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Elevated Paleomagnetic Dispersion at Saint Helena Suggests Long-Lived Anomalous Behavior in the South Atlantic

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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 Yael A. Engbersa,1, Andrew J. Biggina, and Richard K. Bonoa aGeomagnetism Laboratory, Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool L69 7ZE, United Kingdom Edited by Lisa Tauxe, University of California San Diego, La Jolla, CA, and approved June 12, 2020 (received for review January 21, 2020) Earth’s magnetic field is presently characterized by a large and enhanced secular variation is suspected in the region above the growing anomaly in the South Atlantic Ocean. The question of African Large Low Shear Velocity Province (LLSVP; Fig. 1) whether this region of Earth’s surface is preferentially subject to located beneath a part of the South Atlantic region (28). Satellite enhanced geomagnetic variability on geological timescales has and ground-based observations confirm high present-day secular major implications for core dynamics, core−mantle interaction, variation in the Saint Helena area specifically (29). Several studies and the possibility of an imminent magnetic polarity reversal. Here suggest a link between the irregular field behavior and heteroge- we present paleomagnetic data from Saint Helena, a volcanic neous heat flow across the Core Mantle Boundary (CMB) (10, 14, island ideally suited for testing the hypothesis that geomagnetic 25). Lowermost mantle viscosity is sufficiently high [>1020 Pa·s field behavior is anomalous in the South Atlantic on timescales of (30)] to ensure that the margins of LLSVPs should be moderately millions of years. Our results, supported by positive baked contact stable on a timescale of at least 10 million years (My) (31) and and reversal tests, produce a mean direction approximating that potentially much longer (32), suggesting that the South Atlantic expected from a geocentric axial dipole for the interval 8 to 11 region should be showing persistently recurring anomalous be- million years ago, but with very large associated directional dis- havior for at least that timescale. persion. These findings indicate that, on geological timescales, geomagnetic secular variation is persistently enhanced in the vi- Paleomagnetic Data from Saint Helena cinity of Saint Helena. This, in turn, supports the South Atlantic as Existing paleomagnetic datasets (20) recovered from rocks less EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES a locus of unusual geomagnetic behavior arising from core−man- than 10 My old are scarce in the Southern Hemisphere, espe- tle interaction, while also appearing to reduce the likelihood that cially in the South Atlantic (Fig. 1). Here we report a detailed the present-day regional anomaly is a precursor to a global polarity reversal. study of paleosecular variation (PSV) from Saint Helena (Fig. 2), which provides a uniquely ideal opportunity to test the hypoth- South Atlantic Anomaly | reversals | secular variation | core dynamics | esis that geomagnetic variability is enhanced in the South At- core−mantle boundary lantic on a geological timescale. Saint Helena is an island in the South Atlantic consisting of two shield volcanoes that formed between ∼8 Ma and 11 Ma (33, 34). We performed paleodir- aleomagnetic records and geomagnetic observations show us ectional analyses on 51 sites from 46 basalt flows from four that, except for brief intervals associated with reversals and P different shields (SI Appendix, Fig. S1 and Table S1), three from excursions, Earth’s magnetic field is dominated by an axial dipole (1). Indeed, a long-standing hypothesis states that, if averaged over sufficient time (∼104 yto105 y), Earth’s magnetic field Significance approximates a dipole field that is aligned with the rotation axis of Earth, called a geocentric axial dipole (GAD) (2). On shorter Earth’s magnetic field is generated in the outer core by con- timescales and in the present day, there are spatially and tempo- vecting liquid iron and protects the atmosphere from solar rally complex features (3–5) that must be averaged out over time wind erosion. The most substantial anomaly in the magnetic for the GAD hypothesis to be valid. A primary example of these field is in the South Atlantic (SA). An important conjecture is features is the South Atlantic Anomaly (SAA) (Fig. 1), caused by that this region could be a site of recurring anomalies because − a substantial patch of reversed magnetic flux ∼2,900 km below of unusual core mantle conditions, but this has not previously Earth’s surface at the core−mantle boundary (6–10). The effects been tested on geological timescales. With paleodirectional data from rocks from Saint Helena, an island in the SA, we of the SAA in near-Earth space lead to an increase in the particles show that the directional behavior of the magnetic field in the in radiation belts, which causes radiation damage to satellites and SA did indeed vary anomalously between ∼8 million and 11 other spacecraft as well as being a hazard to astronauts (11). The million years ago. This supports the hypothesis of core−mantle fixity and existence of the SAA for times prior to the historical interaction being manifest in the long-term geomagnetic field record remains controversial (12). Additionally, since the growth behavior of this region. of the SAA is associated with an overall decay of the dipole field – (7, 13 15), it has been interpreted by some as a precursor to a Author contributions: A.J.B. designed research; Y.A.E. and A.J.B. performed research; geomagnetic reversal (16–18). Y.A.E. analyzed data; and Y.A.E., A.J.B., and R.K.B. wrote the paper. The discussion about the longevity and locus of the SAA has The authors declare no competing interest. inspired several studies, adding additional data and/or models, This article is a PNAS Direct Submission. – focusing on historic timescales (12, 22 26), or on the South Published under the PNAS license. Atlantic region in the last 10 ky to 300 ky (13, 27, 28). Data from Data deposition: The full dataset, including measurements, site mean directions, and Tristan da Cunha (Fig. 1) from 90 ka to 46 ka show that the VGPs, are available in the MagIC repository at http://earthref.org/MagIC/16824. virtual axial dipole moment measured at this island was weaker 1To whom correspondence may be addressed. Email: [email protected]. than elsewhere (27). Field models extending to 10 ka show This article contains supporting information online at https://www.pnas.org/lookup/suppl/ persistently higher secular variation activity in the Southern doi:10.1073/pnas.2001217117/-/DCSupplemental. Hemisphere relative to the Northern Hemisphere (13). Similarly, www.pnas.org/cgi/doi/10.1073/pnas.2001217117 PNAS Latest Articles | 1of6 Downloaded by guest on September 27, 2021 Angular deviation from geographic pole lava flows record directions that are categorized as transitional by a variable cutoff (Fig. 3B) (36). The flows in the upper shield 02° 04°°0 of the SE volcano (Prosperous Bay) capture a polarity reversal (Fig. 3A and SI Appendix, Fig. S12). Since we captured (at least) seven chrons, we consider it likely that sufficient time was sam- pled overall to adequately represent secular variation. Our mean pole (329.1°E, 81.1°N, A95 = 7.1°) is close to the 4 geographic pole, once this is corrected for the tectonic motion of 1 2 3 Saint Helena since 10 Ma. Therefore, the results are not more 6 5 than marginally inconsistent with the GAD hypothesis being valid in this region (Fig. 3B). 7 Evidence for Enhanced Secular Variation PH AH PH A remarkable feature of the Saint Helena dataset is that, despite directions being well clustered at the within-site level (k > 50), Saint Helena Paleolocation, Saint Helena indicating high-quality measurements, the VGP dispersion is high for its paleolatitude. To assess, quantitatively, the magni- Paleolocation Tristan da Cunha Paleolocations, PSV10 tude of directional variability evident in our dataset, we follow the approach and criteria of a recent global study of PSV over Fig. 1. Present-day magnetic field. Angular distance in degrees from GAD = per VGP from IGRF12 (International Geomagnetic Reference Field) for 2015 the last 10 My (20). Using our preferred dataset (n 34), we (4). The purple dashed lines indicate the boundary between the Atlantic calculate a dispersion, S, of 21.9° ± 3.5° [95% confidence range Hemisphere (AH) and the Pacific Hemisphere (PH). The black line marks a from 10,000 bootstraps (37)] (Fig. 4). Many of the flows were suggested boundary of the African and Pacific LLSVP at the CMB, defined as sampled in sequence, suggesting there is a risk of serial corre- a region with below-average shear wave velocity at 2,850 km depth in at lation (SC), which could underestimate secular variation. We least 10 of 18 mantle tomography models, as interpreted by voting map investigated this by calculating new VGP dispersions for the model (19). Green square marks the paleolocation of Tristan da Cunha dataset after accounting for possible SC (SI Appendix, section 2). (discussed in text). Red dots mark PSV10 locality paleolocations (SI Appendix, section 3 and Table S4) (20). The numbered locations mark specific localities Following investigation of a range of selection criteria, cutoffs, discussed in the text (1, Martinique; 2, Cape Verde; 3, Costa Rica; 4, Gua- and correction of SC, we conclude that the VGP dispersion may deloupe; 5, São Tomé; 6, Fernando de Noronha; 7, Réunion). The magenta have varied between 18.4° and 22.6° (up to 30.4° when no cutoff star marks the present location of Saint Helena, and the blue star marks the is applied). In all cases, the value obtained is substantially higher paleolocation of Saint Helena. All paleolocations are calculated using the than that expected value for this latitude (SI Appendix, Fig.
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