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Enhanced continental phosphorus input into the ocean has been suggested as a potential trigger for the transient oxygenation events during the mid‐Proterozoic; however, the response of phosphorus cycling to these marine oxygenations remains unclear. Here, we report the changes in phosphorus cycling associated with a ∼1.7 Ga transient oxygenation. Abundant authigenic aluminum phosphate–sulfate mineral svanbergite (SrAl3(PO4) (SO4) (OH)6; 8.02 ± 4.92 wt%) is identified within the ∼1.7 Ga Yunmengshan ironstones from the Xiong'er Basin, North China and other contemporaneous basins. This observation provides new evidence to support the suggestion that early diagenetic aluminum phosphate‐sulfate minerals could have represented a critical sink of marine phosphorus during the Proterozoic. We suggest that atmospheric oxygenation and concomitant changes in porewater redox chemistry may have enhanced the formation of early diagenetic phosphates, leading to a negative feedback on the oceanic phosphorus reservoir and atmospheric oxygen levels. Plain Language Summary: It becomes increasingly clear that multiple transient oxygenation events likely punctuated the low background oxygen world during the mid‐Proterozoic. This may imply that a negative feedback could have inhibited a secular rise in atmospheric oxygen, though the deoxygenation mechanisms remain unclear. Phosphorus (P) availability regulated primary production and therefore controlled the atmospheric oxygen levels during this time, and oxygenation would in turn affect phosphorus cycling. Here we investigate the P cycling during a ca. 1.7 Ga transient oxygenation using mineralogical and geochemical methods. The results show an enhanced phosphorus burial during this transient oxygenation. The elevated phosphorus and sulfate inputs from the enhanced continental weathering and oxygenation may have promoted the formation of aluminum phosphate‐sulfate minerals. This, in turn, reduced the bio‐availability of phosphorus in the marine environment, ultimately limiting marine productivity and leading to a negative feedback on the oxygenation event. This study highlights that phosphorus cycling pathway, which was previously overlooked, may have played a role in the deoxygenation during the intermittent oxygenation events in the mid‐Proterozoic. Key Points: Abundant phosphorus and sulfur precipitation as authigenic svanbergite is associated with the ∼1.7 Ga transient oxygenationThe oxygenation resulted in suboxic and acidic porewater conditions rich in P‐ and sulfate, facilitating the authigenesis of svanbergiteThe enhanced P and S burial as svanbergite represents a significant but overlooked negative feedback during the transient oxygenation [ABSTRACT FROM AUTHOR]

The mid‐Proterozoic (∼1.8–0.8 Ga) ocean‐atmosphere system is hypothesized to have experienced fluctuations in redox conditions with transient oxygenation events. One of these happened at ∼1.4 Ga, and it is speculated that this event may link to the emplacement of large igneous province (LIP) at this time. However, direct evidence for this relationship remains to be proved. Here, we report Hg/TOC, P, and trace element concentrations across the ∼1.4 Ga oxygenation event in the Xiamaling Formation of North China. A prominent increase in Hg/TOC is slightly earlier than that of nutrient contents (especially P), pyrite and TOC abundances, suggesting that this distinct oxygenation event was likely the result of LIP activity at ∼1.4 Ga, which increased nutrient and sulfate supply from continental weathering to the ocean, sustaining elevated primary productivity, organic carbon and pyrite burial. This study indicates that LIP weathering could trigger transient oxygenation events during the mid‐Proterozoic. Plain Language Summary: The mid‐Proterozoic (∼1.8–0.8 Ga) retains persistent low oxygen concentrations in atmosphere and shallow seawater but is occasionally punctuated by transient oxygenation events. Multiple lines of evidence from sedimentary geochemistry have argued a global oxygenation event at ∼1.4 Ga, and speculated that this event was closely related to large igneous province (LIP) activity at this time. Direct link between LIP activity and the oxygenation event, however, has rarely been addressed yet. As mercury in sedimentary rocks has been widely used to trace volcanism, we therefore report Hg/TOC, P, and trace element concentrations across the ∼1.4 Ga oxygenation event. The result shows that a prominent increase in Hg/TOC is clearly recognized in the shales that stratigraphically just underly the layers recording the oxygenation event, suggesting a close causal link between LIP emplacement and the oxygenation event. Accompanied with this Hg/TOC ratio peak, increased nutrient contents (especially P), pyrite and TOC abundances are also observed. Based on these observations, we proposed a model that the Xiamaling oxygenation event was the result of the LIP's emplacement at ∼1.4–1.35 Ga, which increased nutrient and sulfate supply from continental weathering to the ocean, sustaining elevated primary productivity, organic carbon and pyrite burial. Key Points: High Hg/TOC ratios associated with large igneous province (LIP) emplacement were detected in the ∼1.4 Ga Xiamaling Formation shales of North ChinaHigh P content accompanied with this interval indicates enhanced phosphorous input from LIP weathering to the oceanA causal link between the LIP weathering and oxygenation event at ∼1.4 Ga was suggested [ABSTRACT FROM AUTHOR]

Abstract Enhanced continental phosphorus input into the ocean has been suggested as a potential trigger for the transient oxygenation events during the mid‐Proterozoic; however, the response of phosphorus cycling to these marine oxygenations remains unclear. Here, we report the changes in phosphorus cycling associated with a ∼1.7 Ga transient oxygenation. Abundant authigenic aluminum phosphate–sulfate mineral svanbergite (SrAl3(PO4) (SO4) (OH)6; 8.02 ± 4.92 wt%) is identified within the ∼1.7 Ga Yunmengshan ironstones from the Xiong'er Basin, North China and other contemporaneous basins. This observation provides new evidence to support the suggestion that early diagenetic aluminum phosphate‐sulfate minerals could have represented a critical sink of marine phosphorus during the Proterozoic. We suggest that atmospheric oxygenation and concomitant changes in porewater redox chemistry may have enhanced the formation of early diagenetic phosphates, leading to a negative feedback on the oceanic phosphorus reservoir and atmospheric oxygen levels.

Abstract The mid‐Proterozoic (∼1.8–0.8 Ga) ocean‐atmosphere system is hypothesized to have experienced fluctuations in redox conditions with transient oxygenation events. One of these happened at ∼1.4 Ga, and it is speculated that this event may link to the emplacement of large igneous province (LIP) at this time. However, direct evidence for this relationship remains to be proved. Here, we report Hg/TOC, P, and trace element concentrations across the ∼1.4 Ga oxygenation event in the Xiamaling Formation of North China. A prominent increase in Hg/TOC is slightly earlier than that of nutrient contents (especially P), pyrite and TOC abundances, suggesting that this distinct oxygenation event was likely the result of LIP activity at ∼1.4 Ga, which increased nutrient and sulfate supply from continental weathering to the ocean, sustaining elevated primary productivity, organic carbon and pyrite burial. This study indicates that LIP weathering could trigger transient oxygenation events during the mid‐Proterozoic.

We have reappraised the age and composition of the mid-Proterozoic Häme dyke swarm in southern Finland. The dominant trend of the dykes of this swarm is NW to WNW. Petrographic observations and geochemical data indicate uniform, tholeiitic lowMg parental magmas for all of the dykes. Nevertheless, the variability in incompatible trace element ratios, such as Zr/Y and La/Nb, provides evidence of changing mantle melting conditions and variable crustal contamination. Our ID-TIMS 207Pb/206Pb ages for four low-Zr/Y-type dykes indicate emplacement at 1639 ± 3 Ma, whereas the most reliable previously published ages suggest emplacement of the high-Zr/Y-type dykes at 1642 ± 2 Ma. We propose that the Häme dyke swarm, and possibly also the other midProterozoic mafic dyke swarms in southern Finland, records a progressive decrease in Zr/Y values due to magma generation under developing areas of thinned lithosphere. We consider that the formation of mafic magmas was most probably associated with the upwelling of hot convective mantle in an extensional setting possibly related to the nearby Gothian orogeny. The generation of tholeiitic magmas below continental lithosphere was probably promoted by the elevated mantle temperature underneath the Nuna supercontinent. We speculate that the origin of most of the relatively small mid-Proterozoic mafic dyke swarms, anorthosites, rapakivi granites, and associated rocks found across Nuna was similarly triggered by extensional plate tectonics and the convection of anomalous hot upper mantle below the supercontinent.

Understanding how new felsic crust is formed and subsequently evolves through time is critical to identifying the geodynamic regimes that have dominated various parts of Earth history, and have important implications for feedbacks between the lithosphere and biosphere, such as controlling the influx of continental detritus into the oceans. In recent years, several trace element-based geochemical proxies have been proposed to allow determination of paleo-crustal thicknesses, which have been calibrated primarily using data collected from modern-day arcs. The application of these proxies through deep time has revealed surprising results, including the suggestion that the mid-Proterozoic continents were atypically thin compared to those in the Archean and the Phanerozoic; however, a range of factors may influence commonly cited trace element ratios (e.g. Sr/Y) rather than just crustal depth, leading to additional and unexpected magnitudes of uncertainty. Here we perform geochemical modelling to deduce the effect of variable bulk-rock composition and geothermal gradient on the trace element signature of felsic melts generated in arc systems. Using a range of protoliths representative of deep arc crust, the results show that considerable care must be taken when analysing simple trace element ratios of granitoid melts and making direct interpretations of the pressure of crystallisation. In particular, changes in geothermal gradients and differences in arc basalt composition impart strong controls on the relative stability of garnet and plagioclase during metamorphism and partial melting, and wide ranges of Sr/Y and La/Yb may be produced in derivative felsic melts produced at the same crustal depth. The interpretation of mid-Proterozoic continental arcs being atypically thin may instead be an artefact of underestimation of the active geothermal gradient at the time of magma formation, which acts to reduce Sr/Y and La/Yb ratios, even in normal thickness (∼35–40 km) crust. Furthermore, we argue that the potentially garnet-free residua during the formation of mid-Proterozoic felsic magmas points to crust formation without lower crustal foundering, and thus, that this commonly invoked paradigm for formation of the continental crust may only be applicable to certain periods of Earth history. • Chemical mohometers Sr/Y and La/Yb are investigated through petrological modelling. • Stability of garnet and plagioclase strongly dependant on protolith during anatexis. • Chemical mohometers should be used with caution, particularly in deep-time. • Mid-Proterozoic may have had normal crustal thickness of 30–40 km. • Foundering of lower crust during arc magmatism may be a temporally restricted process. [ABSTRACT FROM AUTHOR]

We have reappraised the age and composition of the mid-Proterozoic Häme dyke swarm in southern Finland. The dominant trend of the dykes of this swarm is NW to WNW. Petrographic observations and geochemical data indicate uniform, tholeiitic low-Mg parental magmas for all of the dykes. Nevertheless, the variability in incompatible trace element ratios, such as Zr/Y and La/Nb, provides evidence of changing mantle melting conditions and variable crustal contamination. Our ID-TIMS 207Pb/206Pb ages for four low-Zr/Y-type dykes indicate emplacement at 1639 ± 3 Ma, whereas the most reliable previously published ages suggest emplacement of the high-Zr/Y-type dykes at 1642 ± 2 Ma. We propose that the Häme dyke swarm, and possibly also the other mid- Proterozoic mafic dyke swarms in southern Finland, records a progressive decrease in Zr/Y values due to magma generation under developing areas of thinned lithosphere. We consider that the formation of mafic magmas was most probably associated with the upwelling of hot convective mantle in an extensional setting possibly related to the nearby Gothian orogeny. The generation of tholeiitic magmas below continental lithosphere was probably promoted by the elevated mantle temperature underneath the Nuna supercontinent. We speculate that the origin of most of the relatively small mid-Proterozoic mafic dyke swarms, anorthosites, rapakivi granites, and associated rocks found across Nuna was similarly triggered by extensional plate tectonics and the convection of anomalous hot upper mantle below the supercontinent. [ABSTRACT FROM AUTHOR]

Reconstructing the history of biological productivity and atmospheric oxygen partial pressure (pO2) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating pO2 and productivity during the Proterozoic. O-MIF, reported as Δ'17O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS2) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ~1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air-sea gas exchange. Previous analyses of these data concluded that pO2 at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on pO2 is essentially unconstrained by these data. Indeed, pO2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that pCO2 could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O2) between the atmosphere and surface ocean is controlled more by air-sea gas transfer rates than by biological productivity. Improved estimates of pCO2 and/or improved proxies for Δ'17O of atmospheric O2 would allow tighter constraints to be placed on mid-Proterozoic pO2. [ABSTRACT FROM AUTHOR]

Abstract Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments, and have the potential to reflect seawater chemistry indirectly. In fine-siliciclastic settings, they preferentially form in organic-rich mudstones, owing to a significant fraction of the bicarbonate required for carbonate precipitation resulted from the decomposition of organic matter in sediments. In the Member IV of the Xiamaling Formation (ca. 1.40–1.35 Ga), North China, however, carbonate concretions occur in organic-poor green silty shales (avg. TOC = ~ 0.1 wt%). In order to elucidate the mechanism of the concretion formation and their environmental implications, a thorough study on the petrographic and geochemical compositions of the concretions and their host rocks was conducted. Macro- to microscopic fabrics, including deformed shale laminae surrounding the concretions, “cardhouse” structures of clay minerals and calcite geodes in the concretions, indicate that these concretions are of early diagenetic origin prior to the significant compaction of clay minerals. The carbon isotope compositions of the concretions (− 1.7‰ to + 1.5‰) are stable and close to or slightly lower than that of the contemporaneous seawater, indicating that the bicarbonates required for the concretion formation were mainly sourced from seawater by diffusion rather than produced by methanogenesis or anoxic oxidation of methane (AOM); the rare occurrence of authigenic pyrite grains in the concretions likely indicates that bacterial sulfate reduction (BSR) did not play a significant role in their formation either. Almost all the calcite in the concretions has low Mn–Fe in nuclei but high Mn–Fe in rims with average Mn/Fe ratio close to 3.3. The calcite shows positive Ce anomalies (avg. 1.43) and low Y/Ho ratios (avg. 31). This evidence suggests that Mn reduction is the dominant process responsible for the formation of calcite rims while nitrate reduction probably triggered the precipitation of calcite nuclei. Prominence of Mn reduction in the porewater likely indicates that there was sufficient oxygen to support active Mn-redox cycling in the overlying seawater.

Abstract The mid‐Proterozoic (ca. 1.8–0.8 Ga) witnessed a period of stable carbon cycling and long stasis in eukaryotic evolution, which was commonly ascribed to the persistently low oxygen levels in the atmosphere‐ocean system. Recently, several pulsed marine oxygenations were identified from different continents, and presumed to have facilitated the short‐term diversification of eukaryotes in the early Mesoproterozoic Era. To test this hypothesis, an integrated study of iodine species [I/ (Ca + Mg)], paired carbon isotopes (δ13Ccarb and δ13Corg), and elemental abundances was conducted on the fossil‐bearing Tuanshanzi Formation (∼1.64–1.63 Ga), North China. The near‐zero I/(Ca + Mg) ratios (0.00 ± 0.01 μmol/mol) coupled with low δ13Corg (−30.2 ± 1.4‰) in the strata below the fossil‐bearing interval suggests anoxic conditions in shallow seawater. An increase of I/ (Ca + Mg) up to ∼1.54 μmol/mol coupled with a ∼2‰ negative shift in δ13Ccarb and a ∼11‰ positive excursion in δ13Corg in the fossil‐bearing interval point to an increased oxygen concentration. Above the fossil‐bearing interval, I/ (Ca + Mg) decrease to

The mid‐Proterozoic (ca. 1.8–0.8 Ga) witnessed a period of stable carbon cycling and long stasis in eukaryotic evolution, which was commonly ascribed to the persistently low oxygen levels in the atmosphere‐ocean system. Recently, several pulsed marine oxygenations were identified from different continents, and presumed to have facilitated the short‐term diversification of eukaryotes in the early Mesoproterozoic Era. To test this hypothesis, an integrated study of iodine species [I/ (Ca + Mg)], paired carbon isotopes (δ13Ccarb and δ13Corg), and elemental abundances was conducted on the fossil‐bearing Tuanshanzi Formation (∼1.64–1.63 Ga), North China. The near‐zero I/(Ca + Mg) ratios (0.00 ± 0.01 μmol/mol) coupled with low δ13Corg (−30.2 ± 1.4‰) in the strata below the fossil‐bearing interval suggests anoxic conditions in shallow seawater. An increase of I/ (Ca + Mg) up to ∼1.54 μmol/mol coupled with a ∼2‰ negative shift in δ13Ccarb and a ∼11‰ positive excursion in δ13Corg in the fossil‐bearing interval point to an increased oxygen concentration. Above the fossil‐bearing interval, I/ (Ca + Mg) decrease to <0.5 μmol/mol, suggesting deoxygenation and a return to anoxic to suboxic conditions. Combined with relevant data from other mid‐Proterozoic sections of North China and Australia, these data likely indicate dynamic redox conditions in the shallow seawaters of this period, with multiple pulses in oxygenation in otherwise anoxic conditions. These pulsed oxygenations may have facilitated short‐term proliferation and diversification of early eukaryotes in shallow seawaters, but the overall low oxygen levels have impeded their continuous development and the rise to ecological dominance until late Neoproterozoic. Plain Language Summary: The mid‐Proterozoic (ca. 1.8–0.8 billion years ago) is a period characterized by a long stasis in eukaryotic evolution and pervasive ocean anoxia. However, some of the oldest possible examples of multicellular eukaryotic fossils have been interpreted from the strata of ca. 1.65–1.56 billion years ago. To reveal the potential links between eukaryotes and environments, an integrated study of iodine species, carbon isotopes, total organic carbon and element abundances was performed on the ca. 1.65–1.63 billion years old carbonates from North China. The results from iodine contents in marine carbonates indicate dynamic seawater redox conditions, with evidence for oxygenation in the fossil‐bearing interval, and anoxic conditions in the overlying and underlying strata. This was further supported by the prominent shifts in carbon isotopes (both δ13Ccarb and δ13Corg), implying an increased oxygenation in shallow seawater. The co‐occurrence of these features likely suggested that pulsed oxygenation facilitated a short interval of eukaryotic evolution during the terminal Paleoproterozoic, however, the overall low oxygen levels during the mid‐Proterozoic likely impeded their subsequent expansion and diversification. Key Points: A pulsed increase of oxygen in shallow seawater is recorded by elevated I/(Ca + Mg) in the ∼1.64 Ga carbonates of North ChinaThe I/(Ca + Mg) increase from 0 to 1.54 μmol/mol was accompanied by a ∼2‰ negative shift in δ13Ccarb and a ∼11‰ positive excursion in δ13CorgPulsed oxygenation may have facilitated a short evolution of early eukaryotes in shallow seawaters at terminal Paleoproterozoic [ABSTRACT FROM AUTHOR]

The discovery of mid‐Proterozoic (1.8–0.8 billion years ago, Ga) indigenous biomarkers is a challenge, since biologically informative molecules of such antiquity are commonly destroyed by metamorphism or overprinted by drilling fluids and other anthropogenic petroleum products. Previously, the oldest clearly indigenous biomarkers were reported from the 1.64 Ga Barney Creek Formation in the northern Australian McArthur Basin. In this study, we present the discovery of biomarker molecules from carbonaceous shales of the 1.73 Ga Wollogorang Formation in the southern McArthur Basin, extending the biomarker record back in time by ~90 million years. The extracted hydrocarbons illustrate typical mid‐Proterozoic signatures with a large unresolved complex mixture, high methyl alkane/n‐alkane ratios and the absence of eukaryotic steranes. Acyclic isoprenoids, saturated carotenoid derivatives, bacterial hopanes and aromatic hopanoids and steroids also were below detection limits. However, continuous homologous series of low molecular weight C14–C19 2,3,4‐ and 2,3,6‐trimethyl aryl isoprenoids (AI) were identified, and C20–C22 AI homologues were tentatively identified. Based on elevated abundances relative to abiogenic isomers, we interpret the 2,3,6‐AI isomer series as biogenic molecules and the 2,3,4‐AI series as possibly biogenic. The biological sources for the 2,3,6‐AI series include carotenoids of cyanobacteria and/or green sulphur bacteria (Chlorobiaceae). The lower concentrated 2,3,4‐AI series may be derived from purple sulphur bacteria (Chromatiaceae). These degradation products of carotenoids are the oldest known clearly indigenous molecules of likely biogenic origin. [ABSTRACT FROM AUTHOR]

The origin (hydrothermal vs. continental Fe source) and environmental implications in particular for the atmospheric oxygen level of mid-Proterozoic ironstones have been widely studied, but remain highly debated. Here, we present Re-Os, Sr-Nd isotopic and PGE elemental data for ironstones from the ∼1.7 Ga Yunmengshan (YMS) Formation in south margin of the North China Craton to address the controversy. The initial 87Sr/86Sr ratios of the YMS ironstones are ∼0.7063, which are roughly comparable with seawater values around that period, suggesting a seawater signal with minor/minimal direct contributions from submarine hydrothermal fluids. The Nd isotopic compositions (ε Nd (t): −6.6 to −6.0) indicate a dominantly continental crustal signal for the formation of the ironstones. The PGE patterns of the YMS ironstones are distinctly similar to those of hydrogenous Fe-Mn crusts/nodules in modern ocean, also suggesting deposition from normal seawater conditions. Importantly, the YMS ironstones give initial 187Os/188Os ratios of ∼0.6 at ∼1.7 Ga, providing critical constraints for contemporaneous seawater values. This seawater 187Os/188Os value of ∼0.6 strongly suggests a result of oxidative weathering and transportation of Os in continental crust into the ocean, implying a relatively high atmospheric oxygen level at ∼1.7 Ga. • 87Sr/86Sr and PGE-Re patterns of the YMS ironstones suggest a normal seawater signal. • Nd isotopes suggest a primarily crustal source for the YMS ironstones. • YMS ironstones record elevated seawater 187Os/188Os values to ∼0.6 at ∼1.7 Ga. • Os isotopes in YMS ironstones imply a pulsed rise of atmospheric O 2 level at ∼1.7 Ga. • Ironstone is good candidate for Os isotope chemo-stratigraphic studies. [ABSTRACT FROM AUTHOR]

The mid-Proterozoic (ca. 1850–850 Ma) is a peculiar period of Earth history in many respects: ophiolites and passive margins of this age are rare, whereas anorthosite and A-type granite suites are abundant; metamorphic rocks typically record high thermobaric (temperature/pressure) ratios, whereas ultrahigh pressure (UHP) rocks are rare; and the abundance of economic mineral deposits features rare porphyry Cu-Au and abundant Ni-Cu and Fe-oxide Cu-Ag (IOCG) deposit types. These collective observations have been used to propose that a stagnant-lid, or single-lid, tectonic regime operated at this time, between periods of plate tectonics in the Paleoproterozoic and Neoproterozoic. In our reappraisal of the mid-Proterozoic geological record, we not only assess the viability of the single-lid hypothesis for each line of evidence, but also that of the plate tectonic alternative. We find that evidence for the single-lid hypothesis is equivocal in all cases, whereas for plate tectonics the evidence is equivocal or supporting. We therefore find no reason to abandon a plate tectonic model for the mid-Proterozoic time period. Instead, we propose that the peculiarities of this enigmatic interval can be reconciled through the combination of two processes working in tandem: secular mantle cooling and the exceptionally long tenure and incomplete breakup of Earth's first supercontinent, where both of these phenomena had a dramatic effect on lithospheric behaviour and its resulting imprint in the geological record. • Evidence for single-lid vs plate tectonic modes in the mid-Proterozoic are reviewed. • The evidence is equivocal or in favour of plate tectonics in all cases. • The mid-Proterozoic geological record can be explained by two processes. • These are the long tenure and incomplete break-up of the Columbia supercontinent. • In combination with differing plate geodynamics that result from a warmer mantle. [ABSTRACT FROM AUTHOR]

Highlights: [•] Suursaari conglomerate is a remnant of a previously unrecognized ultramature mid-Proterozoic sedimentary cover. [•] Detrital zircon age data and Lu–Hf isotope composition imply rapakivi-related rocks in the source area. [•] Svecofennian orogen had reached peneplane already at 1.65Ga, prior to the onset of the anorogenic rapakivi granite stage. [•] The ∼1.65Ga peneplane was very close to the pre-Vendian 0.65Ga peneplane over large areas in southern Finland. [•] The study supports purely anorogenic setting for the rapakivi granites and related bimodal magmatism in southern Finland. [Copyright &y& Elsevier]

Abstract: Rapakivi granites and the associated suite (anorthosite, mangerite and charnockite) that developed during the Mid-Proterozoic escape the global rules that control usual granitic magma formation. About 40 points that characterise the Proterozoic magmatism confirm the originality of the magmatism. Rapakivi granites developed without a global orogenic context within the supercontinent Columbia. The intrusions extend from the East European shield to the western US, through Fennoscandia, Greenland and Labrador. Other occurrences in the Amazonian shield, Australia or South China are less well documented. They show no definite trend in age or chemistry that would explain large-scale (mantle plume) effects. A mantle upwelling of material is contradictory with the re-assembly of a supercontinent because it occurred before this magmatic episode (1.9–1.8Ga). At ∼1.7Ga, the supercontinent Columbia amalgamated with the collision of the Yapavai and Mazatzal Provinces. A model is suggested that takes into account the supercontinent re-assembly, defining a downwelling flow in the mantle that anchors the continent above it. In contrast to a material flow, heat is still delivered to the base of the continental lithosphere, and is focused toward the juvenile suture zone. The base of the crust must reach 1200–1300°C before producing anorthosite magmas. Under such a high temperature gradient lasting over a long time, magmas are transferred toward the upper crust, giving the thin (5km) and square-shape (100km) that the intrusions presently have. Heat delivery is essentially conductive, leading to long time-spans for intrusions. The presence of the supercontinent, immobile over a descending cell (poloidal mode of convection) developed a tangential force (toroidal mode of convection) that partly split the continent through strike–slip deformation due to plate rotation. It developed progressively between 1.57 and 1.3Ga, starting from Fennoscandia, and then passing to Amazonia, western US and Labrador in a clockwise sense. The associated rotation induced sinistral shear manifested by small-scale shear zones and the orientation of late magmatic facies (topaz-bearing granites) in each province. The Proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons. The present Moho still shows remnants of this process, having bumpy undulations that may reach 22km in amplitude over a distance of 200km. [Copyright &y& Elsevier]

Reconstructing the history of biological productivity and atmospheric oxygen partial pressure ( p O 2 ) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating p O 2 and productivity during the Proterozoic. O-MIF, reported as Δ' 17 O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS 2 ) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air-sea gas exchange. Previous analyses of these data concluded that p O 2 at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on p O 2 is essentially unconstrained by these data. Indeed, p O 2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that p CO 2 could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O 2 ) between the atmosphere and surface ocean is controlled more by air-sea gas transfer rates than by biological productivity. Improved estimates of p CO 2 and/or improved proxies for Δ' 17 O of atmospheric O 2 would allow tighter constraints to be placed on mid-Proterozoic p O 2 ., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

• REEs in the Tieling carbonates imply an oxygenation in shallow waters at ~1.44 Ga. • Oxidative weathering transiently fractionated Cr isotopes in the Tieling carbonates. • Integrated evidence shows higher p O 2 at the Earth surface at ~1.4 Ga. The mid-Proterozoic (1.8–0.8 Ga) may have witnessed persistent and predominant anoxia at the Earth's surface. However, recent studies suggest that there was a period around ~1.4 Ga where oxygen levels might have been transiently elevated, both in the atmosphere and oceans. In order to shed light on this debated topic, we analyzed rare earth elements plus Y (REY) and Cr isotope profiles of the carbonate rocks from the ~1.44-Gyr-old Tieling Formation consisting of the lower Daizhuangzi (DZZ) and upper Laohuding (LHD) Members at the Jixian section, North China. The DZZ samples are characterized by middle REE enriched, hump-shaped REY distribution patterns, while those of the LHD Member display seawater-like REY patterns with slightly negative Ce anomalies (0.67 < Ce/Ce* < 0.87). Although differing greatly, the REY profiles both in the DZZ and LHD Members indicate the presence of Mn oxyhydroxides and thus an oxygenation at least locally in the shallow seawater of the North China Platform at ~1.44 Ga. The middle LHD Member records positive authigenic δ53Cr up to 0.48‰, indicating an enhanced continental oxidative weathering transiently occurred at ~1.44 Ga with p O 2 higher than 1% of the present atmospheric level, and extending the mid-Proterozoic sedimentary record of positive Cr isotope fractionation to as early as ~1.44 Ga. Combining the investigations in this study with other published geochemical evidence from the North China Platform and elsewhere, we suggest that there was a transient and global swing to higher oxygen levels in the atmosphere and oceans at ~1.4 Ga. [ABSTRACT FROM AUTHOR]