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Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO2 was 102 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO2 rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.

The Labrador Trough in northern Quebec and Labrador is a 900 km long Rhyacian-Orosirian orogenic belt containing mixed sedimentary-volcanic successions. Despite having been studied intensively since the 1940s, relatively few chemostratigraphic studies have been conducted. To improve our understanding of the Labrador Trough in the context of Earth history, and better constrain the local record of the Lomagundi-Jatuli carbon isotope excursion, high-resolution sampling and carbon isotope analyses of the Le Fer and Denault formations were conducted. Carbonate carbon isotopes ([delta] [.sup.13]C) in the Le Fer Formation record a large range in values from -4.4[per thousand] to +6.9[per thousand]. This large range is likely attributable to a combination of post-depositional alteration and variable abundance of authigenic carbonate minerals; elemental ratios suggest that the most [.sup.13]C-enriched samples reflect the composition of the water column at the time of deposition. Cumulatively, these data suggest that the Lomagundi-Jatuli Excursion was ongoing during deposition of the Le Fer Formation, approximately 2 km higher in the stratigraphy than previously recognised. However, the possibility of a post-Lomagundi-Jatuli Excursion carbon isotope event cannot conclusively be ruled out. The directly overlying Denault Formation records a range in [delta] [.sup.13]C values, from -0.5[per thousand] to +4.3[per thousand], suggesting that it was deposited after the conclusion of the Lomagundi-Jatuli Excursion and that the contact between the Le Fer and Denault formations occurred sometime during the transition out of the Lomagundi-Jatuli Excursion, ca. 2106 to 2057 Ma. Key words: Lomagundi Excursion, Lomagundi-Jatuli Excursion, Labrador Trough, Le Fer Formation, Denault Formation, carbon isotopes. La fosse du Labrador, dans le nord du Quebec et au Labrador, est une ceinture orogenique rhyacienne-orosirienne de 900 km de long renfermant des sequences sedimentaires-volcaniques mixtes. Bien qu'elle ait fait l'objet de nombreuses etudes depuis les annees 1940, relativement peu d'etudes chimiostratigraphiques s'y sont interessees. Afin d'ameliorer la comprehension de la fosse du Labrador dans le contexte de l'histoire de la Terre et de mieux delimiter le registre local de l'excursion Lomagundi-Jatuli des isotopes du carbone, un echantillonnage de haute resolution et des analyses des isotopes du carbone des Formations de Le Fer et de Denault ont ete effectues. Les isotopes du carbone ([delta][.sup.13]C) de carbonates dans la Formation de Le Fer presentent une grande fourchette de valeurs allant de -4,4[per thousand] à +6,9[per thousand], vraisemblablement attribuable à une combinaison d'alteration post-depot et d'abondance variable de carbonates authigenes; les rapports elementaires donnent à penser que les echantillons les plus enrichis en [.sup.13]C refletent la composition de la colonne d'eau au moment du depot. Collectivement, ces donnees indiqueraient que l'excursion Lomagundi-Jatuli etait en cours durant le depot de la Formation de Le Fer, ce qui la place environ 2 km plus haut dans la colonne stratigraphique que sa position reconnue auparavant. La possibilite d'un evenement enregistre par les isotopes de carbone qui serait posterieur à l'excursion Lomagundi-Jatuli ne peut toutefois être exclue avec certitude. La Formation de Denault immediatement au-dessus presente une fourchette de valeurs de [delta][.sup.13]C allant de -0,5[per thousand] à +4,3[per thousand], ce qui indiquerait qu'elle a ete deposee apres la fin de l'excursion Lomagundi-Jatuli et que le contact entre les Formations de Le Fer et de Denault s'est produit durant la transition au sortir de l'excursion Lomagundi-Jatuli, vers 2106 Ma à 2057 Ma. [Traduit par la Redaction] Mots-cles: excursion Lomagundi, excursion Lomagundi-Jatuli, fosse du Labrador, Formation de Le Fer, Formation de Denault, isotopes du carbone., Introduction The ca. 2.2-1.8 Ga Labrador Trough (Fig. 1) has been studied nearly continuously since the 1940s, yet its geochronology and tectonic history remain poorly constrained, in large part due [...]

The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today.sup.1. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements ([increment].sup.17O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative [increment].sup.17O values (down to -0.880/00) observed in sulfates, except for those from the terminal Cryogenian period.sup.2. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO.sub.2 and O.sub.2 and the photosynthetic O.sub.2 flux at this time.sup.3. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O.sub.2 flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO.sub.2 (2-30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O.sub.2 was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean.sup.4-6 and Phanerozoic primary production.sup.7, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O.sub.2 over geologic time.sup.8.Triple oxygen isotope measurements of 1.4-billion-year-old sedimentary sulfates reveal a unique mid-Proterozoic atmosphere and demonstrate that gross primary productivity in the mid-Proterozoic was between 6% and 41% of pre-anthropogenic levels., Author(s): Peter W. Crockford [sup.1] [sup.2] [sup.3] , Justin A. Hayles [sup.4] [sup.5] , Huiming Bao [sup.5] [sup.6] , Noah J. Planavsky [sup.7] , Andrey Bekker [sup.8] , Philip W. [...]

Abstract: The existence of coherent, large-scale, submarine landslides on modern continental margins implies that their apparent rarity in ancient orogenic belts is due to non-recognition. Two map-scale, coherent, pre-orogenic, normal-sense [...]

Sedimentary rocks deposited across the Proterozoic-Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals (1,2). It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans (3,4). These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances (1,5,6), iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities (7-9)., Proxies such as iron-speciation chemistry record the redox state of local water masses immediately above accumulating sediments. Decades of work on the behaviour of iron in marine sediments underpin the [...]

This study presents accurate and precise iron isotopic data for 16 co-magmatic rocks and 6 pyroxene-magnetite pairs from the classic, tholeiitic Red Hill sill in southern Tasmania. The intrusion exhibits a vertical continuum of compositions created by in situ fractional crystallisation of a single injection of magma in a closed igneous system and, as such, constitutes a natural laboratory amenable to determining the causes of Fe isotope fractionation in magmatic rocks. Early fractionation of pyroxenes and plagioclase, under conditions closed to oxygen exchange, gives rise to an iron enrichment trend and an increase in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII of the melt relative to the Fayalite-Magnetite-Quartz (FMQ) buffer. Enrichment in [Fe.sup.3+]/S[Fe.sub.melt] is mirrored by δ[sup.57]Fe, where [sup.VI][Fe.sup.2+]-bearing pyroxenes partition [sup.57]Fe-depleted iron, defining an equilibrium pyroxene-melt fractionation factor of Δ[sup.57][Fe.sub.px-melt] ≤ -0.25[per thousand] x [10.sup.6]/[T.sup.2]. Upon magnetite saturation, the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] of the melt fall, commensurate with the sequestration of the oxidised, [sup.57]Fe-enriched iron into magnetite, quantified as Δ[sup.57][Fe.sub.mtn-melt] = +0.20[per thousand] x [10.sup.6]/[T.sup.2]. Pyroxene-magnetite pairs reveal an equilibrium fractionation factor of Δ[sup.57][Fe.sub.mtn-px] ≅ +0.30‰ at 900-1,000°C. Iron isotopes in differentiated magmas suggest that they may act as an indicator of their oxidation state and tectonic setting. Keywords Iron isotopes * Tholeiitic differentiation * Fractional crystallisation * Oxygen fugacity * Tasmanian Dolerites * High-temperature stable isotopes, Introduction Since iron is the most abundant polyvalent species in silicate melts, the relative proportion of ferrous to ferric iron control the oxygen fugacity ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) [...]

Global carbon cycle perturbations throughout Earth history are frequently linked to changing paleogeography, glaciation, ocean oxygenation, and biological innovation. A pronounced carbonate carbon-isotope excursion during the Ediacaran Period (635 to 542 million years ago), accompanied by invariant or decoupled organic carbon-isotope values, has been explained with a model that relies on a large oceanic reservoir of organic carbon. We present carbonate and organic matter carbon-isotope data that demonstrate no decoupling from approximately 820 to 760 million years ago and complete decoupling between the Sturtian and Marinoan glacial events of the Cryogenian Period (approximately 720 to 635 million years ago). Growth of the organic carbon pool may be related to iron-rich and sulfate-poor deep-ocean conditions facilitated by an increase in the Fe:S ratio of the riverlne flux after Sturtian glacial removal of a long-lived continental regolith. doi: 10.1126/science.1184508

Significant variability in [[delta].sup.34][S.sub.pyrite] values in Neoproterozoic sedimentary rocks has been attributed to the evolution of nonphotosynthetic sulfide-oxidizing bacteria and the advent of sulfur disproportionation reactions in response to Earth's evolving redox chemistry. We analyzed trace sulfate in carbonates from South Australia and Namibia and reconstructed the sulfur isotope evolution of seawater sulfate. Comparison of our [[delta].sup.34][S.sub.sulfate] record with published [[delta].sup.34][S.sub.pyrite] data from the same or equivalent successions indicates that [[delta].sup.34][S.sub.sulfate] - [[delta].sup.34][S.sub.pyrite] ([[DELTA].sup.34]S) rose gradually through the second half of the Neoproterozoic and fluctuated coincident with episodes of glaciation, but did not exceed 46 [per thousand] before ca. 580 Ma. Large variability in [[delta].sup.34][S.sub.pyrite] in the Neoproterozoic can be explained as a consequence of low sulfate concentrations and rapidly fluctuating [[delta].sup.34][S.sub.sulfate] in seawater rather than the onset of sulfur disproportionation reactions mediated by nonphotosynthetic sulfide-oxidizing bacteria. Keywords: Neoproterozoic, sulfate, [[delta].sup.34]S, South Australia, Namibia.

Earth's surface has undergone a protracted oxygenation, which is commonly assumed to have profoundly affected the biosphere. However, basic aspects of this history are still debated--foremost oxygen (O2) levels in the oceans and atmosphere during the billion years leading up to the rise of algae and animals. Here we use isotope ratios of iron (Fe) in ironstones--Fe-rich sedimentary rocks deposited in nearshore marine settings--as a proxy for O2 levels in shallow seawater. We show that partial oxidation of dissolved Fe(II) was characteristic of Proterozoic shallow marine environments, whereas younger ironstones formed via complete oxidation of Fe(II). Regardless of the Fe(II) source, partial Fe(II) oxidation requires low O2 in the shallow oceans, settings crucial to eukaryotic evolution. Low O2 in surface waters can be linked to markedly low atmospheric O2--likely requiring less than 1% of modern levels. Based on our records, these conditions persisted (at least periodically) until a shift toward higher surface O2 levels between ca. 900 and 750 Ma, coincident with an apparent rise in eukaryotic ecosystem complexity. This supports the case that a first-order shift in surface O2 levels during this interval may have selected for life modes adapted to more oxygenated habitats. [ABSTRACT FROM AUTHOR]

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The Arabian-Nubian Shield (ANS) includes Middle Cryogenian-Ediacaran (790-560 Ma) sedimentary and volcanic terrestrial and shallow-marine successions unconformable on juvenile Cryogenian crust. The oldest were deposited after 780-760 Ma shearing and suturing in the central ANS. Middle Cryogenian basins are associated with ~700 Ma suturing in the northern ANS. Late Cryogenian basins overlapped with and followed 680–640 Ma Nabitah orogenesis in the eastern ANS. Ediacaran successions are found in pull-apart and other types of basins formed in a transpressive setting associated with E-W shortening, NW-trending shearing, and northerly extension during final amalgamation of the ANS. Erosion surfaces truncating metamorphosed arc rocks at the base of these successions are evidence of periodic exhumation and erosion of the evolving ANS crust. The basins are evidence of subsequent subsidence to the base level of alluvial systems or below sea level. Mountains were dissected by valley systems, yet relief was locally low enough to allow for seaways connected to the surrounding Mozambique Ocean. The volcanosedimentary basins of the ANS are excellently exposed and preserved, and form a world-class natural laboratory for testing concepts about crustal growth during the Neoproterozoic and for the acquisition of data to calibrate chemical and isotopic changes, at a time in geologic history that included some of the most important, rapid, and enigmatic changes to Earth’s environment and biota. [ABSTRACT FROM AUTHOR]

A mechanistic understanding of relationships between global glaciation, a putative second rise in atmospheric oxygen, the Shuram carbon isotope excursion, and the appearance of Ediacaran-type fossil impressions and bioturbation is dependent on the construction of accurate geological records through regional stratigraphic correlations. Here we integrate chemo-, litho-, and sequence-stratigraphy of fossiliferous Ediacaran strata in northwestern Canada. These data demonstrate that the FAD of Ediacara-type fossil impressions in northwestern Canada occur within a lowstand systems tract and above a major sequence boundary in the informally named June beds, not in the early Ediacaran Sheepbed Formation from which they were previously reported. This distinction is substantiated by δ13Ccarb chemostratigraphy of the Sheepbed carbonate, which overlies the Sheepbed Formation, and the Gametrail Formation, which overlies the June beds. The Sheepbed carbonate hosts heavy δ13Ccarb values whereas the Gametrail Formation contains a large δ13Ccarb excursion, which we correlate with the globally recognized Shuram excursion. Stratigraphically above the Gametrail excursion, the first bilaterian burrows are present in the basal Blueflower Formation. Together, these data allow us to construct an age model for Ediacaran strata in northwestern Canada and conclude that a purported shift in Fe speciation in the Sheepbed Formation significantly predates the shift recorded above the ca. 582 Ma Gaskiers glaciation in Newfoundland and the first appearance of Ediacaran biota. The Gametrail excursion shares many characteristics with Shuram negative δ13Ccarb excursion: 1) δ13Ccarb and δ18Ocarb covary; 2) δ13Ccarb and δ13Corg do not covary; 3) the excursion is developed during a transgressive systems tract and recovers in an highstand systems tract; and 4) values in some sections are well below mantle δ13C input values but are variable between sections. We relate regional lateral variability in the magnitude and character of this excursion to condensation and diachronous deposition during the transgression and local authigenic carbonate production. In light of these observations, we explore a variety of models for the genesis of the Shuram excursion and suggest that the location and amount of authigenic carbonate production played a role in the excursion., Earth and Planetary Sciences

Neoproterozoic iron formation (NIF) provides evidence for the widespread return of anoxic and ferruginous basins during a time period associated with major changes in climate, tectonics and biogeochemistry of the oceans. Here we summarize the stratigraphic context of Neoproterozoic iron formation and its geographic and temporal distribution. It is evident that most NIF is associated with the earlier Cryogenian (Sturtian) glacial epoch. Although it is possible that some NIF may be Ediacaran, there is no incontrovertible evidence to support this age assignment. The paleogeographic distribution of NIF is consistent with anoxic and ferruginous conditions occurring in basins within Rodinia or in rift-basins developed on its margins. Consequently NIF does not require whole ocean anoxia. Simple calculations using modern day iron fluxes suggest that only models that invoke hydrothermal and/or detrital sources of iron are capable of supplying sufficient iron to account for the mass of the larger NIF occurrences. This conclusion is reinforced by the available geochemical data that imply NIF record is a mixture of hydrothermal and detrital components. A common thread that appears to link most if not all NIF is an association with mafic volcanics., Earth and Planetary Sciences

Multiple eukaryotic clades make their first appearance in the fossil record between ~810 and 715 Ma. Molecular clock studies suggest that the origin of animal multicellularity may have been part of this broader eukaryotic radiation. Animals require oxygen to fuel their metabolism, and low oxygen levels have been hypothesized to account for the temporal lag between metazoan origins and the Cambrian radiation of large, ecologically diverse animals. Here, paleoredox conditions were investigated in the Fifteenmile Group, Ogilvie Mountains, Yukon, Canada, which hosts an 811 Ma ash horizon and spans the temporal window that captures the inferred origin and early evolution of animals. Iron-based redox proxies, redox-sensitive trace elements, organic carbon percentages and pyrite sulfur isotopes were analyzed in seven stratigraphic sections along two parallel basin transects. These data suggest that for this basin, oxygenated shelf waters overlay generally anoxic deeper waters. The anoxic water column was dominantly ferruginous, but brief periods of euxinia likely occurred. These oscillations coincide with changes in total organic carbon, suggesting euxinia was primarily driven by increased organic carbon loading. Overall, these data are consistent with proposed quantitative constraints on Proterozoic atmospheric oxygen being greater than 1% of modern levels, but less than present levels. Comparing these oxygen levels against the likely oxygen requirements of the earliest animals, both theoretical considerations and the ecology of modern oxygen-deficient settings suggest that the inferred oxygen levels in the mixed layer would not have been prohibitive to the presence of sponges, eumetazoans or bilaterians. Thus the evolution of the earliest animals was probably not limited by the low absolute oxygen levels that may have characterized Neoproterozoic oceans, although these inferred levels would constrain animals to very small sizes and low metabolic rates., Earth and Planetary Sciences, Organismic and Evolutionary Biology

Geological mapping and stratigraphic anaylsis of the early Neoproterozoic Fifteenmile Group in the western Ogilvie Mountains of Yukon, Canada, has revealed large lateral facies changes in both carbonate and siliciclastic strata. Syn-sedimentary NNW-side-down normal faulting during deposition of the lower Fifteenmile Group generated local topographic relief and wedge-shaped stratal geometries. These strata were eventually capped by platformal carbonate after the establishment of a NNW-facing stromatolitic reef complex that formed adjacent to the coeval Little Dal Group of the Mackenzie Mountains, Northwest Territories. Correlations between specific formations within these groups are tested with carbon isotope chemostratigraphy. As there are no known 830-780 Ma stratigraphic successions south of 62°N, the basin-forming event that accommodated the Fifteenmile and Little Dal Groups of the Ogilvie and Mackenzie Mountains and equivalent strata in the Shaler Supergroup of Victoria Island was restricted to the northwest margin of Laurentia. Therefore, this event does not represent widespread rifting of the entire western margin of Laurentia and instead we propose that these strata were accommodated in a failed rift generated by localized subsidence associated with the emplacement of the coeval Guibei (China) and Gairdner (Australia) large igneous provinces. The northern margin of Laurentia was reactivated by renewed extension at ca. 720 Ma associated with the emplacement of the Franklin large igneous province. Significant crustal thinning and generation of a thermally subsiding passive margin on the western margin of Laurentia may not have occurred until the late Ediacaran., Earth and Planetary Sciences

The pairing of calcium and magnesium isotopes (δ44/40Ca, δ26Mg) has recently emerged as a useful tracer to understand the environmental information preserved in shallow-marine carbonates. Here, we applied a Ca and Mg isotopic framework, along with analyses of carbon and lithium isotopes, to late Tonian dolostones, to infer seawater chemistry across this critical interval of Earth history. We investigated the ca. 735 Ma Coppercap Formation in northwestern Canada, a unit that preserves large shifts in carbonate δ13C values that have been utilized in global correlations and have canonically been explained through large shifts in organic carbon burial. Under the backdrop of these δ13C shifts, we observed positive excursions in δ44/40Ca and δ7Li values that are mirrored by a negative excursion in δ26Mg values. We argue that this covariation is due to early diagenetic dolomitization of aragonite through interaction with contemporaneous seawater under a continuum of fluid- to sediment-buffered conditions. We then used this framework to show that Tonian seawater was likely characterized by a low δ7Li value of ~13‰ (~18‰ lower than modern seawater), as a consequence of a different Li cycle than today. In contrast, δ13C values across our identified fluid-buffered interval are similar to modern seawater. These observations suggest that factors other than shifts in global seawater chemistry are likely responsible for such isotopic variation. [ABSTRACT FROM AUTHOR]

A central tenet of the Neoproterozoic snowball Earth hypothesis is that glaciations ended synchronously. Although this condition is borne out by U-Pb and Re-Os geochronology, the time scale of deglaciation is much less than the intrinsic error of the highest resolution dating techniques, and consequently calibrating the pace and synchronicity of biogeochemical recovery from Cryogenian glaciations remains a challenge. Given the importance of obtaining a globally synoptic view of paleoenvironmental conditions and biological evolution during these extraordinary transitions, robust correlations and chronologies are imperative. Here we extend the negative triple oxygen isotope (Δ17O) anomaly previously documented in ca. 635 Ma postglacial cap carbonates to two new paleocontinents, Brazil and Norway. The global footprint of this geochemical signal coupled to its short duration provides a unique time datum that can be used to cross-correlate Marinoan postglacial cap carbonate sequences and track the geochemical evolution of the oceans during deglaciation. [ABSTRACT FROM AUTHOR]