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

Abstract

The Precambrian Chernaya Rechka Formation (Igarka Uplift) hosts a high-amplitude positive carbonate carbon and dispersed organic matter δ¹³С isotope anomaly (up to +12.4‰) spanning over 500 m of the section. Variations of δ¹³С_carb and δ¹³С_org are synchronous and do not depend on local sedimentary environments, since the studied anomaly-bearing carbonates were accumulated in different zones of the carbonate ramp. The oxygen isotope composition of these carbonates and other geochemical criteria indicate an insignificant impact of postsedimentary processes on the preservation of isotope systems. Variations of trace elements in the carbonate fraction from the stratotype section of the Chernaya Rechka Formation indicate its accumulation in alternating anoxic and oxic environments that did not affect the carbon isotope composition. It is shown that the limestones, which outcrop on Plakhinskii Island and contain widespread molar-tooth structures, also belong to the Chernaya Rechka Formation in terms of the chemical and isotope composition. The profound positive δ¹³С anomaly was putatively caused by a global deficiency of ¹²С in the paleo-ocean related to the accumulation of methane hydrates and the burial of nonoxidized organic matter. Together with the geochronological and stratigraphic data, minimum 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. The global nature of the positive δ¹³С anomaly provides its correlation with other coeval C-isotope events worldwide.