RATES AND STYLES OF BIOTURBATION BY SHALLOW-WATER ANTARCTIC EPIFAUNA (SCALLOPS AND BRITTLE STARS): IMPLICATIONS FOR THE STRATIGRAPHIC RECORD

Sediments recovered in short cores from the shallow seafloor (7-25 m) under multi-year sea ice in Explorers Cove (EC) near Taylor Valley, Antarctica, consist of massive poorly sorted sands that lack lamination. Absence of bedding in cores is surprising because laminae form readily in EC sediment that has fallen through a few decimeters of seawater. The motile benthic megafauna in EC is dominated by the Antarctic scallop Adamussium colbecki (2-9 ind m^-2), and the ophiuroid Ophionotus victoriae (0.3-1 ind m^-2); there are few large infaunal animals. We tested the hypotheses that disruption of surface sediment by these two epifaunal animals destroys sedimentary lamination and that scallop movement forms divots that are pervasive on the EC seafloor by documenting the behavior of A. colbecki and O. victoriae with time-lapse photography (1 frame sec^-1) in aquaria (30 cm x 30 cm x 15 cm) containing EC sediment in Crary Laboratory, McMurdo Station. One aquarium containing three scallops was filmed for nine days; the other containing one ophiuroid was filmed for four days.

Animal activity reworked the top 2-3 cm in the aquaria in less than two days. O. victoriae disturbed the sediment by burying itself 2-3 cm deep and gashing the surface using breaststroke locomotion; the average ophiuroid bioturbation rate was 0.086 m² day^-1. Scallops moved by expelling jets of water from their mantle cavities. Movements include swimming (propulsion through water column), lateral turning (>90° rotation on sediment surface), clapping (<90° rotation on sediment surface), and flipping (claps turning the scallop over onto its preferred valve). Scallops averaged 14 movements day^-1 ind^-1. Clapping was the most common movement (50%) followed by lateral turning (37%). Water jets ejected during movement eroded sediment to form divots to a depth of 4 cm and reworked sediment at an average rate of 0.12 m² month^-1 ind^-1.

The high bioturbation rate exceeds the sedimentation rate in EC (< few mm/yr). This bioturbation is diffusive and concentrated at the sediment-water interface, leaving no discrete traces or fingerprints of its biologic origin while simultaneously disrupting laminae as they form. Biological activity obliterates sedimentary structures in EC sediments and may similarly affect the sedimentary record in other areas under multi-year sea ice.