Last 160 Ka Paleomagnetic Directional Secular Variation Record from Core MD972151,Southwestern South China Sea

The ground-based magnetic signature of particle precipitation fromthe ring current to low latitude regions during the recovery phase of a geomagnetic storm on 23 December,1995,has been inferred by studying photometric 630.0nm nightglow at a low latitude station,Kolhapur(7.5°N,144.7°E; Geomagnetic,dip latitude 10.6° N) and magnetograms(H,D andZ)of nearby low latitude stations.The maximum optical emission occurredsimultaneously with the maximum positive(north ward)excursions in theH trace of low latitude magnetograms associated with negative H excursions at high latitude observatories.Generally,an increase in Z componentoccurred at all low latitude stations.A large intensity increase of aboutseven times in 630.0nm emission was observed at around 2:15 hrs I.S.T on23 December,1995.The 630.0nm enhancement maximised at the time ofhigh Dst or Kp index during the night.In addition,the ionosonde experiment conducted at Ahmedabad(13.8°N,144.7°E;Geomagnetic)showed theenhancement of peak electron densities in the F-region and sudden lowering of the F layer in the ionosphere.A possible explanation of the effect isgiven in terms of emergence of energetic neutral atoms(Hydrogen or Oxygen) from high energy protons in a typical charge exchange process in thering current depositing energy into the thermosphere-ionosphere system,also possibly due to changes in the composition of the thermosphere-ionosphere during magnetic storms that would enhance the 630.0nm emissions.


INTRODUCTION
Secular variation is one of the major time-spatial dependent behaviors of the earth' s magnetic field. 1t not only plays an important role for investigating the characteristics and origin of the geornagnetic field, but also becomes a very powerful tool for stratigraphie correlation studies.1t has been applied broadly in studying marine and lake sediments and yielded many fruitful results (Barton & MeElhinny, 1981;Creer, 1974;1977;Creer et al., 1976;1980;Hyodo et al., 1993;Turner and Thompson, 1981). During last two deeades, secular variation was applied in paleo-environment change studies. This application provided more precise age data, in addition to the paleontologieal and isotopie methods, to yield much better age constraints for the observed events. The South ehina Sea (SeS; 5-22°N, llO-1200E) located off southeastern Asia (Figure 1) is one of the largest marginal seas in the world. Reports indicate that it has shown a high sedimentation rate, especially since the last glacial period. Very thick sediment depositions were found at its surrounding continental shelf area (Wang et al., 1992). Such thick sediments provide a good opportunity for obtaining high resolution records of paleo-environment changes, paleo-oceanography, paleo-climate et aL In addition, it also provides an excellent chance for paleomagnetic study, inc1uding secular variation analysis.
In June 1997, the IMAGES research group of Taiwan joined the IMAGES III Cruise and retrieved g giant long piston cores from ses. The main purpose of this croise was to study and reconstruct the paleo-oceanography and paleo-climate changes in SCS. To establish good age control, each core was required to undergo paleomagnetic secular variation and rock magnetism studies. In this study, the first secular variation record analyzed from core MD972151 is presented. This will support age data for future stratigraphie correlations and observations about the behavior of the earth magnetic field in the ses area.

GEOLOGICAL SETTING AND LITHOLOGY OF THE CORE
The studied core MD972151 was taken from the southwestern SCS at a location north of the Sunda Shelf. The coordinates of the site position are lO9°52.17'E longitude and g043.73'N latitude ( Figure 1). The water depth ofthis site is about 1,589 m and the total recovered length of the studied core is 26.72 m. The lithology of this core is shown in Figure 2. Major constituents of this core are very fine grain clay and silty clay. Only very few thin sand laminae could be found at depths between 23 to 24 meters and close to the bottom. This figure indicates the presence ofa void of 22.5 cm in length at the depth interval of 1107.5 cm to 1130 cm. So, the actuallength of this core is about 26.5 m.
The Sunda Shelf covers an area extending from the Gulf of Siam to the Java Sea. The northern part accesses large amounts of terrigenous sediments delivered from the Indochina Penin sula by the Mekong River. Although it is the major water way between the SCS and the lndian Ocean, the water depth at its southern part is quite shallow, only 30 or 40 meters. So, this waterway might have been c10sed during the glacial period when the sea level was lower. And, this might have caused the deposition interruption which affected the continuity of the observed data set. However, the coring site of this study is in the northeast part of the Sunda Shelf. The present day water depth and the lithology of the core suggest that the observed record should not have this break in data continuity.
Stratigraphical variation patterns of paleo-declinations and paleo-inc1inations were then set up. In order to investigate the principal behavior of the earth magnetic field, the paleodeclinations had to be corrected for calculating the virtual geomagnetic pole (VOP) positions. This was because of the unknown azimuth of the reference line of the core. The method we used in this study was to put the mean dec1ination of the most stable part (4-25 m; flipped 180 0 for the reversed polarity directions) against that of its present day declination. Finally, the stratigraphical secular variation patterns of the paleo-Iongitude and paleo-Iatitude of VOP were established. The paleomagnetic data of core MD972151 are available electronically at Paleoceanographic Data Center of Core Laboratory -Center for Ocean Research, NSC, at the Institute of Applied Geophysics, National Taiwan

PALEOMAGNETIC SECULAR VARIATION PATTERN
The typical orthogonal plots of the samples are shown in Figure 3. Most of the samples show that secondary components ofNRM could be completely removed after 40 mT demagnetization. Thus, the characteristic remanent magnetizations could be well delimited.
Secular variation patterns of paleo-declination and paleo-inc1ination of samples are presented in Figure 4. The corresponding secular variation patterns of paleo-Iongitude and paleolongitude of VGP are displayed in Figure 5. From these figures, a reversed polarity event could be determined clearly at depths between 21.7 and 23.8 meters. Another smaii reversed event located near the bottoIil of the core was also found. At the depth of 15.3-15.5 meters, an event of negative inclination with north seeking dec1ination occurred. It was most likely an excursion. In addition, an abnormal direction zone appeared at the core top to 3.7 meters: either positive inclination with declination around the NW direction or negative inclination with declination around the NE direction. What these events represent will be discussed below.
In addition, the paleo-declination pattern ( Figure 4) showed a continuous counterc1ockwise variation trend from bottom of the core to 3.7 m in depth, except the reversed events. This trend reflected an eastward drifting phenomenon of VGP at most parts of the core (paleolongitude pattern in Figure 5). This differs greatly from present-day observations of the geomagnetic field which indicate that the magnetic pole displays a westward drift phenomenon. The significance of this phenomenon requires further study.

DISCUSSIONS
Because the reversed polarity event appeared at depths between 21.7 to 23.8 meters indicates it corresponds to the "Blake Event", which has an age interval of about 130-140 ka. This assignment is in close agreement with the results of the oxygen isotope analysis carried out by Lee et al. (Lee et al., 1999). They report that the boundary of stages 5 and 6, which has an age of about 127 ka, is at about 21.5 min depth. This confirmation enables us to estimate the average sedimentation rate of this core to be about 17-18 cm/ky. Based on the estimated sedimentation rate, we concluded that the core under study is able to support information of the last 150-160 ka. In this study, a very short reversed polarity event was also found at the bottom of the core. Past reports made no mention of any polarity revers al during 150-160 ka. However, the paleo-intensity study of the last 4 Ma by Valet and Meynadier (1993) indicated that a low intensity event occurred around 150-160 ka, between the Blake and Jamaica events. We believe that a record of such high resolution as core MD972151 studied in this paper can provide more opportunities to observe the suchshort events. Consequently, we tentatively assign the short reversed polarity event to this low intensity event, and conclude that the core studied will support results for the last 150-160 ka.
The magnetic excursion observed at the depth of 15.3-15.5 m was estimated, by using the estimated sedimentation rate, to have an age of about 80-90 ka. Unfortunately, no such excur- Considering that the sedimentation rate might not have been constant in the past, the observed excursion at 15.3-15.5 min depth in this study could possibly be correlated with this period of low intensity. And this hypothesis could be supported by oxygen isotope data studied by Lee et al. (1999). Abnormal directions found from the core top ta 3.7 m in depth were initially regarded as the Laschamp Event of 40 ka in age. However, this supposition was contradicted by the recent oxygen isotope data studied by Lee et al. (1999). Hence, the initial correlation of this abnormal event to the Laschamp Event was ruled out. According ta that this anomaly appeared at the top of the core, two other possible explanations of its formation could be proposed. One is that it was a previously un-reported excursion that occurred at a time less than 20 ka, the other is that the abnormal paleomagnetic directions resulted from depositional process of the sediments. Which phenomenon the abnormal directions actually represent is a question that will require further data, analysis and speculation.

CONCLUSIONS
Marine sediment cores taken from South China Sea during the IMAGES ID cruise provided a great opportunity for high-resolution study of paleomagnetic secular variation. Through this study, we established a paleomagnetic secular variation pattern for the last 160 ka. One excursions whieh appeared at the depth of 15.3-15.5 m was determined to have an age of about 75 ka. Another, found at the core top to 3.7 m, was shown not to be the 'Laschamp Event'. Rather, it was tentatively considered a previously un-reported excursion. However, the abnormal directions of this excursion indicate an alternate possible explanation based on the deposidon process. Of course, this problem requires further study.
Based on the age assignment, the sedimentation rate of the core studied is estimated to be about 17-18 cmlky on average. Furthermore, the age model, established from the paleomagnetic secular variation pattern of this study, is highly consistent with the oxygen isotope record. Hence, it should play an important role in future stratigraphie correlation work and age control in environmental and paleo-oceanographic studies of the South China Sea area. In addition, eastward drifting of the VGPs is observed in the paleo-longitude pattern. The significance of this phenomenon warrants further investigation.