Chapter 2 - Manganese Carbonates in Modern Sediments

Abstract

This chapter describes the isotope composition and details of the formation of manganese carbonates in sediments in the open oceanic (eg, Guatemala depression of Panama Basin and El Gardo uplift of Central American trough; Pacific Ocean), near-shore marine (eg, Gotland Basin of the Baltic Sea; Onega Bay of the White Sea, Russia), and lakes (eg, Punnus-Yarvi and Konchozero of the Karelian Isthmus, Russia). The isotope data confirm the existing theory of a diagenetic origin of manganese carbonates in lake as well as in sea and ocean sediments by participation of carbon dioxides of microbial origin that formed within the sediment during the process of the oxidation of organic matter during diagenesis. The maximum contents of carbon dioxide formed due to the oxidation of Corg are found in lacustrine carbonates and the carbonate component of Fe-Mn concretions, whereas in Mn carbonates of oceanic sediments the content of carbon dioxide of such origins is minimal. A general regularity has been established: the higher the concentrations of manganese in sediments, the lighter the isotopic composition of the carbon—that is, the greater the quantity of CO2 of microbial origin contained in the carbonate-manganese matter. Sedimentary manganese carbonates—that is, those entering the sediments as a result of direct precipitation within the water column and consequently isotopically equilibrated with the dissolved bicarbonate of the sedimentary water body—to date have been reported from only one fresh-water occurrence.