Palaeoproterozoic, rift-related, ¹³C-rich, lacustrine carbonates, NW Russia. Part II: Global isotope signal recorded in the lacustrine dolostones

Abstract

A comprehensive study of the ∼2200-Ma-old Kuetsjärvi Sedimentary Formation (KSF), NW Russia, was undertaken to contribute to our understanding of palaeoenvironments associated with the global perturbation of the carbon cycle between 2330 and 2060 Ma. Closely spaced drill core samples (n=95) were obtained from a 150-m-thick unit deposited in rift-bound fluvial-deltaic and shallow-water lacustrine settings with a short-term invasion of sea water. Apart from a very few de-dolomitised samples, all other carbonate lithologies are represented by C_org-free, S-poor, quartz-rich dolostones, stromatolites and travertines which have high Sr concentrations (51–1069 ppm) and low Mn/Sr ratios (2·9 ± 2·1). The carbonate succession, excluding travertines, shows high δ¹³C (+7·5 ± 0·6‰, n=95) with a limited variation (+5·8 to +8·9‰). Fluctuating δ¹⁸O values (10·8–20·4‰) were overprinted during diagenesis, regional greenschist-grade and later retrograde metamorphism. Several short-term stratigraphic excursions of δ¹³C were apparently governed by evaporation and CO₂ degassing combined with pulses of¹²C-rich hydrothermal waters precipitating travertines. However, the ¹³C-rich nature of the dolostones reflects the global isotopic signal, which was modified in a shallow water lacustrine environment by evaporation, enhanced uptake of ¹²C by cyanobacteria, and pene-contemporaneous oxidation and loss of organic material. The best proxies to δ¹³C and ⁸⁷Sr/⁸⁶ Sr of coeval sea water recorded in the KSF dolostones are likely to be around +5–6‰ and 0·70406, respectively. The study of the KSF has shown that circumspection is necessary when attempting to model the behaviour and evolution of the global C-cycle in Deep Time. Models which purport to explain global oceanic–atmospheric evolution without first adequately accounting for the possibility that many Precambrian carbonate deposits might be non-marine, or at least influenced by non-marine fluids, should be viewed with caution