BATHYMETRIC VARIATION IN TIME-AVERAGING ACROSS A MODERN SHELF

In a series of projects focused on time-averaging (age mixing), we have assembled a time-series of individually dated shells collected across a depth gradient along the southern Brazilian shelf. This dataset provides an opportunity to assess if temporal mixing in bivalve assemblages varies significantly with water depth.

Shells of the infaunal bivalve mollusk Semele casali were collected from surficial sediments at multiple sites on the Southeast Brazilian Bight, a sub-tropical passive-margin shelf. A total of 275 specimens were individually dated using amino acid racemization [AAR] methods calibrated against 35 radiocarbon dates. The resulting time series revealed a relatively continuous age distribution with shell ages ranging from modern to ~10,600 yrs BP. The age distribution is right skewed with a slowly tapering tail of older specimens extending back to the earliest Holocene. The substantial presence of old shells spanning multiple millennia is suggestive of a long residence time near or at the sediment surface, as would be expected for this sediment-starved shelf that has experienced low net accumulation rates during the Holocene.

Sites were grouped into three bathymetric datasets (<20 m, 20–30 m, and >30 m). The three resulting age distributions are remarkably comparable in terms of skewness (1.32, 1.52, and 2.48), median shell ages (968, 845, and 648 years), and inter-quartile ranges (2467, 1840, and 1777 years). The distributions are indistinguishable statistically using non-parametric rank tests. Monte Carlo simulations suggest that variations observed across the three distributions are comparable to those generated by random resampling of the pooled data under the null model of homogenous time-averaging.

The results suggest that time-averaging is remarkably invariant across the sampled depth gradient. This is a promising outcome suggesting that the temporal resolution of fossil assemblages may be comparable across depositional profiles of sedimentary basins representing shallow-water open shelf settings. Comparative studies of time-averaging across bathymetric gradients are needed for other depositional settings to develop a more robust understanding of spatial variability in temporal resolution of fossil assemblages.