Fig. 4. Secondary electron image (top) of Okeechobee sediment silt collected at station O7 (20–30 cm) (Scale bar = 20 µm). Below this image are scanning electron microscope-energy-dispersive X-ray spectroscope (SEM-EDS) elemental dot maps showing zones of Mg, Si, P, S, Ca, and Fe concentrations based on K characteristic X-ray emissions. Dolomite is indicated by some correspondence between Mg and Ca. Silica bioliths (e.g., sponge spicule fragments, etc.) are evident in the sample, along with a Ca phosphate particle and an iron sulfide crystal. Similar Ca-P associations were confirmed for all six silt-fraction samples that were analyzed by SEM-EDS (Fig. 5).