OLD IDEAS, NEW TECHNIQUES: RECYCLING OF FOSSIL SHELL MATERIAL IN ATLANTIC MARGIN COASTAL SYSTEMS

The distribution of carbonate shell material on Atlantic coast beaches and the inner continental shelf is a balance between production and destruction, combined with sediment transport that varies both spatially and temporally. A significant source of this shelly material is the exhumation of fossil material from the shallow subsurface of the shoreface and inner shelf, particularly in sediment-starved areas. The "pre-modern" age of shells in coastal deposits is recognized using chronologic tools combined with physical (color, abrasion) or chemical (trace element) characteristics (Pilkey et al., 1969; Macintyre et al., 1978), and the distribution of fossil material in coastal settings can be used to trace onshore-offshore sediment dynamics (Thieler et al., 2001), even to abyssal depths (e.g., the Black Shell Turbidite of Elmore et al., 1979).

Amino acid racemization (AAR) studies of beach and shelf shells, although less quantitative than radiocarbon, extend by at least an order of magnitude the observed age range of transported beach/shelf shells (Wehmiller et al., 1995). AAR studies of roughly 40 beach or shelf sites (>400 samples) between Cape Henry VA and Cape Romain SC identify multiple Pleistocene aminozones in results for transported (reworked) beach shells, and for shells from either grab- or core-sampling on the inner shelf. Selected 14C analyses allow calibration and/or evaluation of the AAR age estimates. Corresponding onshore subsurface aminozones identify source units for these reworked Pleistocene shells. These units record multiple Quaternary sea level cycles and are recognized in numerous geophysical studies of the Carolina coastal region (Riggs et al., 1992; 1995). For Mercenaria samples from North Carolina Outer Banks beaches, there is a typical trend in color from chalky white to yellow-orange to grey-black with age (modern, Holocene, Pleistocene, respectively). Darkening is a useful diagnostic tool for shell age, but can be variable with species, host sediment, and geography. At several "hot spots," large abundances of whole darkened Mercenaria valves suggest minimal transport distance and recent or continuing exposure of shoreface units, while other areas may have large abundances of only fragments of Pleistocene shell material, and still other areas lack shell material of any age.