QUANTITATIVE ESTIMATES OF TIME-AVERAGING IN MOLLUSK DEATH ASSEMBLAGES ON THE SOUTHERN BRAZILIAN SHELF

Fossil assemblages are expected to be time-averaged due to temporal (and spatial) mixing of skeletal remains. Our quantitative understanding of time-averaging derives primarily from actualistic studies, in which direct numerical dating of individual specimens is used to assess the scale and structure of age mixing in surficial death assemblages (incipient fossil assemblages). Here, we examine time-averaging in shells collected from surficial sediments at three sites on a passive-margin subtropical shelf (the Southern Brazilian Shelf; 33°S). Specimens of Mactra (Bivalvia: Mollusca) were individually dated using amino acid racemization (AAR) (n = 60) calibrated against AMS radiocarbon ages (n = 15). The amount of time-averaging exceeding the expectation due to uncertainties in dating was determined by subtracting the age-estimation error from the total age-estimate variability, based on 10,000 Bayesian Markov-chain Monte-Carlo replicates. The ¹⁴C-calibrated AAR ages, pooled across all three sites, indicate that individual mollusk shells vary in age from 0 to ~56 cal kyr BP. The age distribution is right skewed (i.e., dominated by young specimens) suggesting that the sampled death assemblages is time averaged over multi-millennial time scales. This age-distribution represents a remarkably complete time series: for the last 6 ka (since the last post-glacial marine transgression) the completeness is 75%, both at 250-yr and at 500-yr binning resolution. The magnitude of time-averaging varied inversely with the water depth, from <15 yr in the deepest site (21 meters) up to 1020-1250 yr in the shallowest site (7 meters). The substantial variation in time-averaging across nearby sites, which are located in a seemingly homogenous depositional setting, indicates the presence of notable (if cryptic) heterogeneities in local taphonomic and/or sedimentological conditions. The multi-millennial scale of time-averaging observed for pooled data and strongly right-skewed nature of age distributions are consistent with other studies of time-averaging in sediment-starved shelf settings. The substantial temporal span of dated specimens points to their potential value as archives of the paleoenvironmental history of the Southern Brazilian shelf over the most recent millennia.