Bahama Carbonate Precipitation Triggered By Saharan Dust

Interesting paper in Geology : The fertilization of the Bahamas by Saharan dust: A trigger for carbonate precipitation? - P.K. Swart, A.M. Oehlert, G.J. Mackenzie, G.P. Eberli and J.J.G. Reijmer

Abstract

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 nutrient-poor environment. Based on measurements of the insoluble material, the Fe and Mn in the carbonate fraction, and the δ15N 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 δ15N 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.

Many carbonate sedimentologists suspect that microbial activity has played an important role in carbonate sediment precipitation and this paper provides more evidence for it from a familiar setting. The geology of the Bahamas has been extensively studied but sometimes basic questions remain "Why such  prolific sediment production by organisms in a nutrient poor environment?".  The answer in this case may be an exogenous source of nitrogen. Whether this is a more general explanation remains to be answered. There may be other controls, for example in the Proterozoic sea water chemistry was different with elevated levels of calcium carbonate saturation. This means that there were lots of Ca and CO3 molecules available to aggregate as CaCO3 crystals. This enabled -with and without the aid of microbial activity-  vast deposits of carbonates to accumulate.