Physical versus chemical processes of “whiting” formation in the Bahamas

Abstract

Whitings are common yet problematic occurrences of highly turbid water parcels among the relatively clear waters of shallow, tropical and subtropical carbonate platforms, banks and seas (e.g. Bahamas, Belize, Gulf of Carpentaria, Persian Gulf). In this paper, existing data on Bahamian whitings are scrutinized in an attempt to identify processes which may account for whiting phenomena worldwide. An examination of the areal distribution of whitings on Great Bahama Bank shows that>90% are associated with either mud or pellet-mud sediment facies. In view of this non-random distribution, several prominent hypotheses of whiting origin are considered: a) resuspension of fine-grained sediment by bottom-feeding fish (fish-muds); b) largescale, instantaneous physicochemical precipitation events; c) large-scale carbonate precipitation incidental to planktonic algal blooms; d) bottom sediment resuspension as a result of turbulent tidal flow. Although none of the above hypotheses can be discounted completely, this paper proposes that the observed geographic and physical characteristics of whitings are consistent with predicted and observed characteristics of turbulent-flow systems; that whitings are the observable manifestation of the “bursting” cycle of turbulence production at flow boundaries. The bursting process is a primary source of turbulence emanating from flow boundaries and can generate velocity excursions which deviate from mean flow velocity by a factor of 4. Published data for mean tidal current velocities on Great Bahama Bank range from 8 to 50 cm s−1. Thus, maximum predictable velocities during the bursting process will range from 32 to 200 cm s−1, quite in excess of empirically determined threshold sand-transport velocities (<25 cm s−1). Four-fold velocity excursions during turbulent flow increase shear stress up to 4-fold and generate 16-fold increases in lift force at the sediment water interface, enhancing bottom-sediment destabilization, particle entrainment and suspension. The visible, roiling nature of whiting water parcels is believed to be the expression of the above turbulent flow processes on Great Bahama Bank. Organization of turbulent boundary-layer flow into alternating low- and high-velocity “streaks” gives rise to alternating zones of active sediment-resuspension and deposition and may account for the development of subparallel digitations frequently observed within whitings. An integrated, conceptual explanation of whiting distribution on Great Bahama Bank is proposed which considers the interaction of physical environmental energy and sediment textural characteristics across the bank-top. Despite high tidal-current, wind and wave energy flux (and sediment resuspension potential) near bank margins, whitings are rare because sediments are nearly devoid of mud-sized particles. Near Andros Island, carbonate-mud is plentiful but whitings are infrequent because deposition occurs in the tidal- and wind-energy shadow of the island. Only on the center of the bank-top is environmental energy (sediment resuspension potential) sufficiently high and carbonate-mud sufficiently abundant for whitings to originate. Evaluation of this proposed whiting generation concept requires an interdisciplinary understanding of chemical, physical, sedimentological and biological aspects of the Great Bahama Bank environment.