Paleointensity of the earth's magnetic field during the Laschamp excursion and its geomagnetic implications

doi:10.1016/0012-821X(88)90058-1

Earth and Planetary Science Letters

Volume 88, Issues 1–2, April 1988, Pages 209-219

https://doi.org/10.1016/0012-821X(88)90058-1 Get rights and content

Abstract

The reversed paleomagnetic direction of the Laschamp and Olby flows represents a specific feature of the geomagnetic field. This is supported by paleomagnetic evidence, showing that the same anomalous direction was recorded at several distinct sites, including scoria of the Laschamp volcano. To examine this anomalous geomagnetic fluctuation, we studied the paleointensity of the Laschamp and Olby flows, using the Thellier method. Twenty-five samples were selected for the paleointensity experiments, and from seven we obtained reliable results. Because the paleointensity results of the Olby and Laschamp flows as well as Laschamp scoria are very similar, they can be represented by a single mean paleointensity,F = 7.7 μT. Considering that this low paleointensity is less than 1/6 of the present geomagnetic field and is more characteristic of transitional behavior, our results suggest that the paleomagnetic directions of the Laschamp and Olby flows were not acquired during a stable reversed polarity interval. A more likely explanation is that the Laschamp excursion represents an unsuccessful or aborted reversal.

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2020, Earth and Planetary Science Letters
Documenting accurately amplitude and rhythms of geomagnetic variations is a prerequisite to understand the mechanisms triggering geomagnetic excursions and reversals. We present new authigenic ¹⁰Be/⁹Be ratio (Be ratio) results covering the 60–20 ka time interval from equatorial core MD05-2920. The most significant Be ratio peak is located 18 cm above the main relative paleointensity (RPI) minimum recorded in this sequence. These are interpreted as two independent recordings of the geomagnetic dipole moment (GDM) decrease linked to the Laschamp excursion dated at ca 41 ka. The stratigraphic offset is assignable to post-depositional magnetization lock-in processes resulting in time delay of 1.6 ka. The Be ratio and RPI records show comparable asymmetric behavior before and after the dipole low. The Be ratio record is combined with three other series to construct a global record of ¹⁰Be paleoproduction variations. This compilation demonstrates the strong coherence of low and mid latitudes data sets and its comparison with ¹⁰Be-flux records from polar regions fully supports the hypothesis of a global atmospheric ¹⁰Be production doubling during the Laschamp. The Be ratio stack is converted in terms of GDM using both a theoretical model and a reconstruction of GDM values using absolute paleointensities measured on lava flows. Both methods provide similar results. The dipole moment record derived from this approach (BeDM_20–60) shows the following characteristics: high field values ( $> 11 \times 10^{22}$ Am²) prior to a sharp two-steps dipole decrease until reaching minimum values ( $1.8 \pm 0.7 \times 10^{22}$ Am²) between 41.6 and 40.9 ka at the time of the Laschamp excursion. The GDM partly recovers up to values of $\sim 6.3 \times 10^{22}$ Am², and oscillates in this range, without showing any sharp decreases that could be associated with excursions reported within the 38–20 ka time interval (e.g. the Mono Lake excursion).
Timing of Quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives
2020, Quaternary Science Reviews
Citation Excerpt :
The most thoroughly documented magnetic excursion is undoubtedly the Laschamp excursion at ∼41 ka with aberrant magnetic directions that have sub-millennial duration (see reviews of Laj et al., 2014 and Singer, 2014). Volcanic rocks close to the village of Laschamps in the Chaîne des Puys region of the Massif Central (France) provided the first credible record of any excursion (Bonhommet and Babkine, 1967), supported by subsequent studies in the same region (e.g., Bonhommet and Zahringer, 1969; Roperch et al., 1988; Guillou et al., 2004; Singer et al., 2009; Laj et al., 2014). The geomagnetic origin of the Laschamp excursion was debated well into the 1980s when its recording in the Chaîne des Puys was interpreted by some authors in terms of a self-reversal process due to negative magnetostatic interactions of two titanomagnetite phases (Heller, 1980; Heller and Petersen, 1982).
We review the ages of Quaternary magnetic polarity reversals and magnetic excursions. Improvements in radioisotopic ⁴⁰Ar/³⁹Ar dating of reversals and excursions reflect the use of multi-collector mass spectrometers, along with ultra-low backgrounds, laser-based incremental heating methods, and adoption of 28.201 Ma for the Fish Canyon sanidine (FCs) standard age (Kuiper et al., 2008). These improvements have brought ⁴⁰Ar/³⁹Ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of δ¹⁸O data to LR04 (Lisiecki and Raymo, 2005) and, in a few cases, correlation to ice-core chronologies. Ages of two polarity reversals, the Matuyama-Brunhes boundary and base Olduvai, are at odds with ages given in popular geomagnetic polarity timescales. Sedimentary records of the Laschamp and Iceland Basin excursion are numerous and widespread, and attest to global manifestation and global synchronicity of these excursions. Records of other Quaternary excursions are less numerous. The Blake excursion (∼115 ka) has been recorded in North Atlantic sediments, Chinese loess, Indian Ocean volcanics, and Spanish speleothems. The Santa Rosa excursion (∼932 ka) has been recorded in North Atlantic and Phillipine Basin sediments, as well as in volcanic rocks from New Mexico and Galapagos. The Punaruu excursion (∼1115 ka) has been observed in sediments from the North Atlantic and California Margin as well as in volcanic rocks on Tahiti. Uniformity of individual excursion ages, using astrochronological, ⁴⁰Ar/³⁹Ar and U–Th methods, argues for synchronous global manifestation of excursions, strengthening the usefulness of magnetic excursions (and reversals) in Quaternary stratigraphy.
Low absolute paleointensity during Late Miocene Noma excursion of the Earth's magnetic field
2019, Physics of the Earth and Planetary Interiors
Citation Excerpt :
During the Mono Lake excursion, volcanic data from the Canary Islands, Auckland (New Zealand) and Massif Central (France) record weak paleointensities (VADMs) of 1.6–4.3 × 1022 Am2, 1.8–2.0 × 1022 Am2 and 1.8–2.1 × 1022 Am2, respectively (Shibuya et al., 1992; Mochizuki et al., 2004b, 2006, 2007; Cassata et al., 2008; Cassidy and Hill, 2009; Kissel et al., 2011; Laj et al., 2014). The Laschamp excursion is studied for volcanic rocks in the French Massif Central and yields two absolute paleointensities for younger (1.0–1.3 × 1022 Am2) and older (2.2–3.3 × 1022 Am2) groups (Roperch et al., 1988; Chauvin et al., 1989; Plenier et al., 2007; Laj et al., 2014). One of the Auckland excursions (Shibuya et al., 1992), which was correlated with the Laschamp excursion (Cassata et al., 2008), gives a weak paleointensity (0.6 × 1022 Am2: Mochizuki et al., 2006).
Paleointensity variation is investigated for a sequence of andesitic lavas which records the Noma geomagnetic excursion at 6.66 ± 0.45 Ma. Sixty-six samples are collected from 11 lava flows at the Kamegaoka Mountain (31° 21′N, 130° 13′E) on the Noma Peninsula, Kyushu Island, Japan. Rock magnetic experiments confirm the presence of Ti-poor titanomagnetite grains with pseudo-single-domain or a mixture of single and multi-domains as the carrier of the remanent magnetization. The application of the Tsunakawa-Shaw method yields reliable geomagnetic paleointensity data set of 29 samples from 9 lavas that cover the Noma excursion. The mean paleointensities of lava flows vary from 1.9 to 10.3 µT. Paleointensity variation reveals a ‘V -shape’ of high–low–high paleointensities during the excursion. The highest value of 10.3 ± 3.5 µT appears prior to the excursion, the paleointensity decreases down to 1.9 ± 0.3 µT as the excursion progresses, and the paleointensity finally regenerates to 5.1 ± 3.0 µT at the end of the excursion. The virtual dipole moment (VDM) during the Noma excursion (0.3–1.1 × 10²² Am²) is less than 16% of the mean VDM during the period between 4 and 8 Ma in Miocene, suggesting that this weak geomagnetic field intensity is ascribed to the nature of excursions rather than the Miocene geomagnetic field properties. The weakest VDM (0.3 × 10²² Am²) in the Noma excursion is detected at the pole position that is located at fairly high latitude (58.9°N) in the opposite hemisphere. The Noma excursion is characterized by a large shift in virtual geomagnetic poles of more than 140° from the geographic pole and by a weak VDM of 1.0 × 10²² Am² or less.
A high-resolution paleosecular variation record from Black Sea sediments indicating fast directional changes associated with low field intensities during marine isotope stage (MIS) 4
2018, Earth and Planetary Science Letters
A total of nine sediment cores recovered from the Archangelsky Ridge in the SE Black Sea were systematically subjected to intense paleo- and mineral magnetic analyses. Besides 16 accelerator mass spectrometry (AMS) ¹⁴C ages available for another core from this area, dating was accomplished by correlation of short-term warming events during the last glacial monitored by high-resolution X-ray fluorescence (XRF) scanning as maxima in both Ca/Ti and K/Ti ratios in Black Sea sediments to the so-called ‘Dansgaard–Oeschger events’ recognized from Greenland ice cores. Thus, several hiatuses could be identified in the various cores during the last glacial/interglacial cycle. Finally, core sections documenting marine isotope stage (MIS) 4 at high resolution back to 69 ka were selected for detailed analyses. At 64.5 ka, according to obtained results from Black Sea sediments, the second deepest minimum in relative paleointensity during the past 69 ka occurred, with the Laschamp geomagnetic excursion at 41 ka being associated with the lowest field intensities. The field minimum during MIS 4 is associated with large declination swings beginning about 3 ka before the minimum. While a swing to 50°E is associated with steep inclinations (50–60°) according to the coring site at 42°N, the subsequent declination swing to 30°W is associated with shallow inclinations of down to 40°. Nevertheless, these large deviations from the direction of a geocentric axial dipole field ( $I = 61 °$ , $D = 0 °$ ) still can not yet be termed as ‘excursional’, since latitudes of corresponding virtual geomagnetic poles (VGP) only reach down to 51.5°N (120°E) and 61.5°N (75°W), respectively. However, these VGP positions at opposite sides of the globe are linked with VGP drift rates of up to 0.2° per year in between. These extreme secular variations might be the mid-latitude expression of a geomagnetic excursion with partly reversed inclinations found at several sites much further North in Arctic marine sediments between 69°N and 81°N. Thus, the pronounced intensity minimum at 64.5 ka and described directional variations might be the effect of a weak geomagnetic field with a multi-polar geometry in the middle of MIS 4.
Paleomagnetic record for the past 80 ka from the Mahanadi basin, Bay of Bengal
2018, Journal of Asian Earth Sciences
High resolution paleomagnetic investigations were performed on a 50.08 m long sediment core (MD161/20) from Mahanadi basin, Bay of Bengal. Core yielded reliable paleomagnetic results for top 20 m below seafloor (mbsf) which spans about 80 ka. Based on the analysis of rock magnetic data, the core is subdivided into five distinct Zones: Zone 1 and Zone 2 cover top 20 mbsf and do not show any abrupt change in magnetic mineralogy, concentration and grain size. Zones 3 and 5 show significant reduction in χ_LF, χ_ARM and SIRM due to dissolution of magnetic minerals. Zone 4 shows moderate values of χ_LF and SIRM. The low value of χ_ARM suggests that magnetic signal is mostly carried by magnetic grains in PSD/MD state. The paleomagnetic data for the top 20 mbsf show four prominent geomagnetic excursions at ∼9 mbsf, ∼13.5 to 15 mbsf, ∼16.3 mbsf and ∼18 to 18.2 mbsf. The age-depth relationship is established using stratigraphic correlation between well-dated sedimentary core NGHP-01-19B and the core MD161/20. The ages of the observed excursions correspond to ∼18 to 20 ka, ∼42 to 49 ka, ∼54 to 57 ka and ∼69 to 70 ka. The excursions at ∼42 to 49 ka, ∼54 to 57 ka, and ∼67 to 70 ka is similar to the known excursions the Laschamp and the split Norwegian-Greenland Sea events (NGS-I and NGS-II). The excursion at 18–20 ka is not observed globally and may be related to lithological/sedimentological changes occurring during last glacial maxima (LGM). The virtual geomagnetic path (VGP) of Laschamp excursion traces clockwise loop. All excursions identified in present study fall in the periods of relatively low paleointensity.
Source of Oceanic Magnetic Anomalies and the Geomagnetic Polarity Timescale
2015, Treatise on Geophysics: Second Edition
Marine magnetic anomalies provide the framework for the geomagnetic polarity timescale for the Late Jurassic to Recent (since 160 Ma). Magnetostratigraphic records confirm that the polarity reversal sequence interpreted from magnetic anomalies is complete to a resolution of better than 30 ky. In addition to this record of polarity reversals, magnetic anomalies also appear to preserve information on geomagnetic intensity fluctuations. The correspondence of coherent near-bottom anomaly variations with independent estimates of field intensity provides strong evidence that geomagnetic intensity modulates the magnetization of the oceanic crust. Indeed, many short-wavelength anomaly variations in sea-surface magnetic profiles over fast-spreading ridges are likely attributable to geomagnetic intensity variations. Although longer-term geomagnetic field behavior may also be reflected in anomaly amplitudes, documenting such a signal requires a better understanding of time-dependent changes in the magnetic source (e.g., from low-temperature alteration) that may also affect magnetic anomalies.
The extrusive layer, with an average remanence of ∼ 5 A m^− 1, is the largest contributor to magnetic anomalies. However, enhanced sampling of oceanic gabbros (average remanence of ∼ 1 A m^− 1) and, to a lesser extent, dikes (average remanence of ∼ 2 A m^− 1) reveals that these deeper (and thicker) layers likely generate anomalies comparable to those from the lavas. Lava accumulation at intermediate- and fast-spreading ridges typically occurs over a narrow (1–3 km) region, and dike emplacement is even more narrowly confined, resulting in a relatively high-fidelity record of geomagnetic field behavior. The slow cooling of the gabbroic layer, however, results in gently dipping polarity boundaries that significantly affect the skewness of the resulting anomalies, which is also a sensitive measure of net rotations of the source layer(s). The magnetizations of the dikes and gabbros are characterized by high stability and are not expected to significantly change with time, although there are insufficient data to confirm this. The lavas, however, typically show evidence of low-temperature alteration, which has been long regarded as a process that progressively reduces the magnetization (and degrades the geomagnetic signal) in the extrusive layer and reduces the amplitude of magnetic anomalies. Sufficient data have become available to examine this conventional wisdom. There is a substantial (∼ 4 ×) reduction in magnetization from on-axis samples to immediately off-axis drill sites (∼ 0.5 My) but little further change in half-dozen or so deep crustal sites to ∼ 160 Ma. High paleointensity that characterizes the last few thousand years may contribute significantly to the high on-axis magnetization. The task of evaluating changes in remanence of the extrusive layer is made more difficult by substantial cooling-rate-dependent changes in magnetic properties and the systematic variation in remanence with iron content (magnetic telechemistry). The commonly cited magnetic anomaly amplitude envelope is in fact not systematically observed – the Central Anomaly is elevated at slow-spreading ridges but is not as prominent at faster-spreading rates. Nonetheless, magnetic anomaly amplitudes are consistent when magnetization change is poorly constrained. Direct determinations of the degree of low-temperature oxidation reveal the presence of highly oxidized titanomagnetite in samples less than 1 My old, suggesting a short (∼ 10⁵ years) time constant though the effects of low-temperature oxidation are quite heterogeneous. While low-temperature oxidation does have some effect on lava magnetization and anomaly amplitudes, there is increasing evidence that marine magnetic anomalies are capable of recording a broad spectrum of geomagnetic field behavior, ranging from millennial-scale paleointensity variations, to polarity reversals, to apparent polar wander, to, more speculatively, long-term changes in average field strength. Several emerging tools and approaches – autonomous vehicles, oriented samples, absolute paleointensity of near-ridge lavas, and measurements of the vector anomalous field – are therefore likely to significantly advance our understanding of the geomagnetic signal recorded in the oceanic crust, as well as our ability to utilize this information in addressing outstanding problems in crustal accretion processes.

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Paleointensity of the earth's magnetic field during the Laschamp excursion and its geomagnetic implications

Abstract

Phys. Earth Planet. Inter.

Phys. Earth Planet. Inter.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Phys. Earth Planet. Inter.

Earth Planet. Sci. Lett.

Sur la présence d'aimantations inversées dans la Chaiˆne de Puys

C. R. Acad. Sci. Paris

Geomagnetic excursions and their paleomagnetic records

Rev. Geophys. Space Phys.

The earth's magnetic field, its history, origin and planetary perspective

Geomagnetic record in Minnesota lake sediments—Absence of the Gothenburg and Eriau excursions

Geology

Geomagnetic excursions: a critical assessment of the evidence as recorded in sediments of the Brunhes Epoch

Philos. Trans. R. Soc. London, Ser. A

Sur l'intensitédu champ magnétique terrestre dans le passéhistorique et géologique

Ann. Géophys.

Sur la direction d'aimantation des laves de la Chaiˆne des Puys et le comportement de champ en France au cours de l'évènement du Laschamp

Thèse d'Etat

Investigations of some magnetic and mineralogical properties of the Laschamp and Olby flows, France

Quaternary Res.

Self-reversal of natural remanent magnetization in the Olby-Laschamp lavas

Nature

The Laschamp excursion

Philos. Trans. R. Soc. London, Ser. A

The determination of paleointensities of the earth's magnetic field with emphasis on mechanisms which could cause nonideal behaviour in Thellier method

J. Geomagn. Geoelectr.

Recherches géomagnétiques sur des coulées volcaniques d'Auvergne

Ann. Géophys.

Minéralogie de la lave de Laschamp

7th Réun. Ann. Sci. Terre