Paleomagnetic study of beachrocks : To detect Holocene geomagntic secular variation in the oceanic region

海滩岩石的古地磁研究：探测海洋地区全新世地磁长期变化

• 批准号: 02640598
• 负责人: TOKIEDA Katsuyasu
• 金额: $ 1.22万
• 依托单位: Shimane University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for General Scientific Research (C)
• 财政年份: 1990
• 资助国家: 日本
• 起止时间: 1990 至 1991
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-02640598/
• 关键词: Beachrock; Paleomagnetism; Geomagnetic secular variation; Oceanic region; Okinawa, Japan; Heron Island, Australia; Chemical Sediments; Carbon 14 dating; Heron Island,Australia; Chemical Sediments

Beachrock is the consolidated deposit which results from lithification by calcium carbonate of sediments in the intertidal zones of tropical and subtropical coasts. Ages of Japanese beachrocks, mostly due to the radio carbon dating of cemented skeletal fragments of some organisms, range from 6500 B. P. to near the present. Rapid cementation is suggested by artifacts incorporated in them : It was reported that beachrocks observed on Pacific atolls in 1951 contained materials used during military action seven years earlier. We made a paleomagnetic study of beachrocks from Heron Island, located at the south end of the Great Barrier Reef, Australia and from Okinawa, Kume-jima and Miyako-jima, Japan.We obtained the following results :(1) The beachrocks treated have NRM intensity ranging from 10^<-7> to 10^<1-6> Am^2/kg, which allows precise measurements of their magnetization by a commercial magnetometer.(2) NRM directions of the beachrocks are more or less scattered in every site. It is no … More ted, however, that they point upward at Heron Island, while downward at Japanese islands corresponding to the geomagnetic field directions is the Southern and Northern Hemisphere. This means that rocks of similar genesis, e. g. silcrete and calcrete, have a capability to record past geomagnetic field reversals.(3) Closely focused NRM directions were observed at 2 sites of Shimajiri, Kume-jima. These rocks have comparatively high intensities of the order of 10^<-6> Am2/kg possibly due to weathered basalt fragments incorporated in them. Magnetic cleaning gave the closest grouping of remanence directions for Shimajiri (A) at 5mT with Im=56.85ﾟ, k=275, A^<95>=4.62ﾟ, N=5 and for Shimajiri (B) at 1.25mT with Im=37.71ﾟ, Dm=0.87ﾟ, k=517, A^<95>=2.27ﾟ, N=9. Under the a. f. demagnetization up to 50mT, four test samples of Shimajiri (B) held their initially focused remanece direction unchanged with smooth decay of their intensity to about one third of the initial values. The mean remanence direction of Shimajiri (B) well agrees with the present field direction (inclination=37ﾟ, declination=-3ﾟ) , but that of Shimajiri (A) clearly deflects toward a deeper inclination. Although ^<14>C ages are unavailable for these rocks at present, (A) is certainly older than (B) because the latter is located near the present sea level, but the former occupies a higher position.We conclude that some beachrocks, whose best could be those occurred on volcanic islands, have a power to detect Holocene geomagnetic secular Variation in the oceanic region where no data has been available, and that paleomagnetic studies of silcrete and calcrete enable to extract geomagnetic polarity record in their remanent magnetizations. We consider that beachrocks acquired their net magnetization through reorientations of the constituent magnetic grains which were advanced along the ambient geomagnetic field during the tidal action twice a day. Less

海滩岩是热带、亚热带海岸潮间带沉积物经碳酸钙岩化作用形成的固结存款。日本海滩岩石的年龄，主要是由于一些生物的胶结骨骼碎片的放射性碳年代测定，范围从6500 B。P，接近现在。据报道，1951年在太平洋环礁上观察到的海滩岩石含有七年前军事行动中使用的材料。对澳大利亚大堡礁南端的苍鹭岛和日本的冲绳、久米岛和宫古岛的海滩岩进行了古地磁研究，得到以下结果：（1）处理后的海滩岩的NRM强度为10^<-7>~ 10^<1-6>Am ^2/kg，可以用商用磁力计精确测量其磁化强度。(2)海滩岩的NRM方向或多或少地分散在每个地点。毫不 ...更多信息 然而，他们指出，它们向上指向苍鹭岛，而向下指向日本岛屿，对应的地磁场方向是南半球和北方半球。这意味着类似成因的岩石，e。G.硅结砾岩和钙结砾岩具有记录过去地磁场逆转的能力。(3)在Kume-jima的Shimajiri的2个地点观察到密切关注的NRM方向。这些岩石具有相对较高的强度，约为10^<-6>Am 2/kg，这可能是由于风化的玄武岩碎片掺入其中。磁清洗给出了最接近的剩磁方向分组：Shimajiri（A）在5 mT时，Im=56.85 μ m，k=275，A^<95>=4.62 μ m，N=5; Shimajiri（B）在1.25mT时，Im=37.71 μ m，Dm=0.87 μ m，k=517，A^<95>=2.27 μ m，N=9。在A下面。F.在高达50 mT的退磁中，四个Shimajiri（B）的测试样品保持其初始聚焦剩磁方向不变，其强度平滑衰减到初始值的约三分之一。岛尻（B）的平均剩磁方向与现在的磁场方向（倾角=37 °，倾角=-3 °）一致，但岛尻（A）的平均剩磁方向明显偏向更深的倾角。虽然<14>目前还没有这些岩石的^ C年龄，但（A）肯定比（B）老，因为后者位于现代海平面附近，但前者所占的位置更高。硅结砾岩和钙质结砾岩的古地磁研究可提取其复磁化中的地磁极性记录。我们认为，海滩岩石获得其净磁化通过重新取向的组成磁性颗粒，这是先进的沿着周围的地磁场在潮汐作用一天两次。少

项目成果

TOKIEDA katsuyasu and ITO haruaki: "Paleomagnetic study of beachrocks from Australia and Japan : Its potentiality to detect Holocene geomagnetic secular variations in the oceanic region" J. G. G.

TOKIEDA katsuyasu 和 ITO haruaki：“澳大利亚和日本海滩岩石的古地磁研究：探测海洋区域全新世地磁长期变化的潜力” J. G. G.

TOKIEDA Katsuyasu ITO Haruaki: "Palaeomagnetic study of beachrocks from Australia and Japan:Its potentiality to detect Holocene geomagnetic secular variations in the oceanic region" J.G.G.(1992)

TOKIEDA Katsuyasu ITO Haruaki：“澳大利亚和日本海滩岩石的古地磁研究：探测海洋区域全新世地磁长期变化的潜力”J.G.G.(1992)

TOKIEA Katsuyasu,ITO haruyuki: "Paleomagmetic study of beachrocks fron Australia and Japan:Its potentiality to detect Holocene geomagmetic secular variations in the oceanic region" J.G.G.(1992)

TOKIEA Katsuyasu、ITO haruyuki：“澳大利亚和日本海滩岩石的古地磁研究：探测海洋区域全新世地磁长期变化的潜力”J.G.G.（1992）