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2017
Shaar, R, Tauxe L, Goguitchaichvili A, Devidze M, Licheli V.  2017.  Further evidence of the Levantine Iron Age geomagnetic anomaly from Georgian pottery. Geophysical Research Letters. 44:2229-2236.   10.1002/2016gl071494   AbstractWebsite

Recent archaeomagnetic data from ancient Israel revealed the existence of a so-called "Levantine Iron Age geomagnetic anomaly" (LIAA) which spanned the first 350years of the first millennium before the Common Era (B.C.E.) and was characterized by a high averaged geomagnetic field (virtual axial dipole moments, VADM>140ZAm(2), nearly twice of today's field), short decadal-scale geomagnetic spikes (VADM of 160-185ZAm(2)), fast field variations, and substantial deviation from dipole field direction. The geographic constraints of the LIAA have remained elusive due to limited high-quality paleointensity data in surrounding locations. Here we report archaeointensity data from Georgia showing high field values (VADM>150ZAm(2)) in the tenth or ninth century B.C.E., low field values (VADM<60 ZAm(2)) in the twelfth century B.C.E., and fast field variation in the fifth and fourth centuries B.C.E. High field values in the time frame of LIAA have been observed so far only in three localities near the Levant: Eastern Anatolia, Turkmenistan, and now Georgia, all located east of longitude 30 degrees E. West of this, in the Balkans, field values in the same time are moderate to low. These constraints put geographic limits on the extent of the LIAA and support the hypothesis of an unusually intense regional geomagnetic anomaly during the beginning of the first half of the first millennium B.C.E., comparable in area and magnitude (but of opposite sign) to the presently active South Atlantic anomaly.

2015
Cromwell, G, Tauxe L, Halldorsson SA.  2015.  New paleointensity results from rapidly cooled Icelandic lavas: Implications for Arctic geomagnetic field strength. Journal of Geophysical Research-Solid Earth. 120:2913-2934.   10.1002/2014jb011828   AbstractWebsite

The Earth's magnetic field is assumed to be a geocentric axial dipole (GAD) when averaged over sufficient time (10(5)-10(6)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 (g) over bar (0)(2) 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 +/- 15.6 mu T (virtual axial dipole moment=78.1 +/- 22.0ZAm(2)), consistent with the present-day field. Thirty-seven sites are between 11ka and 3.35Ma with a median intensity of 33.1 +/- 8.3 mu T (47.0 +/- 11.6ZAm(2)). 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.

2009
Lawrence, KP, Tauxe L, Staudigel H, Constable CG, Koppers A, McIntosh W, Johnson CL.  2009.  Paleomagnetic field properties at high southern latitude. Geochemistry Geophysics Geosystems. 10   10.1029/2008gc002072   AbstractWebsite

Statistical analyses of paleomagnetic data from lava flows are used to study geomagnetic field behavior on million year timescales. Previous paleomagnetic studies have lacked high-latitude measurements necessary to investigate the persistence of geomagnetic anomalies observed in the recent and historical field and replicated in some numerical geodynamo simulations. These simulations suggest that reduced convective flow inside the tangent cylinder may affect the magnetic field at high latitude, whereas lower-latitude observations are expressions of columnar/helical flow outside the tangent cylinder. This paper presents new paleointensity and paleodirectional data from 100 volcanic sites in the Erebus Volcanic Province (EVP), Antarctica, and 21 new age determinations by the (40)Ar/(39)Ar incremental heating method. The new EVP data are combined with previously published paleomagnetic and geochronological results, providing 133 sites, 91 having radioisotopic dates. Modified Thellier-Thellier paleointensity estimates are reported for 47 sites (37 have dates). Ages for the combined data set span 0.03 to 13.42 Ma. The 125 high-quality EVP directional data selected from the merged data set have a non-Fisherian distribution and a mean direction with an inclination anomaly of similar to 3 degrees, but 95% confidence limits include the prediction from a geocentric axial dipole. Virtual geomagnetic pole (VGP) dispersions for Brunhes, Matuyama, and the combined 0-5 Ma data set are consistently high compared with values from middle-to low-latitude regions regardless of the criterion used to determine transitional fields. With VGP latitude cut off at 45 degrees, the dispersion (23.9 +/-2.1 degrees) for the combined 0-5 Ma EVP data set is consistent with earlier high-latitude data and paleosecular variation (PSV) in Model G but not with some more recent statistical PSV models. Mean EVP paleointensity of 31.5 +/-2.4 mu T, derived from 41 high-quality sites, is about half the current value at McMurdo (similar to 63 mu T). The result is essentially independent of data selection criteria. High VGP dispersion and low-intensity values support the global observation of anticorrelation between directional variability and field strength. Simulations of time-varying dipole strength show that uneven temporal sampling may bias the mean EVP intensity estimate, but the possibility of persistently anomalous field behavior at high latitude cannot be excluded.

2008
Johnson, CL, Constable CG, Tauxe L, Barendregt R, Brown LL, Coe RS, Layer P, Mejia V, Opdyke ND, Singer BS, Staudigel H, Stone DB.  2008.  Recent investigations of the 0-5 Ma geomagnetic field recorded by lava flows. Geochemistry Geophysics Geosystems. 9   10.1029/2007gc001696   AbstractWebsite

We present a synthesis of 0 - 5 Ma paleomagnetic directional data collected from 17 different locations under the collaborative Time Averaged geomagnetic Field Initiative ( TAFI). When combined with regional compilations from the northwest United States, the southwest United States, Japan, New Zealand, Hawaii, Mexico, South Pacific, and the Indian Ocean, a data set of over 2000 sites with high quality, stable polarity, and declination and inclination measurements is obtained. This is a more than sevenfold increase over similar quality data in the existing Paleosecular Variation of Recent Lavas (PSVRL) data set, and has greatly improved spatial sampling. The new data set spans 78 degrees S to 53 degrees N, and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time-averaged field (TAF) and paleosecular variation (PSV) for the Brunhes and Matuyama chrons, and for the 0 - 5 Ma interval combined. The Brunhes and Matuyama chrons exhibit different TAF geometries, notably smaller departures from a geocentric axial dipole field during the Brunhes, consistent with higher dipole strength observed from paleointensity data. Geographical variations in PSV are also different for the Brunhes and Matuyama. Given the high quality of our data set, polarity asymmetries in PSV and the TAF cannot be attributed to viscous overprints, but suggest different underlying field behavior, perhaps related to the influence of long-lived core-mantle boundary conditions on core flow. PSV, as measured by dispersion of virtual geomagnetic poles, shows less latitudinal variation than predicted by current statistical PSV models, or by previous data sets. In particular, the Brunhes data reported here are compatible with a wide range of models, from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude. Discriminating among such models could be helped by increased numbers of low-latitude data and new high northern latitude sites. Tests with other data sets, and with simulations, indicate that some of the latitudinal signature previously observed in VGP dispersion can be attributed to the inclusion of low-quality, insufficiently cleaned data with too few samples per site. Our Matuyama data show a stronger dependence of dispersion on latitude than the Brunhes data. The TAF is examined using the variation of inclination anomaly with latitude. Best fit two- parameter models have axial quadrupole contributions of 2 - 4% of the axial dipole term, and axial octupole contributions of 1 - 5%. Approximately 2% of the octupole signature is likely the result of bias incurred by averaging unit vectors.

2006
Tauxe, L, Steindorf JL, Harris A.  2006.  Depositional remanent magnetization: Toward an improved theoretical and experimental foundation. Earth and Planetary Science Letters. 244:515-529.   10.1016/j.cpsl.2006.02.003   AbstractWebsite

The first theoretical predictions for the behavior of magnetic particles in water were that sedimentary magnetizations would be fully aligned with the ambient field, yet redeposition experiments showed a strong (and quasi-linear) dependence on the external field. This empirically observed linearity has served as the fundamental assumption of sedimentary paleointensity studies for decades. We present redeposition experiments which suggest instead that the relationship between depositional remanence (DRM) and applied field may frequently be curved for magnetic fields in the range of the Earth's. Numerical simulations using a flocculation model can explain the redeposition data and suggest that DRM will be significantly non-linear when the floes are small (several microns). There is a strong dependence of floe size on salinity particularly in low salinity environments. Floe size has a profound influence on the efficiency of DRM, hence low salinity environment may give results with poor reproducibility. The size of the floe in which magnetic particles are embedded is not accounted for in current methods of normalization, yet is the most important parameter. On the bright side, however, it now seems possible to quantitatively explain paleointensity in sedimentary systems opening the door to absolute paleointensity estimates from sediments whose key parameters of floe size distribution and settling times can be constrained. (c) 2006 Elsevier B.V All fights reserved.

2004
Tauxe, L, Gans P, Mankinen EA.  2004.  Paleomagnetism and Ar-40/Ar-39 ages from volcanics extruded during the Matuyama and Brunhes Chrons near McMurdo Sound, Antarctica. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000656   AbstractWebsite

Maps of virtual geomagnetic poles derived from international geomagnetic reference field models show large lobes with significant departures from the spin axis. These lobes persist in field models for the last few millenia. The anomalous lobes are associated with observation sites at extreme southerly latitudes. To determine whether these features persist for millions of years, paleomagnetic vector data from the continent of Antarctica are essential. We present here new paleomagnetic vector data and Ar-40/Ar-39 ages from lava flows spanning the Brunhes and Matuyama Chrons from the vicinity of McMurdo Sound, Antarctica. Oriented paleomagnetic samples were collected from 50 lava flows by E. Mankinen and A. Cox in the 1965-1966 austral summer season. Preliminary data based largely on the natural remanent magnetization (NRM) directions were published by Mankinen and Cox [1988]. We have performed detailed paleomagnetic investigations of 37 sites with multiple fully oriented core samples to investigate the reliability of results from this unique sample collection. Of these, only one site fails to meet our acceptance criteria for directional data. Seven sites are reversely magnetized. The mean normal and reverse directions are antipodal. The combined mean direction has (D) over bar =12, (I) over bar=-86, alpha=4, kappa=37 and is indistinguishable from that expected from a GAD field. We obtained reproducible absolute paleointensity estimates from 15 lava flows with a mean dipole moment of 49 ZAm(2) and a standard deviation of 28 ZAm(2). Ar-40/Ar-39 age determinations were successfully carried out on samples from 18 of the flows. Our new isotopic ages and paleomagnetic polarities are consistent with the currently accepted geomagnetic reversal timescales.

Tauxe, L, Luskin C, Selkin P, Gans P, Calvert A.  2004.  Paleomagnetic results from the Snake River Plain: Contribution to the time-averaged field global database. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000661   AbstractWebsite

[1] This study presents paleomagnetic results from the Snake River Plain (SRP) in southern Idaho as a contribution to the time-averaged field global database. Paleomagnetic samples were measured from 26 sites, 23 of which ( 13 normal, 10 reverse) yielded site mean directions meeting our criteria for acceptable paleomagnetic data. Flow ages (on 21 sites) range from 5 ka to 5.6 Ma on the basis of Ar-40/Ar-39 dating methods. The age and polarity for the 21 dated sites are consistent with the Geomagnetic Reversal Time Scale except for a single reversely magnetized site dated at 0.39 Ma. This is apparently the first documented excursion associated with a period of low paleointensity detected in both sedimentary and igneous records. Combining the new data from the SRP with data published from the northwest United States between the latitudes of 40degrees and 50degreesN, there are 183 sites in all that meet minimum acceptability criteria for legacy and new data. The overall mean direction of 173 normally magnetized sites has a declination of 2.3degrees, inclination of 61.4degrees, a Fisher concentration parameter (kappa) of 58, and a radius of 95% confidence (alpha(95)) of 1.4degrees. Reverse sites have a mean direction of 182.4degrees declination, -58.6degrees inclination, kappa of 50, and alpha(95) of 6.9degrees. Normal and reversed mean directions are antipodal and indistinguishable from a geocentric axial dipole field at the 95% confidence level. Virtual geomagnetic pole dispersion was found to be circularly symmetric, while the directional data were elongate north-south. An updated and corrected database for the northwestern U. S. region has been contributed to the Magnetics Information Consortium (MagIC) database at http://earthref.org.

2003
Tauxe, L, Constable C, Johnson CL, Koppers AAP, Miller WR, Staudigel H.  2003.  Paleomagnetism of the southwestern USA recorded by 0-5 Ma igneous rocks. Geochemistry Geophysics Geosystems. 4   10.1029/2002gc000343   AbstractWebsite

The issue of permanent nondipole contributions to the time-averaged field lies at the very heart of paleomagnetism and the study of the ancient geomagnetic field. In this paper we focus on paleomagnetic directional results from igneous rocks of the southwestern U. S. A. in the age range 0-5 Ma and investigate both the time-averaged field and its variability about the mean value. Several decades of work in the southwestern United States have resulted in the publication of paleomagnetic data from over 800 individual paleomagnetic sites. As part of a new investigation of the San Francisco Volcanics, we collected paleomagnetic samples from 47 lava flows, many of which have been previously dated. The new data combined with published data are highly scattered. Contributions to the scatter were considered, and we find that removal of data sets from tectonically active areas and judicious selection according to Fisher's [1953] precision parameter results in an axially symmetric data distribution with normal and reverse modes that are indistinguishable from antipodal. Monte Carlo simulations suggest that a minimum of 5 samples per site are needed to estimate the precision parameter sufficiently accurately to allow its use as a determinant of data quality. Numerical simulations from statistical paleosecular variation models indicate the need for several hundred paleomagnetic sites to get an accurate determination of the average field direction and are also used to investigate the directional bias that results from averaging unit vectors rather than using the full field vector. Average directions for the southwestern U. S. A. show small deviations from a geocentric axial dipole field, but these cannot be considered statistically significant. Virtual geomagnetic pole (VGP) dispersions are consistent with those from globally distributed observations analyzed by McElhinny and McFadden [1997]. However, a systematic investigation of the effect of imposing a cutoff on VGPs with large deviations from the geographic axis indicates that while it may reduce bias in calculating the average direction, such a procedure can result in severe underestimates of the variance in the geomagnetic field. A more satisfactory solution would be to use an unbiased technique for joint estimation of the mean direction and variance of the field distribution.

2000
Tauxe, L, Staudigel H, Wijbrans JR.  2000.  Paleomagnetism and Ar-40/Ar-39 ages from La Palma in the Canary Islands. Geochemistry Geophysics Geosystems. 1   10.1029/2000GC000063   AbstractWebsite

The structure of the time-averaged geomagnetic field has been known for centuries to be approximately dipolar. Significant departures of the time-averaged field from that of an axial geocentric dipole, however, have been reported for decades. The data on which time-averaged field models are based must be of the highest quality in order to document subtle long-term features. We present here new paleomagnetic data and 40Ar/39Ar ages for the island of La Palma, The Canary Islands. Paleomagnetic samples were obtained from 28 lava flows. Of these, 21 met our minimum acceptance criteria for use in time-averaged field models. The 40Ar/39Ar age determinations were successfully carried out on samples from eight of the flows. Our isotopic ages and paleomagnetic polarities are consistent with the currently accepted geomagnetic reversal timescales. The reversed data in the updated database are antipodal to the normal data within the uncertainties, and the time-averaged direction is indistinguishable from that expected from a geocentric axial dipole.