Evidence for a 3.45-billion-year-old magnetic remanence: Hints of an ancient geodynamo from conglomerates of South Africa

金沢大学理工研究域地球社会基盤学系Paleomagnetic and rock magnetic analyses of ̃3445-million-year-old dacite conglomerate clasts and parent body rocks from the Barberton greenstone belt, South Africa, define two dominant components of magnetization. One component, unblocked at low temperature, is an overprint acquired ̃180 million years ago. The other component is unblocked at high temperatures and passes a conglomerate test, indicating that this component is older than the depositional age of the conglomerate (̃3416 Ma). The high unblocking temperature component shows scatter in the parent body rocks that can be explained by the effects of modern lightning strikes, Archean overprinting, and the presence of multidomain magnetic grains that are conducive to carrying secondary magnetizations. Alternatively, this scatter can be explained by exotic magnetization scenarios in the absence of a dynamo, including magnetization by an external field related to solar wind interaction with the atmosphere. Such exotic mechanisms can be tested with the acquisition of paleointensity data. While more scattered than paleomagnetic data recording the more recent geomagnetic field, the high unblocking temperature component in the dacite parent body shows some consistency, and the simplest explanation of the data is that they reflect a geodynamõ3445 million years ago. © 2009 by the American Geophysical Union

Hadaean to Palaeoarchaean stagnant-lid tectonics revealed by zircon magnetism.

Plate tectonics is a fundamental factor in the sustained habitability of Earth, but its time of onset is unknown, with ages ranging from the Hadaean to Proterozoic eons1-3. Plate motion is a key diagnostic to distinguish between plate and stagnant-lid tectonics, but palaeomagnetic tests have been thwarted because the planet's oldest extant rocks have been metamorphosed and/or deformed4. Herein, we report palaeointensity data from Hadaean-age to Mesoarchaean-age single detrital zircons bearing primary magnetite inclusions from the Barberton Greenstone Belt of South Africa5. These reveal a pattern of palaeointensities from the Eoarchaean (about 3.9 billion years ago (Ga)) to Mesoarchaean (about 3.3 Ga) eras that is nearly identical to that defined by primary magnetizations from the Jack Hills (JH; Western Australia)6,7, further demonstrating the recording fidelity of select detrital zircons. Moreover, palaeofield values are nearly constant between about 3.9 Ga and about 3.4 Ga. This indicates unvarying latitudes, an observation distinct from plate tectonics of the past 600 million years (Myr) but predicted by stagnant-lid convection. If life originated by the Eoarchaean8, and persisted to the occurrence of stromatolites half a billion years later9, it did so when Earth was in a stagnant-lid regime, without plate-tectonics-driven geochemical cycling

Current-controlled microtopography in the southern Mozambique Channel illuminated by high-resolution bathymetric and shallow seismic images

The Mozambique Channel (MC) plays an important role in the exchange of water masses between Indian and Atlantic Ocean. During R/V SONNE cruise SO-183 16 lines of multibeam sonar and sub-bottom profiler data were collected in the southern MC. They show a highly variable microtopography on the seafloor. Four main microtopographic zones and several sub-zones have visually been identified. The main zones consider the overall morphology and divide the study area into regions with smooth seafloor, wavy bedforms, seamounts and islands, and the Zambezi Channel. The sub-zones take the reflection pattern and the shape, size and orientation of the bedforms into account. A smooth seafloor occurs on the Mozambican continental slope, north and south of Bassas da India, on the eastern Zambezi Channnel levee and in the Zambezi cone. Wavy bedforms cover the SW, central and NE areas. The most spectacular features are large erosional scours in the SW area. They lie in a region, where the northward flowing Antarctic Bottom Water (AABW) is deflected towards the east due to the shallowing of the MC. Farther eastwards SW-NE trending wavy bedforms are obviously aligned parallel to the deflected AABW and are therefore interpreted as contourite mounds. A W-E trending channel indicates the northernmost extension of the AABW. NW-SE oriented wavy bedforms in the west, hummocky bedforms in the east and arcuate, cross-cutting features in-between reflect a different current regime in the central area. Comparisons with LADCP measurements show, that the western part lies in the range of deep-reaching eddies, so that the wavy bedforms again seem to be contourite mounds aligned parallel to a part of the swirl. The cross-cutting features mark the eastern boundary of the eddy, where a northbound flow direction prevails. The origin of arcuate bedforms and depressions in the NE area is not clear. Deep-reaching eddies which interact with the topography of Bassas da India and the Zambezi Channel may contribute to their formation. All morphological features are draped with sediments indicating, that recent bottom-current velocities are not high enough to erode sediments. This agrees with published velocities of 0.1 m/s. Therefore, the microtopography must originate from a time, when bottom-current velocities were higher. Assuming a published sedimentation rate of 20 m/Myrs and a sediment drape of at least 60 m thickness we propose, that the microtopography developed during Pliocene times or earlier

Detrital zircon in a supercontinental setting: locally derived and far-transported components in the Ordovician Natal Group, South Africa

U–Pb and Lu–Hf signatures of detrital zircon from conglomerates and sandstones of the Ordovician Natal Group, South Africa were determined using laser ablation inductively coupled plasma mass spectrometry. The basal conglomerates are dominated by Palaeo- to Mesoarchaean detrital zircon with εHf values from +3 to −4 with minor Mesoproterozoic input, indicating a proximal source in the Kaapvaal Craton and minor input from rocks of the Natal Sector of the Mesoproterozoic Namaqua–Natal Province. The sandstones are all dominated by a combination of juvenile Mesoproterozoic zircon and Neoproterozoic zircon derived from Mesoproterozoic rocks that were reworked during the Pan-African Orogeny. Several sedimentary sequences from former Gondwana with Neoproterozoic to Permian depositional ages show similar detrital zircon signatures. Sedimentary sequences of such vast temporal and geographical distribution are unlikely to have been fed by a single source, making it more likely that these sequences were fed by several different (Pan-Gondwana) source terranes with closely similar U–Pb and Lu–Hf zircon signatures. The results show that source terrane non-uniqueness can make ascertaining sedimentary provenance from detrital zircon impossible, and should be taken as a reminder when using detrital zircon as evidence for far-reaching conclusions in basin evolution studies and palaeogeography. The final version of this research has been published in the Journal of the Geological Society. © 2016 Geological Societ

Seafloor morphology in the Mozambique Channel: Evidence for long-term persistent bottom-current flow and deep-reaching eddy activity

New high-resolution bathymetric and sub-bottom profiler data collected in the Southern Mozambique Channel along a grid of 16 parallel, non-overlapping lines show a large variety of bedforms which were formed by strong bottom currents. They are classified into four main microtopographic zones and several sub-zones and divide the study area into regions with (1) smooth seafloor, (2) undulating bedforms, (3) seamounts and islands, and (4) the Zambezi Channel. A smooth seafloor occurs on the Mozambican continental slope together with downslope mass-wasting processes, north and south of Bassas da India, on the eastern levee of the Zambezi Channel and in the Zambezi cone. Undulating bedforms of some kilometres wavelength and several tens of metres height cover most of the southern, central and northeastern study area. The most spectacular bedforms are numerous, closely spaced, giant erosional scours of up to ~450 m depth, more than ~20 km length and ~3 - 7 km width in the southwestern part of the study area. Here, northward flowing Antarctic Bottom Water (AABW) is topographically blocked to the north and deflected towards the east due to the shallowing bathymetry of the Mozambique Channel. SW-NE trending undulating bedforms aligned parallel to the deflected AABW and interpreted as small contourite mounds allow to trace the AABW flow path eastwards. An ~100 km long W-E trending channel indicates the northernmost extension of the AABW. NW-SE oriented undulating bedforms in the west, hummocky bedforms in the east and arcuate, cross-cutting features in-between reflect a completely different current regime in the central study area. Comparisons with LADCP sections show, that the western part lies in the range of deep-reaching anticyclonic Mozambique Channel eddies (MCEs), so that the undulating bedforms are again considered to be small contourite mounds aligned parallel to a part of the swirl. The cross-cutting features in the middle mark the eastern boundary of the MCE, where a northbound flow direction prevails. The hummocky bedforms in the east may have developed under the influence of seasonally variable cyclonic East Madagascar Current eddies pretending at least two different flow directions. The origin of arcuate bedforms, sediment ridges and circular or elongate depressions in the northeastern study area is not clear. Bottom currents which interact with the topography of the Bassas da India complex and the Zambezi Channel may contribute to their formation. All morphological features are draped with sediments indicating that the present-day current velocities are not strong enough to erode sediments. This agrees with published LADCP bottom-current velocities of 0.1 m/s. Hence, the microtopography must originate from a time when bottom-current velocities were stronger. Assuming a published sedimentation rate of 20 m/Myrs and a drape of at least 50 m thickness the microtopography may have developed during Pliocene times or earlier

Evidence for current-controlled sedimentation in the Southern Mozambique Channel inferred from high-resolution bathymetric and sub-bottom profiler data

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Morphology of the seafloor in the Southern Mozambique Channel: Evidence for long-term persistent bottom-current flow and deep-reaching eddy activity

New high-resolution bathymetric and sub-bottom profiler data collected in the Southern Mozambique Channel along a grid of 16 parallel, non-overlapping lines show a large variety of bedforms which were formed by strong bottom currents. They are visually classified into four main microtopographic zones and several sub-zones which divide the study area into regions with (1) smooth seafloor, (2) undulating bedforms, (3) seamounts and islands, and (4) the Zambezi Channel. A smooth seafloor occurs on the Mozambican continental slope together with downslope mass-wasting processes, north and south of Bassas da India, on the eastern levee of the Zambezi Channel and in the Zambezi cone. Undulating bedforms of some kilometres wavelength and several tens of metres height cover most of the southern, central and northeastern study area. The most spectacular bedforms are numerous, closely spaced, giant erosional scours of up to ~450 m depth, more than ~20 km length and ~3 - 7 km width in the southwestern part of the study area. Here, northward flowing Antarctic Bottom Water (AABW) is topographically blocked to the north and deflected towards the east due to the shallowing bathymetry of the Mozambique Channel. SW-NE trending undulating bedforms aligned parallel to the deflected AABW and interpreted as small contourite mounds allow to trace the AABW flow path eastwards. An ~100 km long W-E trending channel indicates the northernmost extension of the AABW. NW-SE oriented undulating bedforms in the west, hummocky bedforms in the east and arcuate, cross-cutting features in-between reflect a completely different current regime in the central study area. Comparisons with LADCP sections show, that the western part lies in the range of deep-reaching anticyclonic Mozambique Channel eddies (MCEs), so that the undulating bedforms are again considered to be small contourite mounds aligned parallel to a part of the swirl. The cross-cutting features in the middle mark the eastern boundary of the MCE, where a northbound flow direction prevails. The hummocky bedforms in the east may have developed under the influence of seasonally variable cyclonic East Madagascar Current eddies pretending at least two different flow directions. The origin of arcuate bedforms, sediment ridges and circular or elongate depressions in the northeastern study area is not clear. Bottom currents which interact with the topography of the Bassas da India complex and the Zambezi Channel may contribute to their formation. All morphological features are draped with sediments indicating that the present-day current velocities are not strong enough to erode sediments. This agrees with published LADCP bottom-current velocities of 0.1 m/s. Hence, the microtopography must originate from a time when bottom-current velocities were stronger. Assuming a published sedimentation rate of 20 m/Myrs and a drape of at least 50 m thickness the microtopography may have developed during Pliocene times or earlier

Karoo large igneous province: Brevity, origin, and relation to mass extinction questioned by new 40Ar/39Ar age data

The peak activities of continental flood basalts are currently considered as huge and brief (1 m.y.) magmatic events, with strong implications for geodynamics and biotic turnover. New 40Ar/39Ar dates on the Karoo flood basalts (southern Africa) show a longer duration of magmatism (8 m.y., with 6 m.y. for the main volume) with an apparent south-to-north migration, along with briefer distinctive pulses inside the province. This suggests that the Karoo province does not fit the general plume model invoked for most continental flood basalts (including the Karoo) and may explain the absence of a major contemporaneous mass extinction