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2013年

Rock-magnetic Study for Normalization of theRemanent Magnetization Obtained from an Antarctic
Deep-sea Sediment CoreHaruka MATSUOKA＊Abstract
In order to estimate the relative variation of the geomagnetic field intensity in the Antarctic region,rock-magnetic experiments have been conducted on a deep-sea sediment core obtained from offshore
Wilkes Land, East Antarctica. Acquisition curves of anhysteretic remanent magnetization (ARM) andisothermal remanent magnetization (IRM) indicate that the ARM intensity is not completely saturated
even at the maximum alternating-field (AF) field of 100 mT whereas the IRM intensity is saturated. Thedemagnetization curve of saturation isothermal remanent magnetization (SIRM) is remarkably similar tothat of natural remanent magnetization (NRM), while that of ARM depicts definitely different change inAF field. It implies that the coercivity spectrum of NRM is similar to that of SIRM and far from that ofARM.The normalized ratio of NRM to SIRM is remarkably stable over a broad range of demagnetization
steps, whereas the normalized ratio of NRM to ARM is unstable. Therefore, in relative paleointensity
estimation using the core GC1501, SIRM could be better for normalizing NRM than ARM.

Key words：sediment core, rock-magnetism, relative paleointensity, SIRM

1. Introduction
Deep-sea sediments can provide the continuous records of the past geomagnetic field, anda number of relative paleointensity variations of the geomagnetic field were obtained fromsediments for more than the last 0.8 Ma, which allowed the construction of global reference
paleointensity curves (e.g., Guyodo and Valet, 19991); Yamazaki and Oda, 20052); Valet et al .,20053)) by the technical and methodological development of paleointensity determination.
The sediment core used in this study (GC1501) was obtained from a continental rise site
3060 m deep at the western part of the Antarctic Wilkes Land margin during the TH94 cruise＊

経営情報学部経営情報学科、Tsukuba Gakuin University― 45 ―

筑波学院大学紀要第８集

2013年(1994-1995) of R/V HAKUREI-MARU, which was carried out by the Technology ResearchCenter, Japan National Oil Corporation (Ishihara et al ., 19964)) . The total length of the core is540 cm. Abundant foraminiferal skeletons in good preservation were observed throughoutthe core. The sediment material was siliceous silt and showed brownish gray color. They
clearly indicate oxidized condition, which gives additional stability of magnetic mineral against
alteration throughout burial diagenesis.
A fundamental paleomagnetic studies were conducted on the same core by Sakai etal . (1998) 5) and Matsuoka & Funaki (2003)6). Matsuoka & Funaki (2003)6) observed the NRM
intensities which are 10-100 times higher than those commonly obtained from different
localities and the remarkable high-stability components which survived up to 100 mT in AFdemagnetization. The reported anomalous NRM intensity decay plots in AF demagnetization
are shown in Figure 1.

Figure 1. NRM intensity decay plots in AF demagnetization (Matsuoka & Funaki, 20036)).

The normalization of NRM with using an appropriate proxy is essential for paleointensity
estimates. Sakai et al . (1998) 5) attempted to estimate the relative paleointensity of thegeomagnetic field with using NRM intensity normalized by susceptibility (χ). However, as
pointed out by Levi and Banerjee (1976)7), susceptibility is hard to relate to remanence andhence much less preferable for paleointensity estimation than some form of room temperature
remanence. For detailed discussions, therefore, more reliable paleointensity estimation withusing other normalizer such as ARM or IRM is necessary.
Matsuoka & Funaki (2006) 8) performed the measurements of χ and ARM, which
demonstrated the rock-magnetic homogeneity by the significant low amplitudes of their overallvariations. It commonly allows the relative paleointensity estimation and thus Matsuoka andFunaki (2006)8)estimated a relative paleointensity variation from NRM intensities remaining
after AF demagnetization at 30 mT (NRM 30mT) normalized by ARM intensity at 30 mT
demagnetization (ARM30mT) with checking the absence of correlation between the normalizedintensity (NRM30mT/ARM30mT) and the normalizer (ARM30mT).

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Haruka MATSUOKA：Rock-magnetic Study for Normalization of the Remanent Magnetization Obtained from an Antarctic Deep-sea Sediment CoreFor further study, however, other normalizer also should be determined. SIRM has been
also commonly used as a normalizer for relative paleointensity estimates (e.g., Channell et al .,19979); Sato and Kobayashi, 199810)). The magnitude of SIRM is typically thousands of times
larger than that of NRM, while the magnitude of ARM is a few tens to hundreds of times
that of NRM. Thus SIRM is less preferable than ARM in many cases (e.g., Levi and Banerjee,19767)). However, SIRM is more efficient to estimate the amount of magnetic fractions of highcoercivity (e.g., hematite, maghemite, goethite) than ARM. In this study, therefore, SIRM is
focused as another normalizer by comparison with ARM.
2. Magnetic Measurements
2.1 ARM Experiment
ARM acquisition experiments were carried out by the ARM acquisition coil in the SQUID
magnetometer installed in a low magnetic field room in National Institute of Polar Researchin Tokyo, Japan. ARM was imparted on every sample by superimposing a DC biasing field of
0.1 mT on an increasing AF field up to 100 mT in steps of 10 mT. Since the core was obtained
from the Antarctic polar region, the dominant NRM component is vertical, as supportedby the AF demagnetization results, and it was not demagnetized completely even if thedemagnetization field was 100 mT (Matsuoka & Funaki, 20036)). Therefore, ARM was imparted
toward the horizontal direction of core for minimizing the effect of residual NRM after thedemagnetization. After the ARM acquisition experiments, all samples were demagnetized up
to 100 mT in steps of 10 mT.
2.1 IRM Experiment
The pilot sediment sample was subjected to IRM acquisition experiments with theNIPR Pulse Magnetizer. IRM was imparted on the sample in the field of 50 mT as the firststep, and then in steps of 100 mT up to 800 mT. Same as ARM, IRM was imparted toward
the horizontal direction of core for minimizing the effect of residual NRM after the AF
demagnetization. The acquired SIRM was AF demagnetized up to 100 mT in steps of 10 mTwith using a SQUID magnetometer.
3. Results
A typical result of ARM acquisition experiments was shown in Figure 2a. The obtained
ARM intensity was normalized by its maximum ARM intensity. The acquisition curve shows
10-20 % increase at every step up to 50 mT AF field. Though the ARM intensity increases by
several percent at every step of over 50 mT AF field, it is not completely saturated even at
the AF field of 100 mT.
A typical ARM intensity decay curve in AF demagnetization is shown in Figure 2b.
The decay curve shows strongly convex-down shape. Almost 70 % of whole ARM intensity
disappears at the AF field of 30 mT.

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2013年

A typical result of IRM acquisition experiment was shown in Figure 2c. The intensity ofIRM was normalized by the IRM intensity that was acquired at the DC field of 800 mT. Theacquisition curve shows the drastic increase at every step up to 200 mT AF field. The 92 % of
the IRM intensity acquired at the DC field of 800 mT has been gained up to 200 mT step. The
IRM intensity was saturated in the DC field of 400 mT and thus can be assumed to be SIRM.
The value of SIRM is 22.0 A/m, which is about 200 times larger than the NRM and about 45
times larger than the ARM.
A typical result of AF demagnetization of SIRM was shown in Figure 2d. The decay
curve of IRM shows a remarkable linear change against AF field. The 15 % of the SIRM wassurvived up to 100 mT AF field. The median destructive field of SIRM (MDFSIRM), the valueof the peak AF field necessary to reduce the IRM intensity to half of its initial value, wasestimated as 57.5 mT.

Figure 2. Typical results of ARM and IRM experiments.
(a) ARM acquisition curve on a discrete sample in AF field up to 100 mT.
(b) ARM intensity decay curve in AF demagnetization.
(c) I R M acquisition curve on a discrete sample in DC field up to 800 mT.
(d) I R M intensity decay curve in AF demagnetization.

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Haruka MATSUOKA：Rock-magnetic Study for Normalization of the Remanent Magnetization Obtained from an Antarctic Deep-sea Sediment Core

4. Discussion
The acquisition curves of ARM and IRM are compared in Figure 3a. While the IRM
intensity is saturated, the ARM intensity is not completely saturated even at the maximum
AF field of 100 mT. Since the main carriers of NRM characterized by remarkably highcoercivity may fail to acquire ARM completely, the concentration of NRM carriers cannot be
estimated correctly by using ARM. The demagnetization curves of NRM, ARM and SIRM
are also compared in Figure 3b. The demagnetization curve of SIRM is remarkably similar
to that of NRM, while that of ARM depicts definitely different change in AF field. It indicatesthat the coercivity spectrum of NRM is similar to that of SIRM and far from that of ARM.
A consequence of this dissimilarity in coercivity spectra between ARM and NRM is that theratio of the two is very unstable during demagnetization.

Figure3. Comparison of various remanent magnetizations.
(a) Comparison of acquisition curves of ARM and IRM.
(b) Comparison of the demagnetization curves of NRM, ARM and SIRM.

Figure 4 shows the normalized ratios of NRM to ARM (A*) and of NRM to SIRM (I*)
during demagnetization. The I* is quite stable over a broad range of demagnetization steps,
whereas A* is unstable. Therefore, the author considers the normalization by using SIRM to
be better than ARM for the relative paleointensity estimation.

Figure4. Changes of the ratio of NRM to ARM (A*) and the ratio of NRM to SIRM (I*)
during demagnetization. Solid/open circles show plots of A*/I*.

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筑波学院大学紀要第８集

2013年

5. Conclusion
In order to find the most suitable normalizer for relative paleointensity estimation, tworock-magnetic experiments, ARM experiment and IRM experiment, were carried out. Theacquisition curves of ARM and IRM indicated that the ARM intensity is not completely
saturated even at the maximum AF field of 100 mT while the IRM intensity is saturated. The
comparison of NRM, ARM, and SIRM demagnetization curves revealed that the coercivity
spectrum of NRM is similar to that of SIRM and far from that of ARM. The normalized ratioof NRM to SIRM is significantly stable over a broad range of demagnetization steps, whereas
the normalized ratio of NRM to ARM is unstable. Thus SIRM could be concluded to be betterfor normalizing than ARM.
6. Acknowledgments
I would like to express my sincere thanks to Profs. M. Funaki at National Instituteof Polar Research and Dr. M. Ozima for their constant discussions and encouragements.
I also express my thanks to all the scientists, officers, and crew on the TH94 cruise of R/
V HAKUREI-MARU for obtaining the core used in this study, and the Japan National Oil
Corporation for providing the core.
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