ORIGINAL Seasonality and growth patterns using isotope sclerochronology in shells of the Pliocene scallop Chesapecten madisonius Ann E. Goewert & Donna Surge Received: 15 November 2007 / Accepted: 12 March 2008 # Springer-Verlag 2008 Abstract Growth lines and variation in oxygen and carbon isotope ratios (δ 18 O and δ 13 C) in shells of the Pliocene scallop Chesapecten madisonius preserve seasonal chro- nologies 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 δ 18 O 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. Sea- sonal 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 δ 18 O values). Instead, annual growth lines coincided with the most positive δ 18 O 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. Introduction The Intergovernmental Panel on Climate Change estimates a2–4.5°C increase in global temperature by the 21 st century, and explicitly states that conditions during the middle Pliocene present “a view of the equilibrium state of a globally warmer world” (Jansen et al. 2007). The middle to late Pliocene Warm Interval (3.2–2.4 Ma) marks an episode of globally warm climates in which, relative to today, temperatures were 3.6°C to 8.9°C higher (Zachos et al. 2001; Cronin et al. 2005; Dowsett et al. 2005; Haywood et al. 2005), continents and oceans had similar configurations (Ramstein et al. 1997; Smart et al. 2007), atmospheric CO 2 levels were higher (pCO 2 ~400 ppmv; Kürschner et al. 1996; Raymo et al. 1996), and sea and continental ice were reduced (Raymo et al. 1990; Guo et al. 2004). Moreover, the end of this climate period represents a transition from high sea level, global warmth, and obliquity- driven high-frequency low-amplitude variability to high- frequency high-amplitude cycles of glacial and interglacial that characterize the Pleistocene (Dowsett et al. 2005). Much of our understanding of Neogene paleoclimatology is based on long time series that provide millennial to sub-millennial resolution (Niemitz and Billups 2005), yet little is known about the variation in seasonality during this time, and its concomitant biological implications. Geo-Mar Lett DOI 10.1007/s00367-008-0113-7 A. E. Goewert (*) : D. Surge Department of Geological Sciences, University of North Carolina, Chapel Hill, 104 South Rd., CB#3315, Chapel Hill, NC 27599-3315, USA e-mail: annieg@unc.edu