Late Holocene interaction of natural processes and human activity at two archaeological sites in the Ten Thousand Islands area of southwest Florida, USA

Palaeoenvironmental analysis of Late Holocene stratigraphic sequences recovered from two southwest Florida archaeological sites reveal a synchronous evolution of events caused by natural processes (e.g. sea-level rise) and human activity (e.g. construction of shell works). Basal mangrove peats indicate that a transition from a terrestrial to coastal landscape started similar to 4000 cal BP in conjunction with the Late Holocene sea-level rise. Continued sea-level rise ultimately submerged the area to create estuarine conditions. Commensurate with submergence was the construction of relatively large (0.5 to 1.0 km(2)) shell mounds from 1500 to 1050 cal BP. These activities started in the estuarine environment and over time produced emergent landscape features with significant local relief (>7 m). The overlap in radiocarbon ages of oysters collected from the estuarine interval and the overlying shell mounds suggests construction material was locally harvested. About 1120 cal BP and during the waning of an indigenous presence in the area, red mangrove forests re-emerged along the margins of each site and within the adjacent estuary to create an expansive mangrove-dominated landscape within which both shell works are now embedded. This palaeoenvironmental reconstruction contradicts the prevailing notion that the two sites were purposely constructed on the mainland and adjacent to tidally influenced rivulets. (C) 2022 John Wiley & Sons, Ltd.