The late Holocene has been punctuated with several century-scale variations in climate. These include the Neoglacial, the Medieval Warm Period, and the Little Ice Age. Temperatures varied by as much as 4 degrees Celsius in the Northern Hemisphere from one period to the next. The Seahorse Key Midden was built up from 300 to 800 A.D. This was a transition period from the Neoglacial to the Medieval Warm Period when the Earth warmed abruptly.

 

A debate exists as to the extent of the Medieval Warm Period. How warm was the Medieval Warm Period and how long did it last? Was it a global phenomenon or limited to certain regions? (See Climatic Change, Vol. 26, Nos. 2-3) Evidence from climatic reconstructions from tree-rings, glacial movements, and historical records has shown that a Medieval Warm Period did exist in some regions (for example, northern Europe, Greenland, the Sierra Nevadas, southern South America, and China). The above figure shows warmer than present conditions for Greenland and the Bermuda Rise. West African SST's are similar to present conditions. Other regions show little or no warming trend for that period. Spring rainfall reconstructed from bald cypress tree rings taken from Georgia, South Carolina, and North Carolina show no century-scale variations during the past millienium (Stahle and Cleaveland, Climatic Change 26: 199-212, 1994).

We wanted to know if any variation in mean temperature brought about by medieval warming can be seen in bulk analysis records taken from the clam shells. The bulk analysis samples are samples taken from across all the growth bands of a shell. This gives a lifetime average value from the shell that can range from 5 to 20 years. Any century-scale changes in mean temperature should be recorded in this analysis. The modern mean SST at Cedar Key is around 22 degrees Celsius.

We also wanted to know if there were any changes in variability associated with the Medieval Warm Period in Florida. Stahle and Cleaveland report significant variability in the mean spring precipitation of 20-year subperiods in their Southeast United States tree ring reconstruction.