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Greig A Paterson, Adrian R Muxworthy, Yuhji Yamamoto, Yongxin Pan
Nonideal, nonsingle-domain magnetic grains are ubiquitous in rocks; however, they can have a detrimental impact on the fidelity of paleomagnetic records-in particular the determination of ancient magnetic field strength (paleointensity), a key means of understanding the evolution of the earliest geodynamo and the formation of the solar system. As a consequence, great effort has been expended to link rock magnetic behavior to paleointensity results, but with little quantitative success. Using the most comprehensive rock magnetic and paleointensity data compilations, we quantify a stability trend in hysteresis data that characterizes the bulk domain stability (BDS) of the magnetic carriers in a paleomagnetic specimen...
December 12, 2017: Proceedings of the National Academy of Sciences of the United States of America
Christopher Davies, Catherine Constable
Extreme variations of Earth's magnetic field occurred in the Levant region around 1000 BC, when the field intensity rapidly rose and fell by a factor of 2. No coherent link currently exists between this intensity spike and the global field produced by the core geodynamo. Here we show that the Levantine spike must span >60° longitude at Earth's surface if it originates from the core-mantle boundary (CMB). Several low intensity data are incompatible with this geometric bound, though age uncertainties suggest these data could have sampled the field before the spike emerged...
May 30, 2017: Nature Communications
Aleksey V Smirnov, Evgeniy V Kulakov, Marine S Foucher, Katie E Bristol
Many geodynamo models predict an inverse relationship between geomagnetic reversal frequency and field strength. However, most of the absolute paleointensity data, obtained predominantly by the Thellier method from bulk volcanic rocks, fail to confirm this relationship. Although low paleointensities are commonly observed during periods of high reversal rate (notably, in the late Jurassic), higher than present-day intensity values are rare during periods of no or few reversals (superchrons). We have identified a fundamental mechanism that results in a pervasive and previously unrecognized low-field bias that affects most paleointensity data in the global database...
February 2017: Science Advances
Wei Lin, Greig A Paterson, Qiyun Zhu, Yinzhao Wang, Evguenia Kopylova, Ying Li, Rob Knight, Dennis A Bazylinski, Rixiang Zhu, Joseph L Kirschvink, Yongxin Pan
Microbes that synthesize minerals, a process known as microbial biomineralization, contributed substantially to the evolution of current planetary environments through numerous important geochemical processes. Despite its geological significance, the origin and evolution of microbial biomineralization remain poorly understood. Through combined metagenomic and phylogenetic analyses of deep-branching magnetotactic bacteria from the Nitrospirae phylum, and using a Bayesian molecular clock-dating method, we show here that the gene cluster responsible for biomineralization of magnetosomes, and the arrangement of magnetosome chain(s) within cells, both originated before or near the Archean divergence between the Nitrospirae and Proteobacteria This phylogenetic divergence occurred well before the Great Oxygenation Event...
February 28, 2017: Proceedings of the National Academy of Sciences of the United States of America
Andrey Sheyko, Christopher C Finlay, Andrew Jackson
A striking feature of many natural dynamos is their ability to undergo polarity reversals. The best documented example is Earth's magnetic field, which has reversed hundreds of times during its history. The origin of geomagnetic polarity reversals lies in a magnetohydrodynamic process that takes place in Earth's core, but the precise mechanism is debated. The majority of numerical geodynamo simulations that exhibit reversals operate in a regime in which the viscosity of the fluid remains important, and in which the dynamo mechanism primarily involves stretching and twisting of field lines by columnar convection...
November 24, 2016: Nature
Rakesh K Yadav, Thomas Gastine, Ulrich R Christensen, Scott J Wolk, Katja Poppenhaeger
Earth sustains its magnetic field by a dynamo process driven by convection in the liquid outer core. Geodynamo simulations have been successful in reproducing many observed properties of the geomagnetic field. However, although theoretical considerations suggest that flow in the core is governed by a balance between Lorentz force, rotational force, and buoyancy (called MAC balance for Magnetic, Archimedean, Coriolis) with only minute roles for viscous and inertial forces, dynamo simulations must use viscosity values that are many orders of magnitude larger than in the core, due to computational constraints...
October 25, 2016: Proceedings of the National Academy of Sciences of the United States of America
James Badro, Julien Siebert, Francis Nimmo
No abstract text is available yet for this article.
November 17, 2016: Nature
Mark W Hounslow
Long intervals of single geomagnetic polarity (superchrons) reflect geodynamo processes, driven by core-mantle boundary interactions; however, it is not clear what initiates the start and end of superchrons, other than superchrons probably reflect lower heat flow across the core-mantle boundary compared with adjacent intervals. Here geomagnetic polarity timescales, with confidence intervals, are constructed before and following the reverse polarity Kiaman (Carboniferous-Permian) and Moyero (Ordovician) superchrons, providing a window into the geodynamo processes...
August 30, 2016: Nature Communications
James Badro, Julien Siebert, Francis Nimmo
Recent palaeomagnetic observations report the existence of a magnetic field on Earth that is at least 3.45 billion years old. Compositional buoyancy caused by inner-core growth is the primary driver of Earth's present-day geodynamo, but the inner core is too young to explain the existence of a magnetic field before about one billion years ago. Theoretical models propose that the exsolution of magnesium oxide--the major constituent of Earth's mantle--from the core provided a major source of the energy required to drive an early dynamo, but experimental evidence for the incorporation of mantle components into the core has been lacking...
August 18, 2016: Nature
David W Hughes, Fausto Cattaneo
The earth's magnetic field is generated by dynamo action driven by convection in the outer core. For numerical reasons, inertial and viscous forces play an important role in geodynamo models; however, the primary dynamical balance in the earth's core is believed to be between buoyancy, Coriolis, and magnetic forces. The hope has been that by setting the Ekman number to be as small as computationally feasible, an asymptotic regime would be reached in which the correct force balance is achieved. However, recent analyses of geodynamo models suggest that the desired balance has still not yet been attained...
June 2016: Physical Review. E
Trevor P Almeida, Adrian R Muxworthy, András Kovács, Wyn Williams, Paul D Brown, Rafal E Dunin-Borkowski
The study of the paleomagnetic signal recorded by rocks allows scientists to understand Earth's past magnetic field and the formation of the geodynamo. The magnetic recording fidelity of this signal is dependent on the magnetic domain state it adopts. The most prevalent example found in nature is the pseudo-single-domain (PSD) structure, yet its recording fidelity is poorly understood. Here, the thermoremanent behavior of PSD magnetite (Fe3O4) particles, which dominate the magnetic signatures of many rock lithologies, is investigated using electron holography...
April 2016: Science Advances
Xiangyu Zhao, Ramon Egli, Stuart A Gilder, Sebastian Müller
Sediments continuously record variations of the Earth's magnetic field and thus provide an important archive for studying the geodynamo. The recording process occurs as magnetic grains partially align with the geomagnetic field during and after sediment deposition, generating a depositional remanent magnetization (DRM) or post-DRM (PDRM). (P)DRM acquisition mechanisms have been investigated for over 50 years, yet many aspects remain unclear. A key issue concerns the controversial role of bioturbation, that is, the mechanical disturbance of sediment by benthic organisms, during PDRM acquisition...
2016: Nature Communications
Joseph G O'Rourke, David J Stevenson
Earth's global magnetic field arises from vigorous convection within the liquid outer core. Palaeomagnetic evidence reveals that the geodynamo has operated for at least 3.4 billion years, which places constraints on Earth's formation and evolution. Available power sources in standard models include compositional convection (driven by the solidifying inner core's expulsion of light elements), thermal convection (from slow cooling), and perhaps heat from the decay of radioactive isotopes. However, recent first-principles calculations and diamond-anvil cell experiments indicate that the thermal conductivity of iron is two or three times larger than typically assumed in these models...
January 21, 2016: Nature
Bruce Buffett
No abstract text is available yet for this article.
January 21, 2016: Nature
Hrvoje Tkalčić, Mallory Young, Jack B Muir, D Rhodri Davies, Maurizio Mattesini
The core mantle boundary (CMB) separates Earth's liquid iron outer core from the solid but slowly convecting mantle. The detailed structure and dynamics of the mantle within ~300 km of this interface remain enigmatic: it is a complex region, which exhibits thermal, compositional and phase-related heterogeneity, isolated pockets of partial melt and strong variations in seismic velocity and anisotropy. Nonetheless, characterising the structure of this region is crucial to a better understanding of the mantle's thermo-chemical evolution and the nature of core-mantle interactions...
2015: Scientific Reports
John A Tarduno, Rory D Cottrell, William J Davis, Francis Nimmo, Richard K Bono
Knowing when the geodynamo started is important for understanding the evolution of the core, the atmosphere, and life on Earth. We report full-vector paleointensity measurements of Archean to Hadean zircons bearing magnetic inclusions from the Jack Hills conglomerate (Western Australia) to reconstruct the early geodynamo history. Data from zircons between 3.3 billion and 4.2 billion years old record magnetic fields varying between 1.0 and 0.12 times recent equatorial field strengths. A Hadean geomagnetic field requires a core-mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and/or advective magmatic heat transport...
July 31, 2015: Science
John A Tarduno, Michael K Watkeys, Thomas N Huffman, Rory D Cottrell, Eric G Blackman, Anna Wendt, Cecilia A Scribner, Courtney L Wagner
The dramatic decay of dipole geomagnetic field intensity during the last 160 years coincides with changes in Southern Hemisphere (SH) field morphology and has motivated speculation of an impending reversal. Understanding these changes, however, has been limited by the lack of longer-term SH observations. Here we report the first archaeomagnetic curve from southern Africa (ca. 1000-1600 AD). Directions change relatively rapidly at ca. 1300 AD, whereas intensities drop sharply, at a rate greater than modern field changes in southern Africa, and to lower values...
2015: Nature Communications
Anke Wohlers, Bernard J Wood
Recent (142)Nd isotope data indicate that the silicate Earth (its crust plus the mantle) has a samarium to neodymium elemental ratio (Sm/Nd) that is greater than that of the supposed chondritic building blocks of the planet. This elevated Sm/Nd has been ascribed either to a 'hidden' reservoir in the Earth or to loss of an early-formed terrestrial crust by impact ablation. Since removal of crust by ablation would also remove the heat-producing elements--potassium, uranium and thorium--such removal would make it extremely difficult to balance terrestrial heat production with the observed heat flow...
April 16, 2015: Nature
Julien Aubert, Christopher C Finlay, Alexandre Fournier
Temporal changes in the Earth's magnetic field, known as geomagnetic secular variation, occur most prominently at low latitudes in the Atlantic hemisphere (that is, from -90 degrees east to 90 degrees east), whereas in the Pacific hemisphere there is comparatively little activity. This is a consequence of the geographical localization of intense, westward drifting, equatorial magnetic flux patches at the core surface. Despite successes in explaining the morphology of the geomagnetic field, numerical models of the geodynamo have so far failed to account systematically for this striking pattern of geomagnetic secular variation...
October 10, 2013: Nature
Paul H Roberts, Eric M King
Few areas of geophysics are today progressing as rapidly as basic geomagnetism, which seeks to understand the origin of the Earth's magnetism. Data about the present geomagnetic field pours in from orbiting satellites, and supplements the ever growing body of information about the field in the remote past, derived from the magnetism of rocks. The first of the three parts of this review summarizes the available geomagnetic data and makes significant inferences about the large scale structure of the geomagnetic field at the surface of the Earth's electrically conducting fluid core, within which the field originates...
September 2013: Reports on Progress in Physics
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