Explore the vast range of titles available.

The natural remanent magnetization (NRM) of high sedimentation rate sediments provides significant information about paleomagnetic secular variation of the Earth's magnetic field and can also potentially be used for stratigraphy. However, NRM acquisition depends on conditions inherent to the depositional environment. In addition to recording a precise annual chronology, varved sediments reflect marked annual sedimentary changes. The Earth's magnetic field does not vary significantly over such a short period, so magnetic changes recorded by varves are expected to reflect the influence of depositional parameters on the recording process. We focus here on a sequence of 27 ± 1 varves from the former proglacial Lake Ojibway (∼8.5 ka cal BP) from which individual cm‐thick summer and winter beds were sampled. Paleomagnetic, granulometric and geochemical analyses were conducted on each bed. A mean inclination shallowing of 24.3° is observed in winter beds, along with an 11.3° shallowing in summer beds. Magnetic declinations follow, on average, the expected field direction, but differences of up to 20° occur between successive beds. Summer beds are thicker than winter beds and have stronger magnetic susceptibility, higher Ca/Fe ratios and coarser sedimentary and magnetic grains. This grain size pattern reflects the input of coarser detrital particles during summer, while the finer fraction remained in suspension until it was deposited in winter. A combination of differential compaction between the winter and summer beds, seasonally varying physical and magnetic properties of sediments, and delayed NRM acquisition explains the variable and coercivity‐dependent inclination shallowing. Key Points The seasonal variations have an impact on the remanent magnetization in varved sediments

The main objective of this work is to compare directional (declination and inclination) volcanic and archaeomagnetic data for the last four centuries (~1600–1990) with the historical geomagnetic predictions given by the GUFM1 model which spans from 1590 to 1990. The results show statistical agreement between archaeomagnetic data and directions given by the geomagnetic field model. However, when comparing the volcanic data with the model predictions, marked inclination shallowing is observed. This systematically lower inclination has already been observed in local palaeomagnetic studies (Italy, Mexico and Hawaii) for the 20th century, by comparing recent lava flows with the International Reference Geomagnetic Field (IGRF) model. Here, we show how this inclination shallowing is statistically present at worldwide scale for the last 400 years with mean inclination deviation around 3° lower than the historical geomagnetic field model predictions.

The results of detailed paleomagnetic studies in seven Upper Permian and Lower Triassic reference sections of East Europe (Middle Volga and Orenburg region) and Central Germany are presented. For each section, the coefficient of inclination shallowing f (King, 1955) is estimated by the Elongation–Inclination (E–I) method (Tauxe and Kent, 2004) and is found to vary from 0.4 to 0.9. The paleomagnetic directions, corrected for the inclination shallowing, are used to calculate the new Late Permian–Early Triassic paleomagnetic pole for the East European Platform (N = 7, PLat = 52.1°, PLong = 155.8°, A95 = 6.6°). Based on this pole, the geocentric axial dipole hypothesis close to the Paleozoic/Mesozoic boundary is tested by the single plate method. The absence of the statistically significant distinction between the obtained pole and the average Permian–Triassic (P–Tr) paleomagnetic pole of the Siberian Platform and the coeval pole of the North American Platform corrected for the opening of the Atlantic (Shatsillo et al., 2006) is interpreted by us as evidence that ~250 Ma the configuration of the magnetic field of the Earth was predominantly dipolar; i.e., the contribution of nondipole components was at most 10% of the main magnetic field. In our opinion, the hypothesis of the nondipolity of the geomagnetic field at the P–Tr boundary, which has been repeatedly discussed in recent decades (Van der Voo and Torsvik, 2001; Bazhenov and Shatsillo, 2010; Veselovskiy and Pavlov, 2006), resulted from disregarding the effect of inclination shallowing in the paleomagnetic determinations from sedimentary rocks of “stable” Europe (the East European platform and West European plate).

对华南地块中三叠统巴东组红层样品进行系统的岩石磁学和古地磁学研究,采用高场等温剩磁各向异性(hf-AIR)方法,识别出巴东组红层的磁倾角浅化因子f=0.63.岩石磁学研究结果显示,巴东组红层的主要载磁矿物为赤铁矿和少量磁铁矿;磁化率和高场等温剩磁各向异性组构均指示其具有典型沉积组构特征,表明未遭受后期构造应力改造.高温特征剩磁分量为碎屑赤铁矿所携带,具有单一负极性,并在95％置信水平上通过褶皱检验,与前人在同一剖面不同位置获得的以正极性为主的古地磁方向一致.该特征剩磁方向在地层校正后的平均方向为Ds=222.1°,Is=?27.2°(α95=8.7°),对应古地磁极为48.1°N,215.5°E(A95=8.4°).对包括本文数据在内的华南中三叠世红层高质量古地磁极数据(Q≥5)用f=0.63进行统一校正后,获得平均古地磁极为48.5°N,207.6°E(A95=10.7°).对比华北地块同样经hf-AIR方法浅化校正后的早三叠世古地磁极,两者在其东部参考点上的古纬度完全一致,验证了前人提出的华南华北中生代剪刀式旋转拼合模型.

A combined paleomagnetic, rock magnetic and magnetic fabrics study is conducted on the redbeds of Early Triassic Liujiagou Formation from Qinshui Basin, North China Block (NCB). The E/I (elongation/inclination) method indicates that the characteristic remanent magnetizations (ChRM) was significantly affected by inclination shallowing with a flattening factor f=0.6. Rock magnetic analysis indicates that hematite and magnetite are the main magnetic carriers. Anisotropy of magnetic susceptibility (AMS) result shows typical depositional fabrics in water. The chRM has been successfully isolated from 15 sites with tilt-corrected mean direction of Ds=318.8°, Is=30.9° (α95=6.9°) and a corresponding paleomagnetic pole at 49.3°N, 5.5°E (A95=6.7°). The pole after E/I correction is 53.5°N, 18.0°E. Combining with published high quality Early Triassic pole (Q≥4) for NCB, the mean Early Triassic pole for NCB before and after inclination shallowing correction is 55.3°N, 357.1°E (A95=5.5°) and 60.8°N, 13.4°E (A95=5.8°), respe

One of the key challenges which are traditionally encountered in studying the paleomagnetism of terrigenous sedimentary strata is the necessity to allow for the effect of shallowing of paleomagnetic inclinations which takes place under the compaction of the sediment at the early stages of diagenesis and most clearly manifests itself in the case of midlatitude sedimentation. Traditionally, estimating the coefficient of inclination flattening ( f ) implies routine re-deposition experiments and studying their magnetic anisotropy (Kodama, 2012), which is not possible in every standard paleomagnetic laboratory. The Elongation–Inclination ( E–I ) statistical method for estimating the coefficient of inclination shallowing, which was recently suggested in (Tauxe and Kent, 2004), does not require the investigation of the rock material in a specially equipped laboratory but toughens the requirements on the paleomagnetic data and, primarily, regarding the volume of the data, which significantly restricts the possibilities of the post factum estimation and correction for inclination shallowing. In this work, we present the results of the paleomagnetic reinvestigation of the Puchezh and Zhukov ravine (ravine) reference sections of the Upper Permian and Lower Triassic rocks in the Middle Volga region. The obtained paleomagnetic data allowed us to estimate the coefficient of inclination shallowing f by the E–I method: for both sections, it is f = 0.9. This method was also used by us for the paleomagnetic data that were previously obtained for the Permian–Triassic rocks of the Monastyrskii ravine (Monastirskoje) section (Gialanella et al., 1997), where the inclination shallowing coefficient was estimated at f = 0.6.