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1 Division of Earth Sciences, University of Derby, Derby DE22 1GB, UK A.L.A.Johnson{at}derby.ac.uk
2 Division of Computing, University of Derby, Derby DE22 1GB, UK
3 NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK
4 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
Few data exist on seasonal variation in the temperature (or other aspects of the environment) of late Cenozoic shelf seas in Europe. Ontogenetic records in the shell of Aequipecten opercularis, a widespread, fast-growing and typically well-preserved bivalve, are a potential source. Study of modern forms has shown that oxygen stable isotopes are incorporated in equilibrium with surrounding seawater (hence providing a faithful record of temperature) and data from late Holocene A. opercularis of the southern North Sea Basin (SNSB) indicate that extreme winter (as well as summer) temperatures are registered. Oxygen isotope data from apparently well-preserved, mid-Pliocene shells of the SNSB indicate seasonal temperatures similar to present, whereas microgrowth increment data suggest substantially warmer conditions, in accordance with other evidence. The balance of evidence thus implies cryptic diagenetic corruption of the isotopic temperature signature in mid-Pliocene shells. However, it would be premature to discount the possibility of cooler mid-Pliocene conditions than currently recognized. Ontogenetic variation in carbon isotopic composition within shells is minor and apparently unrelated to environment, but differences between mid-Pliocene, late Holocene and modern shells probably relate to changes in atmospheric CO2 concentration. Unlike certain taxa, seasonal variation is not evident in the strontium or magnesium contents of the A. opercularis shell, but may be displayed by other trace elements, hence affording (together with seasonal increment width variation) a means of independent temporal calibration of isotope profiles. Ontogenetic records of environment from A. opercularis can be complete (and, in the case of increment data, easily recovered) but are of short duration. They must be complemented by (less complete) records from longer lived taxa to obtain the fullest possible environmental history. Seasonal cycles in ontogenetic records afford a means of establishing age and growth rate, and can therefore provide information of value for evolutionary studies and management of living populations.
