In Holocene sediments along the Atlantic coast of eastern United States, the regional carbonate/noncarbonate boundary is located south of Miami, Florida. Study of the post-Cretaceous strata underlying the coastal plain and continental shelf indicates that this boundary has not remained fixed, but has shifted, presumably in response to changes of climate. In Upper Cretaceous strata the carbonate/noncarbonate boundary lies in central Florida. In Oligocene strata this boundary lies north of Cape Hatteras. In Miocene and Pliocene strata the carbonate/noncarbonate boundary is located at various localities in Georgia and northern Florida. Its movement to successively southward positions coincided with the times of cooling climate indicated by paleobotanical, paleobiological, and geochemical evidence.
The carbonate/noncarbonate boundary appears to have shifted not only in response to the long-term climatic trends during the Tertiary but also in response to shorter term climatic fluctuations during the Quaternary. Assuming that climate has been the only factor influencing the location of this boundary during the Cenozoic Era, then position of the boundary enables one to compare Quaternary and pre-Quaternary climates.
Using our analysis of the effects of climate on the carbonate/noncarbonate boundary, and accepting the school of thought that states that warmest Quaternary climates coincided with highest Quaternary sea levels, we would predict that the northernmost position of the boundary should be recorded in the highest emerged marginal marine deposits of Quaternary age on the Atlantic Coastal Plain. Field evidence does not substantiate this prediction. In emerged Quaternary “terraces” the northernmost carbonate/noncarbonate boundary is located at about 27° N. lat, near its position during Late Cretaceous time. This coincidence suggests that during times of maximum Quaternary submergence the climate was only slightly warmer than it is today.
A great northward shift of the carbonate/noncarbonate boundary did occur at least once during the Quaternary. Instead of being on the highest “terrace,” however, the resulting tropical strandline carbonate sediments are located near the outer edge of the continental shelf. The northern limit of these carbonates is Cape Hatteras, near the position of the carbonate/noncarbonate boundary during the time of widespread subtropical climate in the Oligocene Epoch. If the northernmost position of the carbonate/noncarbonate boundary is a reliable indicator of the time of maximum Quaternary warmth, then we must conclude that at least one time of great Quaternary warmth coincided not with a high sea level, but with a time of low sea level. The sea has reached its present level about 13,000 years after the last climatic peak inferred from the northernmost position of the carbonate/noncarbonate boundary.