Skeletal structure, strontium and magnesium content, and mineralogy of the pelecypod Mytilus have been shown to vary with growth temperature and salinity. Based on these skeletal properties, paleotemperatures were determined from fossil specimens of Mytilus from five Pleistocene exposures along the Pacific coast.

Paleotemperatures determined by the various methods do not always agree, suggesting that diagenesis has differentially modified some of the skeletal properties. As skeletal structure is not easily destroyed by diagenesis, skeletal-structure paleotemperatures can be used to evaluate the extent of alteration of shell chemistry and mineralogy. Strontium content appears to be altered at only one of the sites studied. A small amount of conversion of aragonite to calcite seems to have occurred.

Oxygen isotopic composition was also determined for a sample from each locality. When used in conjunction with paleotemperatures determined by the other methods, oxygen isotopic composition of the samples can be used to determine the δO18 value of the water in which the shell grew, yielding information on paleoclimatic conditions.

Paleotemperatures for all late Pleistocene localities are near present-day surface water temperature values for the nearby ocean. The paleotemperature data and oxygen isotope data suggest an interglacial origin for all late Pleistocene localities. The strontium content and δO18 value for the early Pleistocene (upper Merced Formation) locality indicate a lower temperature and slightly reduced salinity.

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