A 12,000-YEAR RECORD OF SUBTROPICAL CLIMATE CHANGE FROM LITTLE SALT SPRING, FLORIDA

An 11-meter sediment core recovered from Little Salt Spring (LSS), in Sarasota County, in southwestern Florida, provided an interpretable record of Holocene subtropical climate variability. Changes in moisture balance and in the paleohydrology of the spring are recorded in the oxygen (ð¹⁸O) and carbon (ð¹³C) isotope values of ostracod calcite, especially the balance between precipitation and evaporation. The record can be generalized into wet conditions in the early and late Holocene, and dry in the middle Holocene, and also during a short time within the overall wet late Holocene. LSS, a 65m deep cover-collapse sinkhole spring and underwater archaeological preserve, may have served as an oasis in Florida during early prehistoric time (Paleoindian through middle Archaic periods). The sediment core used in this study was confirmed to be minimally disturbed and selected wood fragments from three depth-intervals were dated using ¹⁴C-dating techniques, obtaining an age of 12,210 ±190 years at the base of the core. Ostracod abundance in the LSS core was generally high (>300 valves/g of sediment). Due to its cosmopolitan presence in the core, C. ilosvayi, was selected for isotopic analysis.

A rapid 5‰ increase in ð¹³C during the Pleistocene/Holocene boundary represented an abrupt change in water quality as water level in LSS rose because of wet conditions and rapid sea level rise in the early-Holocene. LSS evolved from a closed, shallow freshwater pond to a more open and deeper one with different sources of DIC. The original ostracod assemblage was distinguished by the highest abundances of C. vidua, C. annae, and C. bicilis. During the middle Holocene, the ostracods P. globula and C. ilosvayi became the dominant taxa and the relative abundances of C. vidua and C. annae were reduced to <20%. Dry conditions in the core record were characterized by lighter ð¹⁸O values, contrary to what would be normally expected in drier or more evaporative conditions. However, this apparent anomaly can be explained in terms of groundwater influences.

Seawater intrusion in LSS was evident during the early and late Holocene by the appearance of C. salebrosa and C. floridana in the sediment core during those times. The earlier episode of increased salinity had a short duration (~300 years) and it was followed by a refreshening of LSS with slowing sea level rise as indicated by the ostracod fauna and isotopic values. The later one, in the late Holocene, was of longer duration and magnitude, and was apparently followed by a sudden change to dry climatic conditions as indicated by the stable isotope values and ostracod assemblage composition. At this time, the relative abundance of the more euryhaline species reached close to 80% before dropping to less than 20% at about 800 years BP. The change in ostracod assemblage composition indicated a sharp increase in salinity possibly associated with strong evaporative conditions. The timing and duration of this event is coincident with similar changes in climate records published from other areas of the Caribbean and the Gulf of Mexico. Archaeological evidence from LSS indicates that human occupation at the site ended at about 5300 years BP. At this time, climate had entered into a wet-mode and freshwater was more readily available everywhere in the Florida peninsula, possibly making LSS less valuable to humans.