Is It Modern on the Surface? – Time-Averaging Estimates from the West Florida Shelf Using Amino Acid Racemization from Chione (Bivalvia)

The degree of time averaging of shells in the shallow subsurface (upper 10 cm) of marine nearshore depositional environments is assumed to be minimal. In this study, we measured amino acid racemization in shells dredged from the west Florida shelf to test whether these shallow beds represent timescales shorter than 100 years. Benthic samples were collected at 10 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). Chione bivalve shells representing three taphonomic grades were selected from these samples and analyzed for D/L ratios of aspartic and glutamic acids.

Our analyses show that pristine shells are consistently representative of modern individuals, whereas shells from higher taphonomic grades are not derived from distinct time intervals. Among the higher grade shells, some are very recent, some are likely to be mid-Holocene in age, and others could be as old as late Pleistocene. Radiocarbon analysis, in progress, will be used to refine the time range represented. Our results indicate that 1) surficial deposits of benthic mollusks have undergone substantial reworking and time averaging 2) shallow shell beds on the west coast Florida shelf are not consistently stratified into time-constrained groups and 3) though mostly characterized by late Holocene to modern shells, the shelf surface may include individuals dating as far back as the last interglacial high stand.