Oxygen and carbon isotope ratios of Lampsilis cardium (Unionidae) from two streams in agricultural watersheds of Iowa, USA

Climate archives preserved as seasonal variation in oxygen and carbon isotope ratios (δ18O and δ13C) in unionid shells potentially provide records of pre-instrumental environmental conditions in mid- to high-latitude continental regions. This study measured δ18O and δ13C values of biogenic carbonate and δ18OWATER to investigate the timing of growth line formation and to examine the utility of Lampsilis cardium as an environmental archive. We tested the hypotheses that: (1) L. cardium precipitates its shell in isotopic equilibrium with the environment during biomineralization; and (2) growth lines form seasonally during winter months. These questions were addressed by monitoring water chemistry, by tagging L. cardium to record growth, and by combining sclerochronology and stable isotope geochemistry. Temperature and δ18OWATER were measured fortnightly from September 2002 to July 2004 with a break in sampling from December 2003 to February 2004 in two Iowa rivers, Boone River and Buffalo Creek. These data were used to calculate expected δ18O values to compare to measured shell values. One hundred fifty-four individuals were marked at the postero-dorsal margin in September 2002 and June 2003 to monitor growth. Seven recaptured shells collected in July 2004 were analyzed for shell δ18O and δ13C across three growth lines (including growth lines prior to the initial marking period) from the growing edge toward the umbo. 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. Change in temperature or reaching a threshold temperature appeared to control the onset and cessation of growth. Moreover, the locations of seasonal growth lines corresponded to winter growth cessation allowing the assignment of calendar years to growth lines. Seasonal growth lines were distinguished from non-seasonal disturbance lines by their seasonal location inferred from the δ18O time series recorded in shell growth. Thus, we can faithfully reconstruct ontogeny through time and environmental and climate conditions from mid to high latitudes using L. cardium. Variation in δ13CSHELL followed a more or less sinusoidal trend similar to the seasonal profiles of δ18OSHELL, suggesting that seasonal processes influenced the variation in carbon isotopes. Reduced amplitudes and more negative δ13CSHELL values with increasing age suggest that kinetic and/or metabolic isotope effects become important through ontogeny as observed in other bivalves. The profiles of δ13CSHELL potentially record local differences in landscape vegetation. However, specific factors influencing δ13CSHELL are unknown, and further study of water quality are required to fully assess the variation of δ13CSHELL as an environmental proxy.