Bivalve skeletons record sea-surface temperature and δ18O via Mg/Ca and 18O/16O ratios

Paleotemperatures have been widely deduced from skeletal 18 O/ 16 O ratios, but these are also dependent on salinity. Without an independent measure of salinity, 18 O/ 16 O ratios cannot provide accurate data on past temperature and climate. We grew marine mollusks (bivalves) in the field while real-time data on local seawater temperature and chemistry (Mg, Ca, δ 18 O, and salinity) were gathered. Here we show that for Mytilus trossulus, skeletal Mg/Ca ratios provide an accurate measure of temperature and that weekly sea-surface temperatures may be estimated with an apparent accuracy of approximately ± 1.5 °C. Thus, with analyses of both Mg/Ca and δ 18 O from the same specimen, it is possible to determine seawater temperature and δ 18 O.