Seasonality and Growth Patterns Using Isotope Sclerochronology In Shells of the Pliocene Scallop, Chesapecten Madisonius

Growth lines and variation in oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) in shells of the Pliocene scallop Chesapecten madisonius preserve seasonal chronologies of biological and environmental change. This study evaluated whether: (1) growth rates estimated using isotope sclerochronology were comparable to rates estimated using visual inspection (measuring the width between external growth lines); and (2) prominent growth lines formed annually during summer months. To achieve our first objective, we compared both techniques for estimating growth rates and age on three mid to late Pliocene C. madisonius shells. The first approach located prominent growth lines on the δ¹⁸O time series differentiating annual and non-annual (disturbance) growth lines. The second method assumed all prominent lines were annual. This comparison revealed that visual inspection under-estimated growth rates and over-estimated age. The second objective evaluating the seasonal timing of annual growth line formation using isotope sclerochronology provided unexpected results. We collected the Pliocene shells from the warm-temperate paleobiogeographic province therefore we predicted annual lines formed during summers (most negative δ¹⁸O values). Instead, annual growth lines coincided with the most positive δ¹⁸O values (winter), typical of bivalves from cold-temperate regions. Moreover, shells recorded seasonal temperatures ranging from 3.2-20.8°C, a range lower than the thermal regime defined for warm-temperate environments (8-25°C). Possibly, the Sea Slope Gyre, which mixed eddies and cold filaments of the Labrador Current and warm waters of the Gulf Stream, penetrated the warm-temperate environment in this region. Alternatively, warm-water fauna from the zoogeographic Carolinian subprovince migrated northward and endured by virtue of warm summer temperatures. Regardless of the explanation, our findings provide a glimpse of mid latitude seasonal temperature range for a warm climate episode during the mid Pliocene.