This paper presents a palemagnetic intensity record for late Quaternary time in Taiwan. Samples came from the lower part of an 86-meter sedimentary piston core drilled in the Wukou area of the Taipei Basin. The age interval of the studied portion, as determined by C-14 dating (Liu et al., 1994), was proposed to be between 9,000 yr. B.P. and 21,000 yr. B.P. Natural remanent magnetization (NRM), anhysteresis remanent magnetization (ARM) and low field magnetic susceptibility (χ) of the samples were analyzed.
Results show that the stratigraphical variation patterns of NRM/χ and ARM/χ, after 20 mT alternating field demagnetization treatments on both NRM and ARM of samples, are very similar. This implies that the acquisition of NRM and ARM are under the same magnetic mineralogical conditions. Thus, the ratio of the NRM to ARM at this demagnetization stage could represent the paleointensity secular variation of geomagnetic field in the area studied.
Four major high peak areas were found in the NRM/ARM record (after 20 mT demagnetization). These were at about 60 m, 70 m, 80 m and 90 m in depth. Three of them could be correlated to another intensity record analyzed from a lacustrine sediment core drilled in the Taihu area of China (Wang et al., 1997). Comparing these two records, and the C-14 dated ages of the two cores, the correspondent ages of the four intensity high peaks were proposed to be about 10,000, 13,500, 17 ,500 and 21,000 years before present, respectively. In addition, the age assigned to the peak at about 60 m in depth is also supported by the volcanic records in Japan (Tanaka, 1990). Thus, the record of this study presents a regional secular variation pattern of paleo-intensity of the earth's magnetic field during the last 9,000 to 21,000 years in East Asia.