Nicolas J. Beukes

Atmospheric oxygenation three billion years ago

It is widely assumed that atmospheric oxygen concentrations remained persistently low (less than 10−5 times present levels) for about the first 2 billion years of Earth’s history. The first long-term oxygenation of the atmosphere is thought to have taken place around 2.3 billion years ago, during the Great Oxidation Event. Geochemical indications of transient atmospheric oxygenation, however, date back to 2.6–2.7 billion years ago. Here we examine the distribution of chromium isotopes and redox-sensitive metals in the approximately 3-billion-year-old Nsuze palaeosol and in the near-contemporaneous Ijzermyn iron formation from the Pongola Supergroup, South Africa. We find extensive mobilization of redox-sensitive elements through oxidative weathering. Furthermore, using our data we compute a best minimum estimate for atmospheric oxygen concentrations at that time of 3 × 10−4 times present levels. Overall, our findings suggest that there were appreciable levels of atmospheric oxygen about 3 billion years ago, more than 600 million years before the Great Oxidation Event and some 300–400 million years earlier than previous indications for Earth surface oxygenation.

Facies relations, depositional environments and diagenesis in a major early Proterozoic stromatolitic carbonate platform to basinal sequence, Campbellrand Subgroup, Transvaal Supergroup, Southern Africa

The Campbellrand Subgroup and its correlative, the Malmani Subgroup of the Transvaal Supergroup, represent a major 2300–2600 Ma old carbonate buildup. It is on average between 1500 and 1700 m thick and covers an area of approximately 190,000 km2 on the Kaapvaal craton. The original depository probably extended across the entire 600,000 km2 surface area of the craton and beyond. The buildup consists essentially of two major lithofacies assemblages, namely a basinal, non-stromatolitic, laminated carbonate and shale sequence (with minor chert, iron-formation and mafic tuff interbeds) off the craton to the west, and a shallow-water stromatolitic carbonate sequence on the craton proper. Growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite, oolite and carbonate arenite shoal deposits. These are flanked basinward by “reef-like” giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites. Platform lagoonal deposits consist of dark grey, laminoid fenestrate, stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal, light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites, laminoid fenestrae, tepee structures and flat pebble conglomerates. The buildup consisted primarily of limestone which was replaced by early diagenetic dolomite. The chemical composition of the dolomite is related to depofacies. Platform margin and lagoonal dolomites are manganese-rich, basinal dolomites are iron-rich and intertidal to supratidal deposits are virtually free of iron and manganeses. Indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf. Eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the Kuruman iron-formation.

Tropical laterites, life on land, and the history of atmospheric oxygen in the Paleoproterozoic

The ca. 2.2 Ga Hekpoort paleosol of the Transvaal Supergroup in southern Africa has been considered a type example and the youngest iron-depleted paleosol formed under a reducing atmosphere in the early Precambrian. However, the mineralogical and geochemical data on recently acquired deep drill core intersections indicate that the Hekpoort paleosol represents part of an ancient lateritic weathering profile with an iron-depleted pallid lower zone and an iron-enriched lateritic upper zone. Previous studies of the paleosol took place in areas where only the lower pallid zone was preserved from erosion prior to deposition of cover beds. The laterite profile is comparable to that of modern tropical laterites formed under an oxic atmosphere in the presence of abundant terrestrial biomass. Revised stratigraphic correlation indicates that the Hekpoort laterite profile is a correlative to highly ferruginous laterite profiles of Wolhaarkop in Griqualand West. This information indicates that the oxygen-evolution curve, based on loss or retention of iron in paleosols, should be reexamined.

Pb, O, and C isotopes in silicified Mooidraai dolomite (Transvaal Supergroup, South Africa): implications for the composition of Paleoproterozoic seawater and ‘dating’ the increase of oxygen in the Precambrian atmosphere

Abstract The Mooidraai Dolomite Formation is a unit of marine sedimentary carbonates in the upper Transvaal Supergroup which recorded significant changes in the composition of the Earth’s atmosphere and ocean. Previously available data had suggested that this dolomite was about 2.2 Ga old and showed δ13Ccarb values around 0.8‰ PDB, and hence was an exception to the positive excursion of δ13Ccarb values observed worldwide in marine carbonates deposited between 2.25 and 2.05 Ga ago. We studied the Pb–O–C isotope systematics of drill core samples from a highly silicified and aluminosilicate-free sub-unit of the Mooidraai Dolomite Formation, that yields well-preserved micritic dolomite grains. Selective leaching of the silicified dolomite revealed a significant Pb isotopic contrast between the carbonate and the quartz fraction. The former shows only a narrow range of 208Pb/204Pb (35.35–35.58) which does not correlate with 206Pb/204Pb, indicating the absence of Th-derived detrital Pb. The carbonate-bound Pb defines an isochron that corresponds to an age of 2394±26 Ma (2σ; MSWD=1.8, n=22) which we interpret as the diagenetic age of the Mooidraai dolomite. The δ13Ccarb values range from 0.51 to 0.64‰ PDB and confirm the previous data. The δ18Ocarb values range from −2.08 to +0.18‰ PDB and are amongst the heaviest reported yet from Early Precambrian marine sedimentary carbonates. The quartz fraction is considerably lighter than the carbonate fraction, and shows δ18Oqtz values between −9.22 and −6.73‰ PDB (+21.40 and +23.97‰ SMOW). Together with evidence from fluid inclusion microthermometry suggesting that the quartz formed at minimum temperatures between 130 and 155°C, this indicates that the intense silicification was post-depositional. Comparison of δ18Ocarb values of silicified and non-silicified Mooidraai samples suggests that the post-depositional silicification had sealed the system and thereby protected the primary isotopic compositions of the micritic dolomite. The heavy δ18Ocarb values suggest that 2.4 Ga ago the oxygen isotopic composition of seawater differed only little from that of today. The new Pb-Pb carbonate age for the Mooidraai dolomite indicates that the upper Transvaal Supergroup is about 200 my older than previously thought. This explains the ‘normal’ carbon isotopic composition of the Mooidraai dolomite, and suggests that the pronounced increase of the oxygen content in the Precambrian Earth’s atmosphere that is seen in strata underneath the Mooidraai dolomite occurred before 2.4 Ga ago. On this background, it appears unlikely that the positive excursion of the carbon isotopic ratios of 2.25–2.05 Ga old marine sedimentary carbonates is related to this increase in atmospheric oxygen.

Dating the Oldest Greenstone in India: A 3.51-Ga Precise U-Pb SHRIMP Zircon Age for Dacitic Lava of the Southern Iron Ore Group, Singhbhum Craton

This article reports a precise 3506.8 ± 2.3-Ma U-Pb SHRIMP zircon age for dacitic lava in a well-preserved low-grade metamorphic and low-strained greenstone belt succession of the southern Iron Ore Group, Singhbhum craton, India. This age makes the succession the oldest-known greenstone belt succession in India and one of the oldest low-strain greenstone successions in the world after the 3.51-Ga Coonterunah Group of the Pilbara craton, Western Australia, and the moderately deformed 3.54-Ga Theespruit Formation of the Barberton Greenstone Belt, Kaapvaal craton, South Africa. The geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting. The succession also contains a major ∼120-m-thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the Pilbara and Kaapvaal cratons. This banded iron formation may well be one of the oldest and most well preserved, and together with associated volcanics, it may have immediate implications for understanding >3.5-Ga surface and tectonic processes on Earth.

Filamentous microfossils in the early proterozoic transvaal supergroup: their morphology, significance, and paleoenvironmental setting

Abstract Well preserved filamentous microfossils ( Siphonophycus transvaalensis n. sp.) are described here from the carbonate (Campbellrand Subgroup) to iron-formation (Kuruman Iron Formation) transition of the Transvaal Supergroup, South Africa, estimated to be 2.5-2.3 Ga years old. The microfossils occur in petrographic thin-sections of a core sample of carbonate-chert. They are preserved by permineralization in both chert and in sparry ferroan dolomite. Stratigraphically the fossiliferous core sample occurs as part of an upward transition from a stromatolitic dolomite and limestone sequence (Campbellrand) to the overlying iron-rich sediments of the Kuruman-Griquatown Iron Formations. The average δ 13 C value of the kerogen in the sample is about - 36.9%. The microfossils are filamentous, unbranched, tubular to somewhat flattened, 15–25, μm in diameter and a few to many hundred microns in length. They exhibit a coriaceous, finely granular external surface texture resulting from the presence of adhering, randomly distributed, fine mineralic (carbonate) needles. In salient morphological characteristics they are comparable to the tubular, originally polysaccharide, encompassing sheaths of extant oscillatoriacean cyanobacteria. In comparison with previously described Precambrian microfossils, these fossil filaments are unusual because of their preservation in sparry carbonate (in addition to chert), their relatively large diameter, and their coating by adhering, precipitated, carbonate needles; they appear to be among the oldest assured microfossils now known from Proterozoic-age sediments. The microfossils are interwoven, occurring in subparallel aggregates that form a stromatolitic mat-like fabric; they are considered to be of an endogenetic in situ benthic origin occurring at the proximal margin of a ‘deep shelf’ environment at the front (distal margin) of the Campbellrand carbonate platform. The water depth for this environment, at the break in slope between deep shelf and euxinic basin, is estimated to have been of the order of 40–45 m. Paleomagnetic data support our interpretation that the micro-organisms inhabited a warm water marine environment, probably at low latitudes.

Sedimentology of the Kuruman and Griquatown Iron-formations, Transvaal Supergroup, Griqualand West, South Africa

Abstract Iron-formation was deposited as a distal facies of ferruginous carbonate turbidites in an open shelf environment in front of a shallow-water carbonate platform at the time of deposition of the Campbellrand carbonate sequence. A subsequent transgression resulted in the deposition of open shelf iron-formation on top of the Campbellrand carbonate platform. Progradational sedimentation coupled with shoaling followed, and an iron-formation sequence represented by the Kuruman and Griquatown Iron-formations was deposited. This sequence consists from the base upwards of stacked open-shelf cycles of altered volcanic ash stilpnomelane lutite beds and autochthonous banded ferhythmite units; toe-of-slope greenalite—siderite rhythmites; platform slope greenalite—siderite rhythmites with grainflow bands; platform edge sideritic orthochemical and allochemical iron-formations; epeiric sea orthochemical and allochemical sideritic, hematitic and greenalitic iron-formations; supratidal disclutites and lacustrine banded greenalite lutite. Landwards, the lacustrine felutites were followed by deltaic chloritic claystone, siltstone and quartz wacke of the Koegas Subgroup. Autochthonous ferhythmites of the Kuruman Iron-formation reach a maximum development in a basin near Prieska, whereas orthochemical and allochemical units are more abundant on the Kaapvaal craton. Iron mineral and chert microbanding in the ferhythmites is attributed to seasonal changes in Eh and pH in the depository and may be related to biological activity. Chert mesobanding in the iron-formations is essentially of an early diagenetic origin.

Rapid oxygenation of Earth’s atmosphere 2.33 billion years ago

Continuous multiple sulfur isotope profiles from South African rocks pinpoint the Great Oxygenation Event in the geologic record. Molecular oxygen (O2) is, and has been, a primary driver of biological evolution and shapes the contemporary landscape of Earth’s biogeochemical cycles. Although “whiffs” of oxygen have been documented in the Archean atmosphere, substantial O2 did not accumulate irreversibly until the Early Paleoproterozoic, during what has been termed the Great Oxygenation Event (GOE). The timing of the GOE and the rate at which this oxygenation took place have been poorly constrained until now. We report the transition (that is, from being mass-independent to becoming mass-dependent) in multiple sulfur isotope signals of diagenetic pyrite in a continuous sedimentary sequence in three coeval drill cores in the Transvaal Supergroup, South Africa. These data precisely constrain the GOE to 2.33 billion years ago. The new data suggest that the oxygenation occurred rapidly—within 1 to 10 million years—and was followed by a slower rise in the ocean sulfate inventory. Our data indicate that a climate perturbation predated the GOE, whereas the relationships among GOE, “Snowball Earth” glaciation, and biogeochemical cycling will require further stratigraphic correlation supported with precise chronologies and paleolatitude reconstructions.

Earliest laterites and possible evidence for terrestrial vegetation in the Early Proterozoic

Paleosols preserve information about the composition of the atmosphere and paleoclimatic conditions. Here we report the discovery of the first pisolitic laterites of Precambrian age. The laterites are immediately above the regional unconformity at the base of the Early Proterozoic Gamagara Formation in paleokarst depressions on the Campbellrand dolomite, Transvaal Supergroup, South Africa. The paleosol profiles are very well preserved and show zoning, relict soil textures, and chemical composition virtually identical to those of modern laterites. The pisolitic laterites provide not only evidence for a highly oxygenated atmosphere and possible hot and humid climatic conditions in Early Proterozoic time, but also indicate the presence of terrestrial life on the Kaapvaal craton ∼2.0–2.2 b.y. ago.

Evidence for an early Archaean granite from Bastar craton, India

Abstract: Granitoids in the early Archaean are believed to be potassium-poor tonalite–trondhjemite–granodiorite rocks. Only after continental crust attained sufficient thickness did true (relatively potassium-rich) granites form. No record of true granite prior to 3.4 Ga is available. We report a 3.6 Ga true granite from the Archaean Bastar craton in India. In contrast to the typical early Archaean granitoids, which are commonly deformed into gneisses, this granite is relatively undeformed. The age and composition of the granite implies that continental crust of the Bastar craton attained sufficient thickness to permit intracrustal melting at 3.6 Ga. Supplementary material: Representative major element, trace element and REE composition of the Dalli-Rajhara granite samples and a summary of SHRIMP U-Pb zircon data for the granite sample D-9 are available at http://www.geolsoc.org.uk/SUP18337.