An appraisal of a new method for the full vector reconstruction of the Earth's magnetic field [Elektronische Ressource] : applied to volcanic rocks from Mexico / vorgelegt von Daniel M. Michalk

Michalk, Samuel Daniel

Sujets

• Physik

Informations

• Publié par: universitat_potsdam
• Publié le: 01 janvier 2009
• Nombre de lectures: 4
• Langue: English
• Poids de l'ouvrage: 6 Mo

Extrait

Helmholz-Zentrum Potsdam Deutsches GeoForschungsZentrum,
Sektion 5.2: Klimadynamik und Landschaftsentwicklung






An appraisal of a new method for the full-vector
reconstruction of the Earth’s magnetic field
- applied to volcanic rocks from Mexico







Dissertation
zur Erlangung des akademischen Grades
"doctor rerum naturalium"
(Dr. rer. nat.)
in der Wissenschaftsdisziplin Geophysik










eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät
der Universität Potsdam




vorgelegt von

Daniel M. Michalk

Potsdam, Februar 2009
Doctoral thesis by Daniel M. Michalk

Supervised by Dr. habil. Norbert R. Nowaczyk and Professor Dr. Gerald Haug
Co-supervised by Dr. Harald Böhnel (Geociencias UNAM, Mexico)


Reviewers:

Professor Dr. Gerald Haug (Universität Potsdam)
Professor Dr. Heinrich Soffel (Ludwig-Maximilians-Universität München)
P.D. Dr. Michael Urbat (Universität zu Köln)

thDate of oral defense: 20 of May, 2009














































Published online at the
Institutional Repository of the University of Potsdam:
http://opus.kobv.de/ubp/volltexte/2009/3186/
urn:nbn:de:kobv:517-opus-31868
[http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-31868] Table of contents


Abstract iii

Zusamenfasung v

Acknowledgements vii

1. INTRODUCTION

1. Introduction 1
1.2 The importance of a full-vector geomagnetic field description 4
1.3 Objectives ofthis study 5
1.4 Thesi trucre 5
1.5 Studyarea 6
1.6 Sampling 7 7Methods 9
1.8 Detrmination of the absolute palaeointensity: The most widely used methods, 10
their advantages and limitations
1.9 Databases of absolute palaeointensities 12
1.10Review ofmanuscripts 14
1.11 Conclusions 20
1.12Future outlook 21

2. MANUSCRIPT 1 23-34

The use of mini-samples in paleomagnetism

(Harald N. Böhnel, Daniel M. Michalk, Norbert R. Nowaczyk
and Gildardo Gonzalez)

3. MANUSCRIPT 2 35-51

Evaluation of the multispecimen parallel differential pTRM method:
A test on historical lavas from Iceland and Mexico

(Daniel M. Michalk, Adrian R. Muxworthy, Harald N. Böhnel,
John Maclennan and Norbert R. Nowaczyk)

4. MANUSCRIPT 3 53-77

Evidence for geomagnetic excursions recorded in Brunhes- and
Matuyama – Chron lavas from the Trans-Mexican Volcanic Belt

(Daniel M. Michalk, Norbert R. Nowaczyk, Harald N. Böhnel, Gerardo J.
Aguírre-Diaz, Steven Ownby, Margarita López-Martínez,
and Jörg F.W. Negendank)
i
5. MANUSCRIPT 4 79-105

Application of the multispecimen palaeointensity method to Pleistocene
lava flows from the Trans-Mexican Volcanic Belt

(Daniel M. Michalk, Andrew J. Biggin, Mads F. Knudsen, Harald N. Böhnel, and
Norbert R. Nowaczyk)

Refrnces 107-122

iiAbstract


The Earth’s magnetic field (EMF) is generated by convections in the electrically conducting liquid iron-
rich outer core, modified by the Earth’s rotation. A drastic manifestation of the dynamics of this fluid
body is the occurrence of geomagnetic field reversals in the Earth’s history but also geomagnetic
excursions, which are more frequent features of otherwise stable polarity chrons, but often poorly
constrained in the geological record. Even though numerical models of the highly complex dynamics in
the liquid outer core exist, the computing power needed for these models is still insufficient. To better
understand the origin of the field, we need to know how the field has varied on different geological
timescales. This includes not only information about changes in the ancient field’s direction but also
about the absolute intensity (palaeointensity) and the age. This palaeointensity record is needed for
compiling a full-vector description of the field. A palaeomagnetic and palaeointensity study on lava flows
allows gaining insights about the evolution of the EMF through time and space. However, constraining
the EMF evolution over different geological timescales remains a difficult objective due to the paucity of
available palaeointensity data. This is because most current methods of palaeointensity determination are
based on various modifications of the Thellier method, which only works on material of single-domain
magnetic structure, and which suffers from often high failure rates and time consuming laboratory work.
In consequence the global database of absolute palaeointensities still remains sparse through time and
space. One new alternative approach in palaeointensity studies is the recently proposed multispecimen
parallel differential pTRM (MS) method, which has potentially several advantages over the commonly
used Thellier method, because it is in theory independent of magnetic domain state, less prone to biasing
effects, such as thermal alteration and significantly faster to perform in the laboratory. Thus, in theory, it
offers the possibility to acquire significantly larger data sets of palaeointensities.

A study of highly active volcanic regions, such as the Trans-Mexican Volcanic Belt, seems promising
when attempting a full-vector reconstruction or when looking for field excursions. One aim of this thesis
was to gain new information about the occurrence and global validity of geomagnetic excursions from the
Brunhes- or Matuyama Chron. For this purpose some 75 lava flows from within the Trans-Mexican
Volcanic Belt were sampled for palaeomagnetic analyses. Rock magnetic analyses revealed that in most
cases the remanence was carried by Ti-poor titanomagnetites of pseudo-single domain magnetic grain
size that underwent different degrees of high temperature deuteric oxidation on primary cooling.
Occasionally titanomagnetites with higher Ti-contents and additional contributions of (titano-) haematite
were also present. The scatter of virtual geomagnetic poles from lavas younger than 1.7 Ma was used for
estimating palaeosecular variation and was found to be consistent with latitude dependent Model G and
other high quality palaeomagnetic data from Mexico. The palaeomagnetic mean-vectors of 56 lavas were
correlated to the Geomagnetic Polarity Timescale supplemented with information on geomagnetic
excursions. On the grounds of their associated radioisotopic ages, four lavas were tentatively correlated
with known excursions from marine records. Two lava flows dating of Brunhes Chron were associated
with the Big Lost and Delts/Stage 17 excursions, respectively. From further two flows dating of
Matuyama Chron, one flow was associated with either the Santa Rosa- or Kamikatsura excursions, while
the other could have been emplaced during the Gilsa excursion. The most significant outcome was the
finding that both Brunhes excursional flows display nearly fully reversed directions that deviate almost
180°C from the expected normal polarity direction. This observation could indicate that in particular the
Big Lost and Delta/Stage17 excursions may represent other short periods during which the field
completed a full reversal for a short time, such as was previously found for other older cryptochrons or
tiny wiggles.

Another focus of this thesis was set on estimating the feasibility of the new MS method for routine
palaeointensity determination. This was accomplished by applying the MS method to samples from 11
historical lava flows from Mexico and Iceland from which the actual field intensity was either known
iiifrom contemporary observatory data, or deduced from magnetic field models. Comparing observed with
expected intensity values allowed to test the accuracy of the MS method. It a was found that the majority
of palaeointensity estimates after the MS method yielded results that were very close or indistinguishable
within the range of uncertainty from the expected values. However, a general trend towards an
overestimate in the palaeointensity was also observed, which, on the grounds of corroborating rock
magnetic analyses, was associated with multidomain material. This observation was taken as first
evidence that the MS method is not entirely independent of magnetic domain state, as was originally
claimed. However, a second experiment in which a modification of the most widely used Thellier method
was applied to sister samples from 5 Icelandic flows revealed that, in comparison to the MS method, the
latter produced more accurate and statistically better defined palaeointensities. Thus, from these first
results, the MS method appeared as a viable alternative for future palaeointensity studies.

Subsequently i