Victor A. Melezhik

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 13 C carb is now considered as three positive shifts of δ 13 C 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 13 C 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 13 C of +10‰ to +15‰ cannot be balanced by organic carbon burial (forg) as there is no geological evidence for an enhanced Corg accumulation prior to or synchronous with the excursion. Instead, termination of these excursions is followed by formation of a vast reservoir of 13 C -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 13 C 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 forg was constant: implying that the mechanism involved in the production of Corg was different. Onset of intensive methane cycling resulting in Δc change is another possibility. The majority of sampled 13 C carb -rich localities represents shallow-water stromatolitic dolostones, `red beds and evaporites formed in restricted intracratonic basins, and may not reflect global δ 13 C 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 δ 13 C carb and δ 18 O carb in order to demonstrate that different processes were involved in the formation of 13 C 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 13 C enrichment with δ 13 C values ranging from +5.7 to +17.2‰ vs. V-PDB (mean+9.9±2.3‰) and δ 18 O 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 13 C / 12 C , although it reveals the greatest enrichment in 13 C known from this interval. An external basin(s) is considered to have provided an enhanced Corg burial and global seawater enrichment in 13 C : 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 12 C , 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 δ 13 C 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 13 C and globally enhanced δ 13 C values. We propose that many previously reported δ 13 C values from other localities, where environmental interpretations are not available or have not been taken into account may not represent the global δ 13 C values.

Temporal constraints on the Paleoproterozoic Lomagundi-Jatuli carbon isotopic event

The Paleoproterozoic Lomagundi-Jatuli positive 13C excursion in sedimentary carbonates represents an event whose magnitude and duration is unique in Earth history, although precise absolute chronology of this event remains poorly constrained. In northeastern Fennoscandia, an 1300-m-thick sedimentary-volcanic succession of the Pechenga Greenstone Belt records decline of this isotopic excursion. Zircons from sedimentary rocks that occur within the decline have yielded 207Pb/206Pb dates at 2058 ± 2 Ma (±6 Ma including U decay constant uncertainties) and provide the first maximum age constraint on the termination of the Lomagundi-Jatuli event. Combined with existing constraints, these data indicate an 140 m.y. interval characterized by 13C-rich carbonate accumulation.

Isotopic Evidence for Massive Oxidation of Organic Matter Following the Great Oxidation Event

Analysis of two-billion-year-old rocks reveals an extreme carbon-cycle disruption after atmospheric oxygen increased. The stable isotope record of marine carbon indicates that the Proterozoic Eon began and ended with extreme fluctuations in the carbon cycle. In both the Paleoproterozoic [2500 to 1600 million years ago (Ma)] and Neoproterozoic (1000 to 542 Ma), extended intervals of anomalously high carbon isotope ratios (δ13C) indicate high rates of organic matter burial and release of oxygen to the atmosphere; in the Neoproterozoic, the high δ13C interval was punctuated by abrupt swings to low δ13C, indicating massive oxidation of organic matter. We report a Paleoproterozoic negative δ13C excursion that is similar in magnitude and apparent duration to the Neoproterozoic anomaly. This Shunga-Francevillian anomaly may reflect intense oxidative weathering of rocks as the result of the initial establishment of an oxygen-rich atmosphere.

Emergence of an aerobic biosphere during the Archean-Proterozoic transition: Challenges of future research

The earth system experienced a series of fundamental nupheavals throughout the Archean-Paleoproterozoic transition n(ca. 2500–2000 Ma). Most important were the establishment of nan oxygen-rich atmosphere and the emergence of an aerobic nbiosphere. Fennoscandia provides a fairly complete record nof the hallmark events of that transition: widespread igneous nactivity, its association with a possible upper-mantle oxidizing nevent, the global Huronian glaciation, a rise in atmospheric noxygen, the protracted and large-magnitude Lomagundi-Jatuli ncarbon isotope excursion, a substantial increase in the seawater nsulfate reservoir, changes in the sulfur and phosphorus ncycles, a radical modification in recycling of organic matter, nand the Shunga Event—the accumulation of unprecedented norganic-matter–rich sediments and the oldest known inferred ngeneration of significant petroleum. Current research efforts nare focused on providing an accurate temporal framework for nthese events and linking them into a coherent story of earth system evolution.

Constraints on 87Sr/86Sr of Late Ediacaran seawater: insight from Siberian high-Sr limestones

Abstract: In SE Siberia, carbonate formations with δ13Ccarb values ranging between −12‰ and −7‰ (V-PDB) and Sr concentrations of up to 2.5% occupy an area of 40 000 km2. Several successions exceed 1000 m in thickness and represent the worlds largest known exposures of sedimentary carbonates exhibiting extreme depletion in 13C. The carbonates were deposited on a carbonate platform evolving from a mixed carbonate–siliciclastic ramp to a carbonate ramp, and then from a peritidal rimmed shelf to a deep-water open shelf. All sequences reveal a facies-independent, upward rise in marine δ13Ccarb from −12‰ to −7‰. The trend and magnitude of the values mimic those that are characteristic of the 600–550 Ma Shuram–Wonoka isotope event. A coincident stratigraphic rise in 87Sr/86Sr from 0.70802 to 0.70862 in several sections of limestones, containing 4400 μg g−1 Sr on average, is considered to be by far the best available constraint on a temporal variation of seawater isotopic composition through the Late Ediacaran. If the greatest temporal rate of change in seawater 87Sr/86Sr observed in the Cenozoic is applied to the Siberian sections, the calculated minimum duration for the Suram–Wonoka event is 10 Ma. Supplementary material: XRF and ICP-AES analysis, C, O and Sr isotopic data are available at http://www.geolsoc.org.uk/SUP18324.

Links between Palaeoproterozoic palaeogeography and rise and decline of stromatolites: Fennoscandian Shield

Through a review of literature and new data we have documented two major events in the Palaeoproterozoic history of stromatolites as indicated by palaeontological and palaeoenvironmental studies. With a time resolution of between 40 and 200 Ma we confirm Semikhatov and Raabens, and Awramiks (albeit approximately) maximum in diversity and abundance of stromatolites between 2330 and 2060 Ma ago (Jatulian diversification). We suggest that this taxonomic diversity was driven by a major phase of cratonisation, formation of the Karelian carbonate platform and numerous rift-related shallow-water carbonate basins supersaturated with Ca+2, Mg+2 and CO2. The Jatulian stromatolite explosion is synchronised with a positive δ13Ccarb shift of Jatulian age carbonates. We also document stromatolite decline which occurred on the Fennoscandian Shield somewhere between 2060 and 1900 Ma ago. This decline, both in abundance and in taxonomic diversity, is interpreted as having been caused by the first phase of ‘oceanisation’. The oceanisation led to the considerable reduction in ecological niches that could be utilised by cyanobacteria. The post-Jatulian decline of stromatolites coincides with an abrupt, downward δ13Corg shift from −19%. to −38% and is roughly coeval with the appearance of the first eukaryotic algae documented elsewhere. The systematics of the Fennoscandian diversity of Palaeoproterozoic stromatolites is identical to that reported from India and China and reveals a dissimilarity with abundance and diversity patterns in Australia and Northern America.

Reading the Archive of Earth’s Oxygenation

Part V FAR-DEEP Core Archive and Database.- Part VI FAR-DEEP Core Descriptions and Rock Atlas.

Fractionation of carbon and oxygen isotopes in 13C-rich Palaeoproterozoic dolostones in the transition from medium-grade to high-grade greenschist facies: a case study from the Kola Superdeep Drillhole

Samples from the Kola Superdeep Drillhole (12 262 m), a deep drillhole (1060 m), and from the surface, seaprated by only around 10 km, provided a unique opportunity for direct tracing of δ13C and δ18O changes through a low- to high-grade greenschist-facies transition within impure, 13C-rich Palaeoproterozoic dolostones. The least-altered dolostones have δ13C of +9‰ and δ18O of 22‰. The metamorphic transition is expressed by dolomite + calcite1 + quartz ± K-feldspar ← tremolite + calcite2 ± dolomite ± calcite1 and defined by 13C depletion of calcite2 (c. 3.0‰), calcite1 (1.0–2.0‰) and dolomite (<1‰) which is associated with a Rayleigh distillation process. δ18O shows a considerable resetting in all carbonate components by around 6‰ caused by a Rayleigh distillation process coupled with isotopic exchange between the carbonates and fluids with an external source of oxygen. The retrograde alteration is expressed by the formation of quartz–chlorite veinlets within tectonically bound zones of brecciated and sheared dolostones. The maximum 18O depletion in dolomite (9‰) and calcite1 (c. 4‰) were probably controlled by infiltration into permeable zones of external fluids associated with retrograde alteration; δ13C remains largely unaffected.

Testing for fullerenes in geologic materials: Oklo carbonaceous substances, Karelian shungites, Sudbury Black Tuff

Fullerenes have been reported from diverse geologic environments since their discovery in shungite from Karelian Russia. Our investigation is prompted by the presence of onionskin-like structures in some carbonaceous substances associated with the fossil nuclear fission reactors of Oklo, Gabon. The same series of extractions and the same instrumental techniques, laser desorption ionization and high-resolution mass spectroscopy (electron-impact mass spectroscopy), were employed to test for fullerenes in samples from three different localities: two sites containing putative fullerenes (Sudbury Basin and Russian Karelia) and one new location (Oklo, Gabon). We confirm the presence of fullerenes (C 6 0 and C 7 0 ) in the Black Tuff of the Onaping Formation impact breccia in the Sudbury Basin, but we find no evidence of fullerenes in shungite samples from various locations in Russian Karelia. Analysis of carbonaceous substances associated with the natural nuclear fission reactors of Oklo yields no definitive signals for fullerenes. If fullerenes were produced during sustained nuclear fission at Oklo, then they are present below the detection limit (∼100 fmol), or they have destabilized since formation. Contrary to some expectations, geologic occurrences of fullerenes are not commonplace.

A Chronostratigraphic Division of the Precambrian: Possibilities and Challenges

Abstract: This chapter provides a review of events through Precambrian Earth history, with the aim of providing an up-to-date foundation on which to construct a chronostratigraphic revision of the Precambrian time scale. The guiding principles used to develop a revised Precambrian time scale follow Cloud’s vision to “…seek trend-related events that have affected the entire Earth over relatively short intervals of time and left recognizable signatures in the rock sequences of the globe…”, and apply Gould’s historical principles of directionality and contingency.