Sedimentary facies and climate control on formation of vivianite and siderite microconcretions in sediments of Lake Baikal, Siberia

Authigenic vivianite and siderite microconcretions were found, respectively, in hemipelagic and deltaic facies of 600-m-long BDP-98 sediment section from Lake Baikal. Textural investigations of these microconcretions show that they are typically <1 mm in size, irregular in shape and composed of aggregated crystallites. Dissimilar orientation of vivianite and siderite crystallites suggests formation at different depths in the sediment; up to tens of centimeters for vivianite and tens of meters for siderite. Chemical analyses of both the vivianite and the siderite indicate cation composition characterized by minor amounts of Mn, Ca and Mn apart from the dominating Fe. Rather limited and distinctive carbon isotopic composition of the siderite, with δ ¹³C_VPDB values between about +13 and +16‰, implies formation of the mineral in the methanogenic zone of diagenesis. Isotopic composition of oxygen in the siderite (δ ¹⁸O_VPDB values between about -10 and -11‰ ) is consistent with crystallization temperature at about 10-30°C and water δ ¹⁸O_SMOW values between about -10 and -16‰ . The distribution of the authigenic minerals in the section suggests changes in both sedimentary facies and climate, where vivianite formation was controlled by hemipelagic depositional conditions during the Pliocene and Quaternary, whereas siderite reflects impact of deltaic conditions during the Miocene.