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Oxygen isotopes from Phorcas (Osilinus) turbinatus shells as a proxy for sea surface temperature in the central Mediterranean: A case study from Malta

March 14, 2013

The marine topshell, Phorcus (Osilinus) turbinatus, is a common component of many archaeological sites in the Mediterranean. This species has been successfully used as a palaeoclimate proxy in Italy. To test whether δ18O from P. turbinatus shells can serve as a reliable palaeoclimate archive for other regions of the Mediterranean, we collected live P. turbinatus from the northeast coast of Malta each month for a year. The δ18OSHELL values of the outermost growth increments of these live-collected shells ranged between − 0.4 and + 2.4 ‰. These values correspond to growing temperatures calculated from shell edge δ18O of between 15 °C to 27 °C. Calculated shell edge sea surface temperatures are highly correlated with instrumental records of sea surface temperature recorded over the period of collection. The individuals analysed for this study are smaller than P. turbinatus from populations studied elsewhere in the Mediterranean. Nonetheless, δ18OSHELL provides a robust record of sea surface temperatures, suggesting that smaller/younger shells in archaeological deposits can still provide reliable palaeothermometry records. This study extends the upper growth limit P. turbinatus by 2 °C compared with previous studies of P. turbinatus in the Mediterranean and suggests that, contrary to previous studies, growth shutdown does not occur in all P. turbinatus when sea surface temperatures exceed 25 °C. This may reflect the higher sample resolution that can be obtained from smaller/faster growing shells, or it may reflect actual higher growth tolerances of P. turbinatus populations in Malta. By showing that P. turbinatus precipitate their shells in δ18O equilibrium with surrounding sea water, this study reinforces the potential for the stable isotope chemistry of P. turbinatus shells preserved in Mediterranean archaeological sites to provide a window into the climate and seasonality regimes of the past.

Prendergast A L, Azzopardi M, O’Conell T C, Hunt C, Barker G, Stevens R E, in press. Oxygen isotopes from Phorcas (Osilinus) turbinatus shells as a proxy for sea surface temperature in the central Mediterranean: A case study from Malta, Chemical Geology, 2013, doi:10.1016/j.chemgeo.2013.02.026. Article.

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