EarthRef.org Reference Database (ERR) -- Melezhik et al. 1999

The Palaeoproterozoic positive excursion of delta (super 13) C (sub carb) is now considered as three positive shifts of delta (super 13) C (sub carb) separated by returns to 0 per mil, which all occurred between 2.40 and 2.06 Ma. This isotopic event is unique in terms of both duration (>300 Ma) and (super 13) C enrichment (up to +18 per mil). The mechanism responsible for one of the most significant carbon isotopic shifts in Earth history remains highly debatable. To date, delta (super 13) C of +10 per mil to +15 per mil cannot be balanced by organic carbon burial (f (sub org) ) as there is no geological evidence for an enhanced C (sub org) accumulation prior to or synchronous with the excursion. Instead, termination of these excursions is followed by formation of a vast reservoir of (super 13) C-depleted organic material (-45 per mil at Shunga) and by one of the earliest known oil-generation episodes at 2.0 Ga. None of the three positive excursions of delta (super 13) C (sub carb) is followed by a negative isotopic shift significantly below 0per mil, as has always been observed in younger isotopic events, reflecting an overturn of a major marine carbon reservoirs. This may indicate that f (sub org) was constant: implying that the mechanism involved in the production of C (sub org) was different. Onset of intensive methane cycling resulting in Delta (sub c) change is another possibility. The majority of sampled (super 13) C (sub carb) -rich localities represents shallow-water stromatolitic dolostones, 'red beds' and evaporites formed in restricted intracratonic basins, and may not reflect global delta (super 13) C (sub carb) values. Closely spaced drill core samples (n = 73) of stromatilitic dolostones from the >1980+ or -27 Ma Tulomozerskaya Formation in the Onega palaeobasin, Russian Karelia, have been analysed for delta (super 13) C (sub carb) and delta (super 18) O (sub carb) in order to demonstrate that different processes were involved in the formation of (super 13) C (sub carb) -rich carbonates. The 800 m-thick magnesite-stromatolite-dolomite-'red beds' succession formed in a complex combination of environments on the Karelian craton: peritidal shallow marine, low-energy protected bights, barred basins, evaporative ephemeral ponds, coastal sabkhas and playa lakes. The carbonate rocks exhibit extreme (super 13) C enrichment with delta (super 13) C values ranging from +5.7 to +17.2 per mil vs. V-PDB (mean+9.9+ or -2.3 per mil) and delta (super 18) O from 18.6 to 26.0 per mil vs. V-SMOW (mean 22.0+ or -1.6 per mil). The Tulomozerskaya isotopic excursion is characteristic of the global 2.4-2.06 Ga positive shifts of carbonate (super 13) C/ (super 12) C, although it reveals the greatest enrichment in (super 13) C known from this interval. An external basin(s) is considered to have provided an enhanced C (sub org) burial and global seawater enrichment in (super 13) C: the global background value for the isotopic shift at Tulomozero time (ca. 2.0 Ga) is roughly estimated at around +5 per mil. An explosion of stromatolite-forming microbial communities in shallow-water basins, evaporative and partly restricted environments, high bioproductivity, enhanced uptake of (super 12) C, and pene-contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of CO (sub 2) (and CH (sub 4) ?) are believed to be additional local factors which may have enhanced delta (super 13) C from +5 per mil up to +17 per mil. Such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in (super 13) C and globally enhanced delta (super 13) C values. We propose that many previously reported delta (super 13) C values from other localities, where environmental interpretations are not available or have not been taken into account may not represent the global delta (super 13) C values.