Comment on “Stable carbon isotopes in freshwater mussel shells: Environmental record or marker for metabolic activity?” by J. Geist et al. (2005)

Abstract

In a recent analysis of d 13 C in the aragonitic shells of the bivalve Margaritifera margaritifera, Geist et al. (2005) argue that variation in the carbon isotope composition of contemporaneous shells from one locality over periods of decades cannot be correlated with one another, and thus do not record environmental signals. Instead, they ascribe the observed fluctuations to respiratory loss of light carbon and consequent enrichment of 13 C in the youngest part of the shell. However, we note two significant weaknesses in the study that make it difficult to accept their conclusions. First of all, we note the absence of reliable and long-established sclerochronological techniques in the analysis of their shells needed to determine the timing of shell growth and construct reliable chronologies. Second, the authors do not report relevant environmental and physiological data needed to establish an independent metabolic control on shell d 13 C.