STABLE CARBON ISOTOPE FRACTIONATION IN FRESHWATER MUSSEL SHELLS

Freshwater bivalve shells record geochemical information at the time of aragonite precipitation. Most of the carbon that forms the shell is precipitated from dissolved inorganic carbon (DIC). Therefore, shells may record δ¹³C_DIC values and provide an archive of biogeochemical cycles. However, metabolic C (C_M) that is incorporated into bivalve shells can reduce the precision of the δ¹³C_DIC archive. To constrain the fidelity of freshwater shells as δ¹³C_DIC recorders, we monitored a plethora of water parameters including δ¹³C_DIC and δ¹³C_POC every week for one year adjacent to the Columbus Zoo and Aquarium Freshwater Mussel Conservation and Research Center in the O’Shaughnessy Reservoir in Ohio. Lampsilis cardium that grew in this water were collected and shells and tissues were analyzed for C isotopes. We investigate high-resolution variations in C_M over the year in two shells. Shells were serial sampled over the full year with a resolution of ~1 week. Shell δ¹³C values were more negative than predicted using either the temperature based fractionation from Grossman and Ku (1986; Chemical Geology 59: 59) or the more commonly used temperature independent fractionation from Romenek et al. (1992; GCA 56: 419). Estimates of C_M ranged from 12 to 42% and co-varied with environmental pH and dissolved oxygen. Additionally, we conducted an experiment to determine the δ¹³C value of respired carbon in this species, which illustrated that both δ¹³C values from tissues or POC can be used as a proxy of respired carbon δ¹³C in this system.