Dissolution of ophiuroid ossicles on the shallow Antarctic seafloor: implications for the fossil record and ocean acidification

The brittlestar Ophionotus victoriae is abundant on the seafloor around Antarctica, including in Explorers Cove (EC) at the mouth of the Taylor Valley, but ophiuroid ossicles are rarely reported from sediments recovered in cores from the Ross Sea. To determine if absence of ophiuroid ossicles is a taphonomic artifact, we analyzed: 1) vertebral ossicle dissolution and silhouette area loss during a 2-year in situ experiment in which 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 diver-taken cores from EC.

Ossicles submerged in EC showed significantly higher order dissolution features in SEM analysis and greater loss of silhouette area than pristine control ossicles. Porosity was higher in some experimental than pristine ossicles, but not all; porosity may be strongly controlled by variation in stereom structure. During the 27-day experiment, submerged ossicles lost between 0.07wt% and 1.31wt%; at this rate total dissolution would occur in 6 to 105 years. Search of >7,700 cm3 of sediment from short cores taken by divers in EC yielded few ossicles overall, including only two badly degraded vertebral ossicles; these robust and abundant ossicles should be common. 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 precluding their use in paleoenvironmental, paleoclimatic, and paleoecologic reconstructions. They also provide a harbinger of the effects of impending ocean acidification, predicted to be felt first at high latitudes. If ossicles dissolve when freed of the enclosing organic matrix now before the onset of acidification, the energetic cost of secreting and maintaining the high-Mg calcite ossicles at lower pH may be deleterious to ophiuroids that comprise a major component of polar bottom-dwelling communities.

Walker B. J. Miller M. F., Bowser S. S., Furbish D. J. & Gualda G. A. R., 2012. Dissolution of ophiuroid ossicles on the shallow Antarctic seafloor: implications for the fossil record and ocean acidification. 2012 GSA Annual Meeting in Charlotte (4–7 November 2012). Presentation abstract.

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