AN EVALUATION OF THE EFFECTS OF A CHANGING MIOCENE CLIMATE ON THE GROWTH AND STABLE ISOTOPIC ECOLOGY OF THE SCALLOP CHESAPECTEN NEFRENS

The Middle Miocene was a time of extreme climatic variability, varying from a Middle Miocene Climatic Optimum (MMCO; ~15.5 mya) to an ensuing period of global cooling known as the Miocene Climatic Transition (MCT; ~14.5–13.5 mya). However, accounts of the duration, temperature variation, and extent of cooling differ between studies and geographic locations. Our study uses the scallop, Chesapecten nefrens from MMCO and MCT deposits of the Calvert Cliffs of Maryland to evaluate three objectives. First, to determine if MMCO and MCT scallops record oxygen and carbon stable isotopes consistent with warmer and cooler climates, respectively. Second, to determine if seasonal cycles occur in stable isotopes of MMCO and MCT shells. And lastly, to determine if global cooling affected growth rate and growth cessation of C. nefrens in the Calvert Cliffs.

Contrary to previous climatic studies, the raw d¹⁸O_calcite of C. nefrens indicated an insignificant difference in temperature between MMCO and MCT scallops. Further, when corrected for d¹⁸O_seawater, the d¹⁸O_calcite indicated significantly cooler conditions in the MMCO than MCT (t=2.66, df=436, p< 0.05). Carbon stable isotopes of shell calcite revealed a significantly lighter mean in MMCO than MCT scallops (t= -2.64, df=327.35, p< 0.05), potentially indicating higher productivity during the MMCO than MCT. Transects of d¹⁸O_calcite from umbo to growing edge revealed seasonal cycles in both time periods and indicated a change from consistent summer growth cessation in MMCO C. nefrens to consistent summer and winter growth cessation in MCT C. nefrens. An explanation for the cooler MMCO suggested by d¹⁸O_calcite lies in the season of growth cessation. Shell calcite of MMCO scallops recorded cold winter temperatures, while MCT scallops halted growth in the coldest periods, and therefore do not record the coldest temperatures, biasing MCT d¹⁸O_calcite toward lighter/warmer values. Altered seasonal growth and a significant decrease in primary productivity provide organismal and environmental evidence in support of cooling from MMCO to MCT in the Maryland Miocene.