IS It ALL IN THE LOOKS? TAPHONOMIC GRADES, TIME AVERAGING, AND AGE ESTIMATES FROM CHIONE (BIVALVIA), WEST FLORIDA SHELF

Kowalewski et al (1998) show that there is substantial time-averaging in shells collected from modern surficial sediments, with some shells dating as old as 3,000 years. However the age ranges represented by taphonomic grades within their shell collections was not investigated. In this study, we measure amino acid racemization (AAR) in shells dredged from the west Florida shelf to test whether degrees of time averaging can be distinguished among different taphonomic grades. Benthic samples were collected at 9 m depths using a Capetown dredge at two locations along the southern portion of the shelf (Station IB - 24° 59.977' N, 81° 30.042' W and Station IVC - 27° 15.857' N, 82° 35.063' W). Shells of the bivalve genus Chione, representing three taphonomic grades from pristine to highly degraded, were selected from the dredged samples and analyzed for D/L ratios of aspartic (ASP) and glutamic (GLU) acids.

Radiocarbon measurements show that pristine shells (taph. grade 1) are consistently representative of modern individuals (<58 years). However, more degraded shells (taph. grades 2 and 3) are not representative of a narrow temporal range. Radiocarbon and amino acid racemization analyses indicate ages for more degraded shells can span from recent to mid-Holocene. Additional 14C dating analysis is used to calibrate AAR measurements in more degraded shells. Our results indicate that 1) surficial deposits of benthic mollusks have undergone substantial reworking and time averaging 2) with the exclusion of pristine specimens, time-constrained groups cannot be determined by the taphonomic grade alone 3) pristine shells are indicative of essentially modern assemblages and 4) due to substantial time-averaging, specimens collected in shallow, near shore marine environments have potential value to both modern and paleoecological studies.