Paleotemperatures have been widely deduced from skeletal O-18/O-16 rat
ios, but these are also dependent on salinity, Without an independent
measure of salinity, O-18/O-16 ratios cannot provide accurate data on
past temperature and climate, We grew marine mollusks (bivalves) in th
e field while real-time data on local seawater temperature and chemist
ry (Mg, Ca, delta(18)O, and salinity) were gathered, Here we show that
for Mytilus trossulus, skeletal Mg/Ca ratios provide an accurate meas
ure of temperature and that weekly sea-surface temperatures may be est
imated with an apparent accuracy of approximately +/- 1.5 degrees C, T
hus, with analyses of both Mg/Ca and delta(18)O from the same specimen
, it is possible to determine seawater temperature and delta(18)O.