Average Paleointensity Research Articles

High-resolution U-channel paleomagnetic secular variation and magnetic field excursions from ocean drilling program site 1233 (MIS 1–4; 0–71 ka)

This paper presents new u-channel paleomagnetic secular variation (PSV) data for ODP Site 1233 (Chile margin) for the last 70 ky. This is the highest-resolution, long-term PSV record ever recovered. The u-channel study has carried out detailed af demagnetization of the natural remanence (NRM) and developed a characteristic remanence (ChRM – final PSV direction) for each sampling horizon. The u-channel rock magnetic studies have measured magnetic susceptibility and anhysteretic remanence (ARM) with af demagnetization and used them to normalize the NRM and develop a new, detailed relative paleointensity record. The paleomagnetic record contains three excursions: the Mono Lake Excursion, the Laschamp Excursion, and the Greenland/Norwegian-Sea Excursion. These excursions have nominal sampling intervals of 5–8 years. Both the Mono Lake and Greenland/Norwegian-Sea Excursions involve short duration intervals of excursional directions (<300 years). The excursions are distinctly different from similar excursion records from other parts of the World. The Laschamp Excursion exhibits a full local reversal of the field with fast (<200 yr) movement to/from reversed directions and ∼ 500 years of full reversed polarity directions. This pattern is distinctly different from Laschamp excursion records from other parts of the World. We have carried out a statistical analysis of the Site 1233 u-channel PSV record. Notable specific features of that analysis include documentation that the field spends >1/3 of its time with anomalously large amplitude directional variability (high angular dispersion). These intervals occur at∼25–45 ka and 55–65 ka. These intervals have lower-frequency (more sluggish) directional variability and are associated with lower than average paleointensity.

Relative Paleointensity and Inclination Anomaly Over the Last 8 Myr Obtained From the Integrated Ocean Drilling Program Site U1335 Sediments in the Eastern Equatorial Pacific

AbstractFor understanding the fundamentals of the geodynamo such as the relation between paleointensity and polarity length and time‐averaged field (TAF) structure, continuous records of relative paleointensity (RPI) and inclination anomaly (ΔI) are desired; however, available records older than ~3 Ma are still very limited in time and space. We conducted a paleomagnetic study of the Integrated Ocean Drilling Program Site U1335 sediments in the eastern equatorial Pacific to obtain continuous RPI and ΔI records since ~8 Ma. Slow deposition, ~8.4 m/Myr or less, limits the resolution of the records but did allow for determination of long‐term variations. Rock‐magnetic measurements showed that biogenic magnetite dominates the magnetic mineral assemblages, and the proportion of biogenic to terrigenous magnetic minerals increases prior to ~4 Ma. The average paleointensity between ~4 and 8 Ma is approximately 30% lower than that from 0 to ~4 Ma. The apparent reduction of RPI at ~4 Ma reaches approximately ~50%, but ~20% of this is estimated to be artificial, induced by the increase in the proportion of biogenic magnetite. No relation between paleointensity and polarity length is recognized for the last ~8 Myr. The magnitude of ΔI is slightly larger during reversed polarity chrons (4.43° ± 1.47°) than normal polarity chrons (−0.69° ± 2.98°) over the last ~5 Myr, which agrees with the available TAF models of this time span. Prior to ~6 Ma, the sign of ΔI during the normal chrons might have switched to positive, and ΔI during reversed chrons might have been slightly larger than that after ~5 Ma.

Paleomagnetic study of an historical lava flow from the Llaima volcano, Chile

The understanding of the paleosecular variations (PSV) of the geomagnetic field in South America is still biased by the scarcity of data. Especially, the recent geomagnetic PSV is characterized by the large growth of the South Atlantic Magnetic Anomaly (SAMA) during the last centuries, first documented by the geomagnetic model gufm1 (Jackson et al., 2000). A large amount of data is required to understand the time and geographic distribution of this primary feature, and the Andean Pleistocene and Holocene volcanoes are an excellent recorder of instant local changes in SV. Here we present a preliminary study from 18 paleomagnetic samples collected during 2015 on what it was supposed to be the 1750 or the 1957–58 AD lava flow on the Llaima Volcano (38.692° S; 71.729° W), one of the most active centers of the Chilean Andes, in the Southern Volcanic Zone. A detailed paleomagnetic study was performed in order to recover the Declination and Inclination of the geomagnetic field, obtain the paleointensity and define the magnetic mineralogy. AF demagnetization until 1 T yielded an average vector at Dec/Inc 2.3°/-33.1° with α95 of 2.4°. This direction is carried by titanomagnetite grains with 40–45% ulvospinel as revealed by thermomagnetic curves. Paleointensity estimates were obtained following the IZZI-Thellier protocol. Seven specimens from 5 samples provided reliable results (success rate of 35%), giving an average paleointensity for these specimens of 30.88 ± 2.39 μT. The full magnetic vector obtained here was compared to archaeomagnetic reference curves and the IGRF suggest that the lava flow has the age of 1957–58 AD.

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

The geomagnetic field is predominantly dipolar today, and high-fidelity paleomagnetic mean directions from all over the globe strongly support the geocentric axial dipole (GAD) hypothesis for the past few million years. However, the bulk of paleointensity data fails to coincide with the axial dipole prediction of a factor-of-2 equator-to-pole increase in mean field strength, leaving the core dynamo process an enigma. Here, we obtain a multidomain-corrected Pliocene-Pleistocene average paleointensity of 21.6 ± 11.0 µT recorded by 27 lava flows from the Galapagos Archipelago near the Equator. Our new result in conjunction with a published comprehensive study of single-domain-behaved paleointensities from Antarctica (33.4 ± 13.9 µT) that also correspond to GAD directions suggests that the overall average paleomagnetic field over the past few million years has indeed been dominantly dipolar in intensity yet only ∼ 60% of the present-day field strength, with a long-term average virtual axial dipole magnetic moment of the Earth of only 4.9 ± 2.4 × 10(22) A ⋅ m(2).

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.

Spot Reading of the Absolute Paleointensity of the Geomagnetic Field Obtained from Potsherds (Age Ca. 500-430 AD) in Teotihuacan, Mexico

Archaeointensity data have been obtained successfully using the Thellier-Coe protocol from twelve potsherds recovered from the vicinity of the “Piramide del Sol”, Teotihuacan, Mexico. In order to understand the magnetic behavior of the samples, we have conducted low-field versus temperature (k-T) experiments to determine the magnetic carriers of the pre-Columbian artefacts, as well as Saturation Isothermal Remanent Magnetization (SIRM), hysteresis loops and back-fields TESTS. The Curie temperatures indicate the presence of at least three magnetic mineral phases (238°C - 276°C, 569°C - 592°C, and 609°C - 624°C). The predominant Curie temperatures for these samples are typical of Ti-poor magnetite. The results of the magnetic grain size analyses indicate that if the magnetic mineral in a sample is only magnetite, the distribution on the modified Day et al. (1977) diagram yields specimens in the Single (SD), Pseudo (PSD) and Superparamagnetic (SP) domain ranges. The successfully absolute paleointensity determinations in this study using the Thellier-Coe protocol have yielded an average paleointensity of 38.871 +/- 1.833 m-Teslas (N = 12), and a virtual geomagnetic dipole moment of 8.682 +/- 0.402 × 1022 A/m2 which is slightly lower than the present field strength and which corresponds to an age interval between 500 and 430 AD. Thus, our results correlate well with the recently published CALS3K.4 curve and the incipient archaeointensity reference curve for the Mesoamerican paleo-field results. Therefore, the age of the artefacts would correlate well with absolute the early classic Teotihuacan cultural period.

Magnetic properties and paleointensities as function of depth in a Hawaiian lava flow

The outcome of paleointensity experiments largely depends on the rock-magnetic properties of the samples. To assess the relation between volcanic emplacement processes and rock-magnetic properties, we sampled a vertical transect in a ∼6 m thick inflated lava flow at Hawaii, emplaced in ∼588 AD. Its rock-magnetic properties vary as function of distance from the flow top; the observations can be correlated to the typical cooling rate profile for such a flow. The top and to a lesser extent the bottom parts of the flow cooled faster and reveal a composition of ∼TM60 in which the magnetic remanence is carried by fine-grained titanomagnetites, relatively rich in titanium, with associated low Curie and unblocking temperatures. The titanomagnetite in the slower cooled central part of the flow is unmixed into the magnetite and ulvospinel end-members as evidenced by scanning electron microscope observation. The remanence is carried by coarse-grained magnetite lamella (∼TM0) with high Curie and unblocking temperatures. The calibrated pseudo-Thellier results that can be accepted yield an average paleointensity of 44.1 ± 2.4 μT. This is in good agreement with the paleointensity results obtained using the thermal IZZI-Thellier technique (41.6 ± 7.4 μT) and a recently proposed record for Hawaii. We therefore suggest that the chance of obtaining a reliable paleointensity from a particular cooling unit can be increased by sampling lavas at multiple levels at different distances from the top of the flow combined with careful preliminary testing of the rock-magnetic properties.

Paleomagnetic full vector record of four consecutive Mid Miocene geomagnetic reversals

Seventy Mid Miocene lava flows from flood basalt piles near Neskaupstadur (East Iceland) were sampled, which provide a quasi-continuous record of geomagnetic field variations. Samples were collected along the profile B of Watkins and Walker [Watkins, N., Walker, G.P.L., 1977. Magnetostratigraphy of eastern Iceland. Am. J. Sci. 277, 513–584], which was extended about 250 m farther down in a neighboring stream bed. Published radiometric age determinations [Harrison, C., McDougall, I., Watkins, N., 1979. A geomagnetic field reversal time scale back to 13.0 million years before present. Earth Planet. Sci. Lett. 42, 143–152] range from 12.2 to 12.8 Ma for the sampled sequence. Four reversals were recorded in this profile, with 18 transitional lavas found within or between 17 normal and 30 reversed polarity flows. The large amount of transitional lavas and the large virtual geomagnetic pole dispersion for stable field directions are noteworthy as such features are commonly observed in Icelandic lavas and manifest in a far-sidedness of the average VGP. The reason for this characteristic, which could be related to an anomaly beneath Iceland, a global field phenomenon, local tectonics, and/or non-horizontal flow emplacement, is scrutinized. Non-horizontal flow emplacement is likely in volcanic environments particularly if the sampled lavas are located on the paleoslopes of a central volcano. From the difference of the observed paleomagnetic mean directions to the expected directions assuming a geocentric axial dipole (GAD), a paleoslope which would explain the observed difference was calculated numerically. The obtained dip and dip direction point consistently to a possible volcanic extrusion center of the lavas. The determined paleodip, however, proved to be significantly too high compared to the usual slope of a central volcano, suggesting further reasons for deviations from the GAD. Other datasets of this age from Europe also show enhanced VGP dispersion, suggesting further contributions of geomagnetic origin for this observation. Basically all reversal paths move across the Pacific. Transitions were identified as belonging to C5An.1r–C5Ar.3r based on the Astronomically Tuned Neogene Timescale [Lourens, L., Hilgen, F.J., Laskar, J., Shackleton, N.J., Wilson, D., 2004. A Geological Time Scale. Cambridge University Press]. We selected 122 samples for paleointensity measurements using a modified Thellier method including tests for alteration and multidomain bias. 85 of the measured samples yielded data of sufficient quality to calculate paleointensities for 26 lava flows. The average paleointensity for stable field directions was 23.3 μ T, whereas the intensity drops to a minimum of 5.8 μ T during field transitions. The stable field intensities represent only about half of the present day field. The saw-tooth pattern of intensities, which is characterized by a sharp increase of intensity directly after a reversal and then followed by a gradual decrease towards the next reversal, was not found in this study.

A relative paleointensity record of the geomagnetic field since 1.6 Ma from the North Pacific

Abstract A paleomagnetic study was conducted on a sediment core KR0310-PC1 taken from the central North Pacific in order to obtain a relative paleointensity record in the Matuyama chron from this region. The core reached to about 1.6 Ma. The age control is based on the correlation of the S ratio (S −0.1T) variations with a global oxygen-isotope stack. Isothermal remanent magnetization (IRM) was used as the normalizer of the relative paleointensity estimation; anhysteretic remanent magnetization (ARM) was not adopted because ARM is sensitive to magnetostatic interaction among magnetic particles, which is evidenced in these sediments by an inverse correlation between the ratio of ARM to saturation IRM (SIRM) and SIRM without significant magnetic grain-size changes. For the last 350 kyrs, the record of core NGC65, which was obtained at practically the same site as KR0310-PC1 and covers the Brunhes chron (Yamazaki, 1999), was incorporated because the upper part of KR0310-PC1 was physically disturbed. In the record of NGC65/KR0310-PC1, the average paleointensity in the late Matuyama chron is not lower than that during the Brunhes chron, which does not support the conclusion of Valet et al. (2005) based on their Sint-2000 stack. A spectral analysis on the NGC65/KR0310-PC1 paleointensity record shows a power at the ∼100 kyr eccentricity period. The relative paleointensity and magnetic properties of NGC65/KR0310-PC1 were compared with those of MD982185 from the western equatorial Pacific (Yamazaki and Oda, 2002, 2005). The two sites belong to different oceanographic regimes. Coherent variations in the relative paleointensity despite incoherent changes in the magnetic properties suggest that rock-magnetic contamination to the relative paleointensity is small, if any, and the ∼100 kyr period in the relative paleointensity records would reflect the geomagnetic field behavior.

Cooling rate correction of paleointensity determination for volcanic glasses by relaxation geospeedometry

The influence of the cooling rate on paleointensity estimates was investigated for samples from a vertical profile across a 600 A.D. obsidian lava flow ramp from Lipari, Italy. The natural cooling rates at the glass transition, which were previously determined for the seven investigated samples by relaxation geospeedometry, vary by a factor of more than 4. Rock magnetic investigations indicate a magnetic microlite fraction in the single-domain grain size range and strong magnetic anisotropy. The thermoremanence anisotropy tensor was determined for each specimen to correct the paleointensity results for this anisotropy. The cooling rate dependency of the thermoremanence was determined experimentally. Extrapolation to natural cooling rates indicate an overestimate of the paleointensity by 13% to 20% during experiments with typical laboratory cooling rates. Correcting for the different cooling rates and the cooling rate dependencies within the vertical profile, significantly reduces the standard deviation of the average flow paleointensity. The average paleointensity for the 543 ± 19 A.D. flow ramp results in 52.4 ± 1.1 μT, corresponding to a virtual axial dipole moment of 9.2 ± 0.2 Am2. Uncertainties, introduced by anisotropy correction and cooling rate extrapolation, are considered by error propagation.

Paleointensity of the early geodynamo (2.45 Ga) as recorded in Karelia: A single-crystal approach

Research Article| May 01, 2003 Paleointensity of the early geodynamo (2.45 Ga) as recorded in Karelia: A single-crystal approach A.V. Smirnov; A.V. Smirnov 1Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA Search for other works by this author on: GSW Google Scholar J.A. Tarduno; J.A. Tarduno 1Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA Search for other works by this author on: GSW Google Scholar B.N. Pisakin B.N. Pisakin 2Department of Earth Physics, Saint Petersburg State University, Saint Petersburg 199034, Russia Search for other works by this author on: GSW Google Scholar Geology (2003) 31 (5): 415–418. https://doi.org/10.1130/0091-7613(2003)031<0415:POTEGG>2.0.CO;2 Article history received: 10 Oct 2002 rev-recd: 21 Jan 2003 accepted: 23 Jan 2003 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.V. Smirnov, J.A. Tarduno, B.N. Pisakin; Paleointensity of the early geodynamo (2.45 Ga) as recorded in Karelia: A single-crystal approach. Geology 2003;; 31 (5): 415–418. doi: https://doi.org/10.1130/0091-7613(2003)031<0415:POTEGG>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We report Thellier paleointensity results from single plagioclase crystals derived from ca. 2.45 Ga border dikes of the Burakovka layered intrusion (Karelia, Russia). Rock magnetic and transmission electron microscope analyses indicate that these plagioclase crystals contain pseudosingle- to single-domain magnetite inclusions. Fifteen determinations from four dikes meet reliability criteria and yield an average paleointensity of 43.2 ± 10.8 μT, which corresponds to a virtual dipole moment of 8.43 ± 2.11 × 1022 Am2. Although our result is unlikely to adequately represent the time-averaged field, the mean and range of values are similar to those of the present-day field. These values suggest that the inner core, which may stabilize the geodynamo, had started to grow by Early Proterozoic time. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Repeating waveform initiated by a 180–190 ka geomagnetic excursion in western North America: Implications for field behavior during polarity transitions and subsequent secular variation

New paleomagnetic, lithologic, and stratigraphic data are presented from the sediments of Lake Chewaucan in the Summer Lake Basin, Oregon. The new data place better age constraints on the sediments and improve the accuracy of the previously published paleomagnetic record from this locality. A complex, yet distinct, waveform is observed in all three components of the paleomagnetic vector. The waveform begins as the 180–190 ka Pringle Falls/Long Valley/Summer Lake II geomagnetic excursion and continues for two cycles after the excursion, until the record is interrupted by an unconformity that we correlate to the oxygen isotope stage 6/5e boundary. The waveform's directional morphology in virtual geomagnetic pole (VGP) space is defined by two clockwise loops followed by a distinctive counterclockwise, clockwise, counterclockwise looping sequence. The VGP paths of the two cycles after the excursion are rotated 180° about Earth's spin axis with respect to the VGP paths of the excursion cycle. The waveform also consists of a relative paleointensity variation which repeats during the two cycles after the excursion. The average paleointensity of the postexcursion waveform repetitions is high relative to the extremely low values that occur during the excursion. This observation indicates that excursion‐initiated secular variations can occur after the field fully recovers from the low intensities which commonly typify excursions. Because of the similarities noted previously between this excursion and full polarity transitions (Trie et al., 1991), our new observations constrain models for a wide range of field behavior including polarity transitions, excursions, and secular variation.

The intensity of the geomagnetic field at 3.5 Ga: paleointensity results from the Komati Formation, Barberton Mountain Land, South Africa

Paleointensity measurements have been carried out on 3.5 Ga samples from the Komati Formation type locality using both the Thellier and Van Zijl methods. These samples contain a single steeply-directed negative TRM component acquired during metamorphism of the Komati lavas. Thellier experiments yielded values ranging from 12 to 37 μT but an average paleofield intensity for the four best determinations is 20 ± 3 μT. A slightly lower average paleointensity of 15 ± 3 μT was obtained using Van Zijl experiments. Preheating was used to chemically stabilize seven samples used in Van Zijl determinations and these produced nearly ideal plots with an average paleointensity of 13 ± 2 μT. A single basaltic komatiite sample gave a nearly ideal Van Zijl plot indicating about 21 μT, nearly the same paleointensity as the peridotitic komatiite samples even though its NRM intensity was several orders of magnitude lower. Since the Komati characteristic remanence was acquired during a slow cooling, the data must be reduced by a factor of 1.55 to account for the difference between laboratory and natural cooling rates. Calculation of an equivalent equatorial paleointensity using the paleolatitude implied by the steep Komati characteristic remanence then gives value of 5 μT for the intensity of the geomagnetic field at 3.5 Ga, lower than the present value of about 30 μT.