Dissolution of ophiuroid ossicles on the shallow antarctic shelf: Implications for the fossil record and ocean acidification

摘要

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 silhouette area loss during a 2-year in situ experiment in which whole dead brittlestars were suspended above or placed on the sediment-water interface at water depths of 7-25 m; (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. SEM analysis demonstrates significantly higher levels of dissolution in ossicles submerged for two years than in pristine ossicles. Submerged ossicles also had significant breakage reflected in silhouette area loss. During the 27-day experiment, submerged ossicles lost between 0.07 wt% and 1.31 wt%. At the observed rate of dissolution it would take between 6 and 105 years for vertebral ossicles to dissolve completely. Ossicles submerged for two years had a slightly higher mean porosity than pristine ossicles; porosity is controlled by variability in the porous stereom structure as well as dissolution. Results demonstrate that ossicle dissolution starts soon after death and that the stratigraphic record does not accurately reflect the presence and abundance of ophiuroids, thus complicating their use in paleoenvironmental, paleoclimatic, and paleoecologic reconstructions. These results also provide baseline information about CaCO_3 skeletal dissolution needed to monitor the ocean acidification that is predicted to affect high-latitude benthic ecosystems within decades.