A variety of evidence, including direct human records, indicates fluctuations in climate within the European area over the last few millennia: Medieval times and the period of the Roman empire in western Europe are, for instance, considered to have been somewhat warmer than present. Over shallow areas of the continental shelf such variation should be reflected in seawater temperature. This can be estimated by measuring the ratio of the heavy (¹⁸O) and light (¹⁶O) isotopes of oxygen in the calcium carbonate (CaCO₃) of marine shells. Given seawater of constant ¹⁸O/¹⁶O ratio, the ratio in shell carbonate decreases almost linearly with increasing temperature. The shells of bivalve molluscs are particularly useful for temperature estimation by the isotopic method because they are enlarged by accretion onto existing material and, in the case of scallops, grow more or less continuously and at a rapid rate during the first few years of life. Analysis of serial samples hence enables reconstruction of seawater temperature variation and (in principle) precise characterization of climate.

The carbon isotopic composition (¹³C/¹²C) of shell carbonate is not affected directly by temperature, but is influenced by a variety of other factors (including the amount of biological carbon fixation by phytoplankton, the supply of waters from the deep ocean by upwelling, and the anthropogenic contribution of carbon dioxide to the atmosphere), all of which affect the ¹³C/¹²C ratio of seawater.

Six subfossil Queen Scallop (Aequipecten opercularis) shells were obtained from sediments below the shallow waters of the southern North Sea, carbon dated (age range 2,535 to 965 years before present), and analyzed for oxygen and carbon isotopic composition. Temperature profiles constructed from oxygen isotope data exhibit seasonal fluctuations, but extreme summer and winter temperatures do not correspond to expectation for the dates concerned: Shells from the "Roman" and "Medieval" warm periods yield evidence of temperatures little different from present; shells from supposedly cool intervals provide evidence of relatively warm conditions. It is possible that the profiles obtained are unrepresentative of climate for the periods concerned because they relate to years with weather conditions unlike the average. This possibility requires investigation through analysis of further dated shells.

As in modern Queen Scallop shells from the southern North Sea, profiles of carbon isotopic composition from the subfossil examples exhibit little fluctuation, possibly because of a lack of significant upwelling or of a sharp peak in phytoplankton abundance during the year. By contrast, there is a significant difference between the average compositions of modern and subfossil shells that can be attributed to the greatly increased anthropogenic input of carbon dioxide to the atmosphere following industrialization.

In comparison with subfossil examples, modern Queen Scallops from the southern North Sea show more interrupted growth and are relatively rare. This may be a reflection of pollution.

Jon A. Hickson. Division of Earth Sciences, School of Environmental and Applied Sciences, University of Derby, Derby DE22 1GB, UK.
Andrew L.A. Johnson. Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK.
Tim H.E. Heaton and Peter S. Balson. Coastal Geology and Engineering Group, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK.