OBSERVED VS. MODELED ABUNDANCE OF THE ANTARCTIC SCALLOP ADAMUSSIUM COLBECKI IN THE RECENT SEDIMENTARY RECORD, EXPLORERS COVE ANTARCTICA

Sparse skeletal material in Cenozoic sediment recovered in cores from the Antarctic margin (e.g. Cape Roberts, ANDRILL 1B) contrasts with the prolific modern benthic fauna. Does body fossil abundance in the cores accurately reflect the abundance of shelled animals living at the time of deposition? To answer this question, we are comparing the abundance of the epifaunal Antarctic scallop Adamussium colbecki on the sandy substrate under semi-permanent sea ice in Explorers Cove (EC), Antarctica (offshore of Taylor Dry Valley) with that in shallow cores from the same area. Factors controlling the abundance of shell material retrieved in the recent stratigraphic record include population density, growth rate, life span, recruitment rate, and sedimentation rate. These were used to model shell accumulation. Scallop density based on 20 m² transects (n=16) and 1 m² quadrats (n=21) is 6 m^-2. Average adult scallop size (long axis length) was 75 mm with an estimated cap of 85 mm based on observation. Growth rates from literature show that growth is inversely proportional to size, slowing from 8.5 mm yr^-1 to 0.3 mm yr^-1; a scallop 85 mm long would be ~55 years old. Sexual maturity is reported to occur at ~60 mm. Paucity of small (< 30 mm) scallops indicates intermittent recruitment; we estimate one recruitment event per 10 years, with at least one offspring of each scallop surviving to 85 mm. Rate of sedimentation is inversely proportional with shell density and was estimated to be 1 mm yr^-1. Initial model results indicate that if observed conditions and estimated rates remain constant for 100 years, the resulting 10 cm thick sediment layer should have 69 scallops m^-2. The shells (2 per scallop) would cover an area of 6100 cm² per m². Scaling this estimate to a 7 cm core, each should have 22 cm² of shell in the top 10 cm. Scallop fragments were isolated and measured from 12 cores (10 cm deep) from EC; mean area of fragments recovered per core is 3.4 cm², with no shell in 5 cores. A core from one area (Wales Delta) observed to have twice the mean number of benthic scallops contained 23 cm² of shell. Mean shell area of the other 11 cores was only 1.6 cm². The discrepancy between predicted shell material per core (22 cm²) and observed (mean = 3.4 cm²) suggests significant taphonomic loss between living scallops and the recent stratigraphic record.