THE SCRUB ECOSYSTEM IN SOUTHWEST FLORIDA: VANISHING EVIDENCE OF LATE PLEISTOCENE SEA LEVEL AND CLIMATE

The Florida Scrub Ecosystem is adapted to ancient coastal dune systems where the soil is excessively drained fine-grained quartz sand, devoid of nutrients, and leached of bioclastic carbonate (Kurz, 1942; Myers, 1990). Using vintage aerial photos, U.S.D.A. soil maps, and field study of surviving scrub, we mapped the scrub ecosystem in eastern Collier and southeastern Lee Counties of southwest Florida as it existed mid 20th Century before most of it was lost to development. The mostly aeolian geomorphic features underpinning this scrub ecosystem map range from sand ridges only 0.6 meters high to dunes 13 meters high. They occur in a band 64 km long, 8 km or less from the Gulf of Mexico, but inshore of Holocene barriers and lagoons. The mapped area was submerged 120,000 years ago when sea level stood at about +8 meters.

From this scrub ecosystem map, we identify three types of mostly aeolian geomorphic features and use them to interpret shorelines, episodes of increased aridity, and patterns of differing wind directions in the Late Pleistocene to early Holocene. Type 1 features are identified by linear coast-parallel scrub patterns and suggest that barrier complexes with associated dunes formed when sea level stood at + 2-4 meters. Type 2 features are low stabilized dunes located near streams. These possibly formed during the episodes of marked aridity in south Florida in the late Pleistocene to early Holocene (Watts and Hansen, 1993). Forests now shroud stream banks where bare sand was once exposed to wind. Type 3 features consist of isolated to clumped, arcuate-shaped dunes, yoking shallow depressions, some with floors below the level of present day low tide. The depressions, apparently the source of the sand in the adjacent dunes, may have been eroded at openings in an otherwise vegetated surface. We have identified ten examples of Type 3 dunes. By far, the largest of these are the dunes on Marco and Horr's Islands that partially ring elliptical Barfield Bay. Widmer (1988) and Schindler and Southworth (2004) compared this bay to Carolina Bays. However, unlike the sand rims of highly oriented Carolina Bays that indicate near uniform wind direction, Type 3 dunes in southwest Florida are variable in orientation indicating episodically divergent wind directions. OSL dates and cores from the depressions are needed to better understand them.