ABSTRACT

Bivalve shells have been used in a number of paleoclimate and paleoenvironment studies. In order to investigate oxygen and carbon isotope fractionation between bivalve aragonite and host water, a one year monitoring experiment was conducted. Temperature, δ¹⁸O_WATER and δ¹³C_DIC were measured every week from January 2006 to January 2007. These data were used to calculate expected δ¹⁸O and δ¹³C equilibrium values to compare to measured shell values. At the end of the monitoring experiment, living bivalves (Corbicula fluminea) were collected and sampled for stable isotope analysis (δ¹⁸O_ar and δ¹³C_ar). Comparison of shell δ¹⁸O with expected values revealed precipitation of δ¹⁸O in equilibrium with the ambient environment from early spring to early fall. Winter values were not recorded due to winter growth cessation. Cooler temperatures seem to cause cessation of growth, suggesting that a threshold temperature may control the onset and cessation of growth. Shell δ¹³C are more negative than predicted δ¹³C values, and we observed that all the specimens showed a trend of lower δ¹³C values with increasing age, this suggests that the incorporation of metabolic carbon is the cause of the negative offset in shell δ¹³C. The variable offset from equilibrium δ¹³C values precludes the direct use of Corbicula fluminea shell in the study of the δ¹³C_DIC in ancient bodies of water.