Fe–Mn-(hydr)oxide-carbonate crusts from the Kebrit Deep, Red Sea: Precipitation at the seawater/brine redoxcline

Lithified crusts located within the sediment and recovered from the Kebrit Deep (Red Sea) are composed of Fe- and Mn-(hydr)oxides, carbonates, and sulfides. Mineralogical and geochemical features of these crusts suggest that they are metalliferous layers lithified by carbonates and formed in the narrow band where the transition zone between the anoxic brine and deep seawater crossed the seafloor. Carbonates cementing the Fe–Mn-(hydr)oxides precipitated from normal saline seawater at T = 20–30 °C. Precipitation of Fe–Mn-(hydr)oxides was superimposed the carbonate precipitation in the transition zone. A pumping mechanism involving cyclic Fe- and Mn-(hydr)oxide precipitation/redissolution operated through the transition zone and produced an enrichment of a number of elements in the crusts. The microbiota played an important role in the geochemical cycle of the elements through the redoxcline and in the mineralogy of the sediments underlying the redoxcline waters. Diagenetic processes led to formation of bi- and ternary carbonates, and disulfides.