PHOSPHATIZED CULTURAL DEPOSITS IN A BLACK EARTH MIDDEN MOUND, CENTRAL FL

Platt mound (8BR244) in Brevard County, Florida is located on a remnant fluvial landform in the St Johns River Basin. The mound, approximately 2 m high and 35 m in diameter consists of thick laminae to thin beds of anthropogenic debris and sand. Continuous layers of strata extend across the crest of the mound without evidence of construction by basket dumps.

Three profiles, concretions within pit features and cemented soil horizons were sampled for particle size and soil chemistry. Bulk soil chemistry was analyzed by ICP-MS. Thin sections were prepared and elemental chemistry of cement coatings were analyzed by SEM and EDS.

Black earth layers within the midden have elevated elemental concentrations of P, K, Mg and Ca relative to natural soils. The concentration of K, Mg and Ca correlates with the percent organics measured in different strata. Phosphorous does not appear to change with percent organics and is likely more dependent on concentrations of bone and fish scales. P and Ca, which increases at 95-100 cm depth within the midden coincides with large concretions of bone, fish scales, charcoal, and snail shells within pit features and cultural deposits. Pit features appear to provide a preferential pathway for infiltration of rainwater allowing dissolved P and Ca from surficial strata to cement cultural deposits into large concretions. Elemental chemistry shows that calcium phosphates are the primary cementing agent within the mound. This contrasts with the calcium carbonate cemented horizons documented on tree island middens in Southern Florida.