林田/明

講演予稿:
　国際海洋掘削計画（ODP）167次航海で採取されたカリフォルニア沖1010地点 の堆積物について，磁気層序と古地球磁場強度推定の研究を行った。U-チャ ネル試料のパススルー測定の結果，深度78.5mまでの堆積物が少なくとも27の 磁極帯に分けられ，C3A.n2からブルン・クロン（0〜6 Ma）に対比された。ARM とIRMを用いた残留磁化強度の規格化によって，0.8〜1.1 Maの範囲ではODP Leg 138で報告された古地球磁場強度とよく一致する記録が得られた。しかし 他の層準では規格化した磁化強度と磁性鉱物の濃度や粒径との間に相関が認め

abstract:
　Based on the paleomagnetic data from ODP Leg 138 sediments from the eastern equatorial Pacific Ocean, Valet and Meynadier (1993) suggested that the geomagnetic intensity variation for the last 4 Ma is characterized by asymmetrical saw-tooth patterns with a gradual decrease towards a polarity transition and a rapid recovery after the reversal. Several authors suggested, however, that the saw-tooth patterns are not geomagnetic origin but can be explained by delayed acquisition of the remanence after deposition by either a long-term viscous remanence, a late alignment of magnetic grains, or diagenetic precipitation of magnetic minerals. Another difficulty in obtaining reliable paleointensity records from sediments arises from variations of magnetic mineralogy, concentration and grain size, which sometimes reflect paleoenvironmental factors and are usually difficult to compensate by normalization. During ODP Leg 167, a continuous sequence of pelagic sediments of the Quaternary to Middle Miocene was recovered from Site 1010 off Baja California. In attempt to obtain a magnetic polarity stratigraphy and a long paleointensity record, we made NRM measurements of 59 u-channel samples, which cover the interval from 0 to 78.5 mcd (meters in composite depth) of the spliced section of Holes 1010C and 1010E. The u-channel data were not affected by a drilling-induced magnetization although some of the archive halves measured aboard ship had radial or vertical overprints. The investigated section is divided into at least 27 polarity intervals, which are assigned to the chrons from C3A.n2 to the Brunhes (C1n) chrons spanning the last 6 m.y. The sedimentation rate varies from 10 to 15 mm/kyr, which is the same order as the ODP Leg 138 sites. We used ARM and IRM measurements of the u-channels to evaluate mineral magnetic properties and to assess whether the samples were suitable for relative paleointensity determinations. The ARM-normalized relative paleointensity record displays fluctuations with periods of about 50-100 kyr periods and shows similarities with the paleointensity record from ODP Leg 138 sediments. The agreement is especially good for the time interval between the Cobb Mountain event and the Brunhes/Matuyama boundary. However, the variation in the relative paleointensity with depth can still be correlated with variations in grain-size sensitive parameters, such as the ratio of ARM susceptibility to low-field susceptibility or the ratio of ARM susceptibility to saturation IRM. These results suggest that the normalized paleointensity record of Site 1010 is influenced by variations in the magnetic carriers, possibly associated with paleoclimatic change.

キーワード:
磁気層序，古地球磁場強度，国際海洋掘削計画

Keywords:
magnetostratigraphy, relative paleointensity, Ocean Drilling Program