Seasonality and mean annual sea surface temperatures from isotopic and sclerochronological records

Seasonality is one of the most dramatic climatic variables affecting the physical and biological properties of the surface of oceans. Attempts to deduce the seasonal temperature contrast of past oceans using the fossil record have led investigators to explore the carbonate growth patterns in certain marine invertebrate organisms. In many cases these growth patterns represent annual growth increments analogous to annual rings in trees¹. Just as tree rings contain a record of terrestrial climates², systematic variations in the physical or chemical properties of annual growth increments in the skeletons of marine organisms record the environmental or climatic properties of the marine realm. Several investigations have called attention to the use of stable isotope profiles in growth increments of mollusc shells to determine the seasonal water temperature changes^3-7 and seasonal timing of coastal upwelling⁸. Mollusc shells therefore have the potential of revealing much information about past environmental changes ⁹, particularly along temperate continental shelves where our knowledge of the palaeoceanographic history is very limited. We have now used coordinated isotopic and sclerochronological (growth increment) studies to compare the geochemical history of shell growth in Spisula solidissima with the physical oceanographic history of the Middle Atlantic Bight and determine whether the isotopic records of S. solidissima from different water depths on the continental shelf reflected the seasonally different thermal regimes at those depths. We chose S. solidissima because it is widely distributed along temperate continental shelves, its shell growth and life history are well understood^9,10, and the genus has a fairly wide distribution in Plio-Pleistocene fossil deposits. Jones^11,12 has already established through a detailed sclerochronological analysis of S. solidissima that a 10-20 yr time series of mean sea surface temperature trends can be resolved from the record of annual shell growth. A strong negative correlation exists between growth increment size and yearly mean water temperature¹¹.