We present a revised model for the opening of the South Atlantic Ocean founded on a remapping of the continent—ocean boundaries and Aptian salt basins, the chronology of magmatic activity in and ...around the ocean basin and on the timing and character of associated intraplate deformation in Africa and South America. The new plate tectonic model is internally consistent and consistent with globally balanced plate motion solutions. The model includes realistic scenarios for intraplate deformation, pre-drift extension and seafloor spreading. Within the model, Aptian salt basins preserved in the South American (Brazilian) and African (Angola, Congo, Gabon) continental shelves are reunited in their original positions as parts of a single syn-rift basin in near subtropical latitudes (10°S–27°S). The basin was dissected at around 112 Ma (Aptian—Albian boundary) when the model suggests that seafloor spreading commenced north of the Walvis Ridge—Rio Grande Rise.
The Bangui Magnetic Anomaly (BMA) is one of the largest magnetic anomalies in the world whose origin is still not known. This research investigated the crustal thickness, Curie depths and thermal ...structures in the Central African sub-regions – Cameroon, Central African Republic and adjacent countries – which are largely characterized by the Bangui Magnetic Anomaly. To achieve a better understanding and clearer idea of the location of the possible sources of the BMA, analyses of geothermal structures were conducted. Two potential methods were used: gravity to evaluate the crustal thickness and magnetics for geothermal analysis. Spectral analysis of gravity data shows that crustal thickness range between 14 and 55 km. The highest depths were found in Central African Republic. The lower values of crustal thickness were obtained in South-Chad basin with a minimum of roughly 14 km. Geothermal analysis is carried out using the Curie point depth, thermal gradient and heat-flow evaluations. The results show that the BMA is related to a thick crust of roughly 40 km. Depth to the bottom of possible sources does not exceed the lower crust. The mean Curie point depth estimated is 38 km with an error of ±2 km. Geothermal results also show the difference in the thermal behaviour between the crust in the Pan African and Precambrian domain. The mobile zone which constitutes the Pan African domain is associated with a thin crust of high heat-flow values of 65 mW/m2. However, the Precambrian domain beneath the BMA is associated with a thick crust with lower heat-flow values (roughly 45 mW/m2). The difference between crustal thickness and Curie point depths shows that all the sources of the BMA are crustal. The present results are in favour of a geological origin for the Bangui Magnetic anomaly.
•We estimate the crustal thickness between the Cameroon and the Central African Republic based on ground gravity data.•We analyse the depths to the bottom of magnetic sources and thermal structures based on EMAG2 magnetic data.•We correlate the crustal thickness and the depths to the bottom of magnetic sources.•We locate the possible sources of the Bangui Magnetic anomaly.
Ocean variability at decadal time-scales remains poorly described partly because of the scarcity of high temporal resolution marine records. Here, we present a reconstruction of Sea Surface ...Temperatures (SSTs) over the past two millennia at unprecedented temporal resolution (2 to 5 years), from a marine core located off North Iceland. Alkenone paleothermometry was used to infer SST variability, and tephrochronology to build the age model. Spectral analyses of the SST signal indicate intermittent 20–25 year oscillations, with periods of strong and weak power, that are likely reflecting the ocean response to wind forcing, presumably the North Atlantic Oscillation (NAO). Warmer SSTs and paleo-magnetic proxy data, between 1000 and 1350 year A.D., overlapping the Medieval Warm Period (MWP), suggest enhanced heat transport across the Denmark Strait by the North Icelandic Irminger Current (NIIC). This is in contrast with the subsequent period, which includes the Little Ice Age (LIA), showing continuous cooling towards the 20th century. Reduced NIIC flow through the Denmark Strait likely resulting from higher freshwater and sea ice export from the Arctic would account for the observed colder conditions.
As in other parts of the world, air pollution over West and Central Africa has major health and meteorological impacts. Air quality assessment and its possible sanitary impact have become essential ...even in medium-sized towns, therefore amplifying the need for easy-to-implement monitoring methods with low environmental impact. We present here the potential of magnetic methods to monitor air quality at street level in the medium-sized city of Maroua (northern Cameroon) affected by dust-laden desert winds. More than five hundred (544) samples of bark and leaves taken from Neem trees in Maroua were analyzed. Magnetic susceptibility, saturation remanence, and S-ratio were found to determine the concentration and nature of magnetic particles. They are dominated by magnetite-like particle signals as a part of particulate emissions due to urban activities, including both traffic, composed of a substantial proportion of motorcycles, and wood burning for food preparation. We show that both bark and leaves from Neem trees are adequate passive bio-recorders. The use of both enables different times and heights to be sampled, allowing for the high-resolution monitoring, in terms of spatialization, of various urban environments. Particle emissions require assessment and screening that could be carried out rapidly and efficiently by magnetic methods on bio-recorders, even in cities impacted by dust-laden wind.
The physico-chemical characteristics of particulate matter (PM) in African cities remain poorly known due to scarcity of observation networks. Magnetic parameters of PM are robust proxies for the ...emissions of Fe-bearing particles. This study reports the first magnetic investigation of PM2.5 (PM with aerodynamic size below 2.5 μm) in Africa performed on weekly PM2.5 filters collected in Abidjan (Ivory Coast) and Cotonou (Benin) between 2015 and 2017. The magnetic mineralogy is dominated by magnetite-like low coercivity minerals. Mass normalized SIRM are 1.65 × 10−2 A m2 kg−1 and 2.28 × 10−2 A m2 kg−1 for Abidjan and Cotonou respectively. Hard coercivity material (S-ratio = 0.96 and MDF = 33 mT) is observed during the dry dusty season. Wood burning emits less iron oxides by PM2.5 mass when compared to traffic sources. PM2.5 magnetic granulometry has a narrow range regardless of the site or season. The excellent correlation between the site-averaged element carbon concentrations and SIRM suggests that PM2.5 magnetic parameters are linked to primary particulate emission from combustion sources.
The Earth’s continental crust represents the outermost envelope of the solid Earth, controlling exchanges within the geosphere and reflecting geodynamics processes. One of the fundamental issues of ...Earth Science aims to determine crustal thickness in past geodynamic environments in order to discuss the evolution of certain geodynamic processes through time. Despite presenting a continuing challenge, the evolution of crustal thickness during the last 3 billion years can be investigated using indirect clues yielded by the chemical signature of mafic magmas and associated ferromagnesian minerals (pyroxene, amphibole). Here, we present a new statistical assessment of a global database of magmatic and mineral chemical information. Analysis reveals the increasing occurrence of high-temperature pyroxenes and amphiboles growing in Ca-rich, Fe-poor magma since ~1 Ga, which contrasts with lower temperature conditions of minerals crystallization throughout the Meso- and Palaeoproterozoic times. This is interpreted to reflect temporal changes in the control of Earth’s crust on mantle-derived magma composition, related to changes in lithospheric thickness and mantle secular cooling. We propose that thick existing crust is associated with deeper, hotter magmatic reservoirs, potentially elucidating the mineral chemistry and the contrasting iron content between primary and derivative mafic magmas. Based on both the chemical and mineral information of mafic magma, an integrated approach provides qualitative estimates of past crustal thickness and associated magmatic systems. Our findings indicate that the Proterozoic was characterized by thicker crustal sections (>40–50 km) relative to the Phanerozoic and Archean (<35 km). This period of crustal thickening appears at the confluence of major changes on Earth, marked by the onset of mantle cooling and Plate Tectonics and the assembly of Columbia, the first supercontinent.
Soil contamination linked to abandoned lead–zinc mining wastes represents a serious environmental hazard of worldwide concern, raising the need for cost-effective tools to spatialize the contaminated ...area and assess the risks. In worldwide industrial environments, positive correlation between magnetic susceptibility and heavy metal soil contamination is often reported and used for fast preliminary site assessments. In this study, we report monitoring of an olive grove, situated below one of the abandoned tailings dump of the former Jebel Ressas Pb–Zn mining site (Tunisia), with three different commercial susceptibilimeters (Terraplus KT10, Bartington MS2D and Geonics EM38 induction device). Integration depths of above-mentioned susceptibilimeters vary from 3 to 150 cm. Similar broad-scale magnetic susceptibility maps for the three devices emphasize the effect of topography and runoff transfer as the main mechanisms for tailings material dispersion. Very low susceptibilities (~ 10 × 10
–5
SI) characterize the contaminants magnetic signal, while uncontaminated soils susceptibilities are one order of magnitude higher (100–150 × 10
–5
SI). This unique case of reverse correlation between magnetic susceptibilities and contamination (Pb, Zn, Cd) trace efficiently the degree of contamination and is substantiated by further mineralogical and magnetic properties analyses showing the predominance of iron amorphous oxyhydroxides in the dump, while (titano)maghemite is the main natural magnetic carriers in the uncontaminated soils in this specific carbonated context. The Geonics EM 38, with a proper calibration procedure, reveals a performant tool increasing the depth of investigation and when combined with shallower susceptibilimeter, opens perspectives for qualitative 3D spatialization of metallic contamination.
The Neoproterozoic (1000–542 Myr ago) witnessed the dawn of Earth as we know it with modern-style plate tectonics, high levels of O2 in atmosphere and oceans and a thriving fauna. Yet, the processes ...leading to the fully oxygenation of the external envelopes, its exact timing and its link with the inner workings of the planet remain poorly understood. In some ways, it is a chicken and egg question: did the Neoproterozoic Oxygenation Event (NOE) cause life blooming, low-latitudes glaciations and perturbations in geochemical cycles or is it a consequence of these phenomena? Here, we suggest that the NOE may have been triggered by multi-million years oxic volcanic emissions along a protracted period at the end of the Neoproterozoic when continents were assembled in the Rodinia supercontinent. We report a very oxidized magma source at the upper mantle beneath a ring of subducting margins around Rodinia, and detail here the evidence at the margin of the Arabian shield. We investigate the 780 Ma Biotite and Pink granites and associated rocks of the Socotra Island with rock magnetic and petrographic methods. Magnetic susceptibility and isothermal remanent magnetization acquisitions show that, in these granites, both magnetite and hematite are present. Hematite subdivides magnetite grains into small grains. Magnetite and hematite are found to be primary, and formed at the early magmatic evolution of the granite at very high oxygen fugacity. Massive degassing of these oxidized magmas would reduce the sink for oxygen, and consequently contribute to its rise in the atmosphere with a net O2 flux of at least 2.25 x 107 Tmol. Our conceptual model provides a deep Earth link to the NOE and implies the oxygenation burst has occurred earlier than previously envisaged, paving the way for later changes in the outer envelopes of the planet epitomized on the extreme Neoproterozoic glaciations and the appearance of the first animals.
A paleomagnetic study of 36 sites (315 samples) of Neogene strata from the Peruvian Altiplano and adjacent sub‐Andean belt (Pilcopata area), combined with 40Ar/39Ar dating of 13 lava flows, provide ...new constraints on the Cenozoic deformation history of the northern Bolivian Orocline. In the Peruvian Altiplano, 40Ar/39Ar dating brackets the age of the strata sampled for paleomagnetism between 26.5 and 12.1 Ma for the Huaccochullo basin, 18.6 and 12.1 Ma for the Descanso‐Yauri basin, and 16.8 and 15.5 Ma for the Ayaviri basin. Sub‐Andean sediments are paleontologically constrained as Miocene, yet pre‐9 Ma. Prefolding magnetizations isolated in the Huaccochullo, Descanso‐Yauri, and Pilcopata localities indicate significant counterclockwise vertical axis block rotations of 11.3° ± 5.4°, 31.0° ± 10.2° and 7.8° ± 4.8°, respectively. The pattern of rotations together with mapped structures suggest that deformation of the northern Altiplano was partitioned into large regions experiencing relatively minor rotation versus smaller, isolated basins exhibiting high‐amplitude counterclockwise rotations that lie in a major left‐lateral shear zone. Our new results imply significant transpressional deformation occurred throughout the Peruvian Altiplano since circa 12 Ma and are integrated in a model detailing the tectonic evolution of the northern Bolivian Orocline since 25 Ma.
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