C, D, S, and Sr Isotope Geochemistry and Chemostratigraphy of Ordovician Sediments in the Moyero River Section, Northern Siberian Platform

The Sr-87/Sr-86 ratio in gypsum and limestones of the Ordovician section of the Moyero River decreases from the bottom upward from 0.7091aEuro'0.7095 in the Irbukli Formation (Nyaian Regional Stage, similar to Lower Ordovician Tremadocian Stage) to 0.7080 in the upper part of the Dzherom Formation (Dolborian Regional Stage, similar to Upper Ordovician Katian Stage), which is well consistent with biostratigraphic subdivision of the section and existing concept concerning the strontium isotope evolution of the World Ocean. The most characteristic feature of the carbon isotope curve is decrease of delta D-13 values in carbonates from weakly positive values (0.5aEuro broken vertical bar 1.1aEuro degrees) in the Irbukli Formation (Nyaian Regional Stage) to sharply negative values (-5.4...-5.8aEuro degrees) in the middle part of the Kochakan Formation (top of the Kimaian Regional Stage, similar to end of the Dapingian-base of the Darriwilian Stage). Increase of delta D-18 from 20aEuro'22aEuro degrees to 26aEuro'28aEuro degrees, the negative correlation of delta D-13 and delta D-18, and decrease of delta S-34 in gypsum from 30aEuro'32aEuro degrees to 22aEuro'24aEuro degrees in this interval indicate that the D-13 depletion of carbonates was not related to the sulfate reduction and oxidation of organic matter during diagenesis and that the negative delta D-13 excursion was of primary nature. The presence of negative delta D-13 anomalies at this stratigraphic level in Ordovician sections of the South and North America (Buggish et al., 2003; Edwards and Saltzman, 2014; McLaughlin et al., 2016) indicates the global or subglobal distribution of this event, which was possibly related to the emergence of the oldest ground vegetation. Against the general decrease of delta D-13, the lower part of the section reveals three low-amplitude (1aEuro'2aEuro degrees) positive excursions, the position of which in general confirms the existing correlation scheme of the Moyero River section with the international scale. The upper part of the section is characterized by the alternation of low-delta D-13 intervals (from-2 to-3aEuro degrees) and brief positive excursions with amplitude of 0.5aEuro'1.3aEuro degrees. The positive delta D-13 excursion terminating the Ordovician section of the Moyero River correlates with the delta D-13 excursion in the middle Katian Stage, while the delta D-13 excursion in the lower part of the Baksian Regional Stage correlates with the excursion marking the Katian-Sandbian boundary.