Questioning carbonate diagenetic paradigms: evidence from the Neogene of the Bahamas

Carbonate diagenetic models have been heavily influenced by numerous studies of exposed Quaternary limestones. As a result, meteoric diagenesis is often assumed to be the principle means of altering aragonite-rich sediments into calcitic limestones. However, these models are limited by the scarcity of examples of aragonite-rich sediments buried in seawater that have never been influenced by meteoric fluids. The Bahamas transect cores recovered originally aragonite-rich sediments deposited in deep water beyond the easy reach of meteoric waters and provide an opportunity to test current diagenetic paradigms. The Bahamas transect consists of seven cores drilled in the prograding western margin of Great Bahama Bank. The two proximal cores (Clino and Unda) were drilled on the platform top and recovered shallow-water platform to reef facies overlying deeper margin and proximal slope facies. The five distal cores were drilled by ODP Leg 166 in up to 660 m of water and recovered carbonate slope facies. All studied sections are Neogene to Pleistocene in age. Diagenetic environments were identified based on petrographic and scanning electron microscopy (SEM) observations, XRD mineralogy, carbon and oxygen stable isotopic data, and trace elements. The upper 100–150 m of the two proximal cores were altered in meteoric to mixing-zone diagenetic environments but all other intervals were altered exclusively in marine pore fluids during seafloor, marine-burial, and deep-burial diagenesis. Several of the findings of this study question current carbonate diagenetic paradigms. These include: (1) large-scale sea level lowstands may not have chemically active meteoric lenses as we found no meteoric alteration at the −120 m elevation of the latest Pleistocene lowstand. Rather, phreatic meteoric diagenesis appears restricted to within ≈10 m of the land surface. (2) Mixing-zone diagenesis includes aragonite dissolution and minor LMC cementation but does not show the cavernous porosity or dolomitization predicted by mixing-zone diagenetic models. Current models are based on coastal mixing zones, which do not appear to be applicable to these more inland, and perhaps more typical, locations. (3) Marine-burial diagenesis produces a mature limestone with fabrics formerly considered diagnostic for meteoric diagenesis such as moldic porosity, aragonite neomorphism, blocky calcite spar and calcite microspar. However, oxygen stable isotopic data (average δ 18O=+1‰) indicate alteration in