SHARP SHELLS: APPLICATION OF MOLLUSCAN TAPHONOMY TO BEACH RESTORATION IN ALABAMA

Alabama mollusks were studied during a beach restoration study funded by the Minerals Management Service. Local officials want beach sand without sharp shell fragments. Thus, an overview of local molluscan taphonomy was based on beach samples.

Alabama’s beaches are microtidal; major factors in sediment transport are storms, longshore drift, and influx from the Mobile basin. Storms transport, fragment, and abrade shells; transport is roughly normal to the shore, paralleling sustained wind direction. Longshore drift is mainly westward. Only robust shells are transported for any distance before breaking; each station has its own fauna. A muddy plume enters the Gulf through Mobile Pass and is deflected westward.

Differences in offshore facies are reflected in beach shell samples. East of Mobile Pass, the shelf is veneered by sandy ridges with intervening swales floored by clay to silty sand. Sand ridges yield robust forms such as Anadara and Noetia, whose shells withstand breakage while being abraded. Swales host fragile forms that break quickly (Atrina, Cyrtopleura). West of Mobile Pass, most of Dauphin Island faces a relatively steep, erosional shelf. Offshore Pleistocene deposits yield estuarine Rangia and Crassostrea. Mollusks include some that thrive in variable turbidity (Mulinia, Nuculana, Agriopoma, tellins), but the robust forms that would yield long-lasting sharp fragments are similar to those east of Mobile Pass.

After beach restoration at Gulf Shores, newly broken shells were hazardous because many were oriented with edges and corners pointing up. These include robustly ribbed bivalves that break along ribs (Dinocardium, Trachycardium, Argopecten). Smoother, convex bivalves break irregularly (Macrocallista, Mercenaria, Ostrea); flattish, smooth Dosinia yields sharp triangles. Over months, waves reoriented bivalve fragments to horizontal and eventually rounded off sharp edges. However, robust, strongly curved snails (Oliva, Phalium, Polinices, Strombus) present a sharp surface upward in a range of positions.

Each kilometer has its own fauna of dead shells, underscoring the need for intensive sampling on beaches before restoration. To avoid sharp shells, offshore sampling is desirable, but some idea of the nearshore malacofauna may be gained from beach samples.