SEDIMENTARY GEOCHEMISTRY OF MANGANESE - IMPLICATIONS FOR THE ENVIRONMENT OF FORMATION OF MANGANIFEROUS BLACK SHALES

The sedimentary geochemistry of manganese is dominated by the redox co ntrol of its speciation, higher oxidation states (Mn3+ and (4+)) occur ring as insoluble oxyhydroxides in well-oxygenated environments and th e lower oxidation state (Mn2+) being much more soluble in oxygen-defic ient settings. Its geochemical behavior is therefore quite different i n oxic and anoxic environments, and where oxic and anoxic conditions a re juxtaposed, Mn is recycled between the two environments. In modern marine sediments, Mn is present above its crustal abundance as an oxyh ydroxide in all slowly accumulating (pelagic) sediments of the deep oc ean and in surficial deposits of continental margin environments. Diag enetic recycling of Mn in the latter causes surficial deposits to have larger Mn enrichments than in many pelagic sediments. Bottom sediment s of permanently anoxic basins show no enrichments and have Mn concent rations that are controlled solely by the aluminosilicate fraction. Ma nganese carbonates (kutnohorite and calcic rhodochrosite) are found on ly in anoxic sediments accumulating beneath surface oxic horizons (and therefore under oxygenated bottom waters) in many nearshore environme nts. Such enrichments are due to delivery of Mn by burial of surface o xyhydroxides into the subsurface anoxic environment where they are dis solved. Pore-water Mn levels can reach saturation with respect to a mi xed Mn-Ca carbonate phase in such sediments. The diagenetic origin for these phases is shown by their carbon isotope compositions, which typ ically indicate a carbon source from decomposing organic matter. The p resence of Mn carbonates therefore signifies that the host sediment mu st have accumulated under oxygenated bottom waters. On the basis of th is information it is proposed that, in contrast to several current exp lanations for the formation of Mn carbonates (kutnohorite and rhodochr osite) in ancient organic-rich shales, limestone, and marl sequences a nd in many Mn ore deposits, the occurrence of these mineral phases ind icates that the sediments originally accumulated beneath oxygenated bo ttom waters. By analogy with the present, Mn carbonates could not have formed in the bottom waters of anoxic basins. These diagenetic phases , however, did form where Mn was supplied at a high rate, namely, by t he burial of oxyhydroxide-enriched surface sediments, to a subsurface anoxic environment. This situation could only have occurred under oxyg enated bottom waters. The presence of Mn carbonates in ancient black s hales (and in some carbonate-rich rocks) lends strong support to the n otion that these rocks did not necessarily form in anoxic basins but o we their carbon richness to a high supply of organic matter to sedimen ts deposited under oxygenated bottom waters, probably in continental m argin settings.