Ocean Chemistry and Hydrodynamics as Controls of Mud Production on Great Bahama Bank
Carbonate saturation state, a chemical parameter impacted by ocean acidification, plays an important role in producing whitings - sedimentological processes that contribute mud-sized aragonite crystals in tropical shallow water environments like Great Bahama Bank (GBB). Triggers for whiting formation have been debated for the past 80 yrs. As a result of the contribution of whitings to the sedimentary record, we examined their occurrence in daily MODIS data to understand their seasonality, spatial distribution, and effects of ocean chemistry. The distributions of whitings were compared with a time series of sea surface temperature (SST) and models of hydrodynamics and saturation state for GBB. We demonstrate a link between off- and on-platform circulation and the SST differential between off- and on-platform water masses. Hydrodynamics define the small portion of GBB where whitings occur; >35% of the ~3,000 whitings observed in 2012 originate in a zone covering <1% of the platform. Data suggest that more whitings are triggered in this zone during winter, when the SST differential between the platform top and adjacent off-platform waters is most pronounced. Calculations of saturation state suggest that the winter SST differential acts to increase the saturation of waters in the region of mixing. In contrast, mixing of waters during summer, when a lower SST differential exists, acts to reduce saturation state, possibly providing insight into the seasonality of whitings frequency. This observation has profound implications for the rate and locus of GBB lime-mud production. Our work suggests that the high concentration of mud in the lee of Andros Island is not a result of lethargic currents, as suggested by previous studies, but is the result of the higher frequency of fine-grained sediment production by whitings, whose occurrence can be related to a delicate balance of prevailing hydrodynamics, temperature, and water chemistry. If SST differentials and hydrodynamics exert control on whitings, ocean acidification is expected to suppress their frequency. 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 might have been the only means of producing carbonates. The consequence being that whitings might not have been as widespread in the geological record as previously assumed.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019