Extreme C-13(carb) enrichment in ca, 2.0 Ga magnesite-stromatolite-dolomite- 'red beds' association in a global context: a case for the world-wide signal enhanced by a local environment

The Palaeoproterozoic positive excursion of delta(13)C(carb) is now conside red as three positive shifts of delta(13)C(carb) separated by returns to 0 parts per thousand, which all occurred between 2.40 and 2.06 Ma. This isoto pic event is unique in terms of both duration (> 300 Ma) and C-13 enrichmen t (up to + 18 parts per thousand). The mechanism responsible for one of the most significant carbon isotopic shifts in Earth history remains highly de batable. To date, delta(13)C Of + 10 parts per thousand to + 15 parts per t housand cannot be balanced by organic carbon burial (f(org)) as there is no geological evidence for an enhanced C-org accumulation prior to or synchro nous with the excursion. Instead, termination of these excursions is follow ed by formation of a vast reservoir of C-13-depleted organic material (- 45 parts per thousand at Shunga) and by one of the earliest known oil-generat ion episodes at 2.0 Ga. None of the three positive excursions of delta(13)C (carb) is followed by a negative isotopic shift significantly below 0 parts per thousand, as has always been observed in younger isotopic events, refl ecting an overturn of a major marine carbon reservoirs. This may indicate t hat f(org) was constant: implying that the mechanism involved in the produc tion of C-org was different. Onset of intensive methane cycling resulting i n Delta(c) change is another possibility. The majority of sampled C-13(carb )-rich localities represents shallow-water stromatolitic dolostones, 'red b eds' and evaporites formed in restricted intracratonic basins, and may not reflect global delta(13)C(carb) values. Closely spaced drill core samples ( n = 73) of stromatolitic dolostones from the > 1980 +/- 27 Ma Tulomozerskay a Formation in the Onega palaeobasin, Russian Karelia, have been analysed f or delta(13)C(carb) and delta(18)O(carb) in order to demonstrate that diffe rent processes were involved in the formation of C-13(carb)-rich carbonates . The 800 m-thick magnesite-stromatolite-dolomite-'red beds' succession for med in a complex combination of environments on the Karelian craton: periti dal shallow marine, low-energy protected bights, barred basins, evaporative ephemeral ponds, coastal sabkhas and playa lakes. The carbonate rocks exhi bit extreme 13C enrichment with delta(13)C values ranging from + 5.7 to + 1 7.2 parts per thousand vs. V-PDB (mean + 9.9 +/- 2.3 parts per thousand) an d delta(18)O from 18.6 to 26.0 parts per thousand vs. V-SMOW (mean 22.0 +/- 1.6 parts per thousand). The Tulomozerskaya isotopic excursion is characte ristic of the global 2.4-2.06 Ga positive shifts of carbonate C-13/C-12, al though it reveals the greatest enrichment in C-13 known from this interval. An external basin(s) is considered to have provided an enhanced C-org buri al and global seawater enrichment in C-13: the global background value for the isotopic shift at Tulomozero time (ca. 2-0 Ga) is roughly estimated at around + 5 parts per thousand. An explosion of stromatolite-forming microbi al communities in shallow-water basins, evaporative and partly restricted e nvironments, high bioproductivity, enhanced uptake of C-12, and pene-contem poraneous recycling of organic material in cyanobacterial mats with the pro duction and consequent loss of CO2 (and CH4?) are believed to be additional local factors which may have enhanced delta(13)C from +5 parts per thousan d up to +17 parts per thousand. Such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in C-13 a nd globally enhanced delta(13)C values. We propose that many previously rep orted delta(13)C values from other localities, where environmental interpre tations are not available or have not been taken into account may not repre sent the global delta(13)C values. (C) 1999 Elsevier Science B.V. All right s reserved.