CLAMS AND OYSTERS—APPLES AND ORANGES? COMPARING CONTEMPORARY BIOGEOCHEMICAL ARCHIVES FROM MERCENARIA MEARCENARIA AND CRASSOSTREA VIRGINICA

Understanding the biology of modern organisms is essential to accurately reconstructing past environmental conditions from fossils. Previous work has documented species-specific growth limiting temperatures as well as variable growth rates. These observations suggest reconstructions from different species may reflect biological controls rather than environmental variations. To investigate this potential complication, we compared oxygen isotope (δ¹⁸O_c) profiles from two contemporary marine mollusks (Mercenaria mercenaria and Crassostrea virginica) growing in the same locality (Carteret County, North Carolina). Observed δ¹⁸O_c profiles were collected from shell material deposited between August 2016 and April 2019. These data were calibrated with predicted δ¹⁸O_c profiles (aragonite and calcite) calculated from high resolution environmental records (hourly temperature records and weekly δ¹⁸O water samples). M. mercenaria specimens were periodically stained with calcein during the study interval, facilitating transformation of the δ¹⁸O_c profiles into the time domain. δ¹⁸O_c profiles from C. virginica were fit directly to the predicted δ¹⁸O_c profile. Growth rates from each species were calculated from shell measurements and the dated isotope profiles. Our preliminary results suggest that M. mercenaria and C. virginica have different growth limiting temperatures and variable growth rates. In addition, the species responded differently to rapid environmental changes (e.g., large precipitation events). These events can be identified by slowed or halted growth or changes in observed δ¹⁸O_c. Accordingly, δ¹⁸O_c samples are likely subject to variable degrees of time-averaging. Our findings suggest bioarchives reflect a combination of environmental variation and species-specific biological controls.