HOLOCENE CHANGES IN CAROLINA BAYS

The landscape of the southern Atlantic Coastal Plain has changed profoundly during the Holocene. The coast line shifted with eustatic rise in sea level. In the interior, changes in climate altered fluvial discharge patterns, shifted the dominant upland vegetation from oak to pine, and established numerous wetlands, including isolated wetlands in Carolina bays and other shallow basins. Palaeoenvironmental records from Carolina bays provide evidence for changes in both the landscape and the wetlands. We review evidence for the timing of these transitions and consider implications for development of modern aquatic communities and response to future change.

Carolina bays are shallow, oval, oriented basins; many have distinctive sand rims. Most of the basins contain wetlands. Their organic sediments typically yield Holocene or late Pleistocene basal dates, although OSL dates from bay rims indicate much greater age. We infer that these basal dates reflect significant changes in environmental conditions. Modern records from the Savannah River Site (SRS) in South Carolina indicate that preservation of organic material requires inundation >50% of the year.

Radiocarbon dates from two wetlands on the SRS suggest that the modern hydrologic regimes were established recently, but that the transitions to moist, low energy conditions began in the mid-Holocene. At Flamingo Bay, the deepest part of the basin has been paludified since ~4,500 radiocarbon yr B.P., with significant expansions at ~3,100 and ~300-230 radiocarbon yr B.P. Modern precipitation averages 120 cm annually. A hydrologic model, based on a 20-yr hydrograph for the bay, suggests a prolonged episode of ~100 cm annual precipitation prior to the latest transition. Prior to the mid-Holocene date, precipitation may have been <80 cm annually.

Plant and animal communities, as well as human use, respond strongly to hydrologic conditions in the wetlands. For example, we estimate that marbled salamanders can currently breed in ~80 of 371 Carolina bays and other isolated wetlands on the SRS. A return to the drier conditions that may have prevailed prior to the mid-Holocene would reduce significantly this number; moister conditions predicted by a global warming scenario would greatly increase it.