Paleomagnetic dating of a mysterious lake record from the Kerguelen archi

24 Pages·2016·3.91 MB·English

by  Gert Pettersson

Paleomagnetic dating of a mysterious
lake record from the Kerguelen archi-
pelago by matching to paleomagnetic
field models
Kim Teilmann
Dissertations in Geology at Lund University,
Bachelor’s thesis, no. 480
(15 hp/ECTS credits)
Department of Geology
Lund University
2016

Paleomagnetic dating of a mysteri-
ous lake record from the Kerguelen
archipelago by matching to paleo-
magnetic field models

Bachelor’s thesis
Kim Teilmann
Department of Geology
Lund University
2016
Contents
1 Introduction ....................................................................................................................................................... 7
2 Study area ........................................................................................................................................................... 7
3 Theoretical background .................................................................................................................................... 8
3.1 Magnetic terms 8
3.2 The geomagnetic field 9
3.2.1 Paleomagnetic secular variation 9
3.3 Natural remanent magnetization 9
3.3.1 Depositional remanent magnetization 9
3.3.2 Viscous remanent magnetization 10
3.4 Progressive AF demagnetization 10
3.5 Characteristic magnetization 10
4 Method .............................................................................................................................................................. 10
4.1 Field work 10
4.2 Radiocarbon dating 10
4.3 Sub-sampling for magnetic measurements 10
4.4 Magnetic measurements predating the study 11
4.5 Magnetic measurements 11
4.6 Paleomagnetic field predictions 11
5 Results ............................................................................................................................................................... 11
5.1 Stratigraphy 11
5.1.1 Core L1-1 11
5.1.2 Core L1-3 12
5.1.3 Core L1-4 12
5.1.4 Core L1-5 12
5.2 Core correlation 12
5.3 Radiocarbon dating 12
5.4 Magnetic measurements 12
5.4.1 Declination and inclination 12
5.4.2 Magnetic properties 14
5.5 Paleomagnetic field predictions 14
6 Discussion ......................................................................................................................................................... 14
6.1 Magnetic mineral content 14
6.2 Declination and inclination 16
6.3 Slump deposit hypothesis 17
6.4 Age-depth model 18
6.5 Reliability of the paleomagnetic field models 20
6.6 Complementary studies 20
6.7 Sources of error 21
7 Conclusions ....................................................................................................................................................... 21
8 Acknowledgements .......................................................................................................................................... 21
9 References ......................................................................................................................................................... 21

Cover Picture: Lake Cocytus. Photo by Nathalie Van der Putten.
Paleomagnetic dating of a mysterious lake record from the Ker-
guelen archipelago by matching to paleomagnetic field models
KIM TEILMANN
Teilmann, K., 2016: Paleomagnetic dating of a mysterious lake record from the Kerguelen archipelago by matching
to paleomagnetic field models. Dissertations in Geology at Lund University, No. 480, 22 pp. 15 hp (15 ECTS
credits).
Abstract: The Kerguelen archipelago is a volcanic island group in the southern Indian Ocean. It is located on the
polar front as well as within the Circumpolar Antarctic Current and the Southern Hemisphere Westerly Wind Belt.
This makes it an interesting location for paleoclimate studies. For this reason, a three meter long sediment sequence
from a lake on the archipelago was retrieved in 2013. Radiocarbon dating of the lake record revealed a puzzling
result with the occurrence of several age reversals. This study aims at improving the understanding of the chronolo-
gy of the sequence by establishing an age-depth model based on comparison of the characteristic remanent magnet-
ization of the sediment with the pfm9k.1a and A_FM paleomagnetic models.
Alternating field demagnetization up to 80 mT applied to 153 discrete samples revealed the presence of a vis-
cous component that was removed after demagnetization steps of 10 to 15 mT. Investigation of the sediment stra-
tigraphy and the characteristic remanent magnetization indicates that part of the lake sequence consists of slump
deposits. The declination and inclination of the sediment presumed to be in situ shows similarities with the A_FM
model, and a simple age-depth model for the lower half of the sequence has been constructed by correlation of in-
clination and declination features.
The established age-depth model shows discrepancies of up to a couple of hundred years when compared with
the radiocarbon dates. The discrepancies might be explained by limitations of the paleomagnetic models, as these
are mainly based on paleomagnetic data from the northern hemisphere, but difficulties in the interpretation of the
sediment stratigraphy might play a role as well. Radiocarbon dating of samples from a few identified key horizons
is suggested to improve the understanding of the chronology of the sediment sequence.

Keywords: Paleomagnetism, paleomagnetic field models, paleomagnetic secular variation, Kerguelen archipelago,
lake sediment, slump deposits
Supervisors: Andreas Nilsson & Nathalie Van der Putten
Subject: Quaternary Geology
Kim Teilmann, Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden. E-mail:
[email protected]
Paleomagnetisk datering av ett mystiskt sjösediment från
Kerguelenöarna vid jämförsel med paleomagnetiska fältmodeller
KIM TEILMANN
Teilmann, K., 2016: Paleomagnetisk datering av ett mystiskt sjösediment från Kerguelenöarna vid jämförsel med
paleomagnetiska fältmodeller. Examensarbeten i geologi vid Lunds universitet, Nr. 480, 22 sid. 15 hp.
Sammanfattning: Kerguelenöarna är en vulkanisk ögrupp belägen i det södra Indiska oceanen. Öarna ligger på
polarfronten samt inom den cirkumpolära Antarktiska strömmen och södra halvklotets västvindsbälte. Detta gör
Kerguelenöarna till en intressant plats för klimatstudier. Av denna anledning erhölls en tre meter lång sekvens av
sediment från en sjö på ögruppen 2013. Kol-14 datering av sjösedimenten gav ett gåtfullt resultat med förekomst av
flera åldersomkastningar. Denna studie syftar till att förbättra förståelsen av sedimentens kronologi genom att skapa
en ålders-djup-modell baserad på jämförelse av sedimentens karakteristiska remanenta magnetisering med de pa-
leomagnetiska modellerna pfm9k.1a och A_FM.
Avmagnetisering av 153 prover med alternerande fält upp till 80 mT avslöjade en viskös komponent som av-
lägsnades efter avmagnetiseringssteg på 10 till 15 mT. Undersökning av sedimentstratigrafin och den karaktärist-
iska remanenta magnetiseringen tyder på att delar av sekvensen har avsatts i samband med massflöden. Deklinat-
ionen och inklinationen i de sedimenten som antas vara in situ uppvisar likheter med A_FM, och en simpel ålders-
djup-modell för den nedre halvan av sekvensen har konstruerats genom korrelation av utmärkande deklinations-
och inklinationshändelser.
Den etablerade ålders-djup-modellen visar skillnader på upp till ett par hundra år jämfört med kol-14 date-
ringarna. Avvikelserna kan möjligvis förklaras av begränsningar av de paleomagnetiska modellerna, eftersom dessa
huvudsakligen är baserade på paleomagnetisk data från norra halvklotet, men svårigheter vid tolkningen av sedi-
mentetstratigrafin kan också vara en medverkande faktor. Ytterligare kol-14-dateringar av strategiskt utvalda delar
av sedimentsekvensen rekommenderad för att förbättra förståelsen av kronologin.
Nyckelord: Paleomagnetism, paleomagnetiska fältmodeller, paleomagnetiska sekulära variationer, Kerguelenöar-
na, sjösediment, massflöden
Handledare: Andreas Nilsson & Nathalie Van der Putten
Ämnesinriktning: Kvartärgeologi
Kim Teilmann, Geologiska institutionen, Lunds universitet, Sölvegatan 12, 223 62 Lund, Sverige. E-post:
[email protected]
1 Introduction within the upper practical limit for Holocene paleo-
Studying past climate variability is of great importance magnetic pattern matching, estimated to be around
for understanding the mechanisms of ongoing global 2000 km by Thompson & Oldfield (1982). Instead, a
climate change. Historical records can give some in- potential age-depth model will be established by com-
sight, but are limited in space and time. In order to parison with field predictions according to paleomag-
study historically unrecorded climate variations, it is netic field models pfm9k.1a by Nilsson et al. (2014)
therefore necessary to acquire and study naturally pre- and A_FM by Licht et al. (2013).
served climate proxies. Climate proxies, such as dia-
2 Study area
tom assemblages and oxygen isotopes measured in
sediments have successfully been used to reconstruct The Kerguelen archipelago is a French territory com-
variations in local and global ice volumes and temper- prising more than 300 islands. The total area of the
atures (e.g. Lisiecki & Raymo 2005; Fernandez et al. archipelago is 6500 km² (Damasceno et al. 2002), of
2013). which the main island covers 6000 km² (Hall 1984). It
In December of 2013, three sequences of lake sedi- is located in the Southern Indian Ocean between lati-
ments, consisting of four to five cores each, were re- tudes 48° to 50° south (Fig. 1A) as part of Terres Aus-
trieved from the Kerguelen archipelago, remotely lo- trales et Antarctiques Francaises (French Southern and
cated in the South Indian Ocean. Few terrestrial paleo- Antarctic Lands).
climate records from this area exist, so the lake sedi- The archipelago is part the Kerguelen Plateau, a
ments may provide new insights to the climate history Large Igneous Province that rises more than 2000 me-
of the area. In addition, paleoclimate data from the ters above the surrounding ocean basins (Benard et al.
Kerguelen archipelago might contribute to a better 2010). The bedrock consists mainly of flood basalts of
understanding of past changes in oceanic and atmos- ages between 24 and 29 million years, with the occur-
pheric circulation due to its location at the Polar Front rence of several plutonic complexes throughout the
within the Circumpolar Antarctic Current and the area and of Quaternary deposits in the eastern parts
Southern Hemisphere Westerly Wind Belt (Van der (Damasceno et al. 2002).
Putten et al. 2015). As of 2001, the total ice cover amounted to about
In order to compare and correlate the paleoclimate 500 km² with the biggest contribution from the Cook
data found within the lake sediments with existing ice cap covering about 410 km² (Cogley et al. 2014),
records, establishing a chronology for the sequences is but it is likely that ice covered the entire main island
during the last glacial period (Hall 1984). Evidence of
essential. An attempt to establish a chronology has
widespread ice cover is prevalent, as the archipelagos
been made based on radiocarbon dating of plant
topography is characterized by the presence of numer-
macrofossils and bulk samples. The samples yielded
ages between present day and 2230 uncalibrated 14C ous valleys and fjords produced by glacial erosion
(Damasceno et al. 2002).
years BP (before present), but with the occurrence of
The sediment sequences were retrieved from an
several yet unexplainable reversals. Application of an
unnamed lake, possibly a tarn, on the south eastern
alternative dating method can be helpful to the inter-
lower flank of Mount Carbenay in the central southern
pretation of the enigmatic radiocarbon dates.
area Plaine de Dante (Fig. 1B-C). I suggest the name
One alternative dating method is the use of past
Lake Cocytus1, and will refer to it as such from this
variations of the geomagnetic field, recorded by mag-
point on. The lake is sub-circular with a maximum
netic minerals contained within the lake sediments. An
shore-to-shore length of 240 meters. Shallow areas of
age-depth model of the sediment sequences can be
widths between a few and 60 meters extend from the
obtained by comparing their paleomagnetic data with
lake shore. From the edge of the shallow areas, the
that of an independently dated record from a nearby
lake floor slopes steeply to a maximum depth of ap-
location or with modelled paleomagnetic field predic-
proximately ten meters (Fig. 1D). The catchment area
tions, a method successfully applied to Arctic marine
is restricted to the slopes of the immediate surround-
sediments by Barletta et a. (2010).
ings.
This study aims to establish an age-depth model for
the Kerguelen lake sequences based on the remanent
magnetization of the sediment. Preliminary measure-
ments have shown that the lake record carry a strong 1Lake Cocytus is mentioned by Dante in his Divine Comedy.
and stable magnetization. Unfortunately, there are no Since the lake is located in Plaine de Dante, I considered this
available independently dated paleomagnetic records an appropriate name.
7
Fig. 1. A. Map showing the location of the study area and the Kerguelen archipelago. Adapted from map by Rémi Kaupp at
Wiki Commons. B. Map of the study area showing the location of Lake Cocytus. Adapted from atlas scan downloaded from the
French government’s geoportail website (www.geoportail.gouv.fr). C. Picture of Lake Cocytus with a northerly view. Picture by
Nathalie Van der Putten. D. Transect of Lake Cocytus with location of coring site for sediment sequence L1-1. Background
image from Google Earth.
3 Theoretical background called a magnetic field force (SI-unit: Am-1) (Butler
A basic understanding of the principals involved in the 1992), that either attracts or repels it depending on
acquisition of a remanent magnetization is important polarity.
in order to understand the dating method. A basic de- A material may contain several individual magnet-
scription of the underlying principles is provided in ic dipoles of varying orientation. The term magnetiza-
this section. For more detailed descriptions, the inter- tion (SI-unit: T) is of great importance, and refers to
ested reader is referred to textbooks on the subject the net magnetic moment per unit volume of a materi-
(Thompson & Oldfield 1982; Butler 1992). al. Depending on composition, a material that is within
the range of an external magnetic field may develop an
induced magnetization (Butler 1992) caused by inter-
3.1 Magnetic terms
actions between the external field and the magnetic
A magnet consists of a magnetic dipole with two mag-
dipoles present in the material. The size of the induced
netic poles of opposite polarity. The product of the
magnetization depends on the magnetic susceptibility
size of the magnetic charges and the distance between
(SI-unit: dimensionless if volume specific) of the ma-
them is termed the magnetic moment (SI-unit: Am²),
terial and the strength of the magnetic field force. In
and is a vector pointing in direction from the negative
some materials, a remanent magnetization caused by
to the positive pole (Butler 1992). Any magnetic di-
past magnetic fields may be present as will be dis-
pole is surrounded by magnetic field lines that flow
cussed in section 3.3.
from pole to pole in a radial pattern, and if a magnetic

unit is placed within the field, it encounters what is

8
3.2 The geomagnetic field Declination refers to the angle between the geo-
Earth is surrounded by a magnetic field, originating graphic and the magnetic north pole, while inclination
from flow within the liquid outer core of the planet is the angle between the magnetic field lines and the
(Butler 1992). This flow generates a geomagnetic field horizon (Thompson & Oldfield 1982). Declinations
that to approximately 90% can be explained by a di- towards west are reported as negative values, while
pole placed at the centre of Earth, slightly inclined to easterly declinations are reported as positive. For incli-
Earth’s rotational axis. The poles of the geomagnetic nation values, a downward dip of the north-seeking
dipole are referred to as the magnetic north and south end of a dipole below the horizon is reported as posi-
poles (Fig. 2). tive, while an upwards dip is reported as negative
Averaged over sufficient time, the geomagnetic (Thompson & Oldfield 1982).
dipole coincides with Earth’s rotational axis, a phe-
nomenon conceptually known as the geocentric axial 3.2.1 Paleomagnetic secular variation
dipole (GAD). As mentioned in the previous section, the orientation
The actual geomagnetic dipole is dynamic and of the geomagnetic field varies over time. These varia-
moves relative to the geographical poles. The location tions can be categorized based on their magnitude and
of the magnetic poles at any given time can be de- periodicity. Paleomagnetic secular variations are
scribed by the two location-dependent properties decli- changes that occur over a timespan of one to 10.000
nation and inclination (Fig. 3). years that can be recognized over sub-continental re-
gions (Butler 1992). The main cause of secular varia-
tions is flow-variation in the outer core (Constable &
Constable 2004) with contributions from both the di-
pole and the so-called non-dipole fields (Butler 1992).
Records of past variations can be found in the natural
remanent magnetization of appropriate geological ma-
terials.

3.3 Natural remanent magnetization
A magnetization caused by past magnetic fields that
has been preserved in a material by natural processes
is called a natural remanent magnetization (NRM). A
NRM usually consists of a primary and a secondary
component. Primary components are acquired during
rock formation or sediment deposition, while second-
Fig. 2. Sketch of the dipole part of Earth’s magnetic field.
ary components are acquired later (Butler 1992). De-
Note the inclination of the dipole to Earth’s rotational axis
and the radial pattern of the magnetic field lines flowing pending on the mineral assemblage, a remanent mag-
from the magnetic south pole to the magnetic north pole. netization may decay over time, in a process known as
Adapted from sketch by TStein at Wiki Commons. magnetic relaxation. NRM can result from several
processes, but the processes of greatest interest to this
study are those that govern the acquisition of detrital
remanent magnetization and viscous remanent
magnetization.
3.3.1 Depositional remanent magnetization
Detrital material (e.g. lake sediment) can carry a depo-
sitional remanent magnetization (DRM) if magnetic
minerals are present. As sediment settles in low energy
environments, a fraction of the magnetic minerals are
aligned with the prevailing geomagnetic field at the
time of deposition. According to Butler (1992), the
Fig. 3. Sketch illustrating the concepts of magnetic declina-
alignment may continue post-depositionally in the
tion (Dec) and inclination (Inc). Declination is the angle
uppermost ten to twenty cm of the accumulating sedi-
between magnetic north and geographical north measured at
a given location. Inclination is the angle between a magnetic ment. When further movement of the sediment parti-
field line and horizontal. cles is no longer possible due to dewatering and con-
9
solidation, the DRM is locked-in (Butler 1992), pre- tic remanent magnetization (ChRM) (Zijderveld 1967;
serving a record of the geomagnetic field from deposi- Butler 1992). Under optimal conditions, the remaining
tion to lock-in. high stability component of the NRM corresponds to
the DRM, but since other magnetizations may be pre-
3.3.2 Viscous remanent magnetization
sent as well, ChRM is used to avoid false statements.
Viscous remanent magnetization (VRM) is acquired
The parameters being matched to the paleomagnetic
over time after lock-in of DRM. Whereas DRM is a
models in this study are the declinations and inclina-
physical process of mineral alignment, VRM is pro-
tions of the lake record ChRM.
duced by changes within magnetic minerals with rela-

tively short magnetic relaxation times. In a slow pro-
4 Method
cess, these minerals continuously change their magnet-
4.1 Field work
ization to align with the ambient magnetic field
In December of 2013, field work was carried out by
(Butler 1992). The processes behind these changes are
Nathalie Van der Putten (Department of Geology,
too complex to describe for the purpose of this study,
Lund University), Elisabeth Michel (Climate and En-
but it is important to know that some magnetic grains
vironment Sciences Laboratory, Gif-sur-Yvette,
are more susceptible to viscous magnetization than
France) and Bart Klinck (volunteer). Three sediment
others.
sequences named L1, L2 and L3 were retrieved from
Zijderveld (1967) refers to the smallest strength of
Lake Cocytus within 10 meters of each other, using a
the magnetic field required to change the direction of a
Russian corer at water depths around nine meters.
magnetization as its release force. In the presence of a
Each sequence consists of four to five overlapping
weak magnetic field, such as Earth’s, only magnetic
cores with a total length of around three meters. The
moments with low release forces (short relaxation
cores were collected onboard a boat fixed by lines se-
times) develop a VRM. This implies that some of the
cured by the lake side. Care was taken to minimize
magnetic grains in a sediment sample may remain
orientation variation between cores.
practically unaffected by the geomagnetic field after

lock-in of the DRM.
VRM does not provide information about the geo- 4.2 Radiocarbon dating
magnetic field at the time of deposition, and is com- A total of 11 samples were collected from sequence L3
monly eliminated in the laboratory by exposure to al- and submitted to the Single Stage AMS at Lund Uni-
ternating field demagnetization. versity and the CEA Saclay in France for radiocarbon
dating. The samples were collected by Nathalie Van
3.4 Progressive AF demagnetization der Putten and consist of three bulk samples, three
The method of progressive alternating field (AF) de- samples of mosses and five samples of leafs or seeds
magnetization aims to cancel the VRM by exposure to from the flowering plant Azorella selago.
a series of alternating magnetic field cycles. Each cy-
cle begins with a peak intensity that subsequently de- 4.3 Sub-sampling for magnetic measure-
creases while the field direction alternates between ments
two opposites. From one cycle to the next, the peak
The standard method of discrete sampling by pushing
intensity is increased. With each cycle, the magnetic
sampling cubes into the sediment core, posed a large
dipoles with release forces less than that of the peak
risk of sediment disturbance due to the presence of
AF intensity are aligned randomly in opposite direc-
mosses in sections of the sequence. Instead, sub-
tions, effectively cancelling each other out (Butler
samples were precut with knives of stainless steel and
1992). By increasing the peak intensity of the AF be-
low magnetization (to lower the risk of magnetic con-
tween cycles, a continuously greater proportion of the
tamination).
NRM is cancelled with each step. At sample-specific
Rectangular blocks of sediment with a cross sec-
peak intensities, all VRM is eliminated, while a
tional area of 1.9 x 1.9 cm and a minimum height of 2
stronger magnetization remains.
cm were cut wall-to-wall along the cores. Where

coarseness or lack of cohesion of the sediment pre-
3.5 Characteristic magnetization
vented preservation of internal fabric, sub-sampling
The high stability component of the NRM that remains was omitted. Subsequently, paleomagnetic sampling
after the removal of VRM or other secondary magneti- cubes with inner dimensions of 2 x 2 x 2 cm were
zations of low stability is referred to as the characteris- aligned above the precut rectangles, and pushed down
10