AGE MIXING AMONG SYMPATRIC BIVALVES AND BRACHIOPODS FROM THE BRAZILIAN SOUTH ATLANTIC

Little is known of how time averaging of marine skeletal accumulations compares among different species in a single locality. Here we provide quantitative data on the time averaging magnitude of two species with divergent physical and ecological characteristics, the terebratellidine brachiopod Bouchardia rosea and the tellinacean bivalve Semele casali, collected from two sites (10 m and 30 m ) on a mixed carbonate-siliciclastic shelf off the coast of Brazil in Ubatuba Bay (Southeast Brazilian Bight, SW Atlantic).

Individual shells (n=161) were dated using amino acid racemization (D/L aspartic acid) calibrated with eighteen AMS radiocarbon dates. Both taxa show similar age ranges (modern to 12,000 yrs. BP) with semi-quartile ranges of 1300 years for brachiopods and 1400 years for bivalves. Age distributions for each group are right-skewed and dominated by shells less than 3000 yrs. BP. Comparison of brachiopods at the two sites indicates that specimens from the 30 m site are more time averaged than specimens from the 10 m site, but no significant difference is seen in bivalves from the two sites. The most time averaged species from the 10 m site is S. casali, whereas the most time averaged species from the 30 m site is B. rosea. These differences among sites are attributed to stochastic variation in the intensity of taphonomic processes and frequency of exposure at the sediment-water interface.

Analysis of the completeness of each sample, using a Monte Carlo model that simulates 100% complete uniform or exponential distributions, indicates that brachiopods and bivalves may both have been drawn from 100% complete exponential distributions, although individual sites exhibit appreciable variation in their putative underlying distributions. A broader-scale pattern emerges when these data are considered among other published shell ages from marine settings. As water depth increases, time averaging magnitude for both organisms increases consistently and significantly. This pattern is not unexpected, but its strength when considering taxonomically, environmentally, and geographically pooled data is striking. In summary, these findings imply an independence of the intrinsic physical and biological characteristics of an organism and time averaging magnitude, at least among these commonly fossilized organisms.