Polarity Reversals from Paleomagnetic Observations and Numerical Dynamo Simulations

Amit, Hagay; Leonhardt, Roman

doi:10.1007/978-1-4419-7955-1_11

Cited by 8 publications

(9 citation statements)

References 127 publications

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“…In particular, the rotated Y 0 2 case contains polar cooling that according to most studies may suppress reversals Kutzner and Christensen 2004). Polar cooling results in axial-dipole dominance due to flux concentration at high-latitudes (Amit et al 2010b) and therefore indeed suppresses reversals (a state termed geographic control by Olson and Amit 2014). However, when the dynamo is far from the onset of reversals, Olson and Amit (2014) found that the opposite occurs and polar cooling increases reversal frequency by producing more turbulent conditions (which they termed inertial control).…”

Section: Discussionmentioning

confidence: 97%

Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

Amit

Choblet

Olson

et al. 2015

Prog. in Earth and Planet. Sci.

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Mantle control on planetary dynamos is often studied by imposing heterogeneous core-mantle boundary (CMB) heat flux patterns on the outer boundary of numerical dynamo simulations. These patterns typically enter two main categories: Either they are proportional to seismic tomography models of Earth's lowermost mantle to simulate realistic conditions, or they are represented by single spherical harmonics for fundamental physical understanding. However, in reality the dynamics in the lower mantle is much more complicated and these CMB heat flux models are most likely oversimplified. Here we term alternative any CMB heat flux pattern imposed on numerical dynamos that does not fall into these two categories, and instead attempts to account for additional complexity in the lower mantle. We review papers that attempted to explain various dynamo-related observations by imposing alternative CMB heat flux patterns on their dynamo models. For present-day Earth, the alternative patterns reflect non-thermal contributions to seismic anomalies or sharp features not resolved by global tomography models. Time-dependent mantle convection is invoked for capturing past conditions on Earth's CMB. For Mars, alternative patterns account for localized heating by a giant impact or a mantle plume. Recovered geodynamo-related observations include persistent morphological features of present-day core convection and the geomagnetic field as well as the variability in the geomagnetic reversal frequency over the past several hundred Myr. On Mars the models aim at explaining the demise of the paleodynamo or the hemispheric crustal magnetic dichotomy. We report the main results of these studies, discuss their geophysical implications, and speculate on some future prospects.

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Section: Discussionmentioning

confidence: 97%

Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

Amit

Choblet

Olson

et al. 2015

Prog. in Earth and Planet. Sci.

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“…Biases of estimated ages in sediments result from uncertainties in the age constraints, unknown lock-in depths, and unknown processes of remanence acquisition (Sagnotti, 2018;Sagnotti et al, 2005;Valet & Fournier, 2016). Moreover, spatial differences in the expression of a reversal may result from nondipolar field configurations during reversals (e.g., Amit et al, 2010;Brown et al, 2007;Leonhardt & Fabian, 2007).…”

Section: Introductionmentioning

confidence: 99%

Recordings of Fast Paleomagnetic Reversals in a 1.2 Ma Greigite‐Rich Sediment Archive From Lake Ohrid, Balkans

et al. 2019

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State‐of‐the‐art paleoclimate research strongly depends on the availability of time‐equivalent markers as chronological control to disentangle interrelationships in the climate system from regional to global scale. Geomagnetic reversals are regarded as excellent age constraints because they are global events and independent from climatic conditions. However, spatial variations of timing and internal dynamics of reversals may limit their precision. Our 1.2 Ma high‐resolution (~25 cm/kyr) sediment record from Lake Ohrid is promising to precisely depict the Matuyama‐Brunhes (MB) reversal and the Jaramillo subchron. Two generations of diagenetic ferrimagnetic minerals are present in glacial intervals of the Lake Ohrid record. Early diagenetic greigite acquired a quasi synsedimentary chemical magnetization, while a late diagenetic greigite formation, triggered by the upward diffusion of H2S‐rich waters, obscures the polarity record at the top of the Jaramillo. Interglacial intervals are unaffected by greigite formation, likely due to low iron concentrations. Based on an orbitally tuned age model with tephrostratigraphic markers, the base of Jaramillo can be precisely dated to 1072.4 ka, and the MB reversal to 778.5 ka. Both polarity reversals occurred very rapidly in our record, lasting 2.3 and 1 kyr, respectively. Our results reveal that the dipole component of the Earth's magnetic field fell below the nondipole components only for a short duration in the Mediterranean region. The comparison of the timing of the MB boundary across different archives implies that the onset of the reversal provides a more synchronous age marker compared to often used midpoint ages.

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“…The recent dipole decrease is considered part of a trend that has continued for the last 2000 yr (Merrill and McElhinny, 1983) and a more rapid poleward drift of the dipole axis in the past 50 yr has also been suggested (Amit et al, 2010). Analyzing the past 150 yr of magnetic data, a more significant decay of the geomagnetic dipole intensity was found (Gubbins, 1987;Gubbins et al, 2006), much faster than the rate of free decay in the Earth's core (Olson and Amit, 2006).…”

Section: Introductionmentioning

confidence: 99%

Toward a possible next geomagnetic transition?

Santis

Qamili

2013

Nat. Hazards Earth Syst. Sci.

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Abstract. The geomagnetic field is subject to possible reversals or excursions of polarity during its temporal evolution. Considering that: (a) in the last 83 million yr the typical average time between one reversal and the next (the so-called chron) is around 400 000 yr, (b) the last reversal occurred around 780 000 yr ago, (c) more excursions (rapid changes in polarity) can occur within the same chron and (d) the geomagnetic field dipole is currently decreasing, a possible imminent geomagnetic reversal or excursion would not be completely unexpected. In that case, such a phenomenon would represent one of the very few natural hazards that are really global. The South Atlantic Anomaly (SAA) is a great depression of the geomagnetic field strength at the Earth's surface, caused by a reverse magnetic flux in the terrestrial outer core. In analogy with critical point phenomena characterized by some cumulative quantity, we fit the surface extent of this anomaly over the last 400 yr with power law or logarithmic functions in reverse time, also decorated by logperiodic oscillations, whose final singularity (a critical point t c ) reveals a great change in the near future (2034 ± 3 yr), when the SAA area reaches almost a hemisphere. An interesting aspect that has recently been found is the possible direct connection between the SAA and the global mean sea level (GSL). That the GSL is somehow connected with SAA is also confirmed by the similar result when an analogous critical-like fit is performed over GSL: the corresponding critical point (2033 ± 11 yr) agrees, within the estimated errors, with the value found for the SAA. From this result, we point out the intriguing conjecture that t c would be the time of no return, after which the geomagnetic field could fall into an irreversible process of a global geomagnetic transition that could be a reversal or excursion of polarity.

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Polarity Reversals from Paleomagnetic Observations and Numerical Dynamo Simulations

Cited by 8 publications

References 127 publications

Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

Recordings of Fast Paleomagnetic Reversals in a 1.2 Ma Greigite‐Rich Sediment Archive From Lake Ohrid, Balkans

Toward a possible next geomagnetic transition?

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