Geomagnetic Secular Variation Research Articles

High resolution Holocene paleomagnetic secular variation records from Bay of Bengal

We present high resolution paleosecular variation (PSV) records up to 8cal.kyr BP from three piston cores, MD161/8, MD161/11 and MD161/13 acquired in the Krishna–Godavari (KG) basin, Bay of Bengal. During the Holocene period, high sedimentation rates are recorded at MD161/8 (38.8–248.3cm/kyr), MD161/11 (137–336cm/kyr) and MD161/13 (∼573cm/kyr). Rock magnetic data analysis suggests that the remanence signal is carried by titanomagnetite/titanohematite grains in stable single domain (SSD)/pseudo single domain (PSD) state. The PSV records of MD161/11 and MD161/13 show good correlation in the uppermost sediments despite significant variation in the sedimentation rates; however, poor correlation of PSV records is observed for the core MD161/8 probably due to local effects. Paleoinclination records of MD161/8, MD161/11 and MD161/13 show a low between ∼2.4 and 2.0cal.kyr BP, an increase between 2.0 and 1.4cal.kyr BP and a decrease towards the present. To varying degrees these trends can be observed in the other Asian PSV records of Shuangchiling (SCL) and Biwa lakes. However, the magnitude of the observed inclination anomaly in KG basin is higher (∼40°) compared to those reported from SCL (∼25°) and Biwa (∼10°) lakes. Paleodeclination records of MD161/11 and MD161/13 show a decline between ∼4.0 and 2.9cal.kyr BP, an increase between 2.9 and 2.1cal.kyr BP, a substantial decrease between ∼2.1 and 1.0cal.kyr BP and an increase towards the present. Similar trends can be observed in the other Asian PSV records of SCL and Biwa lakes with a minor age offset of 0.2–0.5kyr. The available models CALS7k.2 and CALS10k.1 are evaluated for their capability in predicting the inclination and declination anomalies from the Asian regions. The CALS7k.2 model can predict most of the inclination anomalies while the CALS10k.1 is unable to predict many of them. The CALS7k.2 model shows that the observed inclination anomalies can be attributed to the presence of a strong non-axial dipolar magnetic field over the Bay of Bengal. More PSV records need to be constructed to better understand the anomalous non-axial dipolar field over the Bay of Bengal.

Paleomagnetic chronostratigraphy of late Holocene Zaca Lake, California

We have developed a paleomagnetic secular variation (PSV) record from Zaca Lake, California, that spans the last ~3200 years. The record is derived from detailed (2 cm) sampling of core 09-1C, which is 873 cm in length. The paleomagnetic remanence is carried jointly by magnetite and greigite. The greigite appears to form in the anoxic bottom water of the lake and/or at the sediment/water interface. The paleomagnetic remanence has a very simple characteristic remanence, which demagnetizes straight to the origin between ~10 and 80 mT. This remanence has strong serial correlation and a pattern of variability that strongly matches 15 other published PSV records from Western North America. In all, 23 radiocarbon-dated PSV features, which were identified in the core, provide an independent timescale for dating the Zaca Lake core and an independent means of correlating Zaca Lake sediments to other Holocene lakes of the Western United States. We have also compared our paleomagnetic chronology with a previously published radiocarbon-based chronology from Zaca Lake (Feakins et al., 2014; Kirby et al., 2014). Our chronology is not significantly different from the radiocarbon-based chronology for the last 2000 years. However, the radiocarbon dates appear to be anomalously young in the older part of the lake record where correlatable PSV features suggest ages that are ~300–500 years older than the radiocarbon dates. Our analysis suggests that the PSV feature ages are a better estimator of lake sediment age in this interval.

Dynamic responses of the Earth’s outer core to assimilation of observed geomagnetic secular variation

Assimilation of surface geomagnetic observations and geodynamo models has advanced very quickly in recent years. However, compared to advanced data assimilation systems in meteorology, geomagnetic data assimilation (GDAS) is still in an early stage. Among many challenges ranging from data to models is the disparity between the short observation records and the long time scales of the core dynamics. To better utilize available observational information, we have made an effort in this study to directly assimilate the Gauss coefficients of both the core field and its secular variation (SV) obtained via global geomagnetic field modeling, aiming at understanding the dynamical responses of the core fluid to these additional observational constraints. Our studies show that the SV assimilation helps significantly to shorten the dynamo model spin-up process. The flow beneath the core-mantle boundary (CMB) responds significantly to the observed field and its SV. The strongest responses occur in the relatively small scale flow (of the degrees L≈30 in spherical harmonic expansions). This part of the flow includes the axisymmetric toroidal flow (of order m=0) and non-axisymmetric poloidal flow with m≥5. These responses can be used to better understand the core flow and, in particular, to improve accuracies of predicting geomagnetic variability in the future.

Relative paleointensity data from the late Holocene deltaic sediments in Korea

A first quasi-successive paleosecular variation data since ∼5.5 kya, including magnetic inclination and relative geomagnetic field intensity variation were obtained from a 55 m-long sediment core in the Nakdong River delta, southeastern Korean Peninsula. Both magnetic inclination curves and relative paleointensity data showed similar trends with those from Lake Biwa, Japan. They also showed close correlation with continuous global geomagnetic field models. For instance, the present study confirms the existence of archeomagnetic jerks. In addition, the present study supports the evidence of globally high geomagnetic intensity circa 3 kya.

Paleomagnetism of Miocene volcanics on Sao Tome: Paleosecular variation at the Equator and a comparison to its latitudinal dependence over the last 5 Myr

AbstractA collection was made in January 2009 of 10 oriented samples from each of 54 sites in lavas on Sao Tome Island (nominal location 0.3ºN, 6.5ºE). Some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation. Overall magnetic properties were excellent (highly stable magnetizations carried by pseudosingle domain magnetite). After principal component analysis of progressive alternating field demagnetization data for the samples, 22 sites had normal polarity magnetizations (D = 0. 6º, I = −8.3º, α95 = 4.3º, κ = 53.1) and 20 had reverse magnetizations (D = 176.0º, I = 4.2º, α95 = 7.3º, κ = 20.8); the directions are within 5° of antiparallel, yielding a positive reversal test. The combined data set of 42 site mean virtual geomagnetic poles converted to common (normal) polarity yields a pole position at 86.0°N, 211.5ºE, A95=3.1º. Ar/Ar and K/Ar dating reveals that these rocks are Miocene in age (∼5–11 Ma), old enough to allow northerly plate motion to help explain the slightly far‐sided pole position. The between‐site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation, Sb, and equaled 11.4° with 95% confidence interval between 9.9° and 13.4°.

Evidence for a new geomagnetic jerk in 2014

AbstractThe production of quasi‐definitive data at Ebre observatory has enabled us to detect a new geomagnetic jerk in early 2014. This has been confirmed by analyzing data at several observatories in the European‐African and Western Pacific‐Australian sectors in the classical fashion of looking for the characteristic V shape of the geomagnetic secular variation trend. A global model produced with the latest available satellite and observatory data supports these findings, giving a global perspective on both the jerk and a related secular acceleration pulse at the core‐mantle boundary. We conclude that the jerk was most visible in the Atlantic and European sectors.

Insights into pre-reversal paleosecular variation from stochastic models

To provide insights on the paleosecular variation of the geomagnetic field and the mechanism of reversals, long time series of the dipolar magnetic moment are generated by two different stochastic models, known as the “domino” model and the inhomogeneous Lebovitz disk dynamo model, with initial values taken from the from paleomagnetic data. The former model considers mutual interactions of N macrospins embedded in a uniformly rotating medium, where random forcing and dissipation act on each macrospin. With an appropriate set of the model’s parameters values, the series generated by this model have similar statistical behaviour to the time series of the SHA.DIF.14K model. The latter model is an extension of the classical two-disk Rikitake model, considering N dynamo elements with appropriate interactions between them. We varied the parameters set of both models aiming at generating suitable time series with behaviour similar to the long time series of recent secular variation (SV). Such series are then extended to the near future, obtaining reversals in both cases of models. The analysis of the time series generated by simulating the models show that the reversals appears after a persistent period of low intensity geomagnetic field, as it is occurring in the present times.

A new high-resolution record of Holocene geomagnetic secular variation from New Zealand

We present the first full Holocene palaeomagnetic secular variation record from New Zealand. The 11 500 year-long record, from the sediments of Mavora Lakes, comprises composite declination, inclination and relative palaeointensity logs, compiled from two six-metre long cores and the uppermost 1.5 m of another. An age model has been developed from 28 AMS radiocarbon age determinations on fragments of southern beech (Lophozonia menziesii and Fuscospora cliffortioides) leaves. The excellent between-core correlation in all three components of the field results in a high-resolution palaeosecular variation record, with precise and accurate age control. The variations change in character from high amplitude in-phase declination and inclination swings in the earliest part of the record to low amplitude variations in the middle part and declination and inclination swings that are 90° out of phase, leading to broad looping of the vector in the upper part of the record, that is consistent with westward drifting sources in the outer core. The present-day field at the site (Dec = 24.2°E, Inc=−70.7°, F=59 μT) represents a rare steep and easterly extreme direction, but close to average intensity. The palaeointensity is inferred to have varied between about 40 and 90 μT, with variations that, to some extent, mirror variations in the virtual axial geomagnetic dipole moment seen from global data, but also show some notable differences, particularly in the past few thousand years.

Drifting and standing fields in the geomagnetic field for the past 400 years

Based on the field model by Jackson et al. (2000), we reanalyzed the data of historical times and decomposed the geomagnetic field into drifting and standing fields. It has been confirmed that the results are consistent with the earlier analyses. When the field is expressed in a spherical harmonic series, the drifting field is characterized by two remarkable features. One is that the drifting field is represented mostly by sectorial harmonics, and the other is that the drift velocity does not differ significantly between the harmonics. The standing field that remains after subtraction of the drifting field from the main geomagnetic field is stronger than the drifting field. The predominance of sectorial terms in the drifting field means that the vertical component, for instance, of the drifting field is symmetric about the equator. On the other hand, the standing field is antisymmetric. Due to this difference, the drifting field largely controls the geomagnetic secular variation in the low latitude region, whereas the standing field dominates in the middle and higher latitudes.

Palaeointensity and Brunhes palaeomagnetic field models

Statistical models of geomagnetic secular variations in terms of a Giant Gaussian Process (GGP) are compared with palaeontensity databases (Brunhes epoch, effusive rocks). We tested two GGP models TK and QC (suggested by Tauxe-Kent and Quidelleur-Courtillot) against 392 volcanic palaeointensity data, and found both models to be incompatible with the data. Even though at present there is no accepted GGP model of the Brunhes chron, we found interesting to clarify the details of this negative test. While such an incompatibility is clear enough for the TK model, because of its too small dipole coefficient, the incompatibility of the QC model was due the 12 per cent of the data with very small intensities; remaining 88 per cent of data is fully compatible with QC model QC. The troublesome, low-palaeointensity records belong to different sites and ages so their excess presence is not an effect of particular location or time. We found no evidence that the marked discrepancy between the empirical data and theoretical calculations on the QC model can be explained by the possibility that the low-intensity values belong to known excursions of the geomagnetic field. As an alternative, we conjecture that selected low-intensity values can possibly stem from acquisition of chemical remanent magnetization instead of the thermoremanence.

Further paleomagnetic results from the ~ 155 Ma Tiaojishan Formation, Yanshan Belt, North China, and their implications for the tectonic evolution of the Mongol–Okhotsk suture

A new paleomagnetic study on well-dated (~155Ma) volcanic rocks of the Tiaojishan Formation (Fm) in the northern margin of the North China Block (NCB) has been carried out. A total of 194 samples were collected from 26 sites in the Yanshan Belt areas of Luanping, Beipiao, and Shouwangfen. All samples were subjected to stepwise thermal demagnetization. After removal of a recent geomagnetic field viscous component, a stable high temperature component (HTC) was isolated. The inclinations of our new data are significantly steeper than those previously published from the Tiaojishan Fm in the Chengde area (Pei et al., 2011, Tectonophysics, 510, 370–380). Our analyses demonstrate that the paleomagnetic directions obtained from each sampled area were strongly biased by paleosecular variation (PSV), but the PSV can be averaged out by combining all the virtual geomagnetic poles (VGPs) from the Tiaojishan Fm in the region. The mean pole at 69.6°N/203.0°E (A95=5.6°) passes a reversal test and regional tilting test at 95% confidence and is thus considered as a primary paleomagnetic record. This newly determined pole of the Tiaojishan Fm is consistent with available Late Jurassic poles from red-beds in the southern part of the NCB, but they are incompatible with coeval poles of Siberia and the reference pole of Eurasia, indicating that convergence between Siberia and the NCB had not yet ended by ~155Ma. Our calculation shows a ~1600-km latitudinal plate movement and crustal shortening between the Siberia and NCB after ~155Ma. In addition, no significant vertical axis rotation was found either between our sampled areas or between the Yanshan Belt and the major part of the NCB after ~155Ma.

Testing the geocentric axial dipole hypothesis using regional paleomagnetic intensity records from 0 to 300 ka

Absolute and relative geomagnetic paleointensity records reveal variations in geomagnetic dipole strength, either via averaging time series of virtual axial dipole moments, or through formal inversion strategies like the penalized maximum likelihood (PML) method used for the PADM2M (Paleomagnetic Axial Dipole Moment for 0–2 Ma) model. However, departures from the most basic geocentric axial dipole (GAD) structure are obvious on centennial to millennial time scales, and paleomagnetic records from igneous rocks suggest small deviations persist on million year time scales. Spatial variations in heat flow at the core–mantle boundary (inferred from large low shear velocity provinces, LLSVPs) are widely suspected to influence both the average geomagnetic field and its regional secular variation. Long term departures from a GAD configuration should be visible from regional differences in paleointensity reconstructions. We use a PML method to construct time-varying models of regional axial dipole moment (RADMs) from a combined set of absolute and relative paleointensity data, and compare results from the last 300 kyr. RADMs are created from sediment records selected from specific latitude and longitude bands. We also test whether grouping records lying above each of the 2 major LLSVPs (centered on Africa and the Pacific) produce RADMs that are distinct from those above regions lacking anomalous seismic structure. Systematic differences appear in the various regional results. In the most recent part of the record regional differences are broadly similar to the Holocene, CALS10k.1b, time-varying geomagnetic field model spanning 0–10 ka. However, lack of Southern hemisphere records prevents direct confirmation of the hemispheric asymmetry present in CALS10k.1b in both average virtual axial dipole moment and its variability. As expected, the 300 kyr RADMs exhibit greater overall temporal field variability than is seen over 0–10 ka. Average RADM is higher in the Pacific and in Equatorial regions than in the Atlantic and in mid–high latitude northern hemisphere regions. Higher average RADMs are associated with lower overall field variability and less pronounced excursional signatures. Notably, the lower variability in the Pacific sector seen here (defined by either longitude band or LLSVP location) suggests that the modern low paleosecular variation there extends over at least the past few hundred thousand years. RADMs identified with LLSVPs show systematic deviations from the non-LLSVP group of records, with distinct characteristics for the African and Pacific provinces. The African LLSVP generates more pronounced RADM minima associated with geomagnetic excursions, and in general paleointensity decreases associated with excursions occur first in the Atlantic longitude sector and over the African LLSVP.

On core convection and the geodynamo: Effects of high electrical and thermal conductivity

Recent theory and experiment suggest the thermal and electrical conductivities of the Earth’s core are 2–4 times higher than previously thought. This has important consequences for the core’s thermal history and behaviour of the geodynamo. The conductivities increase with depth, with a discontinuous jump at the inner core boundary caused by the change in composition and phase change to a solid. Properties of putative core alloys are now sufficiently well known to make it worth exploring the effects of their variation with depth within the core. The magnetic decay times are increased to 58kyr for the whole core, considerably longer than the advection time (the time it takes fluid to traverse the outer core), and 9kyr for the solid inner core. Heat conducted down the adiabat through the core–mantle boundary is in excess of 15TW, which is one third of the Earth’s total heat loss and 2–3 times higher than most estimates. The core can be stirred by chemical convection against a stable thermal gradient, but at a cost that reduces the effective power available for generating magnetic field. We estimate the minimum heat flux required to sustain thermal dissipation alone to be 5–8TW, but this is almost certainly a gross underestimate because it leaves nothing for convective or dynamo processes. Conduction gradients for cooling rates corresponding to these minimum heat fluxes are subadiabatic in the top 740km of the core, which is also unlikely because geomagnetic secular variation requires upwelling somewhere near the core surface. Lateral variations in heat flux at the core–mantle boundary could easily be large enough to exceed the adiabatic value in some places, leading to mixing throughout the upper core. This not only reduces the total heat flux required to produce a well-mixed core, but also explains how mantle anomalies can exert a strong influence on core convection and the form of the geomagnetic field at the core surface. We propose a model of core convection that is vigorous in the lower part and very weak in the upper part.

New paleointensity results from rapidly cooled Icelandic lavas: Implications for Arctic geomagnetic field strength

AbstractThe Earth's magnetic field is assumed to be a geocentric axial dipole (GAD) when averaged over sufficient time (105–106 years). Recent investigations of global paleosecular variation and time‐averaged field behavior on million year timescales generally support a predominantly dipole field in the Northern Hemisphere, but unique field structures at high southern latitudes suggest the presence of a substantial quadrupolar component. Average paleointensity results from Antarctica are approximately half the value predicted by a GAD field; this behavior has not been sufficiently investigated because there is a paucity of absolute paleointensity data from the high latitudes of the Arctic and Antarctic, so no adequate comparisons have been made between the two regions. We collected glassy volcanic material from 129 subaerial and subglacial volcanic units in Iceland in order to provide a suitable intensity data set at high northern latitudes. Forty‐four sites met our very strict specimen and site level selection criteria. Four Holocene sites have a median intensity value of 55.8 ± 15.6μT (virtual axial dipole moment = 78.1 ± 22.0 ZAm2), consistent with the present‐day field. Thirty‐seven sites are between 11 ka and 3.35 Ma with a median intensity of 33.1 ± 8.3 μT (47.0 ± 11.6 ZAm2). This median intensity is indistinguishable from some long‐term global field strength estimates. Reevaluation of existing high‐latitude data suggests a general agreement with our Iceland results, but there are still too few Antarctic sites to adequately compare Arctic and Antarctic field behaviors.

A detailed paleomagnetic and rock-magnetic investigation of the Matuyama-Brunhes geomagnetic reversal recorded in the tephra-paleosol sequence of Tlaxcala (Central Mexico)

Geomagnetic reversals are global phenomena, for about 50 years the paleomagnetists attempted to acquire as many detailed records as possible using the magnetic memory of sediments and lava flows. Yet, transitional field behavior remains poorly characterized largely because of sporadic aspect of volcanic eruptions. In some specific cases, paleosols such as those developed from alluvial or aeolian sediments, may also record the variations of the Geomagnetic Field across the polarity changes. Here, we report a detailed paleomagnetic and rock-magnetic investigation on some radiometrically dated chromic luvisols located in Central Mexico carrying detrital or chemical remanent magnetization. The research was developed in order i) to demonstrate the primary origin of the magnetic remanence and ii) to show that paleosoils are good candidates to provide a high resolution record of the behavior of geomagnetic field during reversals. The lower part of the paleosoil sequence shows a clearly defined reverse polarity magnetization followed by geomagnetically unstable transitional field and ended by normal polarity remanence. Our AMS and rock magnetic data suggest that magnetization is acquired during the initial stage of soil formation in context of active volcanic activity since magnetic fabric is essentially sedimentary and reverse and normal polarity paleodirections are almost antipodal. Titanomagnetites are identified as main magnetic carriers of rock-magnetic measurements including thermomagnetics and hysteresis cycles. We propose that the transition recorded in this study correspond to the B-M boundary, considering the K-Ar datings available at the sequence bottom and that the chromic luvisols are potentially good recorders of the paleosecular variation. The identification of the B-M boundary within the studied sequence has fundamental significance for improving the chronological scale of Tlaxcala paleosol-sedimentary sequence and its correlation with the global proxies.

Paleomagnetic Pole Positions and Geomagnetic Secular Variation from the Cretaceous Ponta Grossa Dike Swarm (Brazil)

A detailed paleomagnetic and rock magnetic investigation has been carried out on the Early Cretaceous Ponta Grossa dike swarm in southern Brazil. This formation seems an excellent target for paleomagnetic study. The dikes are widely distributed over a large area, easy to access, and they record faithfully the geomagnetic field at the time of the eruption. Most of them are fresh and have been dated by K-Ar and Ar-Ar. Thermomagnetic experiments (low-field versus temperature curves) suggest low-Ti titanomagnetites as main remanence carriers, and their domain structure is characterised by a mixture of single-domain and multi-domain grains. Characteristic paleomagnetic directions are retrieved from 28 out of 29 sites (235 standard paleomagnetic cores). 17 sites show normal polarity, 10 sites show reverse polarity and one site shows an oblique direction, with negative inclination, separated 79° of the mean normal directions. The reversal test is positive at the 95% confidence level which ensures that the secondary remanent magnetizations were successfully removed and the sampling adequately averaged the palaeosecular variation. The paleosecular variation parameters values obtained in this study correlate with those determined for the Cretaceous Normal Superchron, between 125 to 84 Ma. The new paleomagnetic pole position (222.0° E, 88.1° S, N = 24, K = 35.68, A95 = 5.0°) agrees well (within uncertainties) with the reference poles determined from Besse and Courtillot (2002), and disagree with those reported in previous studies. The new results should be considered for estimating the Cretaceous paleomagnetic poles for stable South America.

New Paleomagnetic results and evidence for a geomagnetic field excursion during the pleistocene-holocene transition at Pichincha province, Ecuador

Paleomagnetic data from three sedimentary sections in Pichincha province -Quito City (QC), Mullimica (Mu) and El Tingo (ET)- Ecuador (northwestern South America) are reported. Analysis of natural remanent magnetization directions obtained from 109 oriented samples taken at 4 sites, shows that some samples recorded a magnetic component different from the normal present geomagnetic field (GMF). The characteristic remanent magnetization (ChRM) was determined by progressive AF demagnetization. The analysis shows that the sections recorded ChRM of normal, intermediate and reverse polarities during the Pleistocene-Holocene transition and Holocene. Normal directions were recorded in QC, while normal and intermediate polarity directions at Mu and, reverse VGPs at ET. QC and the upper portion of Mu correspond to the paleosecular variation Holocene record for Ecuador during the ∼≤4.7 ka BP. On the other hand, the lower portion of Mu logs represents the transition from normal to intermediate directions occurring at ∼≥5.6 ka BP. Sites from ET recorded two stable oblique reverse records with a large fluctuation far from the present GMF at ∼10.5 ka BP. The transitional virtual geomagnetic poles generally agree with those registered during the possible Pleistocene-Holocene excursion observed in other places of the planet. When plotted in a present world map, VGPs calculated from normal samples at QC are very well clustered in Northern North America, Greenland and Northern Europe; most VGP's calculated from Mu are situated between 30° and 60° northern latitude in Northern North America, Greenland, western Europe, Africa and North Pacific Ocean. Interestingly, the majority of the reverse directions from ET conforms a patch located in southern Africa, and a few ones are situated in central Africa, eastern Australia and Antarctica. An Ecuadorian paleopole was calculated with data resulting from QC and Mu. Also other paleopoles of the same age were processed from other North and South American sites. Remarkably they agree well, although they do not agree with the geographical pole showing ∼15o angular difference in relation to the rotation's axis of the Earth. Finally, is discussed the hypothesis of the global excursional state of the GMF during the last ∼11.0 ka BP and the potential use as dating tool the excursion dated at 10.5 ka BP.

Secular variation of the Earth’s magnetic field and application to paleomagnetic dating of historical lava flows in Chile

The recent geomagnetic secular variation is mainly characterized by the large growth of the South Atlantic Magnetic Anomaly during the last three centuries, first documented by the geomagnetic model gufm1 (Jackson et al., 2000). We report new paleomagnetic results (directions and paleointensities) from several sites in two well dated lava flows in Chile, the 1835 AD eruption of the Osorno volcano and the 1751 AD eruption of the Llaima volcano. In addition, paleointensities were obtained on 14 samples from bricks of shelters built along the main road across the Andes from Santiago (Chile) to Mendoza (Argentina) in 1770±5 AD. The results confirm the high reliability of the global geomagnetic model gufm1 for the last three centuries with a large amplitude of the secular variation in inclination (∼20°) and intensity (∼25μT). Results from three 14C dated volcanic units in the time interval 1400–1750 AD indicate that more paleomagnetic results in well dated lava flows are necessary to improve the robustness of existing global geomagnetic models. At this stage, precise paleomagnetic or archeomagnetic dating in South America using global models should be restricted to the last 3 centuries.To illustrate the potential of paleomagnetic dating in region and time interval with very large geomagnetic secular variation, we report paleomagnetic data from several sites in historical lava flows (1700–1900 AD) from the Antuco, Llaima and Villarrica volcanoes that permit to refine the ages of the major historical effusive volcanic events.

Independently dated paleomagnetic secular variation records from the Tibetan Plateau

Magnetostratigraphy has been serving as a valuable tool for dating and confirming chronologies of lacustrine sediments in many parts of the world. Suitable paleomagnetic records on the Tibetan Plateau (TP) and adjacent areas are, however, extremely scarce. Here, we derive paleomagnetic records from independently radiocarbon-dated sediments from two lakes separated by 250 km on the southern central TP, Tangra Yumco and Taro Co. Studied through alternating field demagnetization of u-channel samples, characteristic remanent magnetization (ChRM) directions document similar inclination patterns in multiple sediment cores for the past 4000 years. Comparisons to an existing record from Nam Co, a lake 350 km east of Tangra Yumco, a varve-dated record from the Makran Accretionary Wedge, records from Lakes Issyk-Kul and Baikal, and a stack record from East Asia reveal many similarities in inclination. This regional similarity demonstrates the high potential of inclination to compare records over the Tibetan Plateau and eventually date other Tibetan records stratigraphically. PSV similarities over such a large area (>3000 km) suggest a large-scale core dynamic origin rather than small scale processes like drift of the non-dipole field often associated with PSV records.

Paleosecular variation of the earth magnetic field at the Canary Islands over the last 15 ka

We report on new paleomagnetic directions obtained from 38 lava flows located at Tenerife and Gran Canaria (Canary Islands, Spain). One flow is a historical one (1706 AD) and 28 other flows are dated by radiocarbon between 1550 AD and about 13 200 BC. Nine other flows are not dated but they have stratigraphic links with the other flows. Thermomagnetic curves, unblocking temperatures and coercivities suggest that the main carrier of the remanent magnetization is titanomagnetite with various titanium contents in the pseudo-single domain range. Paleodirections were obtained by thermal and alternating field demagnetization on more than 400 specimens. The two youngest flows yield directions well consistent with the data previously published from the Canary Islands but only covering the last 500 yr. Comparison with model predictions indicates that the models account on the long-term for most of the data. However, on short-term scale, a better agreement is observed with the archeomagnetic-based model predictions (ARCH3K and SHA.DIF.14k). Two time intervals (between 25 BC and 85 AD and around 600–700 BC), however, are characterized by more variable paleomagnetic directions, suggesting that the variability of the earth magnetic field was faster than predicted by the models. On a wider geographical scale, a rather good consistency is observed between the Canarian dataset and those from Northern Africa, Spain and Azores. Field information is well consistent with paleomagnetic information for the undated sites and they both confirm that the eruption rate of the Gran Canaria volcanic system was high around 600 BC, 1000 BC and 4650 BC. Refined ages could be obtained for two of the undated sites using archeomagnetic dating. Combined with the data previously published for the last 500 yr, this new dataset is the first long PSV record available for the Canary Islands, significantly contributing to the archeomagnetic/paleomagnetic database at latitudes lower than 30°N. Improvement in the time resolution of model predictions is now needed to allow more detailed data-model comparisons on short time scale.