The role of O2 in the evolution of early animals, as represented by some members of the Ediacara biota, has been heavily debated because current geochemical evidence paints a conflicting picture regarding global marine O2 levels during key intervals of the rise and fall of the Ediacara biota. Fossil evidence indicates that the diversification the Ediacara biota occurred during or shortly after the Ediacaran Shuram negative C-isotope Excursion (SE), which is often interpreted to reflect ocean oxygenation. However, there is conflicting evidence regarding ocean oxygen levels during the SE and the middle Ediacaran Period. To help resolve this debate, we examined U isotope variations (δ238 U) in three carbonate sections from South China, Siberia, and USA that record the SE. The δ238 U data from all three sections are in excellent agreement and reveal the largest positive shift in δ238 U ever reported in the geologic record (from ~ -0.74‰ to ~ -0.26‰). Quantitative modeling of these data suggests that the global ocean switched from a largely anoxic state (26%-100% of the seafloor overlain by anoxic waters) to near-modern levels of ocean oxygenation during the SE. This episode of ocean oxygenation is broadly coincident with the rise of the Ediacara biota. Following this initial radiation, the Ediacara biota persisted until the terminal Ediacaran period, when recently published U isotope data indicate a return to more widespread ocean anoxia. Taken together, it appears that global marine redox changes drove the rise and fall of the Ediacara biota.
The Fauske conglomerate represents a rather rare case of a monomict carbonate conglomerate in the Late Neoproterozoic to Silurian, lithostratigraphic successions of the Norwegian Caledonides. Lithological varieties of this conglomerate unit from the Lovgavlen quarry have a highly decorative quality and are well known in both domestic and international markets under trading names such as ‘Norwegian Rose’, ‘Jaune Rose’, ‘Norwegian Green’, ‘Antique Fonce’ and ‘Hermelin’. The Fauske conglomerate is a 60 m-thick unit which rests on either dark grey (‘blue’) calcite marbles or white dolomite marbles. The latter are jointed and fragmented, and also appear as sedimentary collapsebreccia and debris where they are in direct contact with the conglomerate. Although the Fauske conglomerate has been involved in two main pulses of Caledonian tectonic deformation, which produced an early, syn-metamorphic flattening of the clasts and a later folding or rotation of clasts into a spaced cleavage, the overall sedimentary features are still remarkably well preserved. The Fauske conglomerate unit consists of 25 beds (5 cm to 3 metres thick) comprising landslide, carbonate debris and carbonate breccia–conglomero-breccias–greywacke lithofacies. Blocks, fragments, cobbles, pebbles and smaller clasts are mainly of white dolostone and pink, beige, white and ‘blue’ calcite marbles. The matrix has a granoblastic texture and similar range in lithology with variable amounts of quartz, fuchsite, sericite, muscovite and chlorite. Within the unit, an upward fining of the clasts is followed by the gradual development of calcareous greywacke layers which show both cross bedding and channelling. The depositional model involves: (i) a locally developed, tectonically unstable carbonate shelf-margin , (ii) a temporary lowering of sea level, (iii) formation of a high-relief, shore-to-basin fault scarp followed by (iv) the development of a channel, with (v) subsequent, long-distance transport of clasts of pink carbonates from the continent-basin margin, which were (vi) redeposited together with a carbonate debris (white dolomite and ‘blue’ calcite marbles) on the tectonically fragmenting edge of a carbonate shelf. Both matrix and pebbles show a similar range in isotopic values: -1.9 to +0.6‰ (vs. PDB) for δ 13 Ccarb and 0.70896 to 0.70946 for 87 Sr/ 86 Sr. The least altered 87 Sr/ 86 Sr (0.70896) isotopic value plotted on the calibration curve is consistent with a seawater composition corresponding to ages of 470-475, 505-510 and 520, whereas the least altered δ 13 Ccarb (-0.6‰) value matches only 520 Ma.
Carbon and strontium isotope chemostratigraphy (255 δ13Ccarb and δ18O, and 130 87Sr/86Sr analyses of carbonate components in whole-rock samples)
in combination with detailed mapping at 1:20,000 scale in part of the Fauske Nappe in the Rognan area, Nordland, have led to a major revision of
what had previously been considered to represent a continuous lithostratigraphy spanning over 100 million years of geological time: from Cambrian
to Silurian. This new work has indicated that the high-grade, barren, marble and siliciclastic succession occurs in a series of thrust sheets which have
diverse apparent depositional ages ranging from Early Cryogenian to Early Silurian. The new data support earlier interpretations that the rocks
of the Fauske Nappe were deposited along the carbonate platform and adjacent continental slope of the eastern margin of Laurentia during the
Neoproterozoic to Early Palaeozoic time interval. Most of the thrust sheets were generated in Early Ordovician time during the earliest stages of
the main phase of the Taconian orogeny, and then immediately overlain unconformably by a carbonate breccia and conglomerate unit, the Oynes
formation, also of Early Ordovician age. Just one formation (Rognan formation) in the Fauske Nappe is younger, of Early Silurian age, with Sr- and
C-isotopic data that are comparable to those in fossiliferous, Early Silurian, metalimestones farther north in Troms. These particular carbonate rocks
are considered to have accumulated in a post-Taconian successor basin, prior to their transport during the Scandian orogeny into the Uppermost
Allochthon of the Scandinavian 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 87 Sr/ 86 Sr ratios ranging between 0.7062 and 0.7068, and the best preserved δ 13 C values falling 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.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)