Great Oxidation Event

The deposition of major Precambrian phosphorites was restricted to times of global change and atmospheric oxygenation at both ends of the Proterozoic. Phosphorites formed after highly positive carbon isotope excursions in carbonates deposited during the Paleoproterozoic Lomagundi-Jatuli event and the Neoproterozoic Cryogenian and Ediacaran periods. The correlative step-wise rise in atmospheric oxygen over the Proterozoic has been linked to changes in the carbon cycle. However, the postulated relations between carbon isotope events, phosphorites, and atmospheric oxygenation remain unexplained. Paleoproterozoic carbonates of the Aravalli Supergroup, India, preserve evidence for cyanobacterial blooms in the form of tightly packed stromatolitic columns in the world's oldest significant sedimentary phosphate deposit. Restricted basins of the Lower Aravalli Group with stromatolitic phosphorites in Jhamarkotra, Udaipur, Jhabua, and Sallopat exhibit near-zero d 13 C carb values and large ranges of d 13 C org values between À 33.3% and À 10.1%, indicative of a complex carbon cycle. Because phosphate accumulates primarily in oxic sediments, these eutrophic microbial ecosystems likely developed within the photic zone of the shallow, oxygenated marine realm. This is consistent with deposition during the time of increasingly more oxidizing conditions, after the Great Oxidation Event (GOE). Approximately contemporaneous basins without phosphate deposits from Ghasiar, Karouli, Negadia, Umra, and Babarmal exhibit a range of positive d 13 C carb excursions, some with values up to þ 11.2%, that suggest high rates of organic carbon burial, and others with moderately high d 13 C carb values around þ 6% or þ 3%, that suggest smaller carbon cycle perturbations. The d 15 N values of all these rocks vary between À 0.7% and þ3.4%, and are consistent with the predominance of nitrogen fixation during cyanobacterial blooms in all basin types. Such low nitrogen isotope values are interpreted to have arisen from the biological response to high phosphate availability. We conclude that increased phosphate availability during and after the Paleoproterozoic Lomagundi-Jatuli event likely caused cyanobacterial blooms and was a key factor in the oxygenation of Earth's atmosphere. Increasing oxygenation of the shallow ocean seafloor favored the removal of excess phosphate as authigenic apatite, thus dampening effects of weathering increases on organic burial and marine d 13 C carb after about 2.0 Ga.

The rise in atmospheric oxygen between 2.45 and 2.2 Ga has been linked to the demise of a methane-rich atmosphere and the onset of multiple glaciations, culminating in a possible Snowball Earth. The glacial deposits represent possible global marker horizons, however, their correlation is uncertain because key sedimentary successions are undated, hampering paleoenvironmental reconstructions. Three potentially global glaciations have been proposed based on the correlation of glacial deposits in southern Africa with those in North America. However, it has also been suggested that there were four glaciations and that the youngest was a Snowball Earth event recorded only in South African successions. In situ U-Pb zircon ages for tuffs in southern African and North American successions establish the existence of four glaciations between 2.45 and 2.22 Ga. Geochronological and stratigraphic data demonstrate that the three oldest glaciations predate ∼2.31 Ga and that the final glaciation, which is only recognized in South Africa, occurred between 2.26 and 2.22 Ga. The new age-calibrated correlations show that a rise in atmospheric oxygen inferred from sulfur isotope data occurred between the second and third glaciations. At 2.31 Ga, the first marine sulfate evaporites were deposited contemporaneously with 13 C-enriched carbonates, indicating a direct link between perturbations of the carbon and sulfur cycles and rising atmospheric oxygen. The appearance of "red beds" and oxidized palesols after the third glaciation signals a major increase in atmospheric oxygen levels, which culminated with the fourth glaciation between 2.26 and 2.22 Ga, after which the atmosphere remained irreversibly oxygenated.

The rise in atmospheric oxygen between 2.45 and 2.2 Ga has been linked to the demise of a methane-rich atmosphere and the onset of multiple glaciations, culminating in a possible Snowball Earth. The glacial deposits represent possible global marker horizons, however, their correlation is uncertain because key sedimentary successions are undated, hampering paleoenvironmental reconstructions. Three potentially global glaciations have been proposed based on the correlation of glacial deposits in southern Africa with those in North America. However, it has also been suggested that there were four glaciations and that the youngest was a Snowball Earth event recorded only in South African successions. In situ U-Pb zircon ages for tuffs in southern African and North American successions establish the existence of four glaciations between 2.45 and 2.22 Ga. Geochronological and stratigraphic data demonstrate that the three oldest glaciations predate ∼2.31 Ga and that the final glaciation, which is only recognized in South Africa, occurred between 2.26 and 2.22 Ga. The new age-calibrated correlations show that a rise in atmospheric oxygen inferred from sulfur isotope data occurred between the second and third glaciations. At 2.31 Ga, the first marine sulfate evaporites were deposited contemporaneously with 13 C-enriched carbonates, indicating a direct link between perturbations of the carbon and sulfur cycles and rising atmospheric oxygen. The appearance of "red beds" and oxidized palesols after the third glaciation signals a major increase in atmospheric oxygen levels, which culminated with the fourth glaciation between 2.26 and 2.22 Ga, after which the atmosphere remained irreversibly oxygenated.

The marine nitrogen cycle is dominated by redox-controlled biogeochemical processes and, therefore, is likely to have been revolutionised in response to Earth-surface oxygenation. The details, timing, and trajectory of nitrogen cycle evolution, however, remain elusive. Here we couple nitrogen and carbon isotope records from multiple drillcores through the Rooihoogte-Timeball Hill Formations from across the Carletonville area of the Kaapvaal Craton where the Great Oxygenation Event (GOE) and its aftermath are recorded. Our data reveal that aerobic nitrogen cycling, featuring metabolisms involving nitrogen oxyanions, was well established prior to the GOE and that ammonium may have dominated the dissolved nitrogen inventory. Pronounced signals of diazotrophy imply a stepwise evolution, with a temporary intermediate stage where both ammonium and nitrate may have been scarce. We suggest that the emergence of the modern nitrogen cycle, with metabolic processes that approximate their conte...

Twenty-one samples of hematitite and twelve samples of itabirite were collected from different deposits of the Quadrilátero Ferrífero (QF) area and were analyzed for trace and rare earth elements. The purpose of the study is to understand the element distribution in the QF in comparison with other iron formations (IF) around the world. Trace element contents are relatively low with considerable variability, being lower than the contents in the Algoma IF, Anamikie IF, Maru IF in Nigeria and Orissa IF in India. REE's abundance is relatively low, but higher than REE's of Hamersley IF of Western Australia, Surgur belt in India and lower than Kuruman IF in South Africa. Chondrite normalized patterns show slight degrees of fractionation for LREE to HREE and slightly positive Eu anomalies coupled with positive values of (La/Yb) CN and (La/Sm) CN ratios.

The ca. 2.45-2.22 Ga Turee Creek Group, Western Australia, contains carbonaterich horizons that postdate earliest Proterozoic iron formations, bracket both Paleoproterozoic glaciogenic beds and the onset of the Great Oxidation Event (GOE), and predate ca. 2.2-2.05 Ga Lomagundi-Jatuli C-isotopic excursion(s). As such, Turee Creek carbonate strata provide an opportunity to characterize early Paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change. Here, we report on the stratigraphy, sedimentology, petrology, carbon isotope chemostratigraphy, and stromatolite development for carbonate-rich successions within the pre-glacial part of the Kungarra Formation and the postglacial Kazput Formation. Kungarra carbonate units largely occur as laterally discontinuous beds within a thick, predominantly siliciclastic shelf deposit. While this succession contains thin microbialite horizons, most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix. C-isotopic values show marked variation along a single horizon and even within hand samples, reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn-and post-depositional remineralization of organic matter. In contrast, the Kazput carbonates consist of subtidal stromatolites, grainstones, and micrites deposited on a mixed carbonate-siliciclastic shelf. These carbonates exhibit moderate δ 13 C values of-2‰ to +1.5‰ and likely preserve a C-isotopic signature of seawater. Kazput carbonates, thus, provide some of the best available evidence that an interval of unexceptional C-isotopic values separates the Lomagundi-Jatuli C-isotopic excursion(s) from the initiation of the GOE as inferred from multiple sulfur isotopes (loss 4 of mass independent fractionation). The Kazput Formation also contains unusual, m-scale stromatolitic buildups, which are composed of sub-mm laminae and discontinuous, convex upward lenticular precipitates up to a few mm in maximum thickness. Laminae, interpreted as microbial mat layers, contain quartz and clay minerals as well as calcite, whereas precipitate lenses consist of interlocking calcite anhedra, sometimes showing faint mm-scale banding. These cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats, or as local seafloor precipitates that formed on, or within, surface mats. It is possible that both mechanisms interacted to form the unique Kazput stromatolites. These microbialites speak to a distinctive interaction between life and environment early in the Paleoproterozoic Era.

The Paleoproterozoic Lomagundi–Jatuli Event (LJE) is generally considered the largest, in both amplitude and duration, positive carbonate C-isotope (δ13Ccarb) excursion in Earth history. Conventional thinking is that it represents a global perturbation of the carbon cycle between 2.3–2.1 Ga linked directly with, and in part causing, the postulated rise in atmospheric oxygen during the Great Oxidation Event. In addition to new high-resolution δ13Ccarb measurements from LJE-bearing successions of NW Russia, we compiled 14 943 δ13Ccarb values obtained from marine carbonate rocks 3.0–1.0 Ga in age and from selected Phanerozoic time intervals as a comparator of the LJE. Those data integrated with sedimentology show that, contra to consensus, the δ13Ccarb trend of the LJE is facies (i.e. palaeoenvironment) dependent. Throughout the LJE interval, the C-isotope composition of open and deeper marine settings maintained a mean δ13Ccarb value of +1.5 ± 2.4‰, comparable to those settings for mo...

The Matachewan Large Igneous Province (LIP) is interpreted to have formed during the early stages of mantle plume-induced continental break-up in the early Proterozoic. When the Matachewan LIP is reconstructed to its original configuration with units from the Superior Craton and other formerly adjacent blocks (Karelia, Kola, Wyoming and Hearne), the dyke swarms, layered intrusions and flood basalts, emplaced over the lifetime of the province, form one of the most extensive magmatic provinces recognized in the geological record. New geochemical data allow, for the first time, the Matachewan LIP to be considered as a single, coherent entity and show that Matachewan LIP rocks share a common tholeiitic composition and trace element geochemistry, characterized by enrichment in the most incompatible elements and depletion in the less incompatible elements. This signature, ubiquitous in early Proterozoic continental magmatic rocks, may indicate that the Matachewan LIP formed through contamination of the primary magmas with lithospheric material or that the early Proterozoic mantle had a fundamentally different composition from the modern mantle. In addition to the radiating geometry of the dyke swarms, a plume origin for the Matachewan LIP is consistent with the geochemistry of some of the suites; these suites are used to constrain a source mantle potential temperature of c. 1500-1550 C. Comparison of these mantle potential temperatures with estimated temperatures for the early Proterozoic upper mantle indicates that they are consistent with a hot mantle plume source for the magmatism. Geochemical data from coeval intrusions suggest that the plume head was compositionally heterogeneous and sampled material from both depleted and enriched mantle. As has been documented with less ancient but similarly vast LIPs, the emplacement of the Matachewan LIP probably had a significant impact on the early Proterozoic global environment. Compilation of the best age estimates for various suites shows that the emplacement of the Matachewan LIP occurred synchronously with the Great Oxidation Event. We explore the potential for the eruption of this LIP and the emission of its associated volcanic gases to have been a driver of the irreversible oxygenation of the Earth.

This is the first petrographic and chemical study on the origin of co-existing calcite, dolomite, ankerite and sideroplesite in the 2.4 Ga Itabira Group, Brazil. The Itabira Group hosts silicic and dolomitic BIFs, known as quartz itabirites and carbonate itabirites, which are composed of micritic quartz and dolomite with inclusions of nano-and micro-hematite. Nanometric Si-bearing ferrihydrite clusters are observed in the micritic dolomite. Mineralogical and textural features indicate a primary oxidising depositional environment for the Itabira Group at 2.4 Ga. Limestones of the Itabira Group include early diagenetic hematite nodules intergrown with dolomite. Silica in the ferrihydrite structure increases its stability up to 500 • C, and the nano-and micro-hematite included in dolomite and quartz were partially protected from later transformation by being enclosed within the dolomite and quartz crystals. Positive Eu and Y anomalies and heavy rare earth element enrichments of all lithofacies, along with their low terrestrial element abundance (excluding the shales), indicate their primary precipitation from a mixed hydrothermal-marine environment. In the case of the carbonate itabirites, fluids were CO 2 rich. Dolomite precipitated abiotically due to higher temperatures (∼100 • C) and high Mg/Ca ratios. The facies changes from carbonate-rich to quartz-rich are attributed to transgression and regression episodes and/or a change in the upwelling hydrothermal fluid composition. Post depositional reductive dissolution affected the quartz itabirites facilitating the precipitation of ankerite, sideroplesite and magnetite/sulphides from a reducing diagenetic solution, rich in Mn, Mg and Fe, either as lenses or disseminated, in the quartz itabirites and shales. The sideroplesite-richest samples have the most negative ␦ 13 C values (quartz itabirite: −9.49‰; shale: −5.76‰), indicating that at least part of the C is of organic origin, either through the oxidation of organic matter or introduced via the diagenetic fluids. Specularite is replacive and likely related to a post-depositional hydrothermal event. This study shows that careful mineralogical investigations combined with REE + Y and trace element geochemistry is necessary in order to decipher the depositional environments, which are oxidising, mixed marine-hydrothermal. Post-depositional reductive diagenesis masks in part these primary conditions.

Iron-rich rocks of Orosirian Period in the Chilpi Group on the northern margin of the Bastar Craton, Central India, contain an association of hematite-magnetite-greenalite-chamosite-quartz in oxide-silicate facies. Additionally chert (quartz) and siderite occur in chert and carbonate facies. Presence of these mineral assemblages was investigated to infer the redox state of the depositional basin. The results have indicated formation temperature variation of 116-255 °C (average 198 °C) and log P (O2) between-37 and-60 (average-44). A ferruginous state of the shallow water depositional environment, having oxygen content of 10-2 to 10-5 times the present atmospheric level, is inferred. The variations in composition of greenalite-chamosite association indicate development of the mineral phases from the reaction involving kaolinite-illite and magnetite-siderite as end-members. Thermodynamic requirements for the formation of the rare association of magnetite-greenalite-cronstedtite indicate the precipitation of the mineral phases from seawater with enhanced activities of Fe 2+ , Al, Si, Mg and C compared to the level in the present day seawater. The results indicate a steep fall in the atmospheric oxygen content immediately after the Great Oxidation Event of 2400-2000 Ma.

The Paleoproterozoic Lomagundi-Jatuli Event (LJE) is generally considered the largest, in both amplitude and duration, positive carbonate C-isotope (δ 13 C carb) excursion in Earth history. Conventional thinking is that it represents a global perturbation of the carbon cycle between 2.3-2.1 Ga linked directly with, and in part causing, the postulated rise in atmospheric oxygen during the Great Oxidation Event. In addition to new high-resolution δ 13 C carb measurements from LJE-bearing successions of NW Russia, we compiled 14 943 δ 13 C carb values obtained from marine carbonate rocks 3.0-1.0 Ga in age and from selected Phanerozoic time intervals as a comparator of the LJE. Those data integrated with sedimentology show that, contra to consensus, the δ 13 C carb trend of the LJE is facies (i.e. palaeoenvironment) dependent. Throughout the LJE interval, the C-isotope composition of open and deeper marine settings maintained a mean δ 13 C carb value of +1.5 ± 2.4‰, comparable to those settings for most of Earth history. In contrast, the 13 C-rich values that are the hallmark of the LJE are limited largely to nearshore-marine and coastal-evaporitic settings with mean δ 13 C carb values of +6.2 ± 2.0‰ and +8.1 ± 3.8‰, respectively. Our findings confirm that changes in δ 13 C carb are linked directly to facies changes and archive contemporaneous dissolved inorganic carbon pools having variable C-isotopic compositions in laterally adjacent depositional settings. The implications are that the LJE cannot be construed a priori as representative of the global carbon cycle or a planetary-scale disturbance to that cycle, nor as direct evidence for oxygenation of the ocean-atmosphere system. This requires rethinking models relying on those concepts and framing new ideas in the search for understanding the genesis of the grandest of all positive C-isotope excursions, its timing and its hypothesized linkage to oxygenation of the atmosphere. Supplementary material: CO isotope data, figure S1, tables S1-S5 and the dataset for δ 13 C carb values are available at

The Paleoproterozoic Lomagundi–Jatuli Event (LJE) is generally considered the largest, in both amplitude and duration, positive carbonate C-isotope (δ13Ccarb) excursion in Earth history. Conventional thinking is that it represents a global perturbation of the carbon cycle between 2.3–2.1 Ga linked directly with, and in part causing, the postulated rise in atmospheric oxygen during the Great Oxidation Event. In addition to new high-resolution δ13Ccarb measurements from LJE-bearing successions of NW Russia, we compiled 14 943 δ13Ccarb values obtained from marine carbonate rocks 3.0–1.0 Ga in age and from selected Phanerozoic time intervals as a comparator of the LJE. Those data integrated with sedimentology show that, contra to consensus, the δ13Ccarb trend of the LJE is facies (i.e. palaeoenvironment) dependent. Throughout the LJE interval, the C-isotope composition of open and deeper marine settings maintained a mean δ13Ccarb value of +1.5 ± 2.4‰, comparable to those settings for mo...

The oxidation state of volcanic gas emissions influences the composition of the exosphere and planetary habitability. It is widely considered to be associated with the oxidation state of the melt from which volatiles exsolve. Here, we present a global synthesis of volcanic gas measurements. We define the mean oxidation state of volcanic gas emissions on Earth today and show that, globally, gas oxidation state, relative to rock buffers, is a strong function of emission temperature, increasing by several orders of magnitude as temperature decreases. The trend is independent of melt composition and geodynamic setting. This observation may explain why the mean oxidation state of volcanic gas emissions on Earth has apparently increased since the Archean, without a corresponding shift in melt oxidation state. We argue that progressive cooling of the mantle and the cessation of komatiite generation should have been accompanied by a substantial increase of the oxidation state of volcanic gases around the onset of the Great Oxidation Event. This may have accelerated or facilitated the transition to an oxygen-rich atmosphere. Overall, our data, along with previous work, show that there is no single nor simple relationships between mantle, magmas and volcanic gas redox states.

Glacial diamictites deposited in the Mesoarchean, Paleoproterozoic, Neoproterozoic, and Paleozoic eras record temporal variations in their average compositions that reflect the changing composition of the upper continental crust (UCC). Twenty six of the 27 units studied show elevated chemical index of alternation (CIA) and low Sr abundances, regardless of their age, documenting pervasive weathering of the average UCC. Lower abundances of transition metals reflect a shift towards more felsic crustal compositions after the Archean. Superimposed on this chemical difference is the signal of the rise of oxidative weathering of the continents, recorded by changes in the absolute and relative abundances of the redox sensitive elements Mo and V. Neoproterozoic and Paleozoic diamictites show pervasive depletion in Mo and V, reflecting their loss from the continents due to increasing intensity of oxidative weathering, as also recorded in some of the Paleoproterozoic diamictites. A few of the Paleoproterozoic diamictites deposited after the Great Oxidation Event show no depletion in Mo and V (e.g., Gowganda), but such signatures could be inherited from their provenance. In contrast, the pre-GOE Duitschland diamictite (ca. 2.3-2.5 Ga) from South Africa reveals evidence of intense oxidative weathering (i.e., large depletions in Mo), supporting a growing body of observations showing the presence of measurable atmospheric oxygen prior to permanent loss of the mass independent fractionation signal in sulfur isotopes.

This paper analyzes mineralogical, geochemical, and geochronological aspects, along with the effect of hydrothermal/metasomatic overprints, to identify the presence of primary phosphate as well as depositional and paleoenvironmental conditions in marble and calcsilicate sequences recrystallized under transitional amphibolite-granulite metamorphic conditions in the Tanque Novo-Ipirá Complex within the Salvador-Curaçá Orogen, northeastern São Francisco Craton, state of Bahia, Brazil. Petrographic studies have identified up to 10 vol.% disseminated apatite and whole-rock P 2 O 5 contents up to 3.2 wt.%. Post-depositional events affected the lithofacies to varying degrees. Late hydrothermalism did not modify the rare earth element and yttrium (REEY) patterns considerably. When normalized to Post-Archean Australian Shale (PAAS), these lithofacies are marked by flat pattern REEY, true negative Ce anomalies, and positive Y and Gd. The highly variable Eu anomalies were inherited from the source composition but may have been affected by interaction with fluids. U-Pb LA-ICP-MS (laser ablation multicollector inductively coupled plasma mass spectrometry) ages indicate a maximum depositional age of 2128 Ga, as well as Paleoproterozoic and Neoarchean sources. Samples with anomalous phosphorus show Y/Ho ratios >30 and Ce/Ce* anomalies between 0.53 and 1.0 with an average of 0.70, suggesting a sub-oxic environment for phosphate precipitation.

The area northeast of Sudbury, Ontario, is well known for hosting one of the largest unexplained geophysical anomalies in the Canadian Shield, the Temagami Anomaly. In search of a geological explanation for this anomaly, low-grade metamorphic ultrabasic dykes have been discovered in the overlying Huronian Supergroup sedimentary rocks, both in outcrop and in a deep drill core. Here, we report on the first geochemical and geochronological data obtained on these dykes and compare these data with known magmatic units in and around the 1850 Ma impact-generated Sudbury Igneous Complex (SIC). The NW-striking dykes, which cut across sedimentary rocks of the ca. 2.3 Ga Cobalt Group, and which are, in turn, crosscut by pseudotachylitic breccia, are characterized by distinctively high concentrations of Ti, P, Nb, and Zr, highly fractionated rare earth element patterns (La/YbN 7.6–15.5), and a lack of crustal contamination (Nb/Th > 10). Such features are typical of modern ocean island basalt...

This paper analyzes mineralogical, geochemical, and geochronological aspects, along with the effect of hydrothermal/metasomatic overprints, to identify the presence of primary phosphate as well as depositional and paleoenvironmental conditions in marble and calcsilicate sequences recrystallized under transitional amphibolite-granulite metamorphic conditions in the Tanque Novo-Ipirá Complex within the Salvador-Curaçá Orogen, northeastern São Francisco Craton, state of Bahia, Brazil. Petrographic studies have identified up to 10 vol.% disseminated apatite and whole-rock P 2 O 5 contents up to 3.2 wt.%. Post-depositional events affected the lithofacies to varying degrees. Late hydrothermalism did not modify the rare earth element and yttrium (REEY) patterns considerably. When normalized to Post-Archean Australian Shale (PAAS), these lithofacies are marked by flat pattern REEY, true negative Ce anomalies, and positive Y and Gd. The highly variable Eu anomalies were inherited from the source composition but may have been affected by interaction with fluids. U-Pb LA-ICP-MS (laser ablation multicollector inductively coupled plasma mass spectrometry) ages indicate a maximum depositional age of 2128 Ga, as well as Paleoproterozoic and Neoarchean sources. Samples with anomalous phosphorus show Y/Ho ratios >30 and Ce/Ce* anomalies between 0.53 and 1.0 with an average of 0.70, suggesting a sub-oxic environment for phosphate precipitation.

We report evidence for Milankovitch cycles in two drill cores from the Cambro-Ordovician Alum Shale Formation of Scandinavia. The signal is preserved in elemental abundances recorded at high stratigraphic resolution by core scanning XRF analysis (0.2 mm resolution). The new data enable us to establish a floating timeline calibrated to the stable 405 kyr eccentricity cycle for a ~8.7 Myr interval across the Miaolingian-Furongian boundary. This interval spans the Steptoean Positive Carbon Isotope Excursion (SPICE), which is recorded in the d 13 Corg in the studied drill cores. We calculate the durations of the Olenus Superzone to 3.4 ± 0.2 Myr, the Parabolina Superzone to 1.9 ± 0.3 Myr, the Leptoplastus Superzone to 0.33 ± 0.18 Myr, the Protopeltura Superzone to 0.51 ± 0.20 Myr, and the SPICE event straddling the Paibian and lower main part of the Jiangshanian Stage to 3.0 ± 0.2 Myr. The sedimentation rate shows similar trends at both drilling locations and is inversely correlated to eustatic sea level changes in certain time intervals, opening tantalizing new prospects of using cyclostratigraphic analyses of shales to track eustatic sea level variations. The identification of obliquity cycles enables us to calculate the Cambrian Earth-Moon distance as well as the day length at ~493 Ma to 368.9 ± 2.3 •10 6 m and 21.78 ± 0.29 hours, respectively.

The ca. 2.45-2.22 Ga Turee Creek Group, Western Australia, contains carbonaterich horizons that postdate earliest Proterozoic iron formations, bracket both Paleoproterozoic glaciogenic beds and the onset of the Great Oxidation Event (GOE), and predate ca. 2.2-2.05 Ga Lomagundi-Jatuli C-isotopic excursion(s). As such, Turee Creek carbonate strata provide an opportunity to characterize early Paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change. Here, we report on the stratigraphy, sedimentology, petrology, carbon isotope chemostratigraphy, and stromatolite development for carbonate-rich successions within the pre-glacial part of the Kungarra Formation and the postglacial Kazput Formation. Kungarra carbonate units largely occur as laterally discontinuous beds within a thick, predominantly siliciclastic shelf deposit. While this succession contains thin microbialite horizons, most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix. C-isotopic values show marked variation along a single horizon and even within hand samples, reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn-and post-depositional remineralization of organic matter. In contrast, the Kazput carbonates consist of subtidal stromatolites, grainstones, and micrites deposited on a mixed carbonate-siliciclastic shelf. These carbonates exhibit moderate δ 13 C values of-2‰ to +1.5‰ and likely preserve a C-isotopic signature of seawater. Kazput carbonates, thus, provide some of the best available evidence that an interval of unexceptional C-isotopic values separates the Lomagundi-Jatuli C-isotopic excursion(s) from the initiation of the GOE as inferred from multiple sulfur isotopes (loss 4 of mass independent fractionation). The Kazput Formation also contains unusual, m-scale stromatolitic buildups, which are composed of sub-mm laminae and discontinuous, convex upward lenticular precipitates up to a few mm in maximum thickness. Laminae, interpreted as microbial mat layers, contain quartz and clay minerals as well as calcite, whereas precipitate lenses consist of interlocking calcite anhedra, sometimes showing faint mm-scale banding. These cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats, or as local seafloor precipitates that formed on, or within, surface mats. It is possible that both mechanisms interacted to form the unique Kazput stromatolites. These microbialites speak to a distinctive interaction between life and environment early in the Paleoproterozoic Era.

Although no unambiguous biogenic criteria exist to discriminate Precambrian shallow-marine successions from fluvial deposits, physical sedimentological evidence, including an association of primary sedimentary structures and textures, stratigraphic position, and comparison with Phanerozoic and Modern examples has been found useful in identifying these deposit types. Our high resolution sedimentary facies analysis coupled with new mapping clearly indicates shallow-marine to beach-aeolian to fluvial sedimentation in the Paleoproterozoic Koolbye Formation of the Turee Creek Group, Western Australia. A falling stage systems tract within the Koolbye Formation has been documented. Our sedimentary facies analysis in combination with sedimentological analysis of the underlying Kungarra Formation indicates development of at least three falling stage systems tracts within the Turee Creek Group across the rise of atmospheric oxygen (the Great oxidation event).

Partial melting in these mantle sources may have been induced by The 2100 m thick Agnew Intrusion (50 km 2) in central Ontario, 'thermal' plumes. Canada, is a deformed, Palaeoproterozoic, layered leucogabbronoritic to gabbronoritic pluton that is believed to have intruded as a subvolcanic sill between Archaean granitic basement of the Superior Province and overlying Palaeoproterozoic flood basalts. Its emplacement was part of a major magmatic event in the region, which

Chemical signatures of iron oxides from dolomitic itabirite and high-grade iron ore from the Esperança deposit, located in the Quadrilátero Ferrífero, indicate that polycyclic processes involving changing of chemical and redox conditions are responsible for the iron enrichment on Cauê Formation from Minas Supergroup. Variations of Mn, Mg and Sr content in different generations of iron oxides from dolomitic itabirite, high-grade iron ore and syn-mineralization quartz-carbonate-hematite veins denote the close relationship between high-grade iron ore formation and carbonate alteration. This indicates that dolomitic itabirite is the main precursor of the iron ore in that deposit. Long-lasting percolation of hydrothermal fluids and shifts in the redox conditions have contributed to changes in the Y/Ho ratio, light/heavy rare earth elements ratio and Ce anomaly with successive iron oxide generations (martite-granular hematite), as well as lower abundance of trace elements including rare e...

21 samples of hematitite and 12 samples of itabirite were collected through different parts of Quadrilátero Ferrífero (QF) area and were analysed for minor elements with the purpose to understand the source of banded iron formation (BIF). Hematite is the main iron phase and magnetite appears as relict. Tourmaline appears sporadically in minor or trace amounts. Trace element concentrations are relatively low with considerable variability and present anomalous values. REE abundance is relatively low. Chondrite normalized pattern shows relatively high degree of fractionation of LREE to HREE and slight positive Eu anomaly similar to some patterns of BIFs around the world. These features suggest a hydrothermal source of iron. Metamorphic overprinting in the eastern domain suggest supergene enrichment model for QF iron formation origin indicated by anomalous values of minor elements. The source for iron might be near the eastern part of QF region indicated by high values of (La/ Yb)N ratio.

Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (<37.9 pg per gram of rock) and total sterane (<32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyc...

The Paleoproterozoic (2.5-2.0 Ga) is one of the most important periods in Earth's history, and was characterized by a rise in atmospheric oxygen levels and repeated (at least three) severe glaciations (the Huronian glaciations). In this study, we investigate redox conditions in the atmosphere and in shallowmarine environments immediately after the first Huronian glaciation based on the isotopic composition of Os, and the abundance of redox-sensitive elements (Os, Re, and Mo) in sedimentary rocks from the Huronian Supergroup, Canada. We found no significant authigenic enrichment of Os in the sedimentary rocks deposited during the first Huronian deglaciation. The initial isotopic composition of Os in the sediments was close to that of chondrite at the time of deposition (187 Os/ 188 Os = ∼0.11). These results suggest that atmospheric O 2 levels were insufficient to mobilize radiogenic Os through continental weathering (pO 2 < 10 −5-10 −3 present atmospheric level (PAL)). In contrast, we found enrichment of Re in the sedimentary rocks, which suggests the occurrence of oxidative weathering of Re under mildly oxidizing conditions (> 10 −8-10 −5 PAL). Despite the Re enrichment, low abundances of Mo imply possible non-sulfidic conditions in shallow-marine environments at the time of deposition. Together with the results of organic carbon and sulfur analyses, we suggest that atmospheric O 2 remained at relatively low levels of around 10 −8-10 −5 PAL after the first Huronian deglaciation, which contrasts with proposed dramatic increases in O 2 after the second and third Huronian deglaciations. These results imply that the second and third Huronian glaciations may have been global events, associated with climatic jumps from severe glaciations to super-greenhouse conditions and the subsequent blooming of photosynthetic cyanobacteria in the glacial aftermath.

The origin of the iron oxides in Archean and Paleoproterozoic Banded Iron Formations (BIFs) is still a debated question. We report low and high temperature magnetic properties, susceptibility and saturation magnetization results joined with scanning microscope observations within a 35 m section of the Late Archean Boolgeeda Iron Formation of the Hamersley Group, Western Australia. With the exception of two volcanoclastic intervals characterized by low susceptibility and magnetization, nearly pure magnetite is identified as the main magnetic carrier in all iron-rich layers including hematite-rich jasper beds. Two populations of magnetically distinct magnetites are reported from a 2 m-thick interval within the section. Each population shows a specific Verwey transition temperature: one around 120-124 K and the other in the range of 105-110 K. This temperature difference is interpreted to reflect two distinct stoichiometry and likely two episodes of crystallization. The 120-124K transition is attributed to nearly pure stoichiometric magnetite, SEM and microprobe observations suggest that the lower temperature transition is related to chemically impure silician magnetite. Microbial-induced partial substitution of iron by silicon is suggested here. This is supported by an increase in Total Organic Carbon (TOC) in the same interval.

Partial melting in these mantle sources may have been induced by The 2100 m thick Agnew Intrusion (50 km 2) in central Ontario, 'thermal' plumes. Canada, is a deformed, Palaeoproterozoic, layered leucogabbronoritic to gabbronoritic pluton that is believed to have intruded as a subvolcanic sill between Archaean granitic basement of the Superior Province and overlying Palaeoproterozoic flood basalts. Its emplacement was part of a major magmatic event in the region, which

The study of microbial carbonates has acquired new significance with the recognition that they retain valuable information related to biomineralization processes associated with microbial activity throughout geological time. Additionally, microbialites have a demonstrated economic potential to serve as excellent hydrocarbon reservoirs. The Lower Cretaceous Codó Formation, located in the Parnaiba Basin of northeast Brazil, This article is protected by copyright. All rights reserved. comprises a unique stratigraphic sequence of up to 20 m thick, well-preserved carbonate microbialites. Deposited in a continental basin during the initial break up and separation of South America from Africa in the Early Cretaceous, this lacustrine carbonate sequence provides an excellent example to investigate the palaeoenvironmental conditions controlling microbialite facies development. Based on macroscopic and microscopic observations of outcrop and drill core samples, four microbialite facies (stromatolite, lamina, massive and spherulite) were defined and distinguished by textures and microbial fossil content. Changes in facies type are related to alternating palaeo-water depths, as reflected by 87 Sr/ 86 Sr cycles resulting from fluctuations in the sources of meteoric water. Clumped isotope measurements of stromatolitic fabrics yield precipitation palaeo-temperatures with an average value of 35°C. The δ 18 O values of bulk carbonate (-6.8 to-1.5‰ Vienna Pee Dee Belemnite) imply precipitation from water with calculated δ 18 O values between-1.6 and 1.8‰ Vienna Standard Mean Ocean Water, reflecting precipitation from variably modified meteoric waters. The δ 13 C values of bulk carbonate (-15.5 to-7.2‰ Vienna Pee Dee Belemnite) indicate a significant input of carbon derived from aerobic or anaerobic respiration of organic matter. Combined, the data indicate that the evolution of the Codó Formation occurred in a closed lacustrine palaeoenvironment with alternating episodes of contracting and expanding lake levels, which led to the development of specific microbialite facies associations. The results provide new insights into palaeoenvironmental settings, biogenicity and early diagenetic processes involved in the formation of ancient carbonate microbialites and, by extension, improve the knowledge of the reservoir geology of correlative units in deep waters offshore Brazil.

The chemogenic sediments of the Chilpi Group (~2.0-1.8 Ga) of the Bastar Craton have been analysed for the sedimentary environment, geochemistry and redox evaluation of seawater towards the end of the Paleoproterozoic Era. Fabric-retentive fine-grained micritic and oolitic carbonates suggest preservation of primary texture and precipitation from contemporaneous seawater in a shallow water tidal environment. Reducing condition of shallow sea for the Chilpi Group is also recognized by the presence of chamosite associated with Mn-ankerite. The Post-Archean Australian Shale (PAAS) normalized positive Gd and La anomalies, Lu/La, Y/Ho molar ratios and the Y anomaly indicate the preservation of pristine marine nature of the carbonates. Evaluation of multielement concentration indicates no significant effects of detrital contamination, diagenesis, metamorphism and contributions from hydrothermal sources. Positive Ce anomalies observed in all the samples, with maximum Ce/ Ce* up to 2.83, and redox responsive trace element ratio patterns in association with enrichment of Mn and other redox responsive elements such as Zn, Fe, Mo and U indicate a reducing marine environment of deposition of the Chilpi carbonates. Display of positive europium anomalies (Eu/Eu* = 1.02-1.56) can be attributed to the reducing environment as contributions of hydrothermal fluids are ruled out for lack of LREE enrichment compared to HREE. We infer suboxic to anoxic conditions of shallow sea in the Chilpi basin towards the end of the Paleoproterozoic Era, with estimated atmospheric oxygen level of ~10-3 PAL. Reducing conditions in shallow sea indicate that the Great Oxidation Event did not affect seawater composition during late Paleoproterozoic.

The ca. 2.45-2.22 Ga Turee Creek Group, Western Australia, contains carbonaterich horizons that postdate earliest Proterozoic iron formations, bracket both Paleoproterozoic glaciogenic beds and the onset of the Great Oxidation Event (GOE), and predate ca. 2.2-2.05 Ga Lomagundi-Jatuli C-isotopic excursion(s). As such, Turee Creek carbonate strata provide an opportunity to characterize early Paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change. Here, we report on the stratigraphy, sedimentology, petrology, carbon isotope chemostratigraphy, and stromatolite development for carbonate-rich successions within the pre-glacial part of the Kungarra Formation and the postglacial Kazput Formation. Kungarra carbonate units largely occur as laterally discontinuous beds within a thick, predominantly siliciclastic shelf deposit. While this succession contains thin microbialite horizons, most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix. C-isotopic values show marked variation along a single horizon and even within hand samples, reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn-and post-depositional remineralization of organic matter. In contrast, the Kazput carbonates consist of subtidal stromatolites, grainstones, and micrites deposited on a mixed carbonate-siliciclastic shelf. These carbonates exhibit moderate δ 13 C values of-2‰ to +1.5‰ and likely preserve a C-isotopic signature of seawater. Kazput carbonates, thus, provide some of the best available evidence that an interval of unexceptional C-isotopic values separates the Lomagundi-Jatuli C-isotopic excursion(s) from the initiation of the GOE as inferred from multiple sulfur isotopes (loss 4 of mass independent fractionation). The Kazput Formation also contains unusual, m-scale stromatolitic buildups, which are composed of sub-mm laminae and discontinuous, convex upward lenticular precipitates up to a few mm in maximum thickness. Laminae, interpreted as microbial mat layers, contain quartz and clay minerals as well as calcite, whereas precipitate lenses consist of interlocking calcite anhedra, sometimes showing faint mm-scale banding. These cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats, or as local seafloor precipitates that formed on, or within, surface mats. It is possible that both mechanisms interacted to form the unique Kazput stromatolites. These microbialites speak to a distinctive interaction between life and environment early in the Paleoproterozoic Era.

Although no unambiguous biogenic criteria exist to discriminate Precambrian shallow-marine successions from fluvial deposits, physical sedimentological evidence, including an association of primary sedimentary structures and textures, stratigraphic position, and comparison with Phanerozoic and Modern examples has been found useful in identifying these deposit types. Our high resolution sedimentary facies analysis coupled with new mapping clearly indicates shallow-marine to beach-aeolian to fluvial sedimentation in the Paleoproterozoic Koolbye Formation of the Turee Creek Group, Western Australia. A falling stage systems tract within the Koolbye Formation has been documented. Our sedimentary facies analysis in combination with sedimentological analysis of the underlying Kungarra Formation indicates development of at least three falling stage systems tracts within the Turee Creek Group across the rise of atmospheric oxygen (the Great oxidation event).

Significance We present U-Pb ages for the extensive Ongeluk large igneous province, a large-scale magmatic event that took place near the equator in the Paleoproterozoic Transvaal basin of southern Africa at ca. 2,426 Ma. This magmatism also dates the oldest Paleoproterozoic global glaciation and the onset of significant atmospheric oxygenation. This result forces a significant reinterpretation of the iconic Transvaal basin stratigraphy and implies that the oxygenation involved several oscillations in oxygen levels across 10 −5 present atmospheric levels before the irreversible oxygenation of the atmosphere. Data also indicate that the Paleoproterozoic glaciations and oxygenation were ushered in by assembly of a large continental mass, extensive magmatism, and continental migration to near-equatorial latitudes, mirroring a similar chain of events in the Neoproterozoic.

The ca. 2.45-2.22 Ga Turee Creek Group, Western Australia, contains carbonaterich horizons that postdate earliest Proterozoic iron formations, bracket both Paleoproterozoic glaciogenic beds and the onset of the Great Oxidation Event (GOE), and predate ca. 2.2-2.05 Ga Lomagundi-Jatuli C-isotopic excursion(s). As such, Turee Creek carbonate strata provide an opportunity to characterize early Paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change. Here, we report on the stratigraphy, sedimentology, petrology, carbon isotope chemostratigraphy, and stromatolite development for carbonate-rich successions within the pre-glacial part of the Kungarra Formation and the postglacial Kazput Formation. Kungarra carbonate units largely occur as laterally discontinuous beds within a thick, predominantly siliciclastic shelf deposit. While this succession contains thin microbialite horizons, most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix. C-isotopic values show marked variation along a single horizon and even within hand samples, reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn-and post-depositional remineralization of organic matter. In contrast, the Kazput carbonates consist of subtidal stromatolites, grainstones, and micrites deposited on a mixed carbonate-siliciclastic shelf. These carbonates exhibit moderate δ 13 C values of-2‰ to +1.5‰ and likely preserve a C-isotopic signature of seawater. Kazput carbonates, thus, provide some of the best available evidence that an interval of unexceptional C-isotopic values separates the Lomagundi-Jatuli C-isotopic excursion(s) from the initiation of the GOE as inferred from multiple sulfur isotopes (loss 4 of mass independent fractionation). The Kazput Formation also contains unusual, m-scale stromatolitic buildups, which are composed of sub-mm laminae and discontinuous, convex upward lenticular precipitates up to a few mm in maximum thickness. Laminae, interpreted as microbial mat layers, contain quartz and clay minerals as well as calcite, whereas precipitate lenses consist of interlocking calcite anhedra, sometimes showing faint mm-scale banding. These cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats, or as local seafloor precipitates that formed on, or within, surface mats. It is possible that both mechanisms interacted to form the unique Kazput stromatolites. These microbialites speak to a distinctive interaction between life and environment early in the Paleoproterozoic Era.

Major, trace, rare earth elements (REE), and stable isotopic compositions in carbonate rocks of Pila Spi Formation were measured to constrain the source of rare earth elements and depositional environment. The low ΣREE content (average = 6.53 ppm, n = 14) indicates that the carbonates seawater-like REE patterns with (1) LREE depletion (average (Nd/Yb) n = 0.81), (2) HREE enrichment, (3) negative Ce anomaly (average = 0.91), and (4) superchondritic Y/Ho ratio (average = 30.07); slightly lower average value of Y/Ho ratio than the typical seawater value (~44-74) suggests modification of the seawater by input of fresh water. The (Nd/Yb) n and (Dy/Yb) n ratios of the Pila Spi carbonates have similarity with that of the Arabian sea carbonate. The low concentrations of U (0.35-1.0 ppm) suggest the deposition under oxygen-rich environment. This study indicates that the carbonates still preserve their original seawater-like REE pattern, provided that the contamination with terrigenous materials was small, and they serve as a seawater proxy. The Pila Spi carbonates have δ 13 C values ranged from − 6.17 to 0.76‰ PDB, and the δ 18 O values range between − 3.85 and 0.46‰ PDB. The positive correlation between δ 13 C and δ 18 O (r = 0.949, n = 14) values indicates the alteration of original isotopic compositions due to diagenesis. Finally, it is conceived that the Pila Spi carbonate was deposited in the lagoon environment with seawater invading and mixing of the continental (terrigenous) materials to the basin is viable.

Banded iron formations (BIFs) are iron-rich marine chemical sedimentary rocks, and their mineralogy and geochemistry can be used to gain insights into ancient ocean chemistry and biospheric evolution. Magnetite is the major iron-bearing mineral in many BIFs (particularly in the Archean) and is variably interpreted to be of primary, early diagenetic, or metamorphic origin. Different genetic interpretations for magnetite lead to divergent pictures of the Precambrian Earth system and its evolutionary models through time. The Baizhiyan Formation of the Neoarchean Wutai Group (Shanxi, North China) features magnetite-bearing, Algoma-type BIFs deposited ca. 2.52 Ga, in the lead-up to a major period of global iron formation deposition in the Paleoproterozoic. Abundant magnetite crystals found in the silica-rich bands of these BIFs show euhedral, hexagonal morphology. We suggest that this hexagonal magnetite likely represents pseudomorphs after green rust, a mixed-valence iron hydroxy-salt f...

The chemogenic sediments of the Chilpi Group (~2.0-1.8 Ga) of the Bastar Craton have been analysed for the sedimentary environment, geochemistry and redox evaluation of seawater towards the end of the Paleoproterozoic Era. Fabric-retentive fine-grained micritic and oolitic carbonates suggest preservation of primary texture and precipitation from contemporaneous seawater in a shallow water tidal environment. Reducing condition of shallow sea for the Chilpi Group is also recognized by the presence of chamosite associated with Mn-ankerite. The Post-Archean Australian Shale (PAAS) normalized positive Gd and La anomalies, Lu/La, Y/Ho molar ratios and the Y anomaly indicate the preservation of pristine marine nature of the carbonates. Evaluation of multielement concentration indicates no significant effects of detrital contamination, diagenesis, metamorphism and contributions from hydrothermal sources. Positive Ce anomalies observed in all the samples, with maximum Ce/ Ce* up to 2.83, and redox responsive trace element ratio patterns in association with enrichment of Mn and other redox responsive elements such as Zn, Fe, Mo and U indicate a reducing marine environment of deposition of the Chilpi carbonates. Display of positive europium anomalies (Eu/Eu* = 1.02-1.56) can be attributed to the reducing environment as contributions of hydrothermal fluids are ruled out for lack of LREE enrichment compared to HREE. We infer suboxic to anoxic conditions of shallow sea in the Chilpi basin towards the end of the Paleoproterozoic Era, with estimated atmospheric oxygen level of ~10-3 PAL. Reducing conditions in shallow sea indicate that the Great Oxidation Event did not affect seawater composition during late Paleoproterozoic.

Chemical signatures of iron oxides from dolomitic itabirite and high-grade iron ore from the Esperança deposit, located in the Quadrilátero Ferrífero, indicate that polycyclic processes involving changing of chemical and redox conditions are responsible for the iron enrichment on Cauê Formation from Minas Supergroup. Variations of Mn, Mg and Sr content in different generations of iron oxides from dolomitic itabirite, high-grade iron ore and syn-mineralization quartz-carbonate-hematite veins denote the close relationship between high-grade iron ore formation and carbonate alteration. This indicates that dolomitic itabirite is the main precursor of the iron ore in that deposit. Long-lasting percolation of hydrothermal fluids and shifts in the redox conditions have contributed to changes in the Y/Ho ratio, light/heavy rare earth elements ratio and Ce anomaly with successive iron oxide generations (martite-granular hematite), as well as lower abundance of trace elements including rare e...

Heaman (1997) suggested that the Hearst-Matachewan diabase dikes of the Superior Province and the layered mafic intrusions and flood basalts in Karelia may represent the oldest recognized large igneous province (≥2000 km strike length) and that the 2.45 Ga age of this volcanism may reflect a fundamental change in the mechanism of heat transfer within the mantle at about the Archean-Proterozoic boundary. The presence of 2.8 to 2.7 Ga flood basalts elsewhere in the world challenges these inferences. Additionally, the Archean-Proterozoic boundary at 2500 Ma has limited natural significance. Like many other geochronological boundaries, it is one of human convenience or convention. To place as many granite-greenstone and high-grade gneiss terranes in the Archean as possible, and to place as many cover sequences in the Proterozoic as possible, Plumb and James (1986) selected 2500 Ma as the Archean-Proterozoic boundary. This selection was made before many U-Pb dates were available from single zircons from the late granitic intrusions. In most ancient provinces most of these granites now appear to be ≥2.65 Ga. Furthermore, the crystalline basement of the Kaapvaal Province of southern Africa formed by 3.06 Ga (Cheney and Winter, 1995), and, of course, accretion throughout the Phanerozoic welded greenstone and other terranes to continents and caused "cratonization." Blake (1993) described three sequences of flood basalts in the