MESOPROTEROZOIC OCEANS AND MINERAL DEPOSITS

 

The hiatus in banded iron formations between ca. 1.8 and 0.8 Ga is well known.  Two decades ago their absence was ascribed to the oxygenation of much of the deep oceans, the oxidation of hydrothermal Fe²⁺ to Fe³⁺ and its precipitation as a constituent of ferric oxides and oxyhydroxides on the ocean floor.  D.E. Canfield has since proposed that the deep oceans were euxinic rather than oxygenated during the Mesoproterozoic, and that the absence of BIFs is due to the precipitation of hydrothermal Fe as a constituent of FeS₂ rather than as a constituent of oxidized Fe phases.  This proposal has received support from studies of the isotopic composition of Mo in euxinic Mesoproterozoic shales.  The evidence does not, however, rule out a largely oxygenated deep ocean, and the absence of sedimentary Mn deposits during the Mesoproterozoic suggests that the oceans were indeed oxygenated rather than anoxic or euxinic. 

 

The absence of significant Mesoproterozoic phosphorites may explain why the deep oceans were oxygenated despite the relatively low O₂ content of the Mesoproterozoic atmosphere and surface ocean waters.  So little organic matter may have settled into the deep oceans during this period that the O₂ content of deep ocean water was not depleted significantly during its passage along the Mesoproterozoic analogue of the modern “conveyor belt.”   Hence the deep oceans were oxygenated, and the phosphate concentration of upwelling ocean water was too low to permit the formation of phosphorites.  This explanation is somewhat speculative, but it accounts more satisfactorily than the “Canfield ocean” for the absence of BIFs, sedimentary Mn deposits, and phosphorites during the Mesoproterozoic.