Early Ediacaran Positive C-Isotope Anomaly in Limestones of the Chernaya Rechka Formation of the Igarka Uplift (Northwestern Siberian Platform)

Abstract

The Precambrian Chernaya Rechka Formation (Igarka Uplift) hosts a high-amplitude positive carbonate carbon isotope anomaly (up to 12.4‰) spanning through 500 m of the section and occurring simultaneously with a rise of δ13Сorg values. The similar trends of carbon isotope variations in the carbonate fraction and organic matter are not caused by local sedimentary environments since the studied anomaly-bearing carbonates accumulated in different zones of the carbonate ramp and shallow shelf. Oxygen isotope composition of these carbonates and some other geochemical criteria indicate minimal (if any) impact of post-sedimentary processes on preservation of the isotope systems. Concentrations of trace elements in the carbonate fraction indicate alternating anoxic and oxic environments that did not affected the carbon isotope composition during accumulation of the Chernaya Rechka Formation. The profound positive δ13С anomaly was putatively caused by a global deficiency of 12С in the paleo-ocean related to accumulation of methane hydrates and burial of non-oxidized organic matter. Together with geochronological and stratigraphic data, minimal 87Sr/86Sr values (0.7074) in the Chernaya Rechka Formation reveal the lower Ediacaran (lower Vendian) age of the unit (635–580 Ma). Among the closest stratigraphic analogues of the Chernaya Rechka Formation are the Dal’nyaya Taiga Group (Patom Basin) and coeval stratigraphic sequences in the southern Siberian Platform. Global nature of the positive δ13С anomaly provide its correlation with other coeval C-isotope events worldwide.