Genesis of a Petrocalcic Horizon in a Kitchen Midden Tree Island Soil in the Everglades.

The genesis of a petrocalcic horizon in a warm, humid, calcium-rich environment with a fluctuating groundwater table is rare and understudied. We studied a petrocalcic horizon in a tree-island kitchen midden soil in Everglades National Park. The soil is 202-cm deep to limestone with a January water table at 110 cm, about 1 m above the offshore water surface. The 15- to 35-cm thick root-limiting laminar petrocalcic surface was found at 47 cm on the summit position where little erosion occurs. The soil was washed and sieved in distilled water before physical and chemical analyses. The soil water and the water surrounding the island were analyzed for calcium. Calcium in the soil was unusually-high at 5209-5981 ppm, compared to soil water with 114 ppm and offshore water with 73 ppm. Due to the archeological origin of this island, there was a high volume of bones and shells brought onto the island and burned. Initial petrocalcic horizon formation likely resulted from the sequence of: release of calcium from bone and shell, capillary-rise of groundwater to the highest portion of the island, evapotranspiration-driven enrichment of soluble calcium, addition of CO2 and carbonic acid from above to form secondary CaCO3, and plugging of pores. The ratio of CaCO3-cemented sand to in-situ large fragments is higher above the petrocalcic horizon than just below. Sand-sized bones and shells are more susceptible to weathering above the petrocalcic horizon because they are more exposed to weathering and redistribution as secondary CaCO3 after exposure to rainfall and acids. In the often-saturated horizons below the petrocalcic horizon, calcium-rich particles are not exposed to intense weathering, but act as a substrate for dissolved calcium to latch on to by homogeneous nucleation. The petrocalcic horizon appears to persist from a combination of calcium nucleation from below and CaCO3 precipitation from above.