Rare palaeomagnetic evidence of long-term mantle control of the geodynamo and possible role of the NAD field in the reversal process

Hoffman, Kenneth A.; Camps, Pierre; Carlton, Matt

doi:10.1093/gji/ggz480

Cited by 6 publications

(3 citation statements)

References 49 publications

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“…Therefore, this mechanism predicts and requires anomalous secular variation in the southern Atlantic−African region on the timescales that our data support. The concept of the lowermost mantle exerting control on the geodynamo and magnetic field on a timescale of millions of year is supported at least during intervals of reduced dipole moment (50).…”

Section: Implications For the Deep Earthmentioning

confidence: 99%

Elevated paleomagnetic dispersion at Saint Helena suggests long-lived anomalous behavior in the South Atlantic

Engbers

Biggin

Bono

2020

Proc. Natl. Acad. Sci. U.S.A.

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Earth’s magnetic field is presently characterized by a large and growing anomaly in the South Atlantic Ocean. The question of whether this region of Earth’s surface is preferentially subject to enhanced geomagnetic variability on geological timescales has major implications for core dynamics, core−mantle interaction, and the possibility of an imminent magnetic polarity reversal. Here we present paleomagnetic data from Saint Helena, a volcanic island ideally suited for testing the hypothesis that geomagnetic field behavior is anomalous in the South Atlantic on timescales of millions of years. Our results, supported by positive baked contact and reversal tests, produce a mean direction approximating that expected from a geocentric axial dipole for the interval 8 to 11 million years ago, but with very large associated directional dispersion. These findings indicate that, on geological timescales, geomagnetic secular variation is persistently enhanced in the vicinity of Saint Helena. This, in turn, supports the South Atlantic as a locus of unusual geomagnetic behavior arising from core−mantle interaction, while also appearing to reduce the likelihood that the present-day regional anomaly is a precursor to a global polarity reversal.

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Section: Implications For the Deep Earthmentioning

confidence: 99%

Elevated paleomagnetic dispersion at Saint Helena suggests long-lived anomalous behavior in the South Atlantic

Engbers

Biggin

Bono

2020

Proc. Natl. Acad. Sci. U.S.A.

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“…The geomagnetic field intensity data as well as directional records during polarity transitions contain essential information about the Earth's core and its boundary conditions, which are expected to lead to geodynamic models that can explain geomagnetic reversals (e.g., Merrill and McFadden 1999;Leonhardt and Fabian 2007;Nakagawa 2020;Tassin et al 2021). In particular, preferred paths of transitional virtual geomagnetic poles (VGPs) are considered to hold invaluable information on the conditions at the core-mantle boundary, which have been made evident during the transitions (Clement 1991;Laj et al 1991;Hoffman and Mochizuki 2012;Hoffman et al 2020). Importantly, the reversal process is rapid but dynamic compared with the normal process of geomagnetism.…”

Section: Introductionmentioning

confidence: 99%

Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes

Oda

Nakazato

Nanayama

et al. 2022

Earth Planets Space

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We report paleomagnetic records of Matuyama–Brunhes geomagnetic polarity reversal and associated key tephra layers from the Early–Middle Pleistocene marine sedimentary succession in the Boso Peninsula. The outcrop is in Terasaki, Chiba, Japan and ~ 25 km northeast of the Chiba section. The sediment succession consists of a massive siltstone layer of the Kokumoto Formation, Kazusa Group. A tephra layer was identified in the middle of the outcrop with chemical composition comparable to that of the Byk-E tephra layer from the Chiba section defining the base of the Chibanian Stage. Oriented paleomagnetic samples were collected at intervals of 1–10 cm from the siltstone. To identify the primary remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) were conducted on pilot samples. Identification of primary magnetization with PAFD was not successful, especially for reversely magnetized samples. In addition, magnetization during PThD showed sharp drops around 175 °C, which decreased gradually between 175 °C and ~ 300 °C, and became unstable above ~ 350 °C. To extract the primary remanent magnetization while avoiding laboratory alteration by heating, a PThD up to 175 °C followed by PAFD was conducted. Combined analysis of remagnetization circles enables extraction of primary magnetization with improved reliability. Rock magnetic experiments were conducted during stepwise heating to understand the magnetic minerals involved and to evaluate the influence of laboratory heating. During heating, FORC-PCA revealed significant changes of magnetic minerals at 200 °C, 400 °C, 450 °C and 550 °C. Rock magnetic analyses and electron microscopy indicate that titanomagnetite/magnetite are magnetic minerals contributing to primary remanent magnetization. Greigite was also identified preserving secondary magnetizations during sub-seafloor diagenesis. The presence of feroxyhyte is suggested as secondary magnetization through the weathering of pyrite by exposure to the air after the Boso Peninsula uplift. The correlation of relative paleointensity with the Chiba section provides an age model with sedimentation rates of 30 cm/kyr and 18 cm/kr for the intervals above and below the Byk-E tephra. VGP latitudes are highly consistent with those from the Chiba section based on the age model, which assigns the main directional swing from reversed to normal polarities as 772.8 ± 0.5 ka. Graphical Abstract

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“…Merrill and McFadden, 1999;Leonhardt and Fabian, 2007;Nakagawa, 2020;Tassin et al, 2021). In particular, preferred paths of transitional virtual geomagnetic poles (VGPs) are considered to hold invaluable information on the conditions at the core-mantle boundary, which have been made evident during the transitions (Clement, 1991;Laj et al, 1991;Hoffman and Mochizuki, 2012;Hoffman et al, 2020). Importantly, the reversal process is rapid but dynamic compared with the normal process of geomagnetism.…”

Section: Introductionmentioning

confidence: 99%

Matuyama-brunhes Geomagnetic Reversal Record and Associated Key Tephra Layers in Boso Peninsula: Extraction of Primary Magnetization of Geomagnetic Fields From Mixed Magnetic Minerals of Depositional, Diagenesis, and Weathering Processes

Oda¹,

Nakazato²,

Nanayama³

et al. 2021

Preprint

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We describe paleomagnetic records of Matuyama-Brunhes geomagnetic polarity reversal and associated key tephra layers from Early-Middle Pleistocene sediments in the Boso Peninsula. The outcrop is in Terasaki, Chiba, Japan and ~ 25 km northeast of the Chiba Section. The sediment consists of massive silt of the Kokumoto Formation, Kazusa Group, underlaid by thick sand. A tephra layer was identified in the middle of the outcrop with chemical composition comparable to that of the tephra layer Byk-E from the Yoro River section defining the base of the Chibanian Stage. Oriented paleomagnetic samples were taken at intervals of 1–10 cm from the massive silt spanning the vitric tephra layer. To identify the primary remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) were conducted on pilot samples. Identification of primary magnetization with PAFD was not successful, especially for reversely magnetized samples. However, magnetization during PThD showed sharp drops at 175°C, which decreased gradually between 175°C and ~ 300°C, and became unstable above ~ 350°C. To extract the primary remanent magnetization while avoiding laboratory alteration by heating, a PThD up to 175°C followed by PAFD was conducted. Extraction of primary magnetization was significantly improved by applying a combined analysis of remagnetization circles, which was in agreement with the records reported from the Chiba Section. Rock magnetic experiments were conducted during stepwise heating to understand the magnetic minerals involved and to evaluate the influence of laboratory heating. During heating, FORC-PCA revealed significant changes of magnetic minerals at 200°C, 400°C, 450° and 550°C. Rock magnetic analyses and electron microscopy observations indicate that magnetite and titanomagnetite are magnetic minerals contributing to primary remanent magnetization. Greigite was also identified, which preserve secondary magnetizations during sub-seafloor diagenesis. The presence of feroxyhyte is suggested to contribute to secondary magnetization through the weathering of pyrite by exposure to the air after the Boso Peninsula uplift. The similarity of VGP latitude variations between this study and those from the Chiba section was maximized by providing an age model with sedimentation rates of 30 cm/kyr and 18 cm/kr for the intervals above and below the Byk-E tephra.

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Rare palaeomagnetic evidence of long-term mantle control of the geodynamo and possible role of the NAD field in the reversal process

Cited by 6 publications

References 49 publications

Elevated paleomagnetic dispersion at Saint Helena suggests long-lived anomalous behavior in the South Atlantic

Elevated paleomagnetic dispersion at Saint Helena suggests long-lived anomalous behavior in the South Atlantic

Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes

Matuyama-brunhes Geomagnetic Reversal Record and Associated Key Tephra Layers in Boso Peninsula: Extraction of Primary Magnetization of Geomagnetic Fields From Mixed Magnetic Minerals of Depositional, Diagenesis, and Weathering Processes

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