EXTREME 13CCARB 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

Abstract

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