Taphonomy and ages of bivalve shells from San Salvador Island, Bahamas

Paleontological interpretations of taphonomic processes can be complimented by examining abrasion and age of shells in modern settings. A preliminary analysis of the ages of bivalves from Haitian Boat Beach (just east of Barkers Point, northwest coast of San Salvador Island, Bahamas) provides a range of ages of shell material exposed on modern beaches of San Salvador Island and provides estimates of the timing of beach rock lithification. At Haitian Boat Beach Beach, bivalves and gastropods were collected from shell accumulations preserved on the beach as well as within a single 3 to 5-cm thick horizon of beach rock. The ages of 2 valves (Genus Chione) from the beach and 4 valves (Genus Chione and Genus Glycymeris) from the beach rock were analyzed. Two of the most abraded shells were selected for analysis from the beach in an attempt to get a maximum age of beach shell material. These shells were radiocarbon dated by measuring carbon-14 at NOSAMS and converted to calendar years using the CALIB program of Stuiver and Reimer (1993). Two abraded valves collected from the beach yielded calendar year ages BP of 512 and 549 years. The beach rock valves yielded calendar year ages BP of 143, 129, and 1628. These results suggest lithification of beach rock was initiated <120 years ago, and that the valves lithified in the beach rock represent ~1500 years of time-averaging. The ages of shells collected from beach accumulations indicate that either older shells are exhumed from nearby beach rock or underlying sediments or that mollusc shells can persist for ~550 years prior to burial at Haitian Boat Beach. This study also suggests that the degree to which shells are abraded may be indicative of the amount of time at the SWI, at least at Haitian Boat Beach.