PEAT RECORDS OF LATE HOLOCENE CLIMATE AND SEA-LEVEL CHANGE IN SOUTHERN FLORIDA

Pollen, charcoal, and other microfossils from two coastal peat cores are used to reconstruct late Holocene climate and sea-level changes in southern Florida. One core comes from the mangrove fringe of Whitewater Bay and the other from Lignumvitae Key. In addition to the proxy indices, modern analog statistical techniques are used to compare the fossil pollen assemblages to over 189 modern surface samples from peninsular Florida, the southeast US and the Caribbean. Close analogs in the paleorecord are assumed to be similar to the modern vegetation types and by association to the environmental conditions, such as effective precipitation, hydroperiod, and salinity, that control their distribution. Results indicate that plant communities at both coastal locations have responded to climate and sea-level fluctuations over the last 3500 years. The Whitewater Bay site, which now supports a saline dwarf mangrove forest, was a freshwater Cladium marsh with relatively long hydroperiods circa 3400 yr B.P. The core from Lignumvitae Key, with a basal age of 2290 yr B.P., also shows a transition from freshwater or slightly brackish conditions at the base to saline mangrove peat at the top. Sedimentological evidence indicates that sea level rose gradually throughout the late Holocene with a period of accelerated rise, or a level higher than modern, circa 1000 yr B.P. Reduced effective precipitation also occurred about 1000 yr B.P., contemporaneous with the onset of the Medieval Warm period. At 400 yr B.P. changes in plant community reflect another episode of reduced precipitation and possible evidence of freezing temperatures at the Whitewater Bay site. Overall, the late Holocene in southern Florida is characterized by gradually decreasing moisture availability punctuated with century-scale fluctuations. Likewise the eustatic history is that of a gradually rising sea level superimposed by minor oscillations.