The fertilization of the Bahamas by Saharan dust: A trigger for carbonate precipitation?

The enigma of the Bahamas is that this highly productive carbonate system has existed for at least 100 m.y., building a vast edifice of carbonates, thousands of meters thick, in an essentially nutrientpoor environment. Based on measurements of the insoluble material, the Fe and Mn in the carbonate fraction, and the d15N of the sedimentary organic matter, we suggest a paradigm shift in order to explain the formation of the Bahamas and possibly other similar platforms. We propose that the Great Bahama Bank is currently, and may in the past have been, fertilized by atmospheric dust, promoting the fixation of atmospheric N2 by cyanobacteria. These cyanobacteria provided a source of nitrogen to the rest of the community in this nutrient-poor environment. The fixation of N has imparted a characteristic d15N signal and has been responsible, through the drawdown of CO2, for initiating the precipitation of carbonate in the shallow waters. This phenomenon might be responsible for the formation of vast amounts of sediments in the oceans, not only within recent times, but throughout geological history, particularly in the early history of the Earth prior to the existence of calcium carbonate–secreting organisms. INTRODUCTION The Great Bahama Bank (GBB) is a large (>100,000 km2) shallowwater carbonate complex that contains numerous semiemergent islands and is situated to the east of the Florida Straits. These islands are composed primarily of calcium carbonate and have no significant siliciclastic component. Great Bahama Bank has built up over at least the past 100 m.y. (Eberli and Ginsburg, 1987; Schlager et al., 1988) in a nutrient-poor environment. Because high concentrations of nutrients promote macro-algal and micro-algal growth, reducing water transparency and limiting the growth of carbonate producers such as corals and calcareous algae (Hallock and Schlager, 1986), it has been suggested that carbonate platforms flourish in oligotrophic environments. In the Bahamas, the activities of various carbonate-secreting organisms and the inorganic precipitation of calcium carbonate have produced large amounts of sediment, which has been deposited on the platform surface and transported off the platform, allowing the GBB to build laterally. The modern sediments on the GBB are mainly nonskeletal, composed of ooids, peloids, and carbonate muds (Purdy, 1963b; Traverse and Ginsburg, 1966). The muds are ingested by worms, generating pellets that harden and eventually form the central portions of peloids and ooids. All of the surface sediments on the GBB have essentially similar d13C and d18O values, suggesting that at least the nonskeletal materials are genetically related (Swart et al., 2009) and have a common precursor, the mud. The source of Bahamian muds has long been a mystery. One suggested origin is the direct precipitation from surface waters producing intensely white areas of water known as whitings (Black, 1933; Cloud, 1962; Shinn et al., 1989). Conversely, it has been postulated that the whitings might result from sediment reworking (Broecker and Takahashi, 1966; Morse et al., 2003, 1984). The occurrence of whitings is of much more than academic interest, particularly if they result from direct precipitation from seawater. For billions of years, during the early history of Earth, such precipitation may have been the only method of carbonate formation, and therefore the processes occurring on the GBB may provide a valuable insight into carbonate precipitation during early Earth history. A large amount of research has supported both sides of the whitings argument. Proponents of direct precipitation point to the absence of mechanisms whereby the sediments could be stirred up (Shinn et al., 1989), the different structures of the aragonite crystals obtained from the whitings compared to bottom sediments (Loreau, 1982; Reid and MacIntyre, 2000), and the absence of sufficient quantities of possible calcium carbonate–producing organisms on the platform surface (Shinn et al., 1989). Alternatively, the supporters of a reworked origin point to the fact that radiometric dating of the whitings indicates an old, rather than modern, age (Broecker et al., 2000; Morse et al., 1984), and the absence of changes in the carbonate alkalinity between the whiting and the surrounding waters (Morse et al., 2003, 1984) that would be expected if direct precipitation occurred; these workers favor an explanation that the mud is ultimately derived from the breakdown of the skeletons of calcareous green algae (Neumann and Land, 1975). An adaptation of the direct precipitation hypothesis is that the whitings are produced as a result of the photosynthetic activity of cyano bacteria (Robbins, 1992). These organisms remove CO2, raising the saturation state of CaCO3 and inducing the precipitation. Although such a mechanism has been shown to be responsible for precipitation in many locations (Davis et al., 1995; Hodell et al., 1998), carbonate platforms such as the GBB are generally Fe poor despite the fact that this element has proved to be critical for the growth of nitrogen-fixing organisms such as cyanobacteria (Brand, 1991; Chappell et al., 2012), the proposed agent inducing the whitings. One possible source of Fe that might promote such blooms on the GBB is atmospheric dust originating in the Sahara and Sahel regions of Africa. Airborne materials from this and other regions, including North America, are the only likely source of noncarbonate material in the region (Muhs et al., 2007; Prospero et al., 1970). In order to investigate the hypothesis that atmospheric dust might be an important source of Fe in the GBB, we measured the percentage of insoluble residue and the concentration of trace elements (Fe and Mn) characteristic of atmospheric dust (Trapp et al., 2010) in the carbonate-soluble portion of ~250 samples of surface sediments from the GBB. In addition, we measured the d15N values of organic material in these sediments. The results of these analyses were compared with the distribution of whitings as documented by satellite observations (Robbins et al., 1997).