Mollusks have the potential to yield very detailed records of paleoclimate because they excrete a new layer of aragonitic shell daily. The oxygen in the carbonate is isotopically fractionated by the mollusks in response to changes in the sea surface temperature. Corals also contain paleoclimate records, but this record is limited to the tropics and is easily disrupted by boring organisms. They are also susceptible to contamination due to their high porosity. Ocean basin sediment cores are unreliable for high-resolution records because of disruptions caused by burrowing organisms.
We obtained 48 hard clam (Mercenaria campechiensis) shells from an Indian shell midden in Cedar Key, Florida. The midden was deposited by the Weeden Island culture from 300 to 800 AD. Climate in the Northern Hemisphere was in transition from the late Neoglacial to the Medieval Warm Period (MWP) during this time period. The MWP is of significant interest because it is a period when temperatures were similar to or higher than present conditions. We extracted bulk samples of the shells in order to determine the mean oxygen isotope ratio for Cedar Key during the beginning of this period.
A 3-4 mm section of shell running from the umbo to the ventral margin was cut out with a saw. The sections were flattened with a lapidary wheel and epoxied to a 3" by 6", 1/4" thick frosted glass slide. They were then ground to a thickness of 0.5-1.5 mm using a diamond grinder. Samples were extracted with a micromilling machine with a 50 mm milling piece. The samples were taken from a line running across all the growth bands, excluding the outer prismatic and inner nacreous layers.
The data shows that the mean oxygen isotope ratio of seawater at Cedar Key was nearly identical to present conditions. We can conclude from this that the MWP was at least as warm as the last 150 years, assuming that salinity has remained unchanged. Mg/Ca ratio analysis will allow us to determine the relative effects of temperature and salinity because this ratio is unaffected by changes in salinity. Inter-annual variability was greater in the clam record than was observed over the past 150 years.