Seasonality and growth patterns using isotope sclerochronology in shells of the Pliocene scallop Chesapecten madisonius

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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) prominent growth lines were formed annually, and (2) growth rates estimated using isotope sclerochronology were comparable to rates estimated using visual inspection (measuring the width between external growth lines). We compared both techniques for estimating growth rates and age on three late to mid-Pliocene C. madisonius shells. The first approach located prominent growth lines on the δ18O time series, and differentiated between annual and non-annual (disturbance) growth lines. The second approach assumed all prominent lines were annual. This comparison showed that visual inspection underestimated growth rates and overestimated age. Seasonal timing of annual growth line formation using isotope sclerochronology provided unexpected results. Because this region fell within the warm-temperate paleobiogeographic province, 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.