gas chromatography will be collected from the same sites in case there is methane production from buried organic-rich sediments below the surficial sands. Diver collected cores will be subsampled by inserting cut-off syringes through pre-drilled, taped holes in the walls of the core barrels. These subsamples will be expelled into jars with water added, sealed and shaken to equilibrate dissolved and gaseous methane with the headspace. Headspace subsamples will be returned to Chapel Hill for analysis since methane supersaturation is considered unlikely for these sites.
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