Geophysical Journal (1989) 96, 519-528
Palaeomagnetism of stalagmites (speleothems) in SVV Japan
Hayao Morinaga
Division of Environmental Science, The Graduate School of Science and Technology, Kobe University, Nada, Kobe 657,Japan
Hiroo Inokuchi and Katsumi Yaskawa
Faculty of Science, Kobe University, Nada, Kobe 657,Japan
Accepted 1988 September 19.Received 1988 September 19; in original form 1987 November 16
SUMMARY
A palaeomagnetic study has been carried out on three stalagmites from two limestone caves
in SW Japan. Remanent magnetizations of the stalagmite samples are weak, but stable
enough to record the geomagnetic field. The results indicate an internal consistency between
several time-equivalent samples drilled from the same stalagmite. A tentative correlation is
suggested between palaeomagnetic data from two stalagmites collected from the same cave,
and data from two different limestone caves. Combined curves of declination and inclination
variations are constructed on the basis of this tentative correlation between the alternatingfield cleaned results from the three stalagmites. These variation curves can also be tentatively
correlated with those deduced from archaeomagnetic studies in SW Japan. In consequence,
the curves are believed to represent records of past geomagnetic secular variation. The
tentative age of the oldest end of this secular variation curve can be estimated at about 15 OOO
yr BP, based on the correlation with the archaeomagnetic results. A possible long period of
westerly declination and steep inclination is recognized in the older part of the variation
curve.
Key words: limestone cave, palaeomagnetism, remanent magnetization, secular variation,
stalagmite
1 INTRODUCTION
Palaeomagnetic investigations of lake, shallow marine and
cave sediments have been widely used to explore fine-scale
features of the past geomagnetic field, and to establish a
magnetic time-scale for the late Pleistocene to Recent (e.g.
Hyodo & Yaskawa 1980, 1986a, b; Creer & Tucholka 1983;
Creer, Tucholka & Barton 1983; Noel & St Pierre 1984;
Noel 1986a, b).
Unconsolidated sediments sometimes may provide nearcontinuous records of geomagnetic and climatic changes
owing to their near-continuous deposition. They do,
however, acquire remanent magnetizations after deposition,
known as post-depositional remanent magnetization, PDRM
(Irving 1957; Irving & Major 1963; Kent 1973), and,
depending on the nature of the sediment, may possess
magnetic records resulting from a convolution of the
geomagnetic field and a moment fixing function (Hyodo
1984). Therefore, the magnetic recording process for
sediments may be subjected to amplitude attenuation and
phase lag. Using a deconvolution method (Hyodo 1984), a
virtual geomagnetic secular variation record can be derived
from the magnetization records. This method, however, can
be utilized only for homogeneous sediments. Natural
sediments commonly have variously disturbed magnetic
records due to depositional and post-depositional factors,
variable secondary overprinting and sampling disturbance
(e.g. Verosub 1977; Rosenbaum & Larson 1983).
On the other hand, as pottery and bricks from pottery
kilns and ancient fireplaces commonly carry a stable
thermoremanent magnetization, more detailed aspects of
the geomagnetic field direction and intensity may be
deduced from their magnetizations (archaeomagnetism) .
The disadvantage of archaeomagnetism is that it is able to
give only discontinuous records (‘spot readings’) of the
geomagnetic field, and it may fail to reveal fine-scale
behaviour. However, by using a combination of both
archaeomagnetism and palaeomagnetism of sediments and
other materials which can continuously record the
geomagnetic field, it may be possible to establish a
synthesized secular variation curve which expresses more
completely the true behaviour of the field. With this aim in
mind, we have studied the palaeomagnetism of speleothems
(limestone cave formations) to see if they can provide a
reliable palaeosecular variation record.
Stalagmites, which are one type of speleothem, may carry
a stable remanent magnetization due to the presence of fine
grained magnetite (Morinaga, Inokuchi & Yaskawa 1986).
Owing to their continuous growth these formations may play
520
H.Morinaga, H . Inokuchi and K . Yaskawa
an important role as recorders of the past geomagnetic field.
Stalagmites are deposited in limestone caves -through a
chemical precipitation from dripping ground water, making
mound-like, tall cylindrical or conical shapes. The growth of
stalagmites, which takes place as calcium carbonate films are
successively deposited on their surface, is believed
commonly to be slower than that of unconsolidated lake
sediments. As stalagmite samples are able to be easily
divided into disc specimens with a thickness of a few
millimetres, slowness of growth does not become a serious
problem in their sampling for palaeomagnetic investigations.
In addition, the process of magnetization in a calcium
carbonate film deposited on to a stalagmite surface seems to
be simultaneous with its crystallization, differing from that
for unconsolidated sediments (Morinaga et al. 1989).
Latham et al. (1979) showed that the natural remanent
magnetizations (NRMs) of several speleothems were stable
and were apparently unaffected by surface conditions such
as roughness, dripping rates and crystal growth. Possible
secular variations in the geomagnetic field direction for the
last several thousands of years were detected through the
investigations of stalagmites and flowstones in a number of
regions (Latham et al. 1982; Latham, Schwarcz & Ford
1986, 1987).
Palaeomagnetic investigations of speleothems have also
been camed out in Japan with the aim of producing a more
detailed secular variation curve. These investigations
utilized two separate procedures. The first involved repeated
magnetization measurements after successive chipping of
small amounts of carbonate material from a block sample
under analysis (Inokuchi, Morinaga & Yaskawa 1981). The
subtraction vector of the latter from the former measured
values represents the remanence of the chipped materials
from the block sample. The second procedure involved
magnetization measurements of thin disc subsamples sliced
from long cylindrical samples (the 'microsampling' method
of Morinaga et al. 1985, 1986). The disc subsamples were
2.5cm in diameter and about 1.5mm thick. In principle
both procedures provide the opportunity for recording
geomagnetic field behaviour with a high time resolution.
The results of the second procedure indicated that the
surface subsamples, which were being dripped on at the
time of sampling, have much the same remanent direction as
that of the present geomagnetic field in the sampling region.
A geomagnetic secular variation curve for west Japan over
the last 4000yr was obtained through this procedure. This
curve corresponded to other curves obtained from
archaeomagnetic measurements (Hirooka 1971, 1983) and
palaeomagnetic investigations of shallow marine and lake
sediments (Hyodo & Yaskawa 1980; Yamazaki, Joshima &
Saito 1985) in Japan.
The purpose of the present paper is to demonstrate the
from the
reliability of NRM recorded in stalagmites,
combined results of our recent stalagmite palaeomagnetic
investigations, and previously published ones (Morinaga et
uf. 1985, 1986). Although there is no independent dating
control on the investigated samples, we also present a
provisional secular variation curve for the geomagnetic field
in west Japan, which is derived from tentative correlations
of the results from the three stalagmites with archaeomagnetic results.
2
SAMPLES
Two stalagmites were sampled from Komori-Ana cave
(34"13'N, 131"19'E) in Akiyoshi Plateau, Yamaguchi
Prefecture, West Japan. Caves distributed in Akiyoshi
Plateau are divided into six of the development level
categories. Komori-Ana cave is situated on the lowest
development level and is characterized by a horizontal
cavity. The formation age of this cave is believed to be late
Pleistocene to Recent (Fujii, personal communication 1988).
This cave seems to be still active because of the presence of
water flow. Palaeomagnetic, palaeoclimatic and other
investigations have already been performed for one of the
stalagmites, KM1 (Morinaga et al. 1985, 1986). Nine vertical
(time-equivalent) samples were drilled out from the
mound-like stalagmite body (Fig. 1, left). The samples were
cylindrical in form, of 2.5 cm diameter, and about 18 cm in
length. All samples were orientated in situ using a mount
and a magnetic compass. The orientation is considered
accurate to within f5" in inclination and declination. The
other stalagmite (KM2) had a conical shape, with a diameter
less than 20cm and a height of 25cm. Six parallel
(time-equivalent) samples were drilled horizontally from the
outer portion of this stalagmite (Fig. 1, centre). Each
sample was of 2.5 cm diameter and 4-5 cm in length. The
growth layers of parallel samples drilled from two
stalagmites had similar patterns, so that for each stalagmite,
simultaneous growth layers could be identified in respective
samples from their characteristic patterns.
A third stalagmite (RYU) was sampled in Ryuga-Do cave
(33"36'N, 133'45'E) in Kochi Prefecture, SW Japan. This
cave is regarded as having a similar age o r being rather older
than Komori-Ana cave, and also seems to be active at
present because of the presence of water flow. The
stalagmite had a columnar shape, about lOcm in diameter
and 86 cm in height. Seventeen successive (time-serial) core
samples, 2.5 cm in diameter and 5 cm in length, were taken
from the vertical centre part of this stalagmite (Fig. 1,
right). All three of these stalagmites were growing up till
their collection, since their tops were being dripped on at
the time of sampling.
All samples except those from stalagmite KM1 were
wrapped in polyvinyl chloride film and set using plaster of
paris. Setting in plaster of pans prevented the samples from
breaking or crumbling, while the use of polyvinyl chloride
film protected them from possible contamination by the
plaster. Then they were fixed on a non-magnetic mount by
non-magnetic glue and were cut by a diamond blade into
thin disc subsamples of 1.5-2.5 mm thick. The samples from
KM1 were fixed and cut using a similar process without the
wrapping and the setting. The total numbers of disc
subsamples taken from the three stalagmites KM1, KM2
and R W are 311, 68 and 309, respectively.
3 MAGNETIC MEASUREMENTS AND
RESULTS
Magnetic measurements were carried out using a cryogenic
magnetometer whose sensitivity is lo-" Am2
emu).
demagnetization was
ProPessive alternating field (*)
performed on all disc subsamples in order to examine the
Palaeomagnetism of stalagmites
521
RYU
'@
1.5-2.5 0.4mm
ItI-
. .. . .. . . ...
SUBSAMPLES
KMI
E
f
E
0
\o
Qo
.7
*..
E
E
E
E
0
0
0
t
I.
L n
cw
N
I__ 200 mm -4
F3gnre 1. Schematic views of the three stalagmites, KM1, KM2 and RYU, showing their growth layer patterns and positions of subsamples
taken for magnetic measurements.
magnetic stability and to define the characteristic component
of their magnetization. The NRM intensities of disc
subsamples before AF demagnetization were in the range
10-5-10-7 Am2 kg-' (emu g-'). The intensities of samples
from the KM2 and RYU stalagmites were generally weaker
than those from the KM1 stalagmite, by one order. All the
subsamples had very stable components and only weak low
coercivity components, which could be removed at AF
levels up to about 10 mT (Fig. 2). Median destructive fields
fell in the range 10 to 15 mT for almost all the subsamples.
Optimum demagnetizing fields for definition of the stable
characteristic magnetization differed for each subsample
and ranged from around 6 to 12mT. Possible primary
components for most subsamples were very stable, and their
directions changed only slightly during demagnetizing up to
40 mT.
The results of the magnetization determinations obtained
from the three stalagmites are shown in Figs 3 and 4 for
declination and inclination, respectively. In the case of the
parallel samples taken from the two stalagmites KM1 and
KM2, correlation with respect to positions of disc specimens
taken from each cylindrical sample was determined by
observation and comparison of the pattern of the growth
layers. The positions of all subsamples were then adjusted to
distance from the surface of a 'master' sample selected for
each stalagmite by stretching and compressing the records.
In Figs 3(a) and 4(a), the direction (declination and
inclination) data are plotted against distance from the top of
the R W stalagmite. In Figs 3(b), (c), 4(b) and (c), they are
plotted against distance from the surface of the master
sample for stalagmites KM1 and KM2. Sequential five-point
moving average curves are also shown in these figures. The
results for the RYU stalagmite reveal anomalously westerly
declination and steep inclination below 60 cm. However, the
522
H . Morinaga, H . Inokuchi and K . Yaskawa
--
-
KM 1
15-75
--
KM2
3-04
NRM I N T . - ~
=029x10 Am/Kg
A
-
N
SI
0.
--
RYU
111
-- NRM INT -6
=O 21x10 Am /Kg
NRM INT
= 2 4x10-6 Am/Kg
2
S-c
-
Horizontal
;
0
Vertical
Figore 2. Typical orthogonal vector component diagrams for progressive AF demagnetization of stalagmite subsamples. Numbers adjacent to
open circles denote peak demagnetizing AF in mT.
Figure 3. AF-cleaned declinations for the three stalagmites plotted against distance from the top or the surface of the stalagmite (dots and
squares). Sequential five-point moving vector averages are also shown (lines).
Palaeomagnetism of stalagmites
. (b) KM2
'O*]
70'
523
4
( c ) KMI
.: .
- ..
. .
..
*
c
.
F3gure 4. AF-cleaned inclinations for the three stalagmites plotted against distance from the top or the surface (dots and squares). Sequential
five-point moving vector avenges are also shown (lines).
remanences of subsamples near the base show similar NRM
intensities and stabilities to other subsamples during
progressive AF demagnetization.
4
DISCUSSIONS A N D CONCLUSIONS
4.1 Consistency of results from parallel samples from two
stalagmites
The results from six parallel samples taken from stalagmite
KM2 are shown individually in Fig. 5 as a function of
distance from the surface of the master sample. Curves of
declination, inclination and NRM intensity variation for
each parallel sample are, for the most part, consistent with
each other, showing good internal consistency within one
stalagmite. However, there is some scatter in inclination
curves at the top and in declination curves at the bottom.
These inconsistencies may be attributed mainly to the
misfitting of the distance relative to the 'master sample' near
to both ends. Also, because the top and bottom subsamples
are irregularly shaped (not perfect discs), measurement
errors may exist in these subsamples. The generally high
level of internal consistency indicates that the remanent
magnetizations may be attributed to an identical source for
this stalagmite. The precisions of sequential five-point
moving average directions calculated using all KM2
specimens range from k = 41.6 (a,,= 12.0") to k = 1015.0
(a9,= 2.4'). An average value of the precisions is k = 263.1
(a9s
= 5.5"). The internal consistency of data for stalagmite
KM1 is similar to that for KM2 (Morinaga et al. 1986). The
precisions range from k = 28.2 (a,5= 14.7') to k = 1913.0
(agS
= 1.7') and the average value is k = 217.5 (a,,= 6.6')
for the KM1 stalagmite.
4.2 Tentative correlation of results from the three
stalagmites
Sequential five-point moving average curves of direction
(declination and inclination) obtained from the two
Komori-Ana stalagmites, KM1 and KM2, are shown in Fig.
6(a) and (b), respectively. The curves can be tentatively
correlated with each other on the basis of their amplitudes
and variations, showing a possible consistency between two
stalagmites in the same limestone cave. The comparison
between the data for these two stalagmites was made
through direct eye observation so as not to bring about great
discrepancies in the growth rates of the two stalagmites. The
declination of stalagmite KM2 is relative, because this
stalagmite was not orientated owing to its having been
already broken at the sampling time. The declination curve
for stalagmite KM2, thus, is shifted by a constant value, so
as to fit that of stalagmite KM1.
Considering the close proximity of these two stalagmites.
524
H . Morinaga, H . Inokuchi and K . Yaskawa
c
KM2 STALAGMITE
20
-26
O I
I
NRM INTENSITY (AmVKg)
0
1
2
3
4 (cml
Figure 5. Consistency between the palaeomagnetic results for six parallel samples from stalagmite KM2. This shows generally high internal
consistency within the stalagmite.
the degree of correlation between their magnetization
records is surprisingly poor. Factors contributing to this may
include differences of subsample numbers and ratios of
signal to noise between the two stalagmites.
The variation curves of direction (declination and
inclination) obtained from the R W and KM1 stalagmites
are shown in Fig. 7(a) and (b), respectively. The curves are
drawn with sequential five-point moving averages as a
function of distance from the top or from the surface. These
curves can be tentatively correlated with each other,
showing a possible regional consistency between two
different limestone caves. This comparison was also made
through direct eye observation, so as not to bring about
great discrepancies in the growth rate of the two stalagmites.
This tentative correlation suggests that the remanent
magnetizations in the two stalagmites probably reflect the
same magnetization source, most likely the past geomagnetic field.
Based on the tentative correlations of the results from the
three investigated stalagmites, provisional geomagnetic
secular variation curves for SW Japan can be constructed
(Fig. 8) as a function of distance from the top of the RYU
stalagmite. The curves of the declination and the inclination
were drawn with average values every 2mm down to a
distance of about 12cm from the top and with sequential
five-point moving averages in the rest of the range because
of the limited amount of directional data.
4.3 Tentative correlation between the results of this
study and those of arrhaeomagnetic studies in SW Japan
Figure 8 shows variation curves of the past geomagnetic field
direction (declination and inclination, respectively) in SW
Japan obtained from (i) palaeomagnetic results for the three
stalagmites described in the present study and (ii)
archaeomagnetic results (Hirooka 1971, 1983). The curves
for the latter results are based on average values for 50yr
intervals. The curves, especially the inclination curve, show
a reasonably good degree of correlation. The small
inconsistencies between the declination curves may be
attributed, at least in part, to the misfitting of relative
distances for the results from the stalagmites. The relatively
good correlation suggests that the remanent magnetization
of the stalagmites does reflect the past geomagnetic field and
that, under favourable circumstances, stalagmites may be
excellent recorders of the geomagnetic field. It is concluded
that the curves obtained from the palaeomagnetism of the
three stalagmites most probably do represent geomagnetic
secular variation records, although the absolute ages of
these stalagmites have not yet been determined independ-
Palaeomagnetism of stalagmites
525
DECLINATION
0'
-20'
I
I
KM2
-10'
70°
50'
FQwe 6. Tentative correlation between the palaeomagnetic results for two stalagmites (KM1 and KM2) from the same limestone cave
(Komori-Ana cave).
526
H . Morinaga, H . Znokuchi and K . Yaskawa
DECLI NATION
I
IN C L I NAT I ON
Figure 7. Tentative correlation between palaeomagnetic results for two stalagmites from different limestone caves, (i) Ryuga-Do cave and (ii)
Komori-Ana cave. This shows a possible regional consistency between the data from different limestone caves.
ently. The extrapolated age of the oldest end of the curves is
about 15000yr BP, based on correlation with the
archaeomagnetic results, although this estimate is highly
tentative.
4.4 Characteristic featores of the provisional secular
variation curves of the geomagnetic field direction
A long interval of westerly declination is recognized in the
older half (below 60cm in stalagmite R W ) of the
provisional secular variation curve (Fig. 8, upper). The
maximum deflection is about 9OW. Inclination values
during this period are steeper (6@ 80') than expected from
the geocentric axial dipole (Fig. 8, lower). Such a westerly
declination and steep inclination period before 6600yr BP
-
has already been reported from palaeomagnetic studies of
unconsolidated sediments from the inland sea in Japan
(Muroi & Yaskawa 1977; Hyodo & Yaskawa 1986b). The
geomagnetic field behaviour during this period may be
explained by an abnormally large standing component of the
non-dipole field in the vicinity of East Asia, such as the
Mongolian positive anomaly at present (Yukutake &
Tachinaka 1969). It may possibly be related to proposed
short-period geomagnetic excursions during a similar period
(Morner & Lanser 1974, 1975; Noel & Tarling 1975; Morner
1977; Morinaga, Morinaga & Yaskawa 1987), although the
existence of this excursion has been questioned (e.g.
Thompson & Berglund 1976; Jacobs 1984). Further
investigations of the characteristic features of the geomagnetic secular variation records of stalagmites are required
and are now in progress by ourselves.
Palaeomagnetism of stalagmites
527
I NCIL I NAT I ON
50'
30'
c
.
I
.
. .
Figure 8. Tentative correlation between (i) provisional secular variation curves obtained from the three stalagmites described in the present
study and (ii) archaeomagnetic secular variation curves (from Hirooka 1971, 1983).
4.5
Further diecussions and hture works on stalagmites
Stalagmites may be very useful in establishing the
palaeosecular variation curve in the direction as mentioned
above. The palaeomagnetism of stalagmites is a promising
technique which may compensate for the inevitable
problems and potential defects in archaeomagnetic studies
and palaeomagnetic studies of unconsolidated sediments.
Synchroneity between crystallization and magnetization
acquisition of a calcium carbonate film deposited on a
stalagmite surface has been investigated through laboratory
experiments making synthetic stalagmites using sodium
528
H . Morinaga, H . Inokuchi and K . Yaskawa
thiosulfate (Morinaga et al. 1989). The present results
indicate that t h e magnetization acquisition occurs at much
the same time as crystallization an d also that the remanent
directions are parallel to the ambient direct field. The
remanent magnetization is believed to be acquired through
chemical fixation of the magnetic particles along the ambient
direct field and among t h e crystals.
Since stalagmites are predominantly composed of calcium
carbonate, they may be useful in estimating palaeotemperature fluctuations. This, and synchroneity of the
crystallization and magnetization acquisition suggests potential for exploring relationships between the geomagnetic
field and the air temperature in the past using stalagmites
(Morinaga et af. 1985).
ACKNOWLEDGMENTS
We thank Dr M. Ikeya, Faculty of Science, Osaka
University, Dr T. Miki, Technical College, Yamaguchi
University, and D r T. Kuramoto, the Science Museum of
Akiyoshi Plateau, who kindly gave us t h e opportunity t o
perform palaeomagnetic studies of the stalagmites. W e also
thank Mr Y.Nisa, Faculty of Science, Kobe University, for
his assistance a t the time of magnetic measurement on the
KM2 stalagmite. Reviews by Dr M. Noel and one other
anonymous referee greatly improved t h e manuscript.
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