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

Abstract

The ⁸⁷Sr/⁸⁶Sr ratio in gypsum and limestones of the Ordovician section of the Moyero River decreases from the bottom upward from 0.7091?0.7095 in the Irbukli Formation (Nyaian Regional Stage, ~Lower Ordovician Tremadocian Stage) to 0.7080 in the upper part of the Dzherom Formation (Dolborian Regional Stage, ~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 δ¹³С values in carbonates from weakly positive values (0.5…1.1‰) in the Irbukli Formation (Nyaian Regional Stage) to sharply negative values (–5.4...–5.8‰) in the middle part of the Kochakan Formation (top of the Kimaian Regional Stage, ~end of the Dapingian–base of the Darriwilian Stage). Increase of δ¹⁸О from 20?22‰ to 26?28‰, the negative correlation of δ¹³С and δ¹⁸О, and decrease of δ³⁴S in gypsum from 30?32‰ to 22?24‰ in this interval indicate that the ¹³С depletion of carbonates was not related to the sulfate reduction and oxidation of organic matter during diagenesis and that the negative δ¹³С excursion was of primary nature. The presence of negative δ¹³С 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 δ¹³С, the lower part of the section reveals three low-amplitude (1?2‰) 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-δ¹³С intervals (from–2 to–3‰) and brief positive excursions with amplitude of 0.5?1.3‰. The positive δ¹³С excursion terminating the Ordovician section of the Moyero River correlates with the δ¹³С excursion in the middle Katian Stage, while the δ¹³С excursion in the lower part of the Baksian Regional Stage correlates with the excursion marking the Katian–Sandbian boundary.