Paleosecular variations 12–20 Kyr as recorded by sediments from Lake Moreno (Southern Argentina)

We present results of paleomagnetic and sedimentological studies carried out on three cores Lmor1, Lmo98-1, Lmor98-2 from bottom sediments of Lake Moreno (south-western Argentina), and integrate them with data from our previous studies. Measurements of directions (declination D and inclination I) and mass specific intensity of natural remanent magnetization (NRM intensity), magnetic susceptibility (specific, χ and volumetric, κ), isothermal remanent magnetization (IRM), saturation of isothermal remanent magnetization (SIRM), and back field remanent coercivity (B_0CR) were performed. The stability of the NRM was investigated using alternating-field demagnetization. The results show that these sediments meet the criteria required to construct a reliable paleomagnetic record. The cores were correlated very well based on magnetic parameters, such as χ and NRM intensity, as well as with lithological features. Tephra layers were identified from the lithological profiles and magnetic susceptibility logs. We obtained the D and I logs of the characteristic remanent magnetization for the cores as a function of shortened depth. The data from the three cores were combined to form a composite record using the Fisher method. A comparison between stacked inclination and declination records of Lake Moreno and those obtained in previous works on Lake Escondido and Lake El Trébol shows good agreement. This agreement made it possible to transform the stacked curves into time series spanning the interval 12–20 kyr. The results obtained improved our knowledge of SV and the behaviour of the geomagnetic field and also allowed us to determine the range of past inclination variations from −70° to −45° for the southern hemisphere, where data are scarce.     

Paleomagnetic and sedimentological studies at Lake Tahoe,California-Nevada

Three closely spaced 6-m piston cores were taken in the central part of Lake Tahoe. Cores were split into two complete replicates for paleomagnetic study and the remaining sections were used for stratigraphic and mineralogical analysis.Stratigraphic correlation of the cores is based on two distinctive horizons (volcanic ash and diatomite) and upon three different sedimentological regimes dominated by (1) poorly bedded silts and muds, (2) well bedded graded units, and (3) finely laminated silts. These correlations served as the standards for the evaluation of the paleomagnetic data. Extrapolation of¹⁴C dates obtained in the upper sections of the Lake Tahoe sediments suggests that the lower sections of the cores may reach ages of 25,000–30,000 years B.P.X-ray, optical, Curie point, and hysteresis measurements show that magnetite is the only important magnetic mineral in the sediments and occurs in the size range of 10 μm. Hematite is essentially absent. Based on large changes in the declination and inclination of the natural remanent magnetism (NRM) within single graded layers the paleomagnetic signature is a post-depositional remanent magnetism (PDRM). This PDRM is believed to be caused by magnetic orientation during compaction.Paleomagnetic measurements show three regimes that are correlated with the stratigraphic regimes. NRM declination and inclination data show good correlation between the three cores and agree well with the correlations based on sediment character. NRM intensity variations are due largely to the variations in magnetite content and its occurrence as either single detrital grains or as inclusions within the larger silicates. Thus the variation in paleo intensity was not determined.Comparisons of Lake Tahoe data with that from Mono Lake show fair correlations of declination and inclination. The occurrence of a short-wavelength, high-amplitude event in the lower section of the Lake Tahoe cores may provide confirmation of the Mono Lake geomagnetic excursion.     

Rotation of the Iberian arc: Palaeomagnetic results from North Spain

NRM directions measured from 32 sites in Middle Cambrian, Upper Silurian/Lower Devonian and Lower Carboniferous redbeds follow the trend of the Variscan arc in North Spain. Thermal demagnetisation does not significantly alter this pattern. Fold tests show that the NRM is earlier than the ?₁ folds which form the arc; consistency of angle between bedding and the tilt-corrected NRM inclination (22–28°), similarity of the corresponding palaeolatitudes to Carboniferous values and microscopic evidence of Variscan redistribution of hematite indicates that the magnetisation is post-Lower Carboniferous. A statistical plot of the orientation of ?₁ fold traces against angle between ?₁ fold traces and declination of NRM shows that where these folds curve through 165° the NRM has been rotated through 110°: the arc is an orocline. Restoration of this rotation, and that needed to close the Bay of Biscay, brings the calculated mean palaeomagnetic pole reasonably close to the Upper Carboniferous part of the apparent polar wander path for Europe.     

Statistical Analysis of Palaeomagnetic Data from the Last Four Centuries: Evidence of Systematic Inclination Shallowing in Lava Flow Records

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.     

Rotation history of Chios Island, Greece since the Middle Miocene

Superimposed on a regional pattern of oroclinal bending in the Aegean and west Anatolian regions, the coastal region of western Anatolia, shows a complex and chaotic pattern of coexisting clockwise and counterclockwise rotations. Here, we report new palaeomagnetic data from the eastern Aegean island of Chios, to test whether this fits the regional palaeomagnetic pattern associated with the Aegean orocline, or should be included in the narrow zone of chaotic palaeomagnetic directions. Therefore, a combined palaeomagnetic study of Miocene sediments and volcanic rocks has been carried out. Thermal and AF demagnetization of a 130-m thick Middle Miocene succession from the Michalos claypit allowed a stable component of both polarities to be isolated while rock magnetic experiments showed that the main magnetic carrier is magnetite. When compared with the Eurasian reference, the mean declination of 348 ± 5.1° implies 15° of counterclockwise rotation since Middle Miocene times. The obtained shallow inclination of 38 ± 6.7° was corrected to 61.8 ± 3.9°, by applying the elongation/inclination correction method for inclination shallowing. This result is similar to the expected inclination of 58° for the latitude of Chios. The palaeomagnetic analysis (demagnetization treatment and corresponding rock magnetic measurements) of the volcanic rocks identify a stable, predominantly normal, ChRM with poorly constrained mean declination of about 290 ± 19.8° based on five successfully resolved components. The significantly different palaeomagnetic results obtained from an island as small as Chios (and a very short distance), and the relatively large rotation amounts do not fit the regional palaeomagnetic direction of Lesbos and basins in northwestern Turkey which show little or no significant rotation. We thus prefer to include Chios in the coastal zone of chaotic rotations, which may represent a previously inferred tectonic transfer zone that accommodates lateral differences in extensional strain within the Aegean back-arc.     

Evidence for recent palaeomagnetic secular variation in lake sediments from the New Guinea Highlands

Palaeomagnetic declination and inclination data are reported from Mackereth mini cores taken from three lake sites in Papua New Guinea. ¹³⁷Cs, ²¹⁰Pb and ¹⁴C dating provide a timescale for the palaeomagnetic oscillations. Tephrastratigraphy aids correlation between lakes. The palaeomagnetic data resemble the geomagnetic secular variation pattern deduced from historical observations. The new results greatly extend the latitudinal coverage of palaeomagnetic secular variation recorded in the last 10⁴ yr. Their significance in assessing secular variation source models is discussed. The data suggest that the edge of the Pacific region of low non-dipole field activity has remained near New Guinea during the last 10⁴ yr.     

Short-Period Secular Variations (SPSV) of the geomagnetic field recorded in highly scattered palaeomagnetic records of Holocene lake sediments from North Poland

A simple method of calculating running means over different intervals within palaeomagnetic records has been used to identify short-period secular variations of the geomagnetic field. The records from Polish lake sediments reveal variations in declination with a period between 290 and 372 years. The periodicity of variations in the inclination has been estimated to be between 522 and 670 years.     

Rock-magnetic and archeomagnetic survey from some classical settlements at Chapultepec archeological site (western Mesoamerica)

Many archaeological artifacts contain magnetic minerals that may record the direction and strength of the Earth’s magnetic field. The geomagnetic field parameters (declination, inclination and intensity) change through time and then may be used as a dating tool. Over the last three decades, the archeomagnetic method was successfully applied in Europe. Still, no systematic studies are devoted to the American sites. We report here, a detailed rock-magnetic and archeomagnetic investigation of some pre-Columbian settlements at Chapultepec archeological site (western Mesoamerica). Continuous low-field susceptibility vs. temperature curves performed in air point to Ti-poor titanomagnetites as remanence carriers. Hysteresis ratios fall essentially in the pseudo-single-domain region. The twelve oriented samples taken from the furnace located in the habitation area were demagnetized applying pick alternative fields. In most cases, a stable uni-vectorial remanent magnetization was found yielding a reasonably well defined mean characteristic direction: inclination I = 32.8°, declination D = 353.4°, and parameters of Fisherian statistics α₉₅ = 10.4° and k = 14. Still poorly defined directional master curves for Mesoamerica, together with relatively dispersed mean directions obtained in this study, make dating targets difficult. Archaeomagnetic dating was carried out on the basis of Bayessian statistics. This established a most probable time interval from 512 to 634 A.D. with an average of 573 ± 61 A.D.     

Behaviour of the earth's magnetic field as recorded in the sediment of Loch Lomond

A palaeomagnetic record of geomagnetic secular variation during the last 7000 years has been obtained from the sediments of Loch Lomond, Scotland. The magnetic direction fluctuations repeat well between cores and show greater detail, especially over the last 5000 years, than other European records. A time scale has been derived from¹⁴C analyses on the Lomond sediment and comparison with other¹⁴C-dated sediments. Investigation of relative palaeointensity determination methods has shown that the widely used normalization parameter of partial ARM is insensitive to even small sediment grain size fluctuations.The new high-fidelity direction record and improved time scale show that geomagnetic field changes have not followed a simple oscillatory pattern during the last 7000 years. The record enhances the application of palaeomagnetism to dating recent sediments, as the main declination swings are now characterized by fine detail, and paired inclination data are also available. The problem of mismatching swings when correlating with other paired directional records is thus reduced.The palaeomagnetic record agrees well with some archaeomagnetic results. It confirms the period of anticlockwise motion of the geomagnetic field vector, between 1000 and 600 years B.P., which was first documented by English archaeomagnetic investigations. Clockwise motion is shown to predominate during the remainder of the last 5500 years. The VGP path does not correlate with that of Japanese archaeomagnetic results nor North American sediment data from 2000 to 0 years B.P. This suggests that the secular changes are dominated by local non-dipole sources rather than wobbling of the main geomagnetic dipole.     

Palaeomagnetic study of some charnockites of the Palni Hills

The intensity of natural remanence magnetisation (NRM) is measured along the three mutually-perpendicular directions using an astatic magnetometer. The intensity of induced magnetisation is measured using an apparatus fabricated in the laboratory. The Koenigsberger ratio has been calculated for all the samples and the ratio has been used to test the stability of NRM in the samples. The mean direction of magnetisation is determined from the three components of the NRM intensity. The palaeomagnetic pole position of the samples is determined using the direction of magnetisation and the site location. An attempt has been made to fix the geological age of the charnockites using palaeomagnetic methods.     

Palaeomagnetic Reversals in Mississippian Sediments of Missouri

--Normal to reversed polarity transitions have been found in Mississippian limestones of Missouri at longitude 269.7 and latitude 38.6 in an area of approximately 2,000 sq.km centered on the city of St. Louis. The transitions suggest a reversal of the geomagnetic field during the time these limestones were deposited. The mean normal NRM direction of declination 323.2, inclination 14.8, alpha-95 10.0 and corresponding reversed direction declination 144.8, inclination of 11.7 with alpha-95 of 5.2, are each tightly grouped, while in between these two groups the circles of confidence reveal a large scatter. Six normally magnetized groups of sites and corresponding VGP positions were found while seven groups are reversed. The mean normal VGP was at longitude 146.2 and latitude 44.6 and the reversed one at longitude 313.9 and latitude ⪶.8. The large scatter in between these two groups is interpreted as being due to rapid changes in the direction of the geomagnetic field when compared with the rate of deposition of the sediments. Alternating field (AF) and thermal demagnetization (TH) techniques were used to remove secondary components of the NRM.     

Palaeomagnetic secular variation studies of Finnish lake sediment and the carriers of remanence

Palaeomagnetic and mineral magnetism measurements have been carried out on two cores from Lake Vuokonjarvi in Finnish Karelia. The sediment probably covers 5000 years of continuous deposition at a mean sedimentation rate of about 0.8 mm/yr.The magnetic declination exhibits fluctuations of similar amplitude(～20°) and character to those recorded in northern England and northern Ireland. Magnetic inclination variations are of higher amplitude(～15°) than those found in Britain. Matching the palaeomagnetic patterns with the dated British master curves permits an estimate of the rate of deposition of the Finnish sediments, which is suggested to be more reliable than estimates from radiocarbon dating of the Vuokonjarvi sediment.The stable natural remanence is shown to be carried by fine-grained magnetite and titanomagnetite grains and to have grown by post-depositional alignment during a period of the order of 100 years. Laboratory dehydration of the sediment results in loss of around 40% of the stable natural remanence. Such behaviour is also found in lake sediments from central and southern Europe and should be considered in interpreting palaeomagnetic data from dried out lake sections and ocean cores.     

The extent of Greater India,I. Preliminary palaeomagnetic data from the Upper Devonian of the eastern Hindukush,Chitral (Pakistan)

Samples of Upper Devonian sedimentary ironstones from the eastern Hindukush, Chitral (Pakistan), give a characteristic palaeomagnetic direction: declination D = 318°, inclination I = ?6.5°; believed to represent the primary magnetization direction. The samples come from an area which lies north of a major ophiolite zone that recent workers suggest is the southwestern continuation of the Indus Suture. As the present palaeomagnetic results are in fair agreement with palaeomagnetic data from the Siberian platform but not with data from Gondwanaland they can be taken as additional evidence that this suture does indeed constitute the main collision zone between the Gondwanic Indian subcontinent and Asia. The palaeomagnetic data presented here from the Devonian of Chitral suggests additionally: (1) in excess of 100° of counterclockwise rotation of the area, associated most likely with the formation of the regional Hindukush-Pamir-Karakoram syntaxial bend; (2) more than 2000 km of crustal shortening between Chitral and the Siberian platform due to the northward indentation of the Indian Gondwanaland fragment subsequent to collision.     

Magnetostratigraphy of the Lodève Basin, France: Implications for the Permo-Carboniferous reversed superchron and the geocentric axial dipole

New results from the La Lieude Formation now complete the magneto-stratigraphic coverage of the Permian redbeds preserved in the Lodève Basin of southern France. The majority of the samples yield reversed polarity, with a mean of declination D = 187.4°, inclination I = ?0.5° (Fisherian statistics k = 44.2, ??₉₅ = 5.6°, n = 16). Together with previously published results, these data indicate that the entire basin lies within the Permo-Carboniferous Reversed Superchron (PCRS). But the stratigraphically highest sample exhibits normal polarity, suggesting that the transition marking the end of the PCRS may be close. The unusual behaviour of the geodynamo that generates superchrons prompts one to ask if there are concomitant influences on the morphology of the field. The intersecting palaeomeridian method offers a means of pursuing this question. An updated analysis suggests that there are currently no compelling reasons for adding significant higher-order terms to the geocentric axial dipole (GAD) model for the Late Permian.     

Post-depositional remanent magnetization in a re-deposited deep-sea sediment

Re-deposition of some deep-sea clay material in the earth's magnetic field is described. At the time of an induced reversal of the ambient field declination, calcium carbonate layers were introduced for identification purposes. NRM and bulk susceptibility measurements along the still moist sediment cores showed the presence of a time lag of around 10 days between a change in the ambient field direction and a corresponding change in the stable magnetization. AF-demagnetization of discs sampled from a dried core suggested that this time lag was caused by a post-depositional remanent magnetization mechanism. Magnetomineralogical investigations show the presence of the thermally unstable magnetic mineral phase, probably titano-maghemite. A positive correlation between NRM-intensities and water temperature during deposition is discussed. It is suggested that consolidation-rate is a major factor influencing the NRM intensity in deep-sea sediments.     

Model of geomagnetic paleosecular variations for the Northern Hemisphere

Summary A model of radial drifting and oscillating dipoles to represent geomagnetic palaeosecular variations, which showed to be appropriate for the Southern Hemisphere, is evaluated for the Northern Hemisphere. After a new fitting of some involved parameters, its theoretical results are compared with the palaeomagnetic data of Lake de Bouchet (France) and the Black Sea. A time lag between the declination and inclination profiles is observed in both cases. A good cross-correlation coefficient is obtained for the declination and inclination data.Presented at 2nd conference on New Trends in Geomagnetism, Castle of Bechyn, Czechoslovakia, September 24–29, 1990.     

菲律宾海西北部岩芯记录的125 ka以来的地磁场强度及其影响因素

沉积物记录的地磁场强度首先提供了模拟地磁场演化的数据约束,其次提供了沉积物的年龄信息. 本文报道了菲律宾海西北部岩芯记录的地磁场相对强度,并结合岩石磁学和沉积学性质探讨了影响强度的各个因素. 除底部红粘土层的局部磁偏角偏转可能揭示了沉积后改造以外,磁化率各向异性和地磁场方向特征表明沉积物为原状沉积. 岩石磁学性质表明沉积物符合磁性均一性,可以记录可靠的地磁场强度. 由于红粘土层及其下部的磁偏角异常,本文讨论其上部约125 ka的结果. 常规归一方法获得的两个地磁场强度参数NRM/ARM(特征剩磁和非磁滞剩磁比值)和NRM/κ(特征剩磁和磁化率比值)与其它记录对比得到时间-深度对比点,对比点之间的年龄为线性内推或者外推. 地磁场强度时间模型上的岩芯氧同位素与全球氧同位素综合曲线一致证明强度结果的有效性和对比的正确性. 磁化率为归一参数的强度大多低于以非磁滞剩磁为归一参数的强度,频谱和相关分析证明NRM/ARM不与ARM和磁性矿物粒度(ARM/κ)相关,也没有轨道周期性,而NRM/κ却与κ和ARM/κ相关,而且有13~12 ka的周期. 由此我们认为NRM/ARM记录的地磁场强度比NRM/κ更好地消除了气候印记. 进一步探讨了超顺磁含量、碳酸钙含量、磁性矿物组成以及磁性矿物粒度变化与地磁场强度差值的关系,发现末次间冰期较高的超顺磁含量和磁性矿物粒度的较大范围变化造成了地磁场强度差值,后者至少造成了90%差异. 中等含量的碳酸钙和较小的磁性矿物组成变化不是磁场强度差值产生的原因. 如何校正磁性矿物粒度变化的影响将是下一步工作的重点.     

First archaeomagnetic results and dating of Neolithic structures in northern Greece

Archaeomagnetism in Greece has continuously developed during the last decades. Numerous studies have provided high quality data and accurate secular variation curves for the direction and intensity of the geomagnetic field have been constructed. The Greek Secular Variation Curves (SVCs) cover the last 8 millennia for intensity and 6 millennia for direction. The coverage of the archaeological periods remains uneven, with several gaps, mostly in the directional dataset, with only two results for periods older than 2500 B.C. In the present contribution, the first archaeomagnetic results from Neolithic settlements in northern Greece are presented. For the present study, samples were collected from three different archaeological sites: burnt structures in Avgi (Kastoria) and Vasili (Farsala) and one oven from Sosandra (Aridaia). The natural remanent magnetization (NRM) grouping of all specimens indicated that the majority of the samples were burnt in situ, providing thus a reliable direction of the ancient field. Magnetic cleaning (both alternating-field and thermal) revealed the presence of one stable component of magnetisation. Rock magnetic experiments (acquisition of isothermal remanent magnetization (IRM), thermal demagnetisation of the IRM, thermomagnetic curves) have been performed on pilot samples indicating that low coercivity magnetic minerals such as magnetite or Timagnetite are prevailing. The mean directions (declination D, inclination I and parameters of the Fisherian statistics), which arose from the three sites are as follows: Sosandra: D = 343°, I = 55.6°, ??₉₅ = 4.8°; Avgi: D = 10.1°, I = 53.4°, ??₉₅ = 4.2° and Vasili: D = 357.5°, I = 43.1°, ??₉₅ = 4.1°. The obtained data are in a very good agreement with results from Neolithic Bulgaria. This study represents the beginning of an effort to fill the gaps of the Greek secular variation curves and their extension to the Neolithic period.     

Palaeomagnetic directions in Precambrian basic intrusives of the Mount Isa Province,Australia

Palaeomagnetic investigation of basic intrusives in the Proterozoic Mount Isa Province yields three groups of directions of stable components of NRM after magnetic cleaning in fields up to 50 mT (1 mT= 10 Oe). The youngest group (IA) includes results from the Lakeview Dolerite, and yields a palaeomagnetic pole at 12°S, 124°E (A₉₅ = 11°). The second group (IB) has a palaeomagnetic pole 53°S, 102°E (A₉₅ = 11°). The third group (IC) is derived from the Lunch Creek Gabbro and contains normal and reversed polarities of magnetization with a palaeomagnetic pole at 63°S, 201°E (A₉₅ = 9°). Some samples from the gabbro have anomalously low intensities of remanent magnetization in obscure directions attributed to the relative enhancement of the non-dipole component of the palaeomagnetic field during polarity reversal. The present attitude of the igneous lamination is probably of primary, not tectonic origin.