A COMPARISON OF RATES OF TIME AVERAGING BETWEEN THE BIVALVE MACOMA CLERYANA AND BRACHIOPOD BOUCHARDIA ROSEA ON A SHALLOW SUBTROPICAL SHELF

Studies of rates of Holocene time averaging in shallow marine settings are important for establishing estimates of time averaging for the fossil record. Rates currently established using dated shells range from 10² to 10⁴ years. However, with limited exceptions, most research thus far has been on benthic mollusks. Moreover, previous research has primarily focused on only one molluscan species per locality. Here, we analyze rates of time averaging of the tellinid bivalve Macoma cleryana and the terebratulid brachiopod Bouchardia rosea. This research represents the first comparison of time averaging rates for bivalves and brachiopods from the same environment.

The two studied organisms are approximately the same size, but differ in many physical and biological respects. M. cleryana has an aragonitic shell and a shallow infaunal life habit, whereas B. rosea is calcitic and an epifaunal, free-lying suspension feeder. Both organisms are found in high abundances on the mixed carbonate-siliciclastic shelf in Ubatuba Bay (Southeast Brazilian Bight, SW Atlantic), off the coast of the state of São Paulo, Brazil. This setting is especially interesting because the fauna present closely resembles that of a Paleozoic shelf, and thus the findings of this study may be directly applicable to such settings.

Samples were collected along a transect from two shallow (10 and 30 m) nearshore localities. In an ongoing effort, a total of 56 individual bivalve shells and 20 brachiopod shells were dated using amino acid racemization (D/L aspartic acid) calibrated with several AMS-radiocarbon dates for each shell type. The dated shells show remarkably similar age ranges for both taxa at each site. At the 10 m site shells range in age from modern to 3,900 years, while the shells from the 30 m site range in age from modern to 4500 years.

The similarity in age ranges between the two taxa demonstrates that life habit and physical characteristics of the shell may have little to do with time averaging duration. Rather, the most important factor may be burial and exhumation history of each individual shell, which should vary randomly between taxa.