Several lines of geological and geochemical evidence indicate that the level of atmospheric oxygen was extremely low before 2.45 billion years (Gyr) ago, and that it had reached considerable levels ...by 2.22 Gyr ago. Here we present evidence that the rise of atmospheric oxygen had occurred by 2.32 Gyr ago. We found that syngenetic pyrite is present in organic-rich shales of the 2.32-Gyr-old Rooihoogte and Timeball Hill formations, South Africa. The range of the isotopic composition of sulphur in this pyrite is large and shows no evidence of mass-independent fractionation, indicating that atmospheric oxygen was present at significant levels (that is, greater than 10(-5) times that of the present atmospheric level) during the deposition of these units. The presence of rounded pebbles of sideritic iron formation at the base of the Rooihoogte Formation and an extensive and thick ironstone layer consisting of haematitic pisolites and oölites in the upper Timeball Hill Formation indicate that atmospheric oxygen rose significantly, perhaps for the first time, during the deposition of the Rooihoogte and Timeball Hill formations. These units were deposited between what are probably the second and third of the three Palaeoproterozoic glacial events.
Geological, geophysical, and geochemical data support a theory that Earth experienced several intervals of intense, global glaciation ("snowball Earth" conditions) during Precambrian time. This ...snowball model predicts that postglacial, greenhouse-induced warming would lead to the deposition of banded iron formations and cap carbonates. Although global glaciation would have drastically curtailed biological productivity, melting of the oceanic ice would also have induced a cyanobacterial bloom, leading to an oxygen spike in the euphotic zone and to the oxidative precipitation of iron and manganese. A Paleoproterozoic snowball Earth at 2.4 Giga-annum before present (Ga) immediately precedes the Kalahari Manganese Field in Southern Africa, suggesting that this rapid and massive change in global climate was responsible for its deposition. As large quantities of O2are needed to precipitate this Mn, photosystem II and oxygen radical protection mechanisms must have evolved before 2.4 Ga. This geochemical event may have triggered a compensatory evolutionary branching in the Fe/Mn superoxide dismutase enzyme, providing a Paleoproterozoic calibration point for studies of molecular evolution.
The Mesoarchaean (2.96 to 2.91 Ga) Witwatersrand-Mozaan succession of southern Africa contains multiple units that show evidence for the presence of free molecular oxygen in oceanic water columns ...approximately 500 million years prior to the Great Oxidation Event. The lithostratgraphically correlatable Thalu and Brixton formations of the Mozaan and West Rand groups, respectively, now yield further evidence for an oxygen-containing water column. The two formations contain multiple beds of manganese carbonate-bearing mudstone. This study documents these beds and their stratigraphy, mineralogy, petrography, whole rock geochemistry and stable carbon and oxygen isotopes from deep-level drill cores. The manganese carbonate-bearing beds occur towards the base of upward-coarsening units, indicating deposition during higher sea levels, followed by regressions. The mudstones show sharp contacts and compaction around manganiferous carbonate concretions, suggesting early diagenetic growth of the latter. The concretions are composed of either rhodochrosite cores and kutnahorite rims, or kutnahorite cores and ankerite rims, illustrating a decrease in manganese concentration from core to rim. Relative to the surrounding mudstone, the carbonate concretions are markedly enriched in manganese. The rare earth element and yttrium contents in the concretions, normalised to shale, show heavy over light rare earth element enrichment and positive europium and yttrium anomalies, indicating precipitation from mixed marine-hydrothermal water. The carbon and oxygen isotopes of concretionary carbonates are depleted in 13C (-12.2 to -21.30/00) and 18O (-14.9 to -23.90/00) relative to Pee Dee Belemnite, respectively. It is concluded that the main mode of deposition for iron and manganese was by precipitation likely facilitated by iron- and manganese-oxidising bacteria. The iron and manganese were sourced as dissolved Fe2+ and Mn2+ from a distal, high-temperature hydrothermal plume. The stratigraphic position of the studied units suggests that deposition occurred on the middle to outer shelf where iron oxidation and deposition proceeded far enough so that the environment could transition to manganese oxidation and deposition. These precipitates then reacted with organic carbon to form rhodochrosite that nucleated very locally to grow concretions. As the available manganese was all reduced and incorporated into the carbonates, Fe3+-oxyhydroxides were also reduced by any excess organic carbon and incorporated into the concretion rims during later stages of growth. The main implication of the proposed model for concretion formation is that free molecular oxygen was available in the water column of the shelf for manganese-oxidising microaerophyllic chemolithoautotrophs to function. The oxygen concentration was in excess of approximately 5 µM. This concentration, along with the depositional setting of the studied units, falls within the ranges and parameters of previous studies indicating conditions in "oxygen oases" of Archaean oceans.
The ultramafic-mafic layered igneous Molopo Farms Complex straddles the border between South Africa and Botswana. Younger cover obscures this igneous complex and its country rocks, which are ...generally assigned to the Paleoproterozoic Transvaal Supergroup. The area intruded by the complex is characterized by abutting and contrasting successions of the upper Transvaal Supergroup (i.e., the Pretoria and Postmasburg groups), the correlation of which is critical to understanding the first significant build-up in atmospheric oxygen, also known as the Great Oxidation Event. Recent dating of the Postmasburg Group necessitates a reinterpretation of Transvaal Supergroup stratigraphy involving a 200 million year downward revision of the Postmasburg Group relative to the Pretoria Group. The geology of the area intruded by the Molopo Farms Complex may provide important insights into this correlation model. Here we report a 207Pb/206Pb ID-TIMS baddeleyite date of 2054 ± 5 Ma from a gabbroic sample from the Molopo Farms Complex in South Africa, and an U-Pb zircon date of 2056 ± 10 Ma from a highly altered and metamorphosed quartzite in direct contact with ultramafic rocks of the complex in Botswana. We interpret these as crystallization ages of the Complex, which are within error of the 2056 to 2055 Ma age of the Bushveld Complex. Also reported from drill core intersections are U-Pb LA-ICP-MS detrital zircon age data from quartzite samples of both the floor and roof country rock. The roof rock detrital zircon age populations are comparable to those of the Paleoproterozoic Waterberg Group. Age populations in floor rocks are generally similar to those of the Pretoria Group, with the addition of ∼2050 Ma populations, which likely reflect the metamorphic aureole of the complex on distinctly recrystallized country rock. A revised pre-Kalahari regional geology of the Molopo Farms Complex in South Africa, that incorporates the Pretoria Group in the area, implies an unconformable relationship with the Potsmasburg Group. Future recognition of such an unconformable relationship in drill core will ultimately resolve the problem of Transvaal strata correlation.
Abstract Determining the tempo and causality of key palaeoclimate events recorded in sedimentary strata depends on high-resolution numerical ages and well-constrained stratigraphic correlations at ...regional and global scale. This requirement is not necessarily met in Precambrian strata due to poorer age resolution, limited preservation, and secondary overprints. A good example includes the Palaeoproterozoic Rooihoogte and Duitschland formations in South Africa, which document the disappearance of mass-independent fractionation of sulphur isotopes (MIF-S) and contain glacial diamictites at their base. They are thus key records of Earth’s surface oxygenation during the Great Oxidation Event (GOE). However, previous studies have either correlated these units, resulting in a unidirectional oxidation trend; or have regarded them as successive strata, causing an interpretation of oscillating oxidation. This study uses extensive outcrop and new core material to investigate correlation between these two units, and to establish depositional models. Results show that key stratigraphic markers can be traced around the entire Kaapvaal craton both in outcrop and the subsurface. In particular, the basal Bevets breccia and the top Duitschland breccia are here re-interpreted as two separate units that are present at the base and top of both formations, supporting correlation of the formations. Consequently, the base of both formations records a major craton-wide event of uplift and karstification, leading to carbonate dissolution and chert brecciation. Erosion of older rocks from across the craton also delivered material for the basal glacial diamictite. The majority of mixed siliciclastic-carbonate sediments were deposited on a storm- and/or delta-influenced shelf. Depositional packages in both formations reflect post-glacial relative sea level rise, followed by progradation of a deltaic, storm or shoreline depositional system. There is a relatively short-lived depositional hiatus to overlying shales of the Timeball Hill Formation. Both Rooihoogte and Duitschland formations thus record only a single glacial event at their base, and a unidirectional trend of oxidation.
The Puduhush gabbro is located on the western margin of the proto-Kalahari Craton in Southern Africa. This gabbro intrudes the Volop Formation, which conformably overlies the Hartley Formation lava ...of the late Palaeoproterozoic Olifantshoek Group. Here we report a new U-Pb ID-TIMS baddeleyite age as well as petrographic, whole-rock geochemical and palaeomagnetic results for the Puduhush gabbro. The gabbro shows a well-preserved sub-ophitic texture between clinopyroxene and plagioclase, with minor amounts of amphibole, olivine, biotite and Fe-Ti oxides. The new U-Pb ID-TIMS baddeleyite age of 1881 ± 1 Ma reported here for the Puduhush gabbro, together with existing ages for the Hartley Formation, define a ca. 1916 to 1881 Ma age bracket for the Volop Formation. Our 1881 ± 1 Ma age is also within error of ages reported for the oldest episode (so-called Episode 1) of the ca. 1.89 to 1.83 Ga magmatism in the eastern and northern parts of the proto-Kalahari Craton. Our geochemical results also suggest compositional similarities between the Puduhush gabbro and Episode 1 magmatism, particularly the post-Waterberg sills. The virtual geomagnetic pole calculated here for the Puduhush gabbro (VGP: 1.6°N; 352.0°E; A95 = 14.2°) is consistent with the Episode 1 pole. All data are therefore combined to produce a new palaeomagnetic pole (11.7°N; 8.8°E, A95 = 9.3°) for Episode 1 magmatism. The present study provides the first evidence that the ca. 1.89 to 1.83 Ga magmatism had a wider footprint that previously thought, extending to the western margin of the proto-Kalahari Craton. This wide-scale magmatism, previously proposed to be related to a back-arc extension setting, is here reinterpreted in the context of a mantle plume. Our results are consistent with the lithostratigraphic-based notion that at least parts of the red-bed successions (i.e., Olifantshoek and Waterberg Groups) that are hosts to the ca. 1.89 to 1.83 Ga magmatism could be correlative units, representing an extensive sedimentary sequence that once covered large expanses of the proto-Kalahari Craton.
We document the discovery of the first granular iron formation (GIF) of Archaean age and present textural and geochemical results that suggest these formed through microbial iron oxidation. The GIF ...occurs in the Nconga Formation of the ca. 3.0–2.8 Ga Pongola Supergroup in South Africa and Swaziland. It is interbedded with oxide and silicate facies micritic iron formation (MIF). There is a strong textural control on iron mineralization in the GIF not observed in the associated MIF. The GIF is marked by oncoids with chert cores surrounded by magnetite and calcite rims. These rims show laminated domal textures, similar in appearance to microstromatolites. The GIF is enriched in silica and depleted in Fe relative to the interbedded MIF. Very low Al and trace element contents in the GIF indicate that chemically precipitated chert was reworked above wave base into granules in an environment devoid of siliciclastic input. Microbially mediated iron precipitation resulted in the formation of irregular, domal rims around the chert granules. During storm surges, oncoids were transported and deposited in deeper water environments. Textural features, along with positive δ56Fe values in magnetite, suggest that iron precipitation occurred through incomplete oxidation of hydrothermal Fe2+ by iron‐oxidizing bacteria. The initial Fe3+‐oxyhydroxide precipitates were then post‐depositionally transformed to magnetite. Comparison of the Fe isotope compositions of the oncoidal GIF with those reported for the interbedded deeper water iron formation (IF) illustrates that the Fe2+ pathways and sources for these units were distinct. It is suggested that the deeper water IF was deposited from the evolved margin of a buoyant Fe2+aq‐rich hydrothermal plume distal to its source. In contrast, oncolitic magnetite rims of chert granules were sourced from ambient Fe2+aq‐depleted shallow ocean water beyond the plume.
Numerous Mesoproterozoic alkaline intrusions belonging to the Pilanesberg Alkaline Province are present within the Transvaal sub-basin of the Kaapvaal Craton. The Pilanesberg Complex is the ...best-known example; it represents one of the world's largest alkaline complexes, and is associated with a northwest-southeast trending dyke swarm that extends from Botswana to the southwest of Johannesburg. This paper documents the results of a petrological and geochemical study of a thin mafic sill (here referred to as an alkaline igneous body, AIB), which intrudes the ca. 2200 Ma Silverton Formation close to the southernmost part of the Pilanesberg dyke swarm. The AIB has only been observed in cores from a borehole drilled close to Carletonville. It is hypocrystalline, containing randomly oriented elongated skeletal kaersutite crystals and 6 to 8 mm varioles mainly composed of radially oriented acicular plagioclase. These two textures are related to undercooling, probably linked to the limited thickness (70 cm) of the AIB coupled with a probable shallow emplacement depth. Ar-Ar dating of the kaersutite gives an age of ca. 1400 Ma, similar to the age of Pilanesberg Complex. However, the AIB is an alkaline basaltic andesite and is thus notably less differentiated than the Pilanesberg Complex and some of its associated dykes, such as the Maanhaarrand dyke, for which we provide whole-rock geochemical data. Literature data indicate that the Pilanesberg dyke swarm also contains mafic hypabyssal rocks suggesting a link between the dyke swarm and the AIB. The AIB is characterized by strongly negative εNd and εHf, that cannot be related to crustal contamination, as shown by positive Ti and P anomalies, and the absence of negative Nb-Ta anomalies in mantle-normalised trace element diagrams. The AIB magma is interpreted to have been derived from a long-lived enriched, probably lithospheric mantle reservoir. The AIB thus provides important information on the magma source of the Pilanesberg Alkaline Province.
Most geochemical proxies and models of atmospheric evolution suggest that the amount of free O2 in Earth’s atmosphere stayed below 10−5 present atmospheric level (PAL) until the Great Oxidation Event ...(GOE) that occurred between ∼2.2 and 2.4Ga, at which time free O2 in the atmosphere increased to approximately 10−1 to 10−2 PAL. Although photosynthetically produced “O2 oases” have been proposed for the photic zone of the oceans prior to the GOE, it has been difficult to constrain absolute O2 concentrations and fluxes in such paleoenvironments. Here we constrain free O2 levels in the photic zone of a Late Archean marine basin by the combined use of Fe and Mo isotope systematics of Ca–Mg carbonates and shales from the 2.68 to 2.50Ga Campbellrand–Malmani carbonate platform of the Kaapvaal Craton in South Africa. Correlated Fe and Mo isotope compositions require a key role for Fe oxide precipitation via oxidation of aqueous Fe(II) by photosynthetically-derived O2, followed by sorption of aqueous Mo to the newly formed Fe oxides. A dispersion/reaction model illustrates the effects of Fe oxide production and Mo sorption to Fe oxides, and suggests that a few to a few tens of μM free O2 was available in the photic zone of the Late Archean marine basin, consistent with some previous estimates. The coupling of Fe and Mo isotope systematics provides a unique view into the processes that occurred in the ancient shallow ocean after production of free O2 began, but prior to oxygenation of the deep ocean, or significant accumulation of free O2 in the atmosphere. These results require oxygenic photosynthesis to have evolved by at least 2.7Ga and suggest that the Neoarchean ocean may have had a different oxygenation history than that of the atmosphere. The data also suggest that the extensive iron formation deposition that occurred during this time was unlikely to have been produced by anoxygenic photosynthetic Fe(II) oxidation. Finally, these data indicate that the ocean had significant amounts of O2 at least 150Myr prior to previously proposed “whiffs” of O2 at the Archean to Proterozoic transition.
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