Composition and Origin of Ferromanganese Crusts from Equatorial Western Pacific Seamounts

In the equatorial western Pacific, iron-manganese oxyhydroxide crusts(Fe-Mn crusts) and nodules form on basaltic seamounts and on the top of drowned carbonate platform guyots that have been swept free of pelagic sediments. To date, the Fe-Mn crusts have been considered to be almost exclusively of abiotic origin. However, it has recently been suggested that these crusts may be a result of biomineralization. Although the Fe-Mn crust textures in the equatorial western Pacific are similar to those constructed by bacteria and algae, and biomarkers also document the existence of bacteria and algae dispersed within the Fe-Mn crusts, the precipitation, accumulation and distribution of elements, such as Fe, Mn, Ni and Co in Fe-Mn crusts are not controlled by microbial activity. Bacteria and algae are only physically incorporated into the crusts when dead plankton settle on the ocean floor and are trapped on the crust surface. Geochemical evidence suggests a hydrogenous origin of Fe-Mn crusts in the equatorial western Pacific, thus verifying a process for Fe-Mn crusts that involves the precipitation of colloidal phases from seawater followed by extensive scavenging of dissolved trace metals into the mineral phase during crust formation.

Upper Cretaceous oceanic red beds in southern Tibet:Lithofacies,environments and colour origin

Application of mineralogy, geochemistry, sedimentary petrology, and sedimentology methods result in better understanding of the genesis and paleoenvironmens of the Upper Cretaceous oceanic red beds exposed in southern Tibet. The red beds comprise the Chungde Formation. Nine lithofacies recognized within this formation are: red foraminiferal packstone/grainstone, red microfos- sils wackestone, red marlstone with microfossils, red marlstone, red to variegated floatstone and rud- stone (debris flow), red shale, red radiolarite, red chert with radiolaria, and red chert. Sedimentary structures and textures, microfossils, and carbonate content show that the Chuangde Fm was depos- ited near the base of a continental slope in a deep oceanic basin environment, with the basin floor below the carbonate compensation depth (CCD). Red marlstones and limestones intercalated within red shales represent slides and slumps from the upper part of the continental margin. Debris flow and turbidity deposits consist of volcaniclastic, fossilliferous rudstone and floatstone, and very thin cal- careous mudstone, intercalated with red shales. The Upper Cretaceous oceanic red beds in southern Tibet are characterized by high Fe2O3, low FeO, which indicates an oxic diagenetic environment, resulting in precipitation of hematite. The latter occurs as finely, disseminated ferric oxide giving the red color to the rocks. It is concluded that the red beds in southern Tibet were deposited under highly oxygenated bottom conditions in the deep ocean basin. Such conditions not only occurred in a deep ocean basin as indicated by the occurrence of pelagic red shale deposited below the CCD, but also extended up the continental margin as indicated by the presence of red colored marlstones and limestones embedded in the Chuangde Fm. The latter were deposited above CCD, most probably on the continental slope. The oxic bottom conditions are interpreted to be a result of a combination of climate cooling, active bottom ocean circulation, and change in the ocean-atmosphere oxygen budget.