The late Neoproterozoic Araras Group is exposed along the southern border of the Amazon craton and at the northern Paraguay belt, central Brazil. It comprises carbonate rocks over 600 m thick ...overlying Marinoan glaciogenic diamictites, and includes the first well-documented cap carbonate sequence in South America.
δ
13C values for Araras carbonates, interpreted as representative of the original seawater (
δ
18O
>
−
10‰), exhibit a trend from strongly negative
δ
13C values around −
9‰ at the base to enriched
13C compositions towards the top of the succession reaching values above 0‰. The cap carbonate succession is composed of deep platform deposits of dolomite (Mirassol d'Oeste Formation) covered by limestone rich in crusts and cements (Lower Guia Formation). These rocks show
δ
13C values around −
5‰ and anomalous sedimentary structures such as vertical tubes, megaripple bedding (tepee-like structure), and crystal fans of aragonite pseudomorphs. The anoxic deep-platform limestone succession (Guia Formation) that overlies the cap succession presents a homogeneous C-isotope curve, with
δ
13C values around −
2.5 to −
1‰. Upsection, values switch to positive (+
0.1 to +
0.3‰) in platform dolomites and shoreface to shallow-platform deposits (Serra do Quilombo Formation), but exhibit oscillations to slightly negative values in the overlying peritidal facies (Nobres Formation).
87Sr/
86Sr values follow the general C-isotope trend, with values increasing upsection from 0.7074 to 0.7088. Striking shifts in
δ
13C and
87Sr/
86Sr values are observed close to stratigraphic surfaces. The correlation between the isotope record and the relative sea-level curves supports the close association between paleoceanographic changes and paleoenvironmental evolution during the terminal Neoproterozoic on the Araras carbonate platform of Amazon craton.
•Relating the SAA to RFPs is not straightforward.•SAA longitudes dictated by prominent geomagntic flux patches.•SAA latitudes is affected by the strength of the axial dipole and patches.•SAA path can ...be explained by frozen-flux SV scenarios.•SAA intensity decrease requires diffusion.
The South Atlantic Anomaly (SAA) is a region of weak geomagnetic field intensity at the Earth’s surface, which is commonly attributed to reversed flux patches (RFPs) on the core-mantle boundary (CMB). While the SAA is clearly affected by the reversed flux region below the South Atlantic, we show that the relation between the intensity minimum at Earth’s surface and RFPs is not straightforward. We map a field-dependent intensity kernel (Constable, 2007a) to study the relation between the radial geomagnetic field at the CMB and the field intensity at Earth’s surface. Synthetic tests highlight the role of specific patches (reversed and normal) in determining the location of the surface intensity minimum and demonstrate that the SAA can indeed be explained by a few intense patches. We show that the level of axial dipolarity of the field determines the stability of the relation between the SAA minimum and RFPs. The present position of the SAA minimum is determined by the interplay among several robust geomagnetic flux patches at the CMB. The longitude of the SAA minimum appears near the longitude of the Patagonia RFP due to the low-latitude normal flux patches (NFPs) near Africa and mid-Atlantic which diminish the effect of the Africa RFPs. The latitude of the SAA minimum is lower than the Patagonia RFP latitude due to the South Pacific high-latitude NFP and the axial dipole effect. The motion of the SAA minimum is explained by the motions and changes in intensity of these robust geomagnetic flux patches. Simple secular variation (SV) scenarios suggest that while the SAA path can be explained by advection, its intensity decrease requires magnetic diffusion. In addition these SV scenarios provide some speculative predictions for the SAA.
The Vargeão impact structure was formed in the Serra Geral basaltic and rhyodacitic to rhyolitic lava flows of southern Brazil, that belong to the Paraná‐Etendeka large igneous province. The ...Chapecó‐type rhyodacites contain small baddeleyite crystals recently dated at 134.3 (±0.8) Ma, which is regarded as the age of this acid volcanism coeval to the flood basalt eruption. Inside the impact structure, a brecciated rhyodacitic sample displays fine veinlets containing numerous lithic fragments in a former melt. This impact breccia contains newly formed zircons, either in the veins or at the contact between a vein and the volcanic matrix. The zircons are 10–50 μm in length, clear and nearly unzoned. In situ laser‐ablation dating of the zircons provides a concordant Early Aptian age of 123.0 ± 1.4 Ma that is regarded as the age of the impact event. As in situ age determination ensures the best possible selection of the analyzed mineral grains, the methodology employed in this study also represents a promising method for dating other impact structures.
The geomagnetic field's dipole undergoes polarity reversals in irregular time intervals. Particularly long periods without reversals (of the order of 10^{7} yr), called superchrons, have occurred at ...least three times in the Phanerozoic (since 541 million years ago). We provide observational evidence for high non-Gaussianity in the vicinity of a transition to and from a geomagnetic superchron, consisting of a sharp increase in high-order moments (skewness and kurtosis) of the dipole's distribution. Such an increase in the moments is a universal feature of crisis-induced intermittency in low-dimensional dynamical systems undergoing global bifurcations. This implies a temporal variation of the underlying parameters of the physical system. Through a low-dimensional system that models the geomagnetic reversals, we show that the increase in the high-order moments during transitions to geomagnetic superchrons is caused by the progressive destruction of global periodic orbits exhibiting both polarities as the system approaches a merging bifurcation. We argue that the non-Gaussianity in this system is caused by the redistribution of the attractor around local cycles as global ones are destroyed.
Along the Serra Azul (Blue Mountains) in Brazil, a large (∼600-m) incision surface cuts into post-Marinoan cap carbonates and into the underlying glaciogenic diamictites of the Puga Formation. The ...paleovalley is filled with glaciomarine sediments, notably, massive diamictite with multiply striated sandstone and mudstone lonestones of the Serra Azul Formation. Using preexisting mapping, satellite imagery, isotopic data, and new stratigraphic sections, we delineate a paleovalley that we infer to be related to a combination of isostatic rebound due to glacial erosion and sea level fall at the time of the Gaskiers glaciation. The hypothesis that a significant glaciation at this time can be observed in the geological record is consistent with global evidence presented here.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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Paleogeographic connections between São Francisco/Congo, North China and Rio de la Plata Cratons through the Paleoproterozoic have been proposed on the basis of geological and ...paleomagnetic data. We conducted new paleomagnetic analyses for mafic dykes of the 1790 Ma Pará de Minas swarm, occurring in the southern São Francisco Craton. The data revealed south/southwestern, low inclination magnetic directions after alternating field (AF) and thermal demagnetization, providing a paleomagnetic pole at 39.8°S, 196.8°E (A95 = 17.0°). The characteristic remanent magnetization is interpreted as a thermo-remanent magnetization carried by stable ‘pseudo-single domain’ magnetite grains, being acquired during the cooling of the dykes at 1790 Ma, as attested by a positive baked contact test performed on one of the dykes. The new pole in conjunction with 1790–1750 Ma paleomagnetic poles available for other blocks allowed an improved reconstruction of Columbia/Nuna supercontinent at this time frame. Laurentia, Baltica, Siberia, proto-Amazonia, West Africa and proto-Australia were positioned as in previously proposed models. The main difference from our reconstruction concerns the link of the São Francisco/Congo, North China and Rio de la Plata cratons, implying that much of the Columbia supercontinent had already agglutinated around 1850–1800 Ma ago. This configuration is consistent with the onset of eventual taphrogenic episodes over most blocks, accompanied by the emplacement of mafic dyke swarms and granitic intrusions at 1790–1750 Ma.
Geomagnetic field reversals remain as one of the most intriguing problems in geophysics and are regarded as chaotic processes resulting from a dynamo mechanism. In this article we use the polarity ...scale data of the last 170 Myrs collected from the ocean floor to provide robust evidence for an inverse relationship between the complexity of sequences of consecutive polarity intervals and the respective reversal rate. In particular, the variability of sequences of polarity intervals reaches minimum values in the mid-Jurassic when a maximum reversal rate is found, in the early Cretaceous preceding the Cretaceous Superchron, and twice in the last 20 Myrs. These facts raise the possibility of epochs of high regularity in the geomagnetic field reversals. To shed light on this process, we investigate the transition from regular to chaotic regime in a minimal model for geomagnetic reversals. We show that even in chaotic regimes, the system retains the signature of regular behavior near to transitions. We suggest that geomagnetic reversals have switched between different degrees of irregularity, with a dominant periodicity of ≈70 kyrs that results from the occurrence of “ghost” limit cycles (GLCs) or unstable periodic orbits (UPOs) immersed in a chaotic region of phase space.
The Ediacaran Period was an interval of significant global transformation, marked by major changes in the biosphere, cryosphere, hydrosphere and atmosphere, and possibly the solid Earth. A better ...understanding of this interval is thus important to an understanding of the diversification of complex life, the history of long-term climatic change and the evolution of global geochemical cycles. Increasingly detailed temporal records are being acquired from Ediacaran rocks to investigate these changes in time, but we still lack a robust paleogeographic framework to study them in space. Paleomagnetic data—which are used to quantitatively determine the ancient position of continents—appear unusually complex and often contradictory throughout this period. The nature of these complex data remains elusive and four distinct hypotheses have been forwarded to explain them: 1) the tectonic plates were moving especially fast, 2) many of the paleomagnetic data have been corrupted in some as-yet unrecognized way, 3) the solid Earth underwent rapid bouts of true polar wander, or 4) the magnetic field was behaving abnormally. Each of these hypotheses have far-reaching implications. Hypotheses 1, 3 and 4 reflect processes which differ dramatically from their present-day counterparts and defy prevailing paradigms of secular change, whereas hypothesis 2 raises questions about the reliability of existing paleomagnetic interpretations and their paleogeographic derivatives. Significant advances will be garnered through resolution of this enigma, but its endurance reflects its intricacy, and any solution is going to require a collective effort. With the aim to stimulate additional community efforts toward solving it, we probe these multiple working hypotheses, elaborate how they may be further tested and discuss the implications of their possible validation.
Quantum diamond microscope (QDM) magnetic field imaging is a recently developed technique capable of mapping magnetic field sources in geologic samples at 1 micrometer resolution. Applying QDM ...imaging to speleothems can provide high‐resolution time series of detrital input into the cave environment, which, in turn, can yield useful paleoenvironmental information. Here we map the magnetic field over a speleothem from midwest Brazil over a 174 year timespan with annual to sub-annual resolution and perform backfield remanence acquisition experiments to quantify changes in the magnetic grain population through time. We find that magnetic particles occur in highly enriched layers of 10–100 µm thickness that sample the same detrital source population. Combined with petrographic observations and electron microprobe mapping of Mg and Ca, we conclude that detrital enrichment in our sample is caused by drier conditions leading to slow or halted speleothem growth. This interpretation is compatible with oxygen isotopic data and implies that speleothem magnetism can be used to infer the past occurrence of drought and potentially quantify their duration. Future high-resolution magnetic imaging of speleothems may provide additional insight into the mechanism of detrital enrichment and establish their role as a proxy for local moisture and infiltration.
Archeomagnetic field models may provide important insights to the geodynamo. Here we investigate the existence and mobility of reversed flux patches (RFPs) in an archeomagnetic field model. We ...introduce topological algorithms to define, identify, and track RFPs. In addition, we explore the relations between RFPs and dipole changes and apply robustness tests to the RFPs. In contrast to previous definitions, patches that reside on the geographic equator are adequately identified based on our RFPs definition. Most RFPs exhibit a westward drift and migrate toward higher latitudes. Undulations of the magnetic equator and RFPs oppose the axial dipole moment (ADM). Filtered models show a tracking behavior similar to the nonfiltered model, and surprisingly new RFPs occasionally emerge. The advection and diffusion of RFPs have worked in unison to yield the decrease of the ADM at recent times. The absence of RFPs in the period 550–1440 A.D. is related to a low in intermediate degrees of the geomagnetic power spectrum. We thus hypothesize that the RFPs are strongly dependent on intermediate spherical harmonic degrees 4 and above.
Key Points
Most RFPs exhibit westward drift and almost all migrate to higher latitudes
Advection and diffusion of RFPs explain the historical decrease in the ADM
Spherical harmonic degrees 4 and above strongly affect the existence of the RFPs
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