Authigenic mineralization in the early diagenesis of Holocene peatlands in the Baraba forest-steppe (Southern West Siberia)

Postdepositional diagenetic processes and behavior of biogenic and other elements in Holocene peatlands have been studied in Ubinskoe and Sherstobitovo bogs of the Baraba forest-steppe zone. The low pH of the water and an oxic environment in peat were favorable for the formation of goethite and hydrogoethite in upper peat layers. The formation of hematite and andhydride may be related with past wild fires. Early diagenetic processes lead to precipitation of authigenic minerals, especially, pyrite or less often siderite, calcite, and kaolinite. Reduced conditions maintained the formation of metallic Ni and intermetallic Ni-Cr nanometer particles, as well as precipitation of amorphous silica upon dissolution of aluminosilicate minerals in upper peat intervals. Compounds of iron and other elements accumulate in the sampled bogs upon two geochemical barriers: (i) a hydroxide barrier of oxidation and sorption in top peat layers and (ii) alkaline carbonate and reduction sulfide barrier in deeper peat. In the former case, active OM destruction in oxic conditions produces an acidic environment which maintains mobility of elements, including Fe and Al. In the latter case, authigenic minerals, especially pyrite, precipitate in conditions of microbially mediated pH increase from 3.7 to 7.9. Biologically productive surface peat, rich in aquatic plants, contains relatively low percentages of montmorillonite, chlorite, and biotite but higher amounts of obviously diagenetic kaolinite which commonly forms in low-pH environments. Iron-oxidizing bacteria (Fe-OX) found in surface peat layers are active agents in the oxidation of reduced iron, Fe²⁺ to Fe³⁺, from dissolved iron compounds. Precipitation of iron phosphates is obviously mediated by microorganisms, which can extract and mobilize phosphorus from plant remnants. Phosphate-mobilizing bacteria found throughout the peat profiles participate in mineralization of phosphorus bound in organic and mineral compounds and can convert it to the soluble PO4^3-form. The processes of mineral formation in modern peatlands are similar to early diagenesis as both occur in unstable systems, while siderite, vivianite, and goethite belong to the same mineralogical series and are related via variations of Eh, CO2, and pH in bog waters. The incompleteness of mineral formation processes in the sampled bogs is indicated by amorphous state of mineral masses, presence of pseudomorphs, and absence of distinct crystal structure (except for pyrite).

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