SOURCING CHESAPEAKE BAY OYSTERS VIA 87SR/86SR AND TRACE ELEMENT ANALYSIS

Oysters grow by accretion and lay down calcium carbonate shells in chemical equilibrium with the surrounding water. Previous researchers have applied ⁸⁷Sr/⁸⁶Sr in otoliths to trace the changes in salinity of fish nursery habitats, and showed that calcium carbonate records useful information about water chemistry while the organism was alive. Here we propose a method to source oyster shells in the Chesapeake Bay region, by matching the geochemical signature of major Virginia rivers to the geochemistry of oyster shells. Our main tracers were ⁸⁷Sr/⁸⁶Sr and Ba because the majority of Ba in the ocean is derived from continents and is incorporated into oysters during the shell calcification process. Strontium isotopes differ in their sources— with ⁸⁷Sr having a large source from continental weathering. We hypothesize that rivers with small watersheds and a large tidal influence should produce water that is more concentrated in ⁸⁶Sr relative to the other rivers. In contrast, rivers with large watersheds, specifically draining granitic rock, should yield water with a higher concentration of ⁸⁷Sr and higher Ba contents. Shell and water samples were taken from the four main rivers in Virginia that drain into the Chesapeake Bay (James, York, Rappahannock, and Potomac) at the farthest point upstream that oysters grow in abundance. Five to eight oysters of market size and surface water was collected from each site by the Virginia Institute of Marine Science, the Virginia Marine Resource Commission, and the Maryland Department of Natural Resources. Oysters were frozen and water samples were refrigerated until processing. Oyster valves were cleaned and cut with a tile saw to extract the adductor muscle scar, which is a purely aragonitic portion of the shell. These scars were bleached and rinsed in deionized water. The adductor muscle scar was drilled into powder and dissolved in nitric acid. Shell samples were analyzed for ⁸⁷Sr/⁸⁶Sr using Thermo Ionization Mass Spectrometry at Duke University. Samples were analyzed for Sr, Ba, and Ca using atomic absorption and ion chromatography. Preliminary results suggest that oyster shell barium has a strong relationship with salinity, but Sr:Ba values provide the best correlation. These relationships can be used to differentiate oyster shells from other rivers.