Increased seasonality in the Western Mediterranean during the last glacial from limpet shell geochemistry

The seasonal cycle is a fundamental aspect of climate, with a significant influence on mean climate and on human societies. Assessing seasonality in different climate states is therefore important but, outside the tropics, very few palaeoclimate records with seasonal resolution exist and there are currently no glacial-age seasonal-resolution sea-surface-temperature (SST) records at mid to high latitudes. Here we show that both Mg/Ca and oxygen isotope (δ18O) ratios in modern limpet (Patella) shells record the seasonal range of SST in the western Mediterranean — a region particularly susceptible to seasonal change. Analysis of a suite of fossil limpet shells from Gibraltar shows that SST seasonality was greater during the last glacial by ~ 2 °C as a result of greater winter cooling. These extra-tropical seasonal-resolution SST records for the last glacial suggest that the presence of large ice-sheets in the northern hemisphere enhances winter cooling. This result also indicates that seasonality in the Mediterranean is not well-represented in most palaeoclimate models, which typically show little change in seasonal amplitude, and provides a new test for the accuracy of climate models.

Research highlights
► Patella shell δ18O and Mg/Ca allow reconstruction of seasonal SST and seawater δ18O.
► Patella shell δ18O shows a consistent positive offset of 0.72‰ from equilibrium.
► Glacial Patella shells from Gibraltar show SST seasonality ~ 2 °C greater than today.
► Reconstructed SSTs and seasonality contrast with output of palaeoclimate models.