Extreme ¹³C_carb enrichment in ca. 2.0 Ga magnesite–stromatolite–dolomite–`red beds' association in a global context: a case for the world-wide signal enh

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• 作者：Melezhik ; Victor A. ; Fallick ; Anthony E. ; Medvedev ; Pavel V. ; Makarikhin ; Vladimir V.
• 关键词：Palaeoproterozoic ; carbonate platform ; shallow water environment ; stromatolite ; dolomite ; magnesite ; red beds ; evaporates ; strontium ; carbon ; oxygen ; isotopes
• 刊名：Earth-Science Reviews
• 出版年：1999
• 出版时间：October, 1999
• 年：1999
• 卷：48
• 期：1-2
• 页码：71-120
• 全文大小：4.73 M

文摘

The Palaeoproterozoic positive excursion of δ¹³C_carb is now considered as three positive shifts of δ¹³C_carb 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, δ¹³C of +10‰ to +15‰ cannot be balanced by organic carbon burial (f_org) 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 δ¹³C_carb 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 f_org was constant: implying that the mechanism involved in the production of C_org was different. Onset of intensive methane cycling resulting in Δ_c change is another possibility. The majority of sampled ¹³C_carb-rich localities represents shallow-water stromatolitic dolostones, ‘red beds' and evaporites formed in restricted intracratonic basins, and may not reflect global δ¹³C_carb 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 δ¹³C_carb and δ¹⁸O_carb in order to demonstrate that different processes were involved in the formation of ¹³C_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 ¹³C enrichment with δ¹³C values ranging from +5.7 to +17.2‰ vs. V-PDB (mean+9.9±2.3‰) and δ¹⁸O 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 CO₂ (and CH₄?) are believed to be additional local factors which may have enhanced δ¹³C 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 δ¹³C values. We propose that many previously reported δ¹³C values from other localities, where environmental interpretations are not available or have not been taken into account may not represent the global δ¹³C values.