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When Is an "Old" Shell Really Old?

1990; University of Chicago Press; Volume: 98; Issue: 6 Linguagem: Inglês

10.1086/629456

1537-5269

Eric N. Powell, David Davies,

Isotope Analysis in Ecology

Discrimination and comparison between relatively young (recently dead) and relatively old (less recently dead) shells has been an important component of studies on modern death assemblages. We employed amino acid dating to determine whether or not various shell and taphonomic attributes could be used to categorize Donax into age (time-since-death) categories (e.g., old vs. young). As expected, biological interactions and whether the specimen was a right or left valve were not time-dependent. Abraded shells, shells lacking sheen, those showing evidence of dissolution, fragments and chipped shells tended to be relatively old, but many old shells retained the species' original (living) characteristics with regard to all of these attributes except color and sheen. Shell color was the only attribute yielding a satisfactory division of shells into two age categories, old vs. young. The formation of a typical beach assemblage requires the gradual addition and taphonomic alteration of shells over many hundreds of years. No large "age cohort" consisting of many shells of similar age was present on a beach at Port Aransas, Texas. The importance of burial and sediment chemistry as a factor controlling the rates of abrasion, fragmentation, dissolution and dark-staining cannot be minimized. Burial explains the discrepancy between the rapid rates of abrasion typically measured in field experiments, for example, and the much longer time, a hundred years or more, required in a real beach assemblage to obtain heavily abraded shells.