Carbonate Nodule Growth on Florida's Outer Shelf and Its Implications for Fossil Interpretations

The use of carbonate nodules as paleoenvironmental indicators is generally based on examples of modern nodule growth. Although nodules are found in both shallowand deep-water environments, research on their composition, formation, and depositional setting has been limited almost exclusively to shallow depths. The lack of deep-water studies results primarily from the logistical difficulties associated with research below 20-30 m. Consequently, interpretation offossil deposits containing carbonate nodules may be biased by our lack of knowledge regarding nodule formation in relatively deep-water habitats. Recent observations of carbonate nodule growth on Florida's outer shelf reveal the processes by which carbonate nodules may form in a relatively deep (35-65 m), quiet-water setting. Observations in situ and examinations of internal nodule structure indicate that encrusting organisms-primarily the foraminifera, Gypsina vesicularis, and various coralline algae genera-grow on a nodule's upper as well as lower surfaces. Nodule formation may therefore be facilitated by slight repositioning rather than complete overturning. On Florida's outer shelf, repositioning of nodules occurs (1) frequently as a result of intense biologic activity at the sea floor, and (2) infrequently as a consequence of physical transport during hurricane-strength storms or periodic surge events. These findings suggest that in addition to the traditional association with shallow-water habitats and physical turbulence, fossil carbonate nodules may be indicative of a relatively deep, quiet-water setting characterized by persistent bioturbation and occasional near-bottom flow.