BIOTURBATION ON ANTARCTICA'S EXPLORERS COVE SEAFLOOR: WHY ANIMAL ACTIVITY HAS A GREATER IMPACT ON THE SEDIMENTARY RECORD THAN ANIMAL ABUNDANCE

Little is known about the sedimentologic and taphonomic processes under semi-permanent sea ice, a setting sensitive to climate change. These processes are being studied in Explorers Cove (EC) bordering Taylor Dry Valley, Antarctica. We analyzed the amount of bioturbation on the seafloor by point counting disruption on a cm-scale grid superimposed on 21 quadrats (1m²) and by assessing the Bedding Plane Bioturbation Index (BPBI) for each quadrat. All quadrats had BPBI of 5 (60-100% disruption) and averaged 77% points disrupted. Two epifaunal animals, the Antarctic scallop, Adamussium colbecki, and the ophiuroid, Ophionotus victoriae, are responsible for the high level of sediment disruption. A. colbecki produces “divots,” depressions in the substrate ~10 cm in diameter and 3 cm deep. O. victoriae leaves shallow (2 mm) imprints of its arms and central disc. Bioturbation rate is a function both of animal density and animal activity. Mean number of live scallops per 20 m² transect (n=16) is 74; range is 4 to 192 scallops. Mean number of ophiuroids per transect is 16. Rates of animal activity are poorly constrained. The estimated minimum number of divots that A. colbecki would produce is 2 m^-2y^-1, the maximum 6 m^-2y^-1. This translates to 157 to 1884 cm² reworked to a depth of 3 cm per m²y^-1. The more active O. victoriae is estimated to disrupt 281 to 5900 cm² per m²y^-1 to a depth of 2 mm. The activity of both O. victoriae and A. colbecki at the observed density could produce a 100% bioturbated quadrat, commonly observed in EC, in 4 to 57 years. A. colbecki alone could disrupt the top 3 cm of sediment in 5 to 64 years, a rate that exceeds most estimates of sedimentation rates and is consistent with the absence of lamination in cores from EC. Bioturbation by A. colbecki and O. victoriae is pervasive in EC quadrats and decoupled from the abundance of these organisms. As demonstrated in this study, the record of the animal activity is much more likely to be encountered in the stratigraphic record than is the skeletal material, especially in core-based studies. This underscores the importance of linking animals to their patterns of bioturbation in documenting climate change, particularly in climate-sensitive sea-ice-covered areas where rates of sedimentation and reworking are low.