Deciphering annual growth features in Chione elevata shells using isotope sclerochronology

Chione elevata is a venerid bivalve common in modern molluscan assemblages and in the fossil record throughout the southeastern United States. Knowledge of its life span and growth rates can provide insights into paleoenvironmental and paleoecological conditions for a large number of fossil assemblages. We used isotope sclerochronology to decipher the accretionary record in modern shells of C. elevata. Sixty-three live specimens were collected from a soft-substrate intertidal flat near Wilmington, North Carolina. Aragonite samples drilled from the outer shell layer of three specimens were analyzed for oxygen isotope ratios (delta (super 18) O). Previous studies show that dark growth lines in other bivalve taxa from the same geographic area, such as Mercenaria mercenaria, correlated with summer growth cessation. A similar accretionary pattern does not exist in C. elevata. Dark growth lines in C. elevata shells appear randomly distributed. Their occurrence does not always correspond with maxima in delta (super 18) O values (winter growth cessation). Additionally, dark growth lines are not correlated with concentric ornaments on the surface of the shell. Instead, maxima in delta (super 18) O values correspond to indentations, or notches, on the surface of the shell. We characterize these concentric external notches as annual markers of winter growth cessation.