Oxygen and carbon stable isotopic profiles of the fan mussel, Pinna nobilis, and reconstruction of sea surface temperatures in the Mediterranean

Stable oxygen and carbon isotope profiles (δ¹⁸O_skeletal and δ¹³C_skeletal), taken along the direction of growth from the umbo to the shell margin in shells of the pinnid Pinna nobilis, were used to reconstruct sea surface temperatures (SST) in the south-east Mediterranean and ontogenetic records of metabolic CO₂ incorporation. Comparison of the seasonal cycle of SST, predicted from the δ¹⁸O_skeletal record of a small (young) rapidly growing pinnid and temperature measured with a continuous in situ recorder showed that P. nobilis calcifies under isotopic equilibrium with surrounding seawater, thus indicating that P. nobilis shells can be used as a reliable predictor of SST. A 10-year SST record for the south-east Mediterranean was reconstructed from the shell profiles of four pinnid shells of different sizes and ages collected in 1995 and 1996. Reliable resolution of the seasonal SST could only be achieved during the first 4 years of shell growth. As the pinnids grew older, the temperature record was poorly resolved because the shell growth had diminished with age, resulting in time-averaging of the record. The amplitude of the generated seasonal temperature cycle compared favourably (± 2°C) with a long-term temperature record from northern Mediterranean waters. Clear seasonal cycles in δ¹³C_skeletal were observed with an amplitude of ∼1.0‰, similar to the calculated seasonal changes in δ¹³C of seawater (0.6‰) overlying seagrass meadows. An ontogenetic trend towards less positive δ¹³C_skeletal values was too large to be attributed to any decrease in δ¹³C in seawater resulting from the invasion of anthropogenic CO₂. It is suggested that the temporal changes of δ¹³C_skeletal are due to incorporation of respiratory CO₂ into the extrapallial fluid and reflect changes in the metabolic activity of the pinnid rather than changes in the isotopic composition of dissolved inorganic carbon within the surrounding seawater.