Palaeoproterozoic magnesite-stromatolite-dolostone-'red bed' association, Russian Karelia: palaeoenvironmental constraints on the 2.0 Ga-positive carbon isotope shift

The ca. 2000 Ma. Tulomozerskaya Formation, Russian Karelia, is composed of an 800 m-thick magnesite-stromatolite-dolostone-'red bed' succession with t he most C-13-rich dolostones (up to +18 parts per thousand V-PDB) that have ever been reported. Terrigenous 'red beds' are developed throughout the se quence and represent three main depositional settings: (1) a braided fluvia l system over a lower energy, river-dominated coastal plain, (2) a low-ener gy, barred lagoon or bight, and (3) a non-marine, playa lake. A significant component of the sequence consists of biostromal and biohermal columnar st romatolites accreted in shallow-water, low-energy, intertidal zones, barred evaporitic lagoons and peritidal evaporitic environments. Only a small por tion of stromatolites might have been accreted in relatively 'open' marine environments. The red, flat-laminated, dolomitic and magnesite stromatolite s formed in evaporative ephemeral ponds, coastal sabkhas and playa lakes. T epees, mudcracks, pseudomorphs after calcium sulphate, halite casts, and ab undant 'red beds' in the sequence suggest that (1) terrestrial environments dominated over aqueous, and (2) partial or total decoupling took place bet ween the stromatolite-dominated depositional systems and the bordering sea. The greatest enrichment in C-13 occurs in the playa magnesite (up to +17.2 parts per thousand) and in the laminated dolomitic stromatolites accreted in ephemeral ponds (up to +16.8 parts per thousand), whereas the dolostones from more open environments are less rich in C-13 (+5.6 to +10.7 parts per thousand). The isotopic shift (ca. 5 parts per thousand) induced by global factors (i.e. accelerated accumulation of organic material in an external basin) was augmented by that driven by a series of local factors (restricti on, evaporation, biological photosynthesis). The latter enhanced a global d elta(13)C value due to an isotopic disequilibrium between atmospheric CO2 a nd dissolved inorganic carbon in the local aquatic reservoirs precipitating the carbonate minerals. The interplay between global and local factors sho uld be taken into account when interpreting the Palaeoproterozoic carbon is otope excursion and its implications.