Comparing contemporary biogeochemical archives from Mercenaria mercenaria and Crassostrea virginica: Insights on paleoenvironmental reconstructions

Abstract

Reconstruction of past environmental conditions from modern and fossil organisms is predicated on predictable biological responses to environmental variation. Thus, understanding how different species record commonly experienced environmental conditions is critical. This is particularly important in settings where multiple biogeochemical archives are available for study (e.g., a species rich fossil assemblage). Previous work has documented species-specific thermal tolerances and growth rates, which suggests proxy-based reconstructions may reflect a combination of biological controls and environmental variation. To investigate this potential complication, we compared oxygen isotope (δ18O) profiles from shells of two contemporary marine bivalve mollusks (Mercenaria mercenaria and Crassostrea virginica) growing in the same locality (Cape Lookout region, North Carolina, USA). Carbonate oxygen isotope samples (δ18Oc) were collected from shell material deposited between August 2016 and April 2019. These data were calibrated with predicted δ18Oc profiles (aragonite and calcite) calculated from high resolution environmental records (hourly temperature records and weekly water samples). M. mercenaria specimens were periodically stained with calcein during the study interval, facilitating transformation of the δ18Oc profiles into the time domain. δ18Oc profiles from C. virginica were fit directly to the predicted δ18Oc profile. Our results suggest that M. mercenaria and C. virginica living in the Cape Lookout region are capable of depositing shell material throughout the year. Shell growth in both species slows significantly, or halts altogether, during short term large magnitude freshwater input events. M. mercenaria examined in this study show a strong preference for growth during the warmest hours of the day. This observation suggests optimal growth temperatures lie close to the upper limit of temperatures recorded at the study site. C. virginica does not show a preference for shell growth during the warm or cold hours of the day. Thus, a major conclusion of this study is that two different species living in the same place at the same time may record different aspects of commonly experienced environmental conditions. Nevertheless, our findings suggest the fossil record may contain more detailed archives of past environmental conditions than previously thought. Future studies should focus on high-resolution, short-duration calibrations where environmental data is recorded at the time, location, and resolution.