Always a White Christmas in the Bahamas: temperature and hydrodynamics localize winter mud production on Great Bahama Bank

Abstract

ABSTRACTWhitings, or occurrences of fine-grained carbonate in the water column, have been observed in modern environments with salinities ranging from fresh to marine conditions, and thick deposits of lime mud are described throughout the geological record. Despite their ubiquity, the trigger for whitings has been debated for more than eighty years. Satellite data reveal that most whitings are restricted to the northwestern part of Great Bahama Bank (GBB) which occupies < 10% of the platform area. Even here, whitings are further focused. More than 35% of them occur in a zone which occupies just 1% of the platform. We propose a three-step process for the existence of this zone of peak whitings and why the whitings in it are both more frequent and larger in winter than summer. First, the temperature differential between on- and off-platform waters is highest in the winter, setting up a disparity between dissolved CO2 concentrations in the two water masses. Second, hydrodynamic mixing of these two water masses increases the degree of aragonite saturation of the platform-top waters, as colder on-platform waters with theoretically higher concentrations of dissolved gases are warmed via mixing with the warmer off-platform waters. Finally, spatial heterogeneity in the degree of aragonite saturation is higher in the winter, and the zone of peak whitings is situated in an area of locally enhanced saturation state. Hydrodynamic simulation suggests that the whitings zone is located by tidal inflow of off-platform waters across the western margin of GBB, as well as inflow from the Tongue of the Ocean to the north of Andros Island. Despite thermodynamic forcing mechanisms that predict higher frequency of whitings in the summer, the environmental, hydrodynamic, geochemical, and kinetic conditions in the whitings zone appear to support the Goldilocks configuration that enhances the formation of wintertime whitings on Great Bahama Bank. This phenomenon has implications for the interpretation of whitings mud in the geological record, including the geochemical signatures within it.