Chemostratigraphy of Neoproterozoic carbonates: implications for 'blind dating'

The delta C-13(carb) and Sr-87/Sr-86 secular variations in Neoproteozoic seawater have been used for the purpose of 'isotope stratigraphy' but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for 'blind dating'. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local delta C-13(carb) fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative delta C-13(carb) excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and 'blind dating'. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590-544 Myr interval, and two age-groups at 660-610 and 740-690 Myr can be resolved

Enigmatic nature of thick sedimentary carbonates depleted in 13C beyond the canonical mantle value: the challenges to our understanding of the terrestrial carbon cycle

Carbon and oxygen isotope measurements of 66 samples from the 60 m-thick variegated marble in the Upper Allochthon of the Norwegian Caledonides have a mean δ13Ccarb of −8.4 ± 0.9‰ (V-PDB), and a mean δ18O of 20.2 ± 2.2‰ (V-SMOW). The variegated marble is overlain by 150 m-thick pale grey marble characterised by mean δ13Ccarb of −6.5 ± 0.8‰ (n = 25) and underlain by dark grey marbles with a mean δ13Ccarb of +4.8 ± 1.1‰ (n = 61). This tripartite unit of an poorly constrained age—but between Neoproterozoic and Early Silurian—discontinuously developed over a distance of 500 km, is likely to represent one of the largest isotopically anomalous sedimentary carbonate formations yet reported. The marbles depleted in 13C beyond the canonical mantle value of −6‰ show no obvious evidence of post-sedimentary repartitioning of carbon isotopes. Several other carbonate formations deposited within approximately 680–540 Ma time interval (Chenchinskaya, Nikolskaya and Torginskay in Siberia, Ingta in NW Canada, Shuram in Central Oman, Trezona and Wonoka in South Australia) are several hundred meters thick, developed over a distance of hundreds of kilometres, and all show a similar depletion in 13C beyond the mantle value, for reasons that are not well understood. The existence of these carbonates represents a challenging problem for our current understanding of global carbon geodynamics. Changes in the ratio of reduced/oxidised carbon sequestered in sediments, a methane hydrate release or ‘zero’ biological productivity, if applied separately, cannot explain carbon isotope characteristics and formation of these carbonates. We tentatively propose that several factors associated with unusual geodynamic and palaeoclimatic scenarios developed between 600 and 540 Ma might have been involved in the extreme lowering of the isotopic composition of carbon entering the global Earth surface environment. This period was marked by the retreat of Neoproterozoic glaciers and break-up of the supercontinent Rodinia. The late-postglacial warming might have induced a massive release of methane hydrates extremely enriched in 12C. The ‘death’ of Rodinia was marked by unusually rapid (approximately 20 cm/year) motion of newly formed continental plates suggesting vigorous mantle convection and an enhanced restructuring of the lowest compartments of the Earth. This could provide a flux of 12C-rich material from the isotopically light asthenosphere–mantle source (δ13C = −25 to −15‰) to the surface. Considerable reconfiguration of the continental and sea areas of the Earth might have triggered enhanced weathering of previously deposited 12C-rich organic matter. In presenting this working hypothesis we also question the completeness of our understanding of the terrestrial carbon cycle, and the popular assumption that the isotopic composition of carbon entering the global Earth surface environment was always around −6‰

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

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 world's 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

Isotopic evidence for a complex Neoproterozoic to Silurian rock assemblage in the North-Central Norwegian Caledonides

Depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides have been constrained by the application of carbon and strontium isotope stratigraphy. Several marble units intercalated with various schists of the Evenes Group, previously correlated over long distances farther to the north with a late Ordovician-early Silurian, low-grade, fossiliferous succession (Balsfjord Group), have been studied for carbon, oxygen and strontium isotopes. The least altered Sr-87/Sr-86 ratios (0.7066 and 0.7077), and the best preserved delta(13)C values ( + 5.0 and + 8.0parts per thousand) obtained from the lowermost and uppermost marble formations of the Evenes Group are consistent with a seawater composition in the Neoproterozoic (650-600 and 620-610 Ma, respectively). The isotopic data indicate that only one formation (Sr-87/Sr-86 = 0.7083, delta(13)C = + 5.1parts per thousand), forming the middle Evenes Group, is consistent with the previously suggested early Silurian correlation. This is supported by new isotopic data obtained from coral and brachiopod-bearing, Llandovery (443-428 Ma) metalimestones (Sr- 87/Sr-86 = 0.7083, delta(13)C = + 4.3parts per thousand) of the Balsfjord Group. The remaining, fourth formation of the Evenes Group shows isotope data (Sr-87/Sr-86 = 0.7088, delta(13)C = + 2.1parts per thousand) which are consistent with a Cambrian seawater composition. The results obtained have several implications: (i) the late Ordovician-early Silurian Elvenes Conglomerate/ophiolite assemblage has a tectonic contact with the structurally overlying marble formation dated to 650-600 Ma; (ii) the isotopic and geological data do not support the previously proposed correlation of the entire Evenes Group with a late Ordovician-early Silurian, lithostratigraphic succession farther north in the Balsford area; (ii) the Evenes Group is not a lithostratigraphic unit and should be abandoned, as it is composed of a number of marble formations of different age, tectonically emplaced in a non-chronostratigraphic order. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica suggest that a large part of the carbonate rock succession forming the Evenes Group was initially accumulated in warm seas on a continental shelf, probably Laurentia, and was tectonically transported onto Baltica during the early Silurian, Scandian collision, at ca. 425 Ma. The complex tectonic imbrication of the polymetamorphosed and polydeformed Neoproterozoic, Cambrian and early Silurian carbonate Formations also suggests that more than one orogenic episode should be invoked to explain the tectonic juxtaposition of these assemblages. Some of the fault contacts juxtaposing rocks of Neoproterozoic and Cambrian age and the obduction of the ophiolite complex might have been associated with a mid to late Ordovician, Taconian event

Isotopic stratigraphy suggests Neoproterozoic ages and Laurentian ancestry for high-grade marbles from the North-Central Norwegian Caledonides

Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides. Two marble formations hosted by diverse schists from the Bogen Group, all previously correlated over long distances with a Late Ordovician-Early Silurian, low-grade, fossiliferous succession, have been studied for carbon, oxygen and strontium isotopes. The least altered Sr-87/Sr-86 ratios ranging between 0.7062 and 0.7068, and the best preserved delta(13)C values failing between +5.0 and +6.5 % obtained from two marble formations, are consistent with a seawater composition in the time interval 700-600 Ma. The results obtained do not support the previously proposed correlation of the Bogen Group with an Ordovician-Silurian lithostratigraphic succession further north. The apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the region requires revision. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica imply that carbonates were initially accumulated in seas on a continental shelf, probably Laurentia, and were tectonically transported onto Baltica during Early Silurian, Scandian collision, at c. 425 Ma. Prospecting for new dolomite marble deposits of the Hekkelstrand type and carbonate-hosted manganese-iron ores should be restricted to 700-600 Ma sequences in the Uppermost Allochthon of the Norwegian Caledonides

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

<p>In SE Siberia, carbonate formations with δ¹³C_carb 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 world's 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 δ¹³C_carb 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. </p