Bill Watts and colleagues have studied the vegetation and climate history of Florida’s Lake Wales Ridge by pollen and plant macrofossil analysis of lake sediments. They try to find long time-sequences in order to compare the vegetation record of the Holocene, roughly the last 10,000 years, with the vegetation of the Last Glacial Period which preceded it. Sediments dating from the present to about 60,000 years ago occur at Lake Tulane (see photo, this page), Avon Park, Florida (28 mi. N of Archbold; see map, this page). This very detailed record, well-dated by many radiocarbon dates, is the most comprehensive known from the eastern United States. Archbold’s Lake Annie (see photo, this page) also contains an old but less complete sedimentary record. Old sediments are difficult to find because lake water-levels fell during the Glacial Period, due to lowered sea-levels (to which upland aquifers are linked) and perhaps to reduced precipitation. For example, Buck Lake (3 mi. N of Lk. Annie) now holds 19 m of water and 6 m of sediment which began to be deposited 8,500 years ago as a peat with shallow-water aquatic plants. This evidence, paralleled in other sites, suggests that the regional water table was 18-20 m below present before 8,500 years ago. Most Florida lakes are relatively shallow and their basins were dry until the early Holocene. We find that lakes of at least 40 ha with water depths of 18 m or more are the most likely to yield long sedimentary records. Few such lakes are known but probably there are more to be found.

Pollen diagrams from Ridge lakes are pine-dominated for the last 5,000 years, but, in the earlier Holocene, oak and herbs dominate, suggesting a drier climate than now. In the preceding 50,000 years at Lake Tulane, pine and oak/herbs alternate in dominance. Thus pine dominated 36,000-44,000 years ago, oak/herbs 46,000-58,000 years ago. Shorter periods of dominance by either also occur. We assume that pine-dominance means a wetter climate similar to the present. Oak/herb assemblages mean drier climate and lower water tables. This is supported at Lake Tulane by sedimentary evidence. Pine dominance is associated with organic sediments with few macrofossils, oak/herb with sediment with a mineral component and numerous seeds and fruits of shallow-water and shore plants. Notably, there is no significant presence of tropical plants or northern species at any level of sediment. Thus, we are seeing, in the pollen layers, alternations in importance of different components of the present Ridge flora.

Eric Grimm and George Jacobson discovered that the striking alternations in Last Glacial vegetation at Lake Tulane correlate with changes recorded in ocean cores from the North Atlantic. The sudden discharges of icebergs into the North Atlantic (Heinrich Events) are contemporary with changes in the Ridge’s vegetation and may also correlate with melt water discharges from the Great Lakes Region to the Gulf of Mexico. The presentation of a more detailed analysis of Lake Tulane events based on a much larger data base is our team’s present major concern. The fact that our Florida data can be linked to continental-scale events and contribute to the understanding of world climate change, moves our work from a local regional study to center stage in climate studies.