TAPHONOMIC CONDITION AS A PROXY FOR MOLLUSCAN TIME AVERAGING IN SYDNEY HARBOUR (NSW, AUSTRALIA)

Previous studies have documented a lack of correlation between molluscan shell age and taphonomic grade in coastal marine environments. These findings suggest that the extent of taphonomic damage observed within a bulk sample cannot be used as a proxy for the amount of time represented by that assemblage. However, few studies have revisited the question since advancements in amino acid racemization (AAR) dating have increased the capacity to quantify time-averaging from bulk samples. The purpose of this study is to determine if the extent of shell breakage is related to extent of time averaging in otherwise similar bulk samples. High rates of sediment mixing and/or low rates of sedimentation are thought to increase the extent of time averaging in a sample, but both of these processes could result in specimens spending longer periods of time in the taphonomically active zone. Increased rates of time averaging should correlate with increased taphonomic damage, unless shell damage results in destruction.

We obtained death assemblages from two sites in Sydney Harbour (New South Wales, Australia) using a diver-operated airlift to sample the top 20 cm of sediment. Sites were selected based on water depth, marine salinity, a sandy bottom substrate, and overlapping species composition. Bivalves collected from each site were identified to the species level, then partitioned into complete versus broken specimens. Five commonly occurring genera (Callista, Dosinia, Fulvia, Glycymeris, Solen) were chosen to represent a range of shell durabilities. Genus-specific breakage was quantified as the proportion of identifiable shell mass that was unbroken. Time-averaging at each site was quantified using calibrated AAR ages for 12 right valves of the venerid Fulvia tenuicostata, which is the most abundant local bivalve in the last 6,500 years. While initial results only show a significant difference for Fulvia, all five genera had a lower median portion of complete shell at the site with more time-averaging. At the higher time-averaged site, the most robust genus (Glycymeris) was > 95% complete. At the lower time-averaging site, the least robust genus (Solen) was only ~ 10% complete. This suggests that inherent shell durability is more likely to drive the proportion of complete shells than the processes that promote time-averaging.