Dissolution of ophiuroid ossicles (Ophionotus victoriae) in Explorers Cove, Antarctica: implications for the Antarctic fossil record

Abstract

The brittlestar, Ophionotus victoriae, is the third most abundant epifaunal animal in Explorers Cove (EC; mouth of Taylor Valley). However its ossicles, composed of high-Mg calcite, rarely occur in either short cores of recent EC sediment or long cores of Cenozoic deposits in McMurdo Sound (e.g. MIS-ANDRILL 1B). To identify taphonomic processes we analyzed 1) ossicle abundance and microstructural damage in EC cores, 2) ossicle dissolution in a 27-day, in situ experiment using ossicles freed from organic matter, and 3) soft-tissue decay and ossicle dissolution over a 2-year in situ experiment where whole dead brittlestars were suspended above or shallowly buried in the sediment within the taphonomically active zone (TAZ). Seafloor experiments showed a greater maximum percent silhouette area loss (max: 68%) and greater microstructural damage than those suspended (max: 24% loss). From 7,775cm3 of sediment searched from EC, one highly dissolved ossicle was found. Scanning electron microscopy shows the highest levels of dissolution textures, as well as the greatest degradation of the distal portions. During the 27-day experiment ossicles lost between 0.07wt% and 1.31wt%. At observed rate of dissolution and continued exposure in the TAZ it would take between 6 and 110 years for vertebral ossicles to completely dissolve. These results suggest that dissolution affects ossicles soon after death and that the stratigraphic record does not accurately reflect the presence and abundance of ophiuroids, which are major components of shallow Antarctic epifaunal communities.