Hydrodynamic control of fluvial oncoid formation in a modern high-energy tropical cascade system

Oncoids are microbially-coated carbonate grains that form in diverse aquatic environments, yet their modern fluvial examples remain comparatively understudied. This study documents modern fluvial oncoids from Dunn's River Falls, Jamaica, a dynamic high-energy tropical cascading waterfalls and stepped pools with active tufa deposition over Miocene and Pleistocene limestones. Oncoids from seven plunge pools were analyzed for size, shape, and internal structure using granulometry and petrography, and selected cortices were microdrilled for X-ray diffraction, stable-isotope analysis, and radiocarbon dating. Overwhelmingly gravel-size oncoids are spherical to ellipsoidal, with their internal fabric strongly influenced by nucleus size and geometry. Oncoids display concentric lamination of alternating light and dark sets, with individual laminae often poorly developed, discontinuous, and variably thick. Oncoids are commonly truncated, with local rehealing, indicating repeated abrasion and frequent overturning under turbulent flow. Mineralogy is dominated by low-Mg calcite. Stable isotope values are narrowly clustered (mean δ¹³C = - 9.97‰; mean δ¹⁸O = - 5.09‰), consistent with precipitation from well-mixed freshwater, short residence times, and strong soil-zone CO₂ contribution. Conventional radiocarbon ages (3095-3225 BP) yield older apparent ages in outer lamina sets, suggesting incorporation of ¹⁴C-depleted dissolved inorganic carbon and a freshwater reservoir effect. The findings demonstrate that spherical morphology in fluvial carbonates does not imply low-energy conditions, and that their radiocarbon ages cannot be used as straightforward chronological archives.