scholarly journals Geomagnetic Polarity Reversals in a Late Tertiary Lava Sequence from the Akaroa Volcano, New Zealand

1970 ◽  
Vol 21 (2) ◽  
pp. 163-183 ◽  
Author(s):  
A. L. Evans
Keyword(s):  
New Zealand ◽  
Late Tertiary ◽  
Lava Sequence ◽  
Polarity Reversals ◽  
Geomagnetic Polarity

A record of the lower Mammoth geomagnetic polarity reversal from a marine succession in the Boso Peninsula, central Japan

2021 ◽  
Author(s):  
Yuki Haneda ◽  
Makoto Okada
Keyword(s):  
Geomagnetic Field ◽  
Central Japan ◽  
High Accumulation ◽  
Significant Information ◽  
Boso Peninsula ◽  
Northern Latitudes ◽  
Lava Sequence ◽  
Directional Change ◽  
Polarity Reversals ◽  
Geomagnetic Polarity

Summary Palaeomagnetic records from geological archives provide significant information about the nature of geomagnetic polarity reversals; however, there are few detailed palaeomagnetic records of pre-Pleistocene reversals. The lower Mammoth Subchron boundary (late Pliocene) is recorded in a 10-m interval of a marine succession deposited at high accumulation rates (9–66 cm/kyr) in the Boso Peninsula, central Japan. Here, we report a continuous palaeomagnetic record of the lower, normal to reverse boundary interval of the Mammoth Subchron, including the geomagnetic field direction and relative palaeointensity, with an average temporal resolution of ca 800 years. A hybrid method of thermal demagnetization at 200° C and progressive alternating field demagnetization were used to effectively extract the primary palaeomagnetic component, which is carried by magnetite. The lower Mammoth transition is characterized by palaeomagnetic direction of instability and decay of the relative palaeointensity, and occurred from late Marine Isotope Stage MG3 (3351 ka) to MG2 (3336 ka) or MG1 (3331 ka), spanning 15–20 kyr. Virtual geomagnetic poles (VGPs), calculated from primary palaeomagnetic directions, rapidly rebounded twice from southern latitudes to northern latitudes within the transition. In contrast to the complex lower Mammoth reversal behavior recorded in the Boso Peninsula succession, records from a lava sequence in O'ahu (Hawai'i) reveal a rebound following a 180° directional change, and those from a marl succession in Sicily (Italy) indicate a single rapid directional change. Diverse geomagnetic field evolution among these three sections is reflected resolution difference among the records likely in combination with an influence of non-axial dipole field.

scholarly journals Possible configurations of the magnetic field in the outer magnetosphere during geomagnetic polarity reversals

2000 ◽  
Vol 18 (1) ◽  
pp. 11-27 ◽  
Author(s):  
D. M. Willis ◽  
A. C. Holder ◽  
C. J. Davis
Keyword(s):  
Magnetic Field ◽  
Spherical Cavity ◽  
Exact Equation ◽  
Outer Magnetosphere ◽  
Special Cases ◽  
Magnetospheric Configuration And Dynamics ◽  
Polarity Reversals ◽  
Field Lines ◽  
Geomagnetic Polarity ◽  
The Magnetic Field

Abstract. Possible configurations of the magnetic field in the outer magnetosphere during geomagnetic polarity reversals are investigated by considering the idealized problem of a magnetic multipole of order m and degree n located at the centre of a spherical cavity surrounded by a boundless perfect diamagnetic medium. In this illustrative idealization, the fixed spherical (magnetopause) boundary layer behaves as a perfectly conducting surface that shields the external diamagnetic medium from the compressed multipole magnetic field, which is therefore confined within the spherical cavity. For a general magnetic multipole of degree n, the non-radial components of magnetic induction just inside the magnetopause are increased by the factor {1 + [(n + 1)/n]} relative to their corresponding values in the absence of the perfectly conducting spherical magnetopause. An exact equation is derived for the magnetic field lines of an individual zonal (m = 0), or axisymmetric, magnetic multipole of arbitrary degree n located at the centre of the magnetospheric cavity. For such a zonal magnetic multipole, there are always two neutral points and n-1 neutral rings on the spherical magnetopause surface. The two neutral points are located at the poles of the spherical magnetopause. If n is even, one of the neutral rings is coincident with the equator; otherwise, the neutral rings are located symmetrically with respect to the equator. The actual existence of idealized higher-degree (n>1) axisymmetric magnetospheres would necessarily imply multiple (n + 1) magnetospheric cusps and multiple (n) ring currents. Exact equations are also derived for the magnetic field lines of an individual non-axisymmetric magnetic multipole, confined by a perfectly conducting spherical magnetopause, in two special cases; namely, a symmetric sectorial multipole (m = n) and an antisymmetric sectorial multipole (m = n-1). For both these non-axisymmetric magnetic multipoles, there exists on the spherical magnetopause surface a set of neutral points linked by a network of magnetic field lines. Novel magnetospheric processes are likely to arise from the existence of magnetic neutral lines that extend from the magnetopause to the surface of the Earth. Finally, magnetic field lines that are confined to, or perpendicular to, either special meridional planes or the equatorial plane, when the multipole is in free space, continue to be confined to, or perpendicular to, these same planes when the perfectly conducting magnetopause is present.Key words. Geomagnetism and paleomagnetism (reversals-process, time scale, magnetostratigraphy) · Magnetospheric physics (magnetopause, cusp, and boundary layers; magnetospheric configuration and dynamics)

scholarly journals Polar caps during geomagnetic polarity reversals

2018 ◽  
Vol 216 (2) ◽  
pp. 1334-1343 ◽  
Author(s):  
Bruno Zossi ◽  
Mariano Fagre ◽  
Hagay Amit ◽  
Ana G Elias
Keyword(s):  
Polar Caps ◽  
Polarity Reversals ◽  
Geomagnetic Polarity

Evolution and biogeography ofLyalliaandHectorella(Portulacaceae), geographically isolated sisters from the Southern Hemisphere

2007 ◽  
Vol 19 (4) ◽  
pp. 417-426 ◽  
Author(s):  
Steven J. Wagstaff ◽  
Françoise Hennion
Keyword(s):  
New Zealand ◽  
Southern Hemisphere ◽  
Common Ancestor ◽  
Growth Habit ◽  
Intergenic Spacer ◽  
Plant Evolution ◽  
Late Tertiary ◽  
Close Relationship ◽  
Divergence Estimates ◽  
Transoceanic Dispersal

AbstractThe Southern Hemisphere contains many monotypic taxa, for which phylogenetic relationships are important to illuminate biogeographical history. The monotypic genusLyalliais endemic to the sub-Antarctic Iles Kerguelen. A close relationship with another monotypic taxon, the New Zealand endemicHectorella, was proposed. They share a dense cushion growth habit with small coriaceous leaves that lack stipules. The solitary flowers are bicarpellate with two sepals, 4–5 petals, 3–5 stamens and a bifid style. The fruit is an indehiscent capsule with 1–5 seeds. The flowers ofLyallia kerguelensisare hermaphroditic with four petals and three stamens whereas the flowers ofHectorella caespitosaare female, male or hermaphroditic, with five petals and five stamens.Lyallia kerguelensisis rare on Kerguelen, whereasHectorella caespitosais confined to the South Island of New Zealand. Our phylogenetic analysis oftrnK/matK intergenic spacer andrbcL sequences provides evidence supporting a close relationship betweenLyalliaandHectorella. The two species form a well-supported clade that is nested within the Portulacaceae. Divergence estimates suggest they shared a common ancestor during the late Tertiary long after the fragmentation of Gondwana. Such relationships underscore the importance of transoceanic dispersal and extinctions for plant evolution in the Southern Hemisphere.

scholarly journals Studies in Middle and Late Tertiary Foraminifera From New Zealand

2021 ◽  
Author(s):  
◽  
Paul Vella
Keyword(s):  
New Zealand ◽  
Deep Water ◽  
New Genera ◽  
Upper Oligocene ◽  
Late Tertiary

<p>Many Uvigerinidae are important zone fossils in deep-water massive sandstones and mudstones of upper Oligocene and Miocene age in Raukumara Peninsula. Twenty-nine species and six subspecies are described, of which nineteen species and three subspecies are new. Subspecific classification is revised, five new genera and four new subgenera being established, partly on morphology and partly on lineage sequences. Time ranges are given in terms of eleven local zones which are correlated approximately with New Zealand stages and with European stages.</p>

scholarly journals A simplified correlation between vertebrate evolution and Paleozoic geomagnetism

2019 ◽  
Author(s):  
John P Staub
Keyword(s):  
Atmospheric Oxygen ◽  
Low Frequency ◽  
Strong Relationship ◽  
Evolutionary Patterns ◽  
Geologic Time Scale ◽  
Polarity Reversals ◽  
Low Levels ◽  
Geomagnetic Polarity ◽  
Taxonomic Groups ◽  
Biologic Factors

Background. Despite a fifty-year failure of paleontologists to find a viable connection between geomagnetic polarity reversals and evolutionary patterns, recent paleobiology databases show that the early appearance, radiation, and diversification of Paleozoic vertebrates tends to occur during periods having frequent collapses of the Earth’s geomagnetic field. The transition time during the collapse of the Earth’s protective magnetic shield can last thousands of years, and the effects on biota are unknown. Solar and cosmic radiation, volcanism, climate alteration, low-frequency electromagnetic fields, depletion of ozone, the stripping of atmospheric oxygen, and increasing production of Carbon14 in the stratosphere have been proposed as possible causes, but previous studies have found no effects. Methods. Using published databases, we compiled a spreadsheet showing the first appearance of 2104 genera with each genus assigned to one of 8 major taxonomic groups. From Gradstein’s Geologic Time Scale 2012, we delineated 17 Paleozoic zones with either high or low levels of polarity reversals. Results. From our compilation, 727 Paleozoic vertebrates represent the initial radiation and diversification of individual Paleozoic vertebrate clades. After compensating for sample-size and external geologic and sampling biases, the resulting Pearson’s correlation coefficient between the 727 genera and geomagnetic polarity zones equals 0.8, a result that suggests a strong relationship exists between Paleozoic vertebrates and geomagnetism. Discussion. The question: is this apparent connection between geomagnetism and the evolution of Paleozoic vertebrate due to environmental or biologic factors. If biologic, why are vertebrates the only biota effected? And after an indeterminate period of time, how do vertebrates become immune to the ongoing effects of polarity reversals?

scholarly journals Polarity Reversals in the Earth's Magnetic Field

Eos ◽  
2016 ◽  
Vol 97 ◽  
Author(s):  
Fabio Florindo
Keyword(s):  
Magnetic Field ◽  
Solid Earth ◽  
Earth’S Magnetic Field ◽  
Earth's Magnetic Field ◽  
Polarity Reversals ◽  
Geomagnetic Polarity

Studies of geomagnetic polarity reversals have generated some of the biggest and most interesting debates in the paleomagnetic and wider solid Earth geophysics communities over the last 25 years.

Sign in / Sign up

Export Citation Format

Share Document