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 Corg-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

Palaeoproterozoic evaporites in Fennoscandia: implications for seawater sulphate, the rise of atmospheric oxygen and local amplification of the δ13C excursion

“…, 1999). The TF is typical of such deposits where δ 13 C ratios very likely represent a signal amplified by local environmental factors, these include: evaporation, CO 2 degassing, enhanced uptake of 12 C by cyanobacteria and pene‐contemporaneous oxidation of organic material in cyanobacterial mats with the production and consequent loss of CO 2 in subaerial and shallow‐water environments (Melezhik et al. , 2005).…”

Palaeoproterozoic, rift-related, 13C-rich, lacustrine carbonates, NW Russia. Part I: Sedimentology and major element geochemistry

“…The 13 C global signal might have been modified by local factors associated with the Kuetsjärvi lake environment; namely, evaporation, enhanced biological uptake of 12 C, and penecontemporaneous oxidation of organic material in cyanobacterial mats. problem will be considered in a companion paper (Melezhik et al 2005). The isotopic part of this Table 6.…”

Palaeoenvironmental and diagenetic reconstruction of a closed‐lacustrine carbonate system – the challenging marginal setting of the Miocene Ries Crater Lake (Germany)

“…While δ 13 C and δ 18 O chemostratigraphy is widely used to characterize Pleistocene–Holocene glacial and interglacial periods, it has been only rarely applied for the palaeoenvironmental reconstruction of ancient lacustrine successions (e.g. Melezhik et al ., ; Montgomery et al ., ).…”