Prehistoric Human Effects on Everglades Tree Island Soils and Petrocalcic Horizon Formation

The origin and properties of fixed tree islands in the Florida Everglades have been extensively studied. However, opinions vary as to their age and method of formation. The objective of this study was to characterize the soil on two islands and to look for evidence of prehistoric human influence on island soil formation. These islands were used intensively by prehistoric peoples for hunting, fishing, and temporary residences.

The soils on the head of Crandon's Island and the sideslope of Heartleaf Hammock were studied as examples of 43 investigated islands. Descriptions and lab analysis showed artifact content in middens above and below a petrocalcic horizon all the way to bedrock on the head of Crandon's Island. There were artifacts in every horizon on Heartleaf Hammock above bedrock, but no petrocalcic horizon. With depth, both soils showed irregular decrease of organic matter, changing texture, and variable bone, shell, tools, pottery shards, petrocalcic and rock fragment content. Radiocarbon dates indicate that fixed tree islands began their development sometime around 4500 to 5000 years BP, when water was at a lower level. The later formation of petrocalcic horizons on island heads in humid to aquic environments can be narrowed to a 1000 year timespan.

In almost all fixed tree islands investigated, dense prehistoric middens containing bone P, shell, and artifacts are present and make up a large volume of the soil material, possibly equal to additions of precipitated calcium carbonate, food waste, P and guano from innumerable foraging birds. Occasional high-energy fluvial or eolian events may have added sandy sediments. Droughts may have interrupted human visitation and hastened petrocalcic horizon formation. Modern humans have altered fixed tree island development by altering the flow, depth and volume of water, thereby causing a large decrease in bird populations, possible release of P, and decreased island size.