The late Quaternary vegetational and climatic history of the unglaciated southeastern Coastal Plain of the USA remains inadequately understood, despite its importance as a current center of biodiversity as well as a presumed glacial refugium for many eastern North American species. Natural basins are sparse north of peninsular Florida, and first-generation pollen studies are limited by bulk-sediment radiocarbon chronologies, low stratigraphic sampling-density of both pollen and radiocarbon dates, and a dearth of charcoal and independent paleoclimate proxies. Correlation of vegetation transitions among paleoecological sites, and with ice-core and ocean-sediment chronologies, is currently difficult, but necessary to assess spatiotemporal patterns of vegetation change and underlying mechanisms.
We obtained new sediment cores from two classic Coastal Plain sites [previously studied as part of William A. Watts’ pioneering efforts]: White Pond (South Carolina) and Sheelar Lake (Florida). We are developing robust sediment chronologies based on AMS-dated terrestrial plant macrofossils (minimum of one date per 2000 years), with high-density pollen sampling (minimum of one sample per 150-200 years) and contiguous macroscopic charcoal sampling. We are also obtaining paleotemperature estimates from organic biomarkers (branched glycerol dialkyl tetraethers (brGDGTs)) from sediment sequences that span the last deglaciation.
Results/Conclusions
Our record from White Pond (SC) indicates that open pine-spruce forest grew under cold and dry conditions from 30,000-20,000 yr BP. The record during this interval shows no vegetational changes correlated with Dansgaard-Oeschger cycles and Heinrich events recorded in the North Atlantic, but elevated oak before 26,000 yr BP suggest relatively warm conditions preceding the last glacial maximum (LGM), possibly corresponding to a pre-LGM interstadial widely recorded in the Great Lakes region. Warming between 20,000 and 10,400 yr BP was accompanied by a transition to mesic hardwood forest. Charcoal accumulation rates are low before 10,400 BP, indicating that fire was not an important factor in maintaining conifer-forests nor in mediating transitions to hardwoods. Pine increased after 10,400 yr BP, accompanied by a 10-fold increase in charcoal, and pine increased again after 6400 yr BP. Low-intensity surface fires increased after 1200 yr BP, possibly related to regional establishment of the Mississippian culture. Preliminary studies at Sheelar Lake (FL) indicate open forests dominated by oak and hickory between 19,000 and 18,000 yr BP, with a gradual transition between 18,000 and 16,000 yr BP towards closed hardwood forest with mesic taxa (Ostrya/Carpinus, ash, elm, beech). Pine-dominated forests appear to have developed after 14,000 yr BP.