Axel Hofmann

Geodynamo, Solar Wind, and Magnetopause 3.4 to 3.45 Billion Years Ago

Early Origin of Earths Magnetic Field Earths magnetic field protects us from stellar winds and radiation from the Sun. Understanding when, during the Earths formation, the large-scale magnetic field was established is important because it impacts understanding of the young Earths atmosphere and exosphere. By analyzing ancient silicate crystals, Tarduno et al. (p. 1238; see the Perspective by Jardine) demonstrate that the Earths magnetic field existed 3.4 to 3.45 billion years ago, pushing back the oldest record of geomagnetic field strength by 200 million years. This result combined with estimates of the conditions within the solar wind at that time implies that the size of the paleomagnetosphere was about half of that typical today, but with an auroral oval of about three times the area. The smaller magnetosphere and larger auroral oval would have promoted loss of volatiles and water from the early atmosphere. Analysis of ancient silicate crystals indicates that Earth’s magnetic field existed 3.40 to 3.45 billion years ago. Stellar wind standoff by a planetary magnetic field prevents atmospheric erosion and water loss. Although the early Earth retained its water and atmosphere, and thus evolved as a habitable planet, little is known about Earth’s magnetic field strength during that time. We report paleointensity results from single silicate crystals bearing magnetic inclusions that record a geodynamo 3.4 to 3.45 billion years ago. The measured field strength is ~50 to 70% that of the present-day field. When combined with a greater Paleoarchean solar wind pressure, the paleofield strength data suggest steady-state magnetopause standoff distances of ≤5 Earth radii, similar to values observed during recent coronal mass ejection events. The data also suggest lower-latitude aurora and increases in polar cap area, as well as heating, expansion, and volatile loss from the exosphere that would have affected long-term atmospheric composition.

Sulfur record of rising and falling marine oxygen and sulfate levels during the Lomagundi event

Carbonates from approximately 2.3–2.1 billion years ago show markedly positive δ13C values commonly reaching and sometimes exceeding +10‰. Traditional interpretation of these positive δ13C values favors greatly enhanced organic carbon burial on a global scale, although other researchers have invoked widespread methanogenesis within the sediments. To resolve between these competing models and, more generally, among the mechanisms behind Earth’s most dramatic carbon isotope event, we obtained coupled stable isotope data for carbonate carbon and carbonate-associated sulfate (CAS). CAS from the Lomagundi interval shows a narrow range of δ34S values and concentrations much like those of Phanerozoic and modern marine carbonate rocks. The δ34S values are a close match to those of coeval sulfate evaporites and likely reflect seawater composition. These observations are inconsistent with the idea of diagenetic carbonate formation in the methanic zone. Toward the end of the carbon isotope excursion there is an increase in the δ34S values of CAS. We propose that these trends in C and S isotope values track the isotopic evolution of seawater sulfate and reflect an increase in pyrite burial and a crash in the marine sulfate reservoir during ocean deoxygenation in the waning stages of the positive carbon isotope excursion.

Cyclicity of Triassic to Lower Jurassic continental red beds of the Argana Valley, Morocco: implications for palaeoclimate and basin evolution

Abstract Cyclical playa deposits form a prominent part of the continental clastic succession of the Argana Valley, Western High Atlas of Morocco. The red beds formed in Triassic to Lower Jurassic times during rifting of the North American and African plates. Detailed stratigraphic work revealed asymmetrical, metre-scale cycles in mudstone-dominated successions that constitute the intermediate and upper portion of the basin fill. Sedimentary cycles commonly comprise ephemeral lake shales at the base, playa mudflat mudstones in the intermediate part, and both fluvial and aeolian sandstones at the top. Cycles of the Aglegal Member (T4) are mainly characterized by analcime-rich playa mudflat deposits displaying features indicative of palaeo-Vertisols. Cycles of the Sidi Mansour (T7) and Hasseine (T8) Members are represented by ephemeral lake or dry playa mudflat deposits grading into saline mudflat mudstones that are overlain by sheet flow and minor aeolian sandstones. Sedimentary cycles of the Imerhrane Member (T9) comprise sheet delta sandstones at the base overlain by ephemeral lake and dry playa mudflat mudstones. Upper cycle portions consist of aeolian dune and sand sheet sandstones. Cyclicity is attributed to palaeoclimatic and associated palaeohydrological fluctuations, probably within the Milankovitch frequency band. Sedimentary facies of each cycle provide evidence for a gradual climatic drying with time. The asymmetrical drying-upward cycles possibly record the precession cycle, since they are modulated by symmetrical (i.e., oscillating drying/wetting) cycles of a lower frequency indicative of eccentricity. A long-term change in palaeoclimate ranging from semi-arid conditions with seasonal precipitation (T4) towards an arid, non-seasonal climate (T8) is preserved within the cyclical units spanning a time interval of several million years. Individual cycles can be traced laterally for tens of kilometres. Correlative sections are almost identical in thickness, suggesting a phase of tectonic quiescence during their deposition. Sediment accumulation was not restricted to small-scale grabens, but took place continuously throughout the exposed area of the Argana Valley, implying that accommodation space has not been controlled by syndepositional tectonism.

The geochemistry of Archaean shales derived from a mafic volcanic sequence, Belingwe greenstone belt, Zimbabwe: Provenance, source area unroofing and submarine versus subaerial weathering

Shales of the ∼2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2–2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1–100 m thick units intercalated with shallow–water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K2O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (LaN/YbN = 2.84–4.45) and no significant Eu anomaly (Eu/Eu* = 0.93–0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (LaN/YbN = 0.69–2.19) and a small, negative Eu anomaly (average Eu/Eu* = 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering.

Pb- and Nd-isotope systematics of stromatolitic limestones from the 2.7 Ga Ngezi Group of the Belingwe Greenstone Belt: constraints on timing of deposition and provenance

Abstract Pb–Pb isochrons have been obtained for stromatolitic limestones from the late Archaean Belingwe Greenstone Belt of Zimbabwe, providing direct age constraints on the deposition of these shallow water marine sediments. Samples from the Manjeri Formation and stratigraphically higher Cheshire Formation yield age estimates of 2706±49 Ma (MSWD=11.5) and 2601±49 Ma (MSWD=0.93), respectively. These data are in agreement with published U–Pb zircon and Pb–Pb whole rock ages of associated volcanics, and we, therefore, interpret our Pb–Pb ages as representing the timing of early diagenesis, thus providing a minimum age for carbonate precipitation. A 2543±70 Ma age (MSWD=5.1) for one sample from the Cheshire Formation is considered to reflect either late-stage diagenesis, a craton wide thermal/chemical disturbance or tectonic activity along the crustal-scale Mtshingwe fault. Calculated model μ1-values for Manjeri and Cheshire limestones are 8.40±0.02 and 9.02±0.01, similar to values for approximately 3.5 and 2.9 Ga Archaean basement units adjacent to the Belingwe Belt. Negative eNd (Tdeposition) and fSm/Nd suggest derivation of the REE from old, LREE enriched continental crust. Two-stage Nd model ages for the carbonates indicate that precursor rocks were extracted from the mantle at around 3.5 Ga (Cheshire Formation) and 3.3 Ga (Manjeri Formation), in good agreement with mantle-extraction ages for local basement units (model TDM: 3.5–3.3 Ga).

Chapter 5.5 Silicified Basalts, Bedded Cherts and Other Sea Floor Alteration Phenomena of the 3.4 Ga Nondweni Greenstone Belt, South Africa

Publisher Summary This chapter elaborates the silicified basalts, bedded cherts, and other sea floor alteration phenomena of the 3.4 Ga Nondweni Greenstone belt, South Africa. The Nondweni greenstone belt is one of a number of Palaeoarchaean supracrustal fragments preserved in the southeastern part of the Kaapvaal Craton, south of the Barberton Greenstone Belt. These supracrustal fragments are associated with migmatites and granitoid basement rocks and are unconformably overlain by the cratonic cover succession of the ca. 3.0 Ga Pongola Supergroup. Contacts of the greenstone fragments with older basement granitoid rocks are not observed, and the Archaean rocks occur as inliers within an extensive Mesozoic cover. The preserved Archaean supracrustal fragments exhibit a wide variety of rock types and include clastic, chemical, and biological sedimentary rocks, pyroclastic rocks, and volcanic rocks ranging from komatiites and basalts to andesites. This range of rock types precludes simple lithostratigraphic correlations. The volcanic rocks of the Toggekry Formation contrast with those of the other two formations in that basalts are only a minor component, and the sequence is made up of massive rhyolite and rhyodacite flows, and felsic tuffs that include graded air-fall tuffs.

Shallowing-Upward Carbonate Cycles in the Belingwe Greenstone Belt, Zimbabwe: A Record of Archean Sea-Level Oscillations

ABSTRACT Shallowing-upward carbonate cycles in the 2650 Ma Cheshire Formation, Belingwe greenstone belt (Zimbabwe), closely resemble their Proterozoic and Phanerozoic counterparts. The cycles form part of a karstified carbonate-ramp sequence that is overlain by, and grades basinward into, siliciclastic turbidites. A single section of 74 cycles (1.5 m average thickness) was studied in detail. Two basic cycle types are recognized, both with an asymmetric facies stacking pattern. One cycle type contains open marine, subtidal shale at the base. Shale is intercalated with storm-generated sandstone and grainstone beds that become more common and thicken upward, indicating progressive shallowing. Wave-rippled ooid-intraclast grainstone beds and bedsets overlie shale or form the base of the second cycle type. Grainstone formed at or above fair-weather wave base as shoreface sand sheets in an agitated, shallow subtidal setting. Microbial laminites constitute the top of both cycle types and are interpreted as peritidal deposits. In the upper part of the studied section, microbial boundstones with aragonite pseudomorphs are intercalated with, or overlie, laminites and formed in a supratidal environment. The vertical facies distribution within a cycle is indicative of rapid submergence followed by gradual shallowing of relative sea level. High-frequency eustatic sea-level changes are favored over an autocyclic mechanism and tectonically induced allocyclicity as the controlling mechanism for the cyclicity. Hierarchies of stratigraphic cyclicity occur on different scales and may be a result of the combined effects of several orders of sea-level oscillations. Cycle recurrence ratios correspond well to the Milankovitch frequencies calculated for the Late Archean, suggesting that orbital climatic forcing may have been in operation in Archean times.

The Belingwe Greenstone Belt: Ensialic or Oceanic?

Publisher Summary The Belingwe belt in Zimbabwe is probably the best preserved late Archaean greenstone belt known. The basal unit that rests unconformably on upto 3.5 Ga old granitoid gneisses and Mtshingwe Group rocks, consists of fluvial to shallow-marine sedimentary rocks, and is similar to cratonic cover successions. The overlying volcanic unit is a submarine lava plain sequence of massive and pillow basalts with komatiites near the base and andesites near the top. Many workers have interpreted the Zimbabwe craton as vertically accreted crust. Coherent units of volcano–sedimentary rocks were laid down in rifts on top of older continental basement (ensialic model) and underwent little deformation until the late-stage emplacement of granite-gneiss complexes. The Belingwe greenstone belt is situated in the southern part of the Zimbabwe craton, north of the Limpopo belt and east of the Great Dyke. It contains a well preserved, 2.9–2.65 Ga old volcano–sedimentary sequence which is surrounded by granitoid–gneiss terrains, the 3.5 Ga Shabani gneiss complex to the east and the 2.9 Ga Chingezi gneiss complex to the west.

Titanium isotopic evidence for felsic crust and plate tectonics 3.5 billion years ago

An early call for plate tectonics The composition of continental crust far back in Earths history gives us insight into when plate tectonics ramped up and has influenced ocean chemistry. Greber et al. looked at titanium isotopes in shales, which form from eroded continental crustal sediments, to estimate the composition 3.5 billion years ago, closer to the origins of Earth. They found a silica-rich composition, which indicates that plate tectonics was happening deep in our distant past. Other changes in crustal composition might be linked to changing ocean chemistry and major events such as the oxygenation of our atmosphere. Science, this issue p. 1271 Titanium isotopes in shale rock require the occurrence of plate tectonics on Earth 3.5 billion years ago. Earth exhibits a dichotomy in elevation and chemical composition between the continents and ocean floor. Reconstructing when this dichotomy arose is important for understanding when plate tectonics started and how the supply of nutrients to the oceans changed through time. We measured the titanium isotopic composition of shales to constrain the chemical composition of the continental crust exposed to weathering and found that shales of all ages have a uniform isotopic composition. This can only be explained if the emerged crust was predominantly felsic (silica-rich) since 3.5 billion years ago, requiring an early initiation of plate tectonics. We also observed a change in the abundance of biologically important nutrients phosphorus and nickel across the Archean-Proterozoic boundary, which might have helped trigger the rise in atmospheric oxygen.

A tectonic origin for ironstone horizons in the Zimbabwe craton and their significance for greenstone belt geology

Metre-thick horizons of ironstone lithologically similar to ‘sulphide-facies’ banded iron formation, but interpreted as silicified and sulphide-impregnated shear zones, are a common component of the greenstone stratigraphy of the Archaean Zimbabwe craton. Such tectonic ironstones separate different lithostratigraphic units commonly regarded as autochthonous rock sequences. On outcrop scale, shearing along ironstone horizons is indicated by anastomosing foliation domains, folding, boudinage and mylonitic fabrics, and, on a regional scale, by truncation of bedding and/or foliation, an anastomosing geometry of the horizons and duplication or juxtaposition of lithostratigraphic units. Tectonic ironstone formation is attributed to in situ silicification and iron (sulphide)-impregnation of rocks, commonly sediments, by mineralized fluids that penetrated the shear zones and their immediate wall rocks. The shear zones formed as a result of thin-skinned thrust tectonics that gave rise to horizontal accretion, imbrication and juxtaposition of volcanic and sedimentary rock units before deformation associated with granitoid diapirism. As a result, ‘layer-cake’ stratigraphic models of greenstone sequences containing tectonic ironstone ‘layers’ have to be treated with care, and the use of ironstone horizons as stratigraphic markers should be discouraged.