One of the most common shells found in the middens of the south-east United States is that of the hard clam, Mercenaria mercenaria and Mercenaria campechiensis. These species range from the Gulf of St. Lawrence to the Gulf of Mexico. The species mercenaria lives about 5-10 years and grows to about half the size of the species campechiensis, which can live up to 20-25 years. The clams grow continuously throughout their lifetime, laying down a new layer of shell each day.
The middle layers of the clamshell are composed of aragonite, CaCO3. These are the layers that we sampled. The clam changes the ratio of oxygen-18 to oxygen-16 in the aragonite in a predictable manner depending on the temperature of the water that the clam is living in. Higher water temperatures cause the clam to precipitate aragonite with a higher oxygen isotope ratio. This ratio is also affected by the oxygen isotope ratio of the water. Freshwater has a much lower oxygen isotope ratio than saltwater, so a change in salinity due to precipitation or evaporation changes the oxygen isotope ratio of the water. An equation that relates the oxygen isotope ratio of the aragonite to sea surface temperature and the oxygen isotope ratio of the water was developed by Grossman and Ku (Chemical Geology 59: 59-74, 1986). This is a slightly modified version of that equation.
With modern temperature and salinity observations from Cedar Key, this equation can be used to produce a predicted oxygen isotope ratio curve for moderncampechiensis shells. A modern campechiensis shell is currently being sampled for comparison with the predicted oxygen isotope ratios.