LA-ICP-MS LINE SCANS OF SHELL MG/CA RATIOS AS A RAPID, COST-EFFECTIVE ALTERNATIVE TO STABLE ISOTOPE ANALYSIS FOR OYSTER SCLEROCHRONOLOGY

Analysis of stable isotope profiles is one of the few methods of producing retrospective estimates of life history traits (e.g., growth rate, lifespan, phenology) from the shells of mollusks that lack reliable morphological indicators of annual accretionary growth (e.g., growth lines), such as oysters. This fact has made population-level studies of the shells of these organisms difficult, because stable isotope analyses tend to be time consuming and expensive. To address this challenge, we tested Mg/Ca profiles produced via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) as a faster, less expensive alternative to stable isotope profiles for oyster sclerochronology. We produced growth profiles of both δ¹⁸O values and Mg/Ca ratios from 21 oyster specimens, and compared the profiles and resulting lifespan interpretations based on the two methods. We analyzed specimens of two species—Crassostrea virginica and Magallana gigas (formerly Crassostrea gigas)—covering a wide geographic and temporal range, including the Atlantic, Gulf, and West Coasts, and from live-collected to the Pleistocene, respectively, in order to test the method on shells that grew in a variety of seasonal temperature regimes. We found that Mg/Ca profiles from laser ablation line scan analyses were very similar to the more common δ¹⁸O profiles. Almost all (97%, n=102) of the seasonal peaks and troughs in the δ¹⁸O profiles were detectable in the corresponding Mg/Ca profiles for each specimen, suggesting that LA-ICP-MS analyses of Mg/Ca performs well for oyster sclerochronology. This result is important because: 1) the lower cost of LA-ICP-MS analyses may lessen the financial barrier to population-level sclerochemical studies, 2) this method likely will work for other molluscan taxa that lack reliable annual morphological features in addition to oysters, and 3) Mg/Ca ratios tend to be less sensitive to salinity variations than δ¹⁸O values, suggesting LA-ICP-MS analyses may also be useful for studies of estuarine taxa.