Paleotemperatures have been widely deduced from skeletal 18O/16O ratios, but these are also dependent on salinity. Without an independent measure of salinity, 18O/16O 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, δ18O, 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 δ18O from the same specimen, it is possible to determine seawater temperature and δ18O.