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

滩岩是热带和亚热带海岸潮间带沉积物由碳酸钙岩化作用形成的固结矿床。日本海滩岩石的年龄，主要是由于对一些生物的胶结骨骼碎片的放射性碳定年，范围从6500 b.p.到接近现在。其中所包含的人工制品表明它们具有迅速的胶结作用：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。我们对位于澳大利亚大堡礁南端的Heron岛和日本冲绳、久米岛和宫古岛的海滩岩石进行了古地磁研究。我们得到了以下结果：(1)处理的滩岩的NRM强度范围为10^<-7> Am^2/kg至10^<1-6> Am^2/kg，可以通过商用磁力计精确测量其磁化强度。(2)滩岩的NRM方向在各站点或多或少地分散。更确切地说，它们向上指向苍鹭岛，而向下指向日本岛屿，与地磁场方向对应的是南半球和北半球。这意味着类似成因的岩石，如硅质和钙质，有能力记录过去的地磁场倒转。(3)在岛尻、久米岛的2个地点观测到紧密集中的NRM方向。这些岩石的强度相对较高，约为10^<-6> Am2/kg，可能是由于风化的玄武岩碎片掺入其中。磁清洗结果表明，岛尻(A)在5mT时，Im=56.85, k=275, A^<95>=4.62， N=5；岛尻(B)在1.25mT时，Im=37.71, Dm=0.87, k=517, A^<95>=2.27， N=9。在高达50mT的a.f.f退磁作用下，4个岛尻(B)试样的剩磁方向保持不变，强度平滑衰减至初始值的三分之一左右。岛尻(B)的平均剩余物方向与当前磁场方向（倾角=37，赤纬=-3）吻合较好，但岛尻(A)的剩余物方向明显偏向较深的倾角。虽然目前无法得到这些岩石<14>的年龄，但(A)肯定比(B)更老，因为后者位于现在的海平面附近，但前者占据更高的位置。我们认为，在没有资料的海洋地区，某些滩岩具有探测全新世地磁长期变化的能力，其中最好的是火山岛上的滩岩，对硅结和钙质的古地磁研究可以提取其剩余磁化的地磁极性记录。我们认为滩岩的净磁化是通过在潮汐作用期间每天两次沿着环境地磁场前进的组成磁粒的重新定向获得的。少

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