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‰