ENIGMATIC SILICA SPHERES AND THEIR PALEOENVIRONMENTAL IMPLICATIONS: A CASE STUDY FROM FLORIDA’S PLIOCENE SHELL BEDS

Paleoenvironmental proxies, ranging in category from isotope data, sequence stratigraphy, to micro/macro-fossil analyses, are vital to the assessment of past habitats. Fossil assemblages are most often utilized to assess past environments Florida’s Pliocene and Pleistocene shell beds, in particular the Tamiami Formation (Pinecrest beds). However, the sheer volume of fossil data in the shell beds and their complex taphonomic histories frequently overwhelm standard paleoenvironmental techniques, providing differing habitat and water-depth estimates. The majority of fossils collected in these units are macro-faunal (> 2 cm) bivalves and gastropods, generally recognized as warmer water marine taxa. Conversely, benthic foraminiferal studies and the remains of aquatic and terrestrial vertebrates in the shell beds seem to suggest a much shallower, and possibly more freshwater influenced, depositional environment. This mismatch of material and paleoenvironments has led to a search for additional environmental proxies. Here, using micro-geochemical analysis, we describe small sodium silicate spheres as paleoenvironmental indicators of periodic hypersaline conditions, and, therefore, very nearshore, shallow conditions from the Pinecrest beds of Florida. Precipitation of sodium silicate occurs under a very specific set of conditions and the incorporation of sodium ions (and any other ions) into the silica lattice is dependent on the temperature of the solution when supersaturation takes place; this allows for a paleo-temperature estimate (and thereby a potential water-depth estimate) of the waters when the spheres formed. As the sodium silica spheres are highly stable after formation, this novel method of paleoenvironmental investigation may also allow for the identification of hypersaline conditions in other regions where a record of such environments does not exist even though they may have at one time been present.