Timescales of the Paleomagnetic Field

320 Pages·2004·38.043 MB·English

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Geophysical Monograph 145
Timescales of the Paleomagnetic Field
James E. T. Channell
Dennis V. Kent
William Lowrie
Joseph G. Meert
Editors
88 American Geophysical Union
Washington, DC
Published under the aegis of the AGU Books Board
Jean-Louis Bougeret, Chair; Gray E. Bebout, Carl T. Friedrichs, James L. Horwitz, Lisa A. Levin, W. Berry Lyons,
Kenneth R. Minschwaner, Andy Nyblade, Darrell Strobel, and William R. Young, members.
Library of Congress Cataloging-in-Publication Data
Timescales of the paleomagnetic field / James E.T. Channell... [et al.], editors,
p. cm. -- (Geophysical monograph ; 145)
Includes bibliographical references.
ISBN 0-87590-410-6
1. Paleomagnetism. 2. Stratigraphic correlation. I. Channell, J. II. Series.
QE501.4.P35T56 2004
538'.727-dc22
2004057403
ISBN 87590-410-6
ISSN 0065-8448
Copyright 2004 by the American Geophysical Union
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Front Cover: Snapshot from a geodynamo simulation by Gary A. Glatzmaier (University of
California, Santa Cruz) and Paul H. Roberts (University of California, Los Angeles).
Back Cover: Adapted from E. Irving, Figure 7, page 18, this volume.
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CONTENTS
Preface
James E. T. Channell, Dennis V Kent, William Lowrie, and Joseph C. Meert vii
Parti.
The Geocentric Axial Dipole (GAD) Hypothesis, Continental Reconstruction and
Long-Term Geomagnetic Field Behavior
Geocentric Axial Dipole Hypothesis: A Least Squares Perspective
Michael McElhinny 1
The Case for Pangea B, and the Intra-Pangean Megashear
E. Irving 13
The Quality of the European Permo-Triassic Paleopoles and Its Impact on Pangea Reconstructions
Rob Van der Voo and Trond H. Torsvik 29
On the Origin and Distribution of Magnolias: Tectonics, DNA and Climate Change
R. J. Hebda and E. Irving 43
A Long-Term Octupolar Component in the Geomagnetic Field? (0-200 Million Years B.P.)
Vincent Courtillot and Jean Besse 59
The Paradox of Low Field Values and the Long-Term History of the Geodynamo
John A. Tarduno and Alexei V Smirnov ..75
Intensity and Polarity of the Geomagnetic Field During Precambrian Time
David J. Dunlop and Yongjae Yu 85
A Simplified Statistical Model for the Geomagnetic Field and the Detection of Shallow Bias in
Paleomagnetic Inclinations: Was the Ancient Magnetic Field Dipolar?
Lisa Tauxe and Dennis V Kent 101
Part 2.
Magnetic Polarity Stratigraphy and Acquisition of Magnetization
Geomagnetic Polarity Timescales and Reversal Frequency Regimes
William Lowrie and Dennis V. Kent 117
A Middle Eocene-Early Miocene Magnetic Polarity Stratigraphy in Equatorial Pacific
Sediments (ODP Site 1220)
Josep M. Pares and Luca Land 131
Astronomical Tuning and Duration of Three New Subchrons (C5r.2r-1n, C5r.2r-2n and C5r.3r-1n)
Recorded in a Middle Miocene Continental Sequence From NE Spain
Hayfaa Abdul Aziz and Cor G. Langereis 141
Non-Uniform Occurrence of Short-Term Polarity Fluctuations in the Geomagnetic Field? New Results
from Middle to Late Miocene Sediments of the North Atlantic (DSDP Site 608)
Wout Krijgsman and Dennis V Kent 161
40Ar/39Ar Chronology of Late Pliocene and Early Pleistocene Geomagnetic and Glacial Events in
Southern Argentina
Brad S. Singer, Laurie L. Brown, Jorge O. Rabassa, and Herve Guillou 175
After the Dust Settles: Why Is the Blake Event Imperfectly Recorded in Chinese Loess?
Josep M. Pares, Rob Van der Voo, Maodu Yan, and Xiaomin Fang 191
The Matuyama Chronozone at ODP Site 982 (Rockall Bank): Evidence for Decimeter-Scale
Magnetization Lock-in Depths
J.E.J. ChannellandY. Guyodo 205
Part 3.
Short-Term Field Behavior: The Reversal Process, Secular Variation and Paleointensity
The Complexity of Reversals
Robert Coe and Jonathan M.G. Glen 221
Regionally Recurrent Paleomagnetic Transitional Fields and Mantle Processes
Kenneth A. Hoffman and Brad S. Singer 233
Paleomagnetic Intensity Data as a Time Sequence: Opening a Window Into Dynamics of Earth's
Fluid Core?
Ross Baker and Keith Aldridge 245
High Resolution Global Paleointensity Stack Since 75 kyr (GLOPIS-75) Calibrated to
Absolute Values
Carlo Laj, Catherine Kissel, and Juerg Beer 255
Historic Archaeomagnetic Results From the Eastern U.S., and Comparison with Secular
Variation Models
Stacey Lengyel and Rob Sternberg 267
Low Pacific Secular Variation
David Gubbins and Steven J. Gibbons 279
Intensity-Inclination Correlation for Long-Term Secular Variation of the Geomagnetic Field and Its
Relevance to Persistent Non-Dipole Components
Toshitsugu Yamazaki and Hirokuni Oda 287
An Equivalent Source Model for the Geomagnetic Field
C.G.A. Harrison 299
Earth's Magnetic Field
Neil D. Opdyke and Victoria Mejia 315
PREFACE
To mark the 70th birthday of Neil D. Opdyke, a Chapman at Rice, they traveled under a Fulbright fellowship to Ted Irv­
Conference entitled "Timescales of the Internal Geomagnetic ing's laboratory in Canberra, Australia, and from there to Sal­
Field" was held at the University of Florida in Gainesville isbury (now Harare, Zimbabwe). Neil's paleomagnetic work
on March 9-11, 2003. This AGU Chapman Conference was in Africa in the early 1960s contributed significantly to the
sponsored by the U.S. National Science Foundation, Univer­ eventual documentation of continental drift, and this work
sity of Florida, Florida Museum of Natural History, and 2G still forms an integral part of African apparent polar wander
Enterprises. Forty-one talks and twenty-three posters were paths.
presented during the three-day meeting. This monograph con­ One of Neil's most important contributions to Earth Science
tains twenty-four of those papers, and is a balanced subset was his pioneering use of magnetic polarity stratigraphy as a
of the papers presented at the conference. The monograph is means of global correlation. His magnetostratigraphic stud­
divided into three parts. Part 1 deals with the geocentric axial ies on marine piston cores done at Lamont Doherty Geolog­
dipole (GAD) hypothesis, continental reconstruction, and ical (now Earth) Observatory in the mid-1960s remain a model
long-term geomagnetic field behavior. Part 2 comprises papers of how biomagnetostratigraphy should be done, and estab­
on magnetic polarity stratigraphy and the acquisition of sed­ lished the importance of magnetic stratigraphy as an integral
imentary magnetization. Part 3 deals with secular variation, component of geologic timescales. In 1969, Opdyke and
paleointensity, and short-term geomagnetic field behavior. Henry used marine core data for a convincing test of the GAD
These are all topics that have been substantially impacted by hypothesis that is central to the use of paleomagnetism in
Neil's scientific work. continental reconstruction. Neil's work with N.J. Shackleton
Some 40 years after the 1960s revolution in the Earth Sci­ in 1973 marked the beginning of the integration of oxygen iso­
ences, most of the practitioners of that time have retired from tope stratigraphy and magnetostratigraphy that has led to cur­
the scene. One who has not yet retired and maintains his irre­ rent methods of tuning timescales. Neil pioneered magnetic
pressible zest for our science is Neil Opdyke. A brief synop­ stratigraphy in terrestrial (non-marine) sediments and pro­
sis of Neil's research accomplishments (outlined below) does duced some of the most impressive records, notably from
not, in itself, do justice to the inspiration that many of us con­ Pakistan and southwestern USA. These studies led to a vastly
tinue to receive through Neil's good-natured enjoyment of improved time frame for vertebrate evolution and allowed
sound scientific debate. He has been a regular participant and the documentation of mammal migration.
mainstay of the GP section at AGU for about half a century! Since his move from Lamont to the University of Florida in
After graduating in geology from Columbia College in 1981, Neil has constructed the first polarity timescale for the
New York, Neil began his career in paleomagnetism follow­ Carboniferous and Early Permian using magnetostratigraphic
ing a chance meeting with S.K. Runcorn in the early sum­ data from Colorado, Pennsylvania, NE Canada and Australia.
mer of 1955. Work as a field assistant with Runcorn in Arizona He has been involved in paleomagnetic studies in China that
during that summer led to his recruitment to Cambridge as a refined our knowledge of the paleogeography and tectonic
graduate student. When, in 1956, Runcorn moved to the history of the vast Asian landmass. Neil is presently engrossed
Department of Physics at King's College, Durham Univer­ in studies of magnetic directions and paleointensities in young
sity (later to become the University of Newcastle-upon-Tyne), (<5 Ma) volcanic rocks along the American Cordillera from
Neil moved out of the drafty rooms of Gonville and Caius Patagonia to Alaska, as well as from Australia. These projects
College (Cambridge) to the damp North Sea climes of New­ are designed to determine the precise time-averaged struc­
castle. The seeds of quantification of continental drift were ture of the geomagnetic field, central to the refinement of the
being sown during the late 1950s both at Cambridge and at GAD hypothesis and for models of the geodynamo.
Newcastle, and Neil's research aimed to establish the case This brings us to the topics covered in this volume.
for continental drift from both paleomagnetic data and paleo- Part 1 is mainly concerned with the GAD hypothesis, the
wind directions. In 1958, Neil moved from Newcastle to Rice idea that the time-averaged geomagnetic field closely approx­
University marrying Margie Wilson on the way. After a year imates the field of a geocentric axial dipole, which has served
us well for reconstruction of the mosaic of continents and
plates through time. At the next level of reconstruction, how­
Timescales of the Paleomagnetic Field
Geophysical Monograph Series 145 ever, inconsistencies are apparent and have been difficult to
Copyright 2004 by the American Geophysical Union reconcile. The GAD hypothesis has been challenged on two
10.1029/145GM0O counts. First, paleomagnetic data may imply persistent con-
vii
tributions of significant non-axial dipole (NAD) fields, par­ short-lived (~5 kyr duration) polarity subchrons or excur­
ticularly for the Tertiary of central Asia, for the Permo-Triassic sions in the Brunhes and Matuyama Chrons, coupled with
of Pangea, and possibly for much of the Precambrian. In addi­ high-quality relative paleointensity records, indicate that peri­
tion, analyses of paleomagnetic data for the last 5 Myr indi­ ods of low paleointensity are frequently accompanied by
cate small but significant NAD contributions in the short-lived but major perturbations of the direction of the
time-averaged field. The series of papers in Part 1 take different geomagnetic field. The study of sub-Milankovitch-scale pale-
views on these issues, ranging from evidence for significant oclimate requires stratigraphic correlation at an appropriate
NAD fields, to explanations of apparent time-averaged NAD (millennial-scale) resolution. As correlation at this scale is
features in terms of artifacts of data distribution or recording not easily achieved through traditional isotopic methods, geo­
process. magnetic paleointensity records and associated directional
Part 2 deals with magnetic polarity stratigraphy, chrono- perturbations will become significant for this purpose. In
stratigraphy, and the acquisition of magnetization in sedi­ addition, understanding this short-term geomagnetic behav­
ments. The geomagnetic polarity record is central to the ior is important for constraining models of the geodynamo.
construction of geologic timescales, and provides the princi­ This volume is dedicated to Neil Opdyke on the occasion
pal tool for calibration of marine and terrestrial biozonations. of his 70th birthday. In recognition of the importance of his
The polarity record continues to evolve with the recognition work to the Earth Sciences, he has been awarded the George
of brief polarity subchrons, and the limitations to this evolu­ P. Woollard Award of the Geological Society of America
tion may lie with the sedimentary recording process. The (1987), the Fleming Medal of the American Geophysical
polarity record of the geomagnetic field indicates changes in Union (1996), and has been elected to the National Academy
reversal frequency on 107-108 year time-scales that are incom­ of Sciences (1996) and the American Academy of Arts and Sci­
patible with core processes and must, therefore, be attributed ences (1998).
to prolonged interactions between the mantle and core.
Part 3 deals with secular variation and short-term field James E.T. Channel!
behavior, toward the other end of the variability spectrum. Dennis V Kent
High-sedimentation-rate marine and lake sediments have, in William Lowrie
the last few years, revolutionized our understanding of the Joseph G. Meert
behavior of the geomagnetic field. The presence of ubiquitous Editors
viii
Geocentric Axial Dipole Hypothesis: A Least Squares
Perspective
Michael McElhinny
Gondwana Consultants, Port Macquarie, New South Wales, Australia
Departures from the geocentric axial dipole (GAD) model of the time-averaged
paleomagnetic field have been proposed both for the time-interval 0-5 Ma and for
the Mesozoic and Paleozoic. At present the basic problem is that there are not
enough data of sufficient quality to be able to determine second order terms other
than a small persistent geocentric axial quadrupole (GAQ). Even for the interval 0-5
Ma using the lava flow database, the majority of the data were derived around 25 years
ago when demagnetization analytical procedures were not as robust as those currently
in use. There are many ways in which an artificial geocentric axial octupole (GAO)
term can arise from data artifacts and these are discussed in some detail using a
least squares approach. No differences between the normal and reverse fields are seen
in the data. Application of the random paleogeography test suggests that persistent
GAO terms are present in the Mesozoic, Paleozoic and Precambrian. However it has
now been shown that the method is flawed because the basic assumption of ran­
dom paleogeography is not valid. It has been proposed that the problem of the recon­
struction of Pangea (Pangea A versus Pangea B) during the early Mesozoic and
Paleozoic can be resolved if persistent GAO terms are present. However, many of
the data used for this conclusion were derived in the 1970s and need to be redone
using modern methods. At present the only distinguishable second order feature is
a persistent GAQ of about 4% of the GAD field.
1. INTRODUCTION hypothesis when putting forward the concept of the apparent
polar wander path for the interpretation of paleomagnetic
The Geocentric Axial Dipole (GAD) hypothesis represents results from Great Britain. This use of the GAD hypothesis is
the fundamental assumption used when calculating paleo­ now standard procedure in paleomagnetism.
magnetic poles for use in determining apparent polar wander It was originally thought that the validity of the GAD
paths. It was first introduced into paleomagnetism by Hos- hypothesis was demonstrated by the fact that paleomagnetic
pers [1954], who made use of the new statistical methods poles for the past few million years centered about the pres­
developed by Fisher [1953]. This was the first demonstration ent geographic pole [Cox and Doell, 1960; Irving, 1964;
that the average of virtual geomagnetic poles (VGP) over sev­ McElhinny, 1973]. Unfortunately this observation is not by
eral thousand years in Recent times centered about the geo­ itself sufficient to demonstrate that the time-averaged field is
graphic pole. Creer et al. [1954] explicitly invoked the GAD o
purely that of a geocentric axial dipole (g\). Any geocentric
Timescales of the Paleomagnetic Field
axial field represented by zonal harmonics (g ,g>g3 etc)
Geophysical Monograph Series 145 x 2
will also produce paleomagnetic poles centered about the
Copyright 2004 by the American Geophysical Union
geographic pole. Opdyke and Henry [1969], using the incli­
10.1029/145GM01
nations observed in 52 deep-sea sediment cores world-wide,
1
2 GEOCENTRIC AXIAL DIPOLE HYPOTHESIS
(a) All Data
showed that the plot of mean inclination versus latitude was
180°E
consistent with the latitude variation of inclination expected
from the GAD hypothesis for the past few million years
82-
(Figure 1). However, this was only a first-order solution and Reverse
showed that the GAD was the dominant term. Wilson [1970, 84- 2488
/—X
1971, 1972] noted that the paleomagnetic poles for the past
86-
few million years tended to plot too far away from the obser­
vation site along a great circle joining the site to the geo­ 88- Normal
graphic pole. Also the poles tended to plot to the right of 90°N 4455 90°E|
the geographic pole when viewed from the observation site.
(b) All Igneous Rocks (c) Lava Flows
These effects were referred to as the far-sided effect and the
180°E 180°E
right-handed effect.
Wilson [1971] introduced the concept of the common-site
82- 82-
longitude pole position in which all observation sites are
placed at zero longitude. This was a convenient way to analyze 84- Reverse \ 84- Reverse \
the far-sided and right-handed effects. A recent analysis of 1422 \ \971 A
86- 86-
observations covering the time interval 0-5 Ma using this
'<f>} \
method by Quidelleur et al. [1994] and McElhinny et al. 88 V^SZ Normal A 88- K_y Normal A
[1996] is shown in Figure 2 for separated normal and reverse 90°N 2986 90oE| 90°N 1976 90oE|
polarity data. The mean of the reverse polarity data is signif­ Figure 2. Common site longitude global mean pole positions for
icantly more far-sided and right-handed than that for the nor­ 0-5 Ma plotted on a polar stereographic projection (latitudes >80°N)
mal polarity data. This difference between the means of the with their 95% circles of confidence. The numbers of sites used in
detennining each mean are indicated, (a) and (b) are from global data
normal and reverse polarity data is not significant when only
analyzed by McElhinny et al. [1996]. (c) is from a global lava data­
data from igneous rocks are considered, although the far-sided
base analyzed by Quidelleur et al. [1994].
and right-handed effects remain. Wilson [1970, 1971, 1972]
modeled the far-sided effect as originating from an axial dipole
source displaced northward along the axis of rotation. How­
ever, this is a non-unique solution for modeling geomagnetic
90-r
field sources and it is more appropriate to use spherical har­
monics to describe these effects. In this case the offset dipole
model is equivalent to a geocentric axial dipole (gf) plus a
geocentric axial quadrupole (g2). Egbert [1992] has shown
that there is a natural sampling bias in VGP longitudes in
which their distribution peaks 90° away from the sampling
longitude. However, the observed right-handed effect is prob­
ably too large to be explained in this way. It now appears that
the right-handed effect may represent an artifact resulting
from inadequate magnetic cleaning or arise from the poor
geographical distribution of the data.
The first attempts to carry out spherical harmonic analyses
of the time-averaged field over the past few million years
were carried out by Wells [1973], Creer et al. [1973] and
Georgi [1974]. However, the use of poor quality or unevenly
distributed data can result in very inaccurate spherical har­
monic descriptions as reflected in the widely different results
obtained by these authors. Wells [1973] concluded that only
the zonal harmonics were significant after a careful consid­
eration of all the errors involved. However, Creer et al. [1973]
Figure 1. Mean inclinations observed in 52 deep-sea sediment cores
and Georgi [1974] concluded that there were significant non-
covering the past few million years plotted as a function of latitude.
The solid curve is the variation expected from a GAD field. Redrawn zonal harmonics present, some of them of comparable size
from Opdyke and Henry [1969]. to the zonal ones.