A COMPARATIVE ANALYSIS OF TIME AVERAGING FOR BIVALVES AND BRACHIOPODS FROM A MODERN TROPICAL SHELF

Estimates of the duration of time averaging in modern marine shell accumulations have been obtained from several sites and for a variety of species in recent years. Lacking thus far has been a comparative analysis of time averaging for major groups of organisms collected from the same site. To that end, this study utilized shells dredged from two sites (10m, 30m) on a tropical mixed carbonate/siliciclastic shelf (Southeast Brazilian Bight, SW Atlantic). From each site, a suite of specimens of the aragonitic bivalve Semele casali (10m: n=36; 30m: n=36), and the calcitic brachiopod Bouchardia rosea (10m: n=30; 30m: n=28), were chosen. An estimate of the age of each shell was obtained using amino acid racemization (D/L Aspartic Acid), calibrated with 18 AMS-radiocarbon dates spread amongst both species and each site.

The shells of both species range in age from modern to ~8000 BP. B. rosea attain their greatest range at the 30m site (mean=3815 BP; standard deviation=2548 BP) S. casali attain their greatest range at the 10m site (mean=1964 BP; standard deviation=2417 BP) Quantitative comparisons of time averaging were done by compiling shell ages into age-frequency distributions (AFDs) grouped by site and by species. Specimens from the 10m site have statistically indistinguishable AFDs (Kruskal-Wallis, p=0.79; Kolmogorov-Smirnov, p=0.43), whereas AFDs from the 30m site are significantly different (K-W, p<0.001; K-S, p<0.001). This is caused by the brachiopods from the 30m site which have a strongly platykurtic and multi-modal AFD, rather than the highly right-skewed AFD of bivalves from both sites and brachiopods from the 10m site. The dissimilarity of AFDs for brachiopods between sites indicates a biological signal in the data. Variability of AFDs between sites has been noted previously in B. rosea and may indicate significant spatio-temporal fluctuations in abundance, possibly due to variation in upwelling intensity on millennial time-scales. The similarity of AFDs for bivalves and brachiopods at the 10m site suggests a taphonomic overprint. These two species differ in several physical attributes such as shell thickness and mineralogy. Yet, their AFDs at this site are similar suggesting commonalities in the taphonomic process that transcend the physical differences between them.