Hydrodynamic Control of Whitings and Mud Production on Great Bahama Bank

Abstract

As a result of the potential contribution of whitings to the sedimentary record in the Bahamas and by analogy to ancient carbonate platforms, considerable effort has been applied to understand the triggers and mechanisms of carbonate precipitation atop Great Bahama Bank (GBB). By developing a hydrodynamic model for GBB and mapping whitings occurrences from satellite data, we demonstrate a plausible link between off-platform circulation and on-platform currents, which in turn serves to define the small portion of the GBB where whitings occur. We propose the periodic impingement of the Florida Current atop the GBB to be a precondition for whitings, because these events serve to nourish the platform-top with normal marine water, which is substantially more supersaturated with respect to CaCO₃ than waters stagnated atop the platform.

Alongside conducive water chemistry, a rather specific configuration of platform-top hydrodynamics is needed to form whitings. The currents driving whiting formation occur in the lee of Andros Island, a portion of the platform traditionally believed to be sheltered from currents caused by winds. The zone of most intense whitings corresponds to the limit of the most pronounced incursion of the Florida Current atop GBB, as identified in the hydrodynamic model. More than 35% of the ~3,000 whitings observed in 2012 originated in this zone, which has an area of only 1,000 km² (<1% of the platform area). Whitings are more prevalent and more widespread in winter than summer, patterns that might be linked to seasonality in platform-top current direction. These findings do not contradict the possible fertilizing effect of aerosol dust from Africa on whitings, but highlight a seasonal incongruence between peak whitings (winter) and peak dust (summer).

It appears, therefore, that the high concentration of mud in the portion of GBB in the lee of Andros Island, is not a result of the lower currents as suggested in previous studies, but is the result of the higher frequency of fine-grained sediment production by whitings. This observation has particular relevance to the production of carbonate muds in early Earth history - prior to the evolution of the myriad of carbonate-secreting organisms, abiotic precipitation of aragonite might have been the only means of producing and accumulating carbonates. The consequence being that whitings might not have been as widespread across platforms in the geological record as previously assumed.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018