magnetic properties of lake soppen: how climate history ...€¦ · −1 x 10−4 y vrm ≈ 5 mt...
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Conclusion:Climatic conditions influence the sedimentology, hence iron mineralogy in the lakes. This in turn influ-ences the magnetic properties. Our results show that the sediments, whose ferromagnetic mineralogy is controlled by SD magnetite reflect variations in the Earth’s magnetic field. Areas in which production of MTB is suppressed or where iron mineralogy is chemically altered, do not record field behaviour. This example provides evidence that high-resolution records of periods with varying climatic conditions have to be critically evaluated.
References:1 A. Lotter (2001) The paleolimnology of Soppensee (Central Switzerland), as evidence by diatoms, pollen, and fossil pigments analyses. Journal of Paleolimnology 25, 65-79
2 L. Tauxe, T. Pick, Y.S. Kok (1995) Relative paleointensity in sediments: a pseudo-Thellier approach. Geo-physical Research Letter, 22, 2885-2888
0 2 4
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RPI
-X Z MDF 30 - 40 mT
Figure 3: left: AF demagnetization of the NRM with median destructive fields (MDF) between 30 and 45 mT. right: FORC is typical for SD magnetite. Both curves are representative for Z4
Figure 2: Downcore logs of 1) concentration parameters: χ, ARM intensity 2) NRM intensity 3) composition parameters: S-ratio 4) grain size indica-tor: ARM/IRM 5) Fe counts from XRF core scanner
-40x10-3
-20
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40
Hu
(T
)
0.120.100.080.060.040.020.00Hc (T)
65
43
2
10
-1
x10-3
Figure 4: Relative paleo- intesity record. The marked part (Z4) fulfills all criteria for a good record. Swedish lakes (red and blue) match well during
the Holocene.
Results:
0 10 20 30 40 50 60 70 800
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1
�eld (mT)
Nor
mal
ized
NRM
First order reversal curve (FORC)
Magnetic properties reflect changes in the lithological composition (Fig. 2): Z1: deglaciation: mixed response of MD magnetite and hematite Z2: Bolling/Allerod, Younger Dryas: predominated by single domain (SD) magnetite Z3: calcitic varves: low concentration of Fe and ferromagnetic minerals Z4: organic rich zone: dominated by biogenic SD magnetite (Fig. 3) Z5: antropogenic influenced zone: mixed response of SD magnetite and hematite
Z1
Z2
Z3
Z5
Z4 0 2 4 6 8x 10−4
−8−7−6−5−4−3−2−1
x 10−4
Y
VRM ≈ 5 mT
Horizontal
Vertical
From the above zones, only Z4 fulfills the magnetic mineral criteria to establish a reliable paleointensity record (Fig. 4). The application of the pseudo-Thellier method provides a robust paleofield for the time interval 6.5-1.7 cal kyr BP (Fig. 7)
FurskogstjärnetFrangsjön
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χR T
(10−07 m3 kg−1)
aver
aged
age
(cal
yr B
P)
2 4 6 8ARM
0.5 1 1.5 2NRM
0.8 1S−ratio
0.05 0.15ARM/IRM
2 4 6Fe
total counts(10-04Am2kg-01)
1) 1) 2) 3) 4) 5)
Table 1: Magnetic method used and their settings
Investigation Instrument Remarks number of samples
Low field magnetic susceptibility (χ)
AGICO Kappabridge KLY-2 operating frequency 875 Hz at room temperature and 77 K
220
Natural Remanent Magnetization (NRM)
2G Enterprise model 755R, 3-axis DC-SQUID rock magnetometer
220
Anhysteretic Remanent Magnetization (ARM)
2G Enterprise model 755R, 3-axis DC-SQUID rock magnetometer
produced along z-axis, static field: 1 mT, AF amplitude: 100 mT
220
NRM demagnetization in-line AF demagnetizer 2G-755-SRm
15 steps up to 80 mT 220
ARM demagnetization in-line AF demagnetizer 2G-755-SRm
23 steps up to 120 mT, 1 mT bias field
220
Imprinting of Isothermal Remanent Magnetization (IRM)
ASC Scientific Pulse magnetizer model IM-10-30
produced along z-axis, 5 steps up to 1.2 T
220
Measurement of IRM 2G Enterprise model 755R, 3-axis DC-SQUID rock magnetometer
at room temperature and 77 K 220
Hysteresis parameter: Ms, Mr, Hc, Hcr
Alternating Gradient magnetometer Micromag model 2900 (Prinston)
100
First order reversal curves (FORC)
Alternating Gradient magnetometer Micromag model 2900 (Princeton)
50
Methods:
The relative paleointensity (RPI) of the ancient magnetic fields was determined by the pseudo-Thellier method 2 and cal-culated by a trapezoidal numerical inte-gration:
RPI =
120mT
20mTNRM (H AF )dHAF
120mT
20mTARM (H AF )dHAF
Figure 1: Magnetic susceptibility for all four piston cores with a GEOTEK Multi-sensor core logger and the composite profile, measured on
discrete samples.0 10 20 30
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com
posi
te d
epth
(cm
)
χ So08−010 10 20 30χ So08−02
0 10 20 30χ So08−03
0 10 20 30χ So08−04
0 10 20 30composite
pro�l
Introduction:Rapidly deposited lake sediments provide high-resolution records of the Earth’s magnetic field and can reflect paleoclimate in great detail 1. Pleisto-cene and Holocene magnetic signatures, influenced by environmental and climatic changes have been investigated in sediments from Soppensee, a small lake in central Switzerland. Magnetotactic bacteria (MTB) are the magnetic carrier during the Holocene warm period, allowing for a high fidel-ity record of the Earth’s magnetic field as input for magnetic field models. To achieve a robust record the magnetic susceptibility (χ) and the sedimen-tology of the four cores are used to define the composite profile (Fig. 1).
Magnetic suscepti-bility was measured by the core logger (blue curves) and on discrete samples
(red curve).
zoning
Magnetic properties of Lake Soppen: How climate history affects paleointensity estimation
Jessica Kind1 Ann Hirt