HIGH RESOLUTION ENVIRONMENTAL RECONSTRUCTION USING STABLE ISOTOPES IN BIVALVE MOLLUSKS SHELLS: A CLOSER LOOK AT SAMPLING STRATEGIES

The interpretation of physical and chemical patterns in bivalve mollusk shells provides a valuable tool in (paleo)environmental reconstruction and (paleo)ecological analysis. Environmental information is preserved in two distinct types of archives: 1) periodic and episodic growth lines and increments, and 2) biogeochemical variations. While inference from sclerochronological records has become a commonplace component of paleoenvironmental research, the initial design and execution of environmental monitoring and shell sampling often receives much less attention. Because shell growth is accretionary, bivalve mollusk shells essentially function as biological chart recorders; they record environmental conditions experienced during shell accretion. Variable growth rates and cessations of growth however, conspire to cause this chart to run at different speeds. Decreasing growth rates through ontogeny reflect slower growth rates and/or a shorter growth interval in successive years. Furthermore, intra-annual growth rate variation is controlled by several hierarchical patterns of environmental variability including, but not limited to, the annual temperature cycle, tidal cycles, and diurnal temperature variability. Taken together, these biological controls and physical factors can modulate when, and how fast, shells grow, which highlights the importance of carefully-designed sampling strategies. In addition, spatiotemporal environmental variation, at multiple scales, may further complicate interpretation of biogeochemical archives. Here we document the importance of carefully-designed, high-resolution sampling strategies using high-resolution records of environmental variation, computer simulated models of biogeochemical data, and actual sclerochronological profiles from freshwater, estuarine and marine settings. Finally, we conclude with specific recommendations for environmental monitoring in the field and shell sampling in the lab. These sampling “best practices” will likely maximize the veracity of both modern calibration studies and ancient environmental reconstructions.