Abstract
A variety of sedimentological criteria and direct field observations indicate that deposits of shallow carbonate platforms and mud banks are extensively transformed during megafaunal bioturbation by deep-burrowing crustaceans. 210 Pb dating of surficial sediment and burrow fills dissected from the upper nearly equal 1-3 m of sediments at four sites on the Caicos Platform and in South Florida corroborates sedimentologic descriptions of rapid biogenic alteration of entire facies. Deep burrow networks are either progressively filled when vacated or catastrophically filled during storms. 210 Pb distributions from the study sites show that at least some infill is predominantly surficial sediment (0-30 cm). Assuming that all identifiable deep burrow fills containing excess 210 Pb derive from the uppermost 0-5 cm interval, an estimate of facies replacement by "nonlocal" transport can be made based on measured excess 210 Pb values of fill and the corresponding total discernible fill volume in cores. Although there is a large uncertainty in some cases, calculations indicate that at the sites studied, burrow excavation and infilling can completely transform the upper nearly equal 1-2 m, and possibly 3.5 m, of deposits in nearly equal 100-600 yr, depending on the exact sediment interval. More rapid transformation of deposits is required if fill is derived from below nearly equal 5 cm. Biogenic transformation rates are sufficiently fast compared to net sedimentation that burrow infills, not primary physical deposition, determine the composition, porosity, fabric, and texture of the preserved facies. The 210 Pb profiles in the deepest regions of deposits in the present cases are further complicated by basal enrichments of 226 Ra, which apparently diffuses upwards from Pleistocene calcrete surfaces into overlying Holocene sediment. This diffusion requires careful documentation of supported 210 Pb near this contact, but also offers the potential for an additional transport tracer internal to the deposits.
