MID-HOLOCENE AGE FOR NATURAL LAKE OKEECHOBEE SHORELINE?

Recent AMS radiocarbon ages from shallow cores taken east of Lake Okeechobee provided mid-Holocene ages on freshwater shell fragments. Low ridges immediately north and east of the Hoover Dike, which encircles the modern lake, have been cored. They are thought to be natural shorelines of the lake, formed during high stages, with sand bought from the Kissimmee River via littoral drift. Stratigraphy shows the ridges to be medium- to fine-grained quartz sand interbedded with shells and organics, superimposed upon peat and muck layers typical of the Everglades. Dated shells were from 140 cm and 32 cm below the surface - remarkably shallow for their apparent age. Although the shallower sample was somewhat older (by 400 years) the samples are from different cores separated by over 2 km in distance. While their fragmented state suggested deposition in a swash zone, the size of these shells did not indicate excessive reworking. It is possible that they are in deposits of mid-Holocene age.

These are believed to be the first AMS dates from the northerly shoreline of Lake Okeechobee; previously published ages (from the 1970s) indicated very late Holocene deposits. Ongoing studies will show their exact importance, as additional coring, surveying, pollen analysis and radiocarbon dating will provide limits for these ages.

In addition, these mid-Holocene ages are the first from the northern end of the lake. Early studies have placed the origin of the Everglades at 4000-6000 BP, largely based on ages from the southern rim of Lake Okeechobee. The confirmed existence of freshwater shells in a more northerly location 5000 years ago argues for an extensive and long-lived lake, and has implications for the associated Kissimmee River-Lake Okeechobee-Everglades hydrological system.