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

海滩岩是热带和亚热带海岸潮间带沉积物经碳酸钙岩化后形成的固结矿床。日本海滩岩石的年龄，主要是由于对一些生物胶结的骨骼碎片进行的放射性碳测年，范围从6500年前到现在。据报道，1951年在太平洋环礁上观察到的海滩岩石中含有七年前军事行动中使用的材料，这表明了快速胶结作用的存在。我们对澳大利亚大堡礁南端的苍鹭岛、日本冲绳、久美岛和宫古岛的海滩岩进行了古地磁研究，得到了以下结果：(1)处理后的海滩岩的NRM强度在10^&lt；-7~10^&lt；1-6 Am^2/kg之间，这使得商业磁力仪可以精确地测量它们的磁化强度。(2)每个地点的NRM方向都或多或少地分散。这不是…然而，更重要的是，它们向上指向苍鹭岛，而向下指向与地球磁场方向相对应的日本岛屿是南半球和北半球。这意味着类似成因的岩石，如硅灰岩和钙灰岩，具有记录过去地磁场反转的能力。(3)在库姆吉马岛津的两个地点观察到了非常集中的NRM方向。这些岩石具有较高的强度，约为10^&lt；-6&gt；Am2/kg，可能是由于其中含有风化玄武岩碎屑所致。磁选最接近的剩磁方向群在5mT，Im=56.85ﾟ，k=275，A^&lt；95&gt；=4.62ﾟ，N=5；对于Shimajiri(B)，1.25mT，Im=37.71ﾟ，Dm=0.87ﾟ，k=517，A^&lt；95&gt；=2.27ﾟ，N=9。在50mT的磁场退磁下，4个岛津(B)样品的初始聚焦剩磁方向保持不变，强度衰减到初始值的三分之一左右。岛池(B)的平均剩磁方向与现在的磁场方向(倾角=37ﾟ，赤纬=-3ﾟ)很好地吻合，而岛池(A)的平均剩磁方向明显偏向更深的倾角。虽然目前无法获得这些岩石的^&lt；14&gt；C年龄，但(A)肯定比(B)更古老，因为后者位于现今海平面附近，但前者所处的位置较高。我们得出结论，一些海滩岩，其中最好的可能是那些出现在火山岛上的海滩岩，在没有资料的海洋地区具有探测全新世地磁长期变化的能力，对硅质岩和钙灰岩的古地磁学研究能够在其剩余磁化作用中提取地磁极性记录。我们认为，海滩岩的净磁化是通过在潮汐作用期间沿周围地磁场前进的组成磁性颗粒的重定向而获得的。较少

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