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6 The History of Environmental Change in South Florida The static view of the physiography and ecosystem described in the previous chapter may not have pertained to the past, since paleoenvironmental and geological conditions were much different in the Late Pleistocene through Middle Holocene than currently found in the coastal area of southwest Florida. The coastal zone of southwest Florida was influenced by geological and ecological processes to a much greater degree than other coastal zones of eastern North America. There are two reasons for this. The first is the very broad, relatively shallow and gently inclined continental shelf, which extends west from the present Florida peninsula. Global warming trends associated with the beginning of the Holocene resulted in the melting of the massive polar ice sheets of the Wisconsin period of the Late Pleistocene. This, in tum, resulted in the submergence of this coastal shelf with a transgressing sea level and a retreating coastline. The other important factor is the proximity of the tropical southwest Florida coast to the subtropical zone to the north, since minor fluctuations in global temperature patterns can dramatically affect the environment and possibly alter the area's ecosystems. Both of these factors act as dynamic geological processes which alter the ecological setting, including the spatial distribution and relative areal extent of the physiographic environments and their associated ecosystem types present on the coast. Thus, the specific history of human utilization of the resources of the southwest Florida coast is conditioned in part by these dynamic geological and paleoenvironmental processes. An understanding of these specific histories is necessary for knowing the types of ecosystems present in the past and their spatial location. The History of Environmental Change in South Florida 139 A number of geological periods will be referred to in this chapter. Many of these are standard in usage, such as Wisconsin and Sangamon. The Holocene, however, will be partitioned into units which do not exactly follow normal usage, although the terminology followed here is similar in many ways to Gagliano 's (1977). Table 12 has been constructed to present my usage explicitly and to provide a chronology of some of the important geological events which have taken place in south Florida. I have also included the scheme used by Gagliano (1977) and the sea-level positions for these periods. Table 13 summarizes the paleoenvironmental history of south Florida. The Effect of Post-Pleistocene Sea Level on Coastal Configuration Sea level was at a number of different positions during the Pleistocene, resulting in a series of beach terraces throughout the world. These varying sea levels were due to the expansion and shrinkage of ice sheets in the polar regions of the earth. The maximum advance of these ice sheets occurred during the Late Wisconsin glacial period, dated at 18,000 years B.P. (Watts 1975). At this time, the Laurentide ice sheet, centered over Hudson Bay, obtained its maximum thickness of 3,500 m. A similar ice sheet, the Fennoscandian, was located in Eurasia. These ice sheets bound up much of the the earth's moisture, resulting in a sea level of up to 120m below its present position (Milliman and Emery 1968;J. A. Clark et al. 1978). Lower sea level due to glacial expansion occurred at earlier times during the Pleistocene (Cronin et al. 1981),but only the Late Wisconsin glacial advance and its associated sea levels will be considered here. Although it is possible that earlier (Le., pre-Wisconsin) human occupation of the New World occurred in the long undifferentiated interstadial from 73,000 to 30,000 B.P. (MacNeish 1976), the sea level at this time was 7 m above its present position (Cronin et al. 1981), and all of south Florida, owing to its low elevation, would have been inundated. The effect of this sea-level position on the peninsula of Florida has been illustrated by mapping the shoreline position of the Sangamon sea (see fig. 17). Regardless of the initial date of human entry into the New World, therefore, south Florida could not have been occupied by human beings prior to 30,000 B.P. This date also marks the beginning of the deteriorating climate, which eventually led to the maximum expansion of glacial ice (Watts 1980:389). The amount of water bound in the Wisconsin ice sheets corresponds inversely with the ocean volume, and so the 18,000 B.P. glacial maximum also corresponds with the maximal exposure of coastal shelf area, and possible areal occupation, of...

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