Dissolution of Ophiuroid Ossicles on the Shallow Antartic Shelf
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The brittlestar, Ophionotus victoriae, is abundant in Explorers Cove, offshore Taylor Valley. However its ossicles, composed of high-Mg calcite, have not been reported from Cenozoic cores taken from McMurdo Sound. To identify taphonomic processes we analyzed (1) ossicle dissolution and silouette area loss during a two-year in situ experiment in which whole dead brittlestars were suspended above or placed on the sediment-water interface at water depths of 7-25m; (2) ossicle dissolution in a 27-day, in situ experiment using ossicles freed of soft tissue; (3) porosities of experimental and pristine ossicles; and (4) abundance of ossicles in short cores taken at shallow depths in Explorers Cove.
Above : Profusion of ophiuroids (Ophionotus victoriae), filter-feeding scallops (Adamussium colbecki), and sea urchins (Sterechinus neumayeri) on and among plates of anchor ice extending upward from the sediment surface. Eight km south of Explorers Cove. Scallop shells are ∼8 cm across.
SEM analysis demonstrates significantly higher levels of dissolution in ossicles submerged for two years than for pristine ossicles. X-ray Tomography was used to determine the porosity of the ossicles, 24 samples were imaged at the GeoSoilEnvironCARS beamline (sector 13) of the Advanced Proton Source (APS) at Argonne National Laboratoryin Chicago, Illinois. X-ray tomography was used because it is a nondestructive way to create a three-dimensional (3-D) map of the linear attenuation coefficient (for details, see Rivers et al., 1999; Landis and Keane, 2010) that can be used to calculate the total volume of the ossicles. For mapping, four ossicles were stacked in a plastic pipette tip, separated by foam and enclosed with transparent adhesive tape. The vials were placed on a rotating stage; a radiograph was taken at every 0.25u step, with a total of 720 radiographs taken for each tomogram. Tomograms were obtained at 19keV with resulting cubic volume elements (voxels) of 6.1 or 8.9 micrometers in each linear dimension. Reconstructions were performed using tomo_display (Rivers and Gualda, 2009).