Publications

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2014
Paterson, GA, Tauxe L, Biggin AJ, Shaar R, Jonestrask LC.  2014.  On improving the selection of Thellier-type paleointensity data. Geochemistry Geophysics Geosystems. 15:1180-1192.   10.1002/2013gc005135   AbstractWebsite

The selection of paleointensity data is a challenging, but essential step for establishing data reliability. There is, however, no consensus as to how best to quantify paleointensity data and which data selection processes are most effective. To address these issues, we begin to lay the foundations for a more unified and theoretically justified approach to the selection of paleointensity data. We present a new compilation of standard definitions for paleointensity statistics to help remove ambiguities in their calculation. We also compile the largest-to-date data set of raw paleointensity data from historical locations and laboratory control experiments with which to test the effectiveness of commonly used sets of selection criteria. Although most currently used criteria are capable of increasing the proportion of accurate results accepted, criteria that are better at excluding inaccurate results tend to perform poorly at including accurate results and vice versa. In the extreme case, one widely used set of criteria, which is used by default in the ThellierTool software (v4.22), excludes so many accurate results that it is often statistically indistinguishable from randomly selecting data. We demonstrate that, when modified according to recent single domain paleointensity predictions, criteria sets that are no better than a random selector can produce statistically significant increases in the acceptance of accurate results and represent effective selection criteria. The use of such theoretically derived modifications places the selection of paleointensity data on a more justifiable theoretical foundation and we encourage the use of the modified criteria over their original forms.

2013
Shaar, R, Tauxe L.  2013.  Thellier GUI: An integrated tool for analyzing paleointensity data from Thellier-type experiments. Geochemistry Geophysics Geosystems. 14:677-692.   10.1002/ggge.20062   AbstractWebsite

Thellier-type experiments are a method used to estimate the intensity of the ancient geomagnetic field from samples carrying thermoremanent magnetization. The analysis of Thellier-type experimental data is conventionally done by manually interpreting data from each specimen individually. The main limitations of this approach are: (1) manual interpretation is highly subjective and can be biased by misleading concepts, (2) the procedure is time consuming, and (3) unless the measurement data are published, the final results cannot be reproduced by readers. These issues compound when trying to combine together paleointensity data from a collection of studies. Here, we address these problems by introducing the Thellier GUI: a comprehensive tool for interpreting Thellier-type experimental data. The tool presents a graphical user interface, which allows manual interpretation of the data, but also includes two new interpretation tools: (1) Thellier Auto Interpreter: an automatic interpretation procedure based on a given set of experimental requirements, and 2) Consistency Test: a self-test for the consistency of the results assuming groups of samples that should have the same paleointensity values. We apply the new tools to data from two case studies. These demonstrate that interpretation of non-ideal Arai plots is nonunique and different selection criteria can lead to significantly different conclusions. Hence, we recommend adopting the automatic interpretation approach, as it allows a more objective interpretation, which can be easily repeated or revised by others. When the analysis is combined with a Consistency Test, the credibility of the interpretations is enhanced. We also make the case that published paleointensity studies should include the measurement data (as supplementary files or as a contributions to the MagIC database) so that results based on a particular data set can be reproduced and assessed by others.

2012
Dallanave, E, Muttoni G, Agnini C, Tauxe L, Rio D.  2012.  Is there a normal magnetic-polarity event during the Palaeocene-Eocene thermal maximum (similar to 55 Ma)? Insights from the palaeomagnetic record of the Belluno Basin (Italy) Geophysical Journal International. 191:517-529.   10.1111/j.1365-246X.2012.05627.x   Abstract

In the lowermost Eocene sedimentary record of Ocean Drilling Program Site 1262 (Leg 208, Walvis Ridge, South Atlantic Ocean), the presence of a similar to 53-kyr-long normal polarity event has been recorded within the similar to 2.55-Myr-long reverse polarity Chron C24r (similar to 53.355.9 Ma) and termed PalaeoceneEocene magnetic reversal (PEMR). The origin of the PEMR has been speculatively related to a change in the Earth's rotation rate that was in turn influenced by an abrupt overturning of the ocean-atmosphere circulation that occurred during the PalaeoceneEocene thermal maximum (PETM) at similar to 55 Ma. Such provocative genesis for a magnetic-polarity reversal demands the PEMR to be confirmed (or refuted) in additional PETM sections. Here, we present detailed palaeomagnetic and rock-magnetic data from the Forada and Cicogna sections of the Belluno Basin in NE Italy, which contain an expanded and continuous record of the PETM termed clay marl unit (CMU). Our data indicate that these sediments were deposited during a continuous interval of reverse geomagnetic field polarity. We therefore conclude that no magnetic-polarity reversals occurred throughout the PETM. In addition, we studied the origin of the high degree of flattening affecting the characteristic magnetic component directions of the sediments, which we interpret as due to a combination of depositional inclination shallowing typical of detrital haematite, and post-depositional compaction of clays, particularly abundant in the carbonate-depleted CMU.

2011
Donadini, F, Elming SA, Tauxe L, Halenius U.  2011.  Paleointensity determination on a 1.786 Ga old gabbro from Hoting, Central Sweden. Earth and Planetary Science Letters. 309:234-248.   10.1016/j.epsl.2011.07.005   AbstractWebsite

Paleointensities from Precambrian rocks are rare and might be biased by remagnetization processes. Here we present new analyses of samples from a 1.786 Ga gabbro near Hoting, Central Sweden. Rock magnetic and mineralogical analyses indicate that one of the sites (site 5) may be pristine, whereas the others exhibit evidence of alteration. Characteristic remanent magnetization was determined using principal component analysis for each sample and was compared with results obtained in a previous study of Elming et al. (2009). Intensity measurements from site 5 show higher values compared to those of the other sites, suggesting that alteration processes may lead to underestimation of the field intensity. After cooling rate and anisotropy correction, the field moment at 1.786 Ga was estimated to be 25.6 +/- 33 ZAm(2) and 15.2 +/- 6.1 ZAm(2) from site 5 only and from all sites respectively. We consider the result from site 5 to be more accurate owing to the lack of evidence for alteration: our estimates agree well with the Proterozoic VDM values suggested by Biggin et al. (2009). (C) 2011 Elsevier B.V. All rights reserved.

Shaar, R, Ron N, Tauxe L, Kessel R, Agnon A.  2011.  Paleomagnetic field intensity derived from non-SD: Testing the Thellier IZZI technique on MD slag and a new bootstrap procedure. Earth and Planetary Science Letters. 310:213-224.   10.1016/j.epsl.2011.08.024   AbstractWebsite

Experimental techniques to determine paleomagnetic field intensity are based on a theoretical framework that is valid only for single-domain (SD) ferromagnetic particles. Yet, most of the available materials exhibit distinctly non-SD properties. Designing the optimal paleointensity methodology for non-SD is, therefore, a fundamental challenge in paleomagnetism. The objective of this study is to experimentally test the IZZI Thellier absolute paleointensity method on small MD recorders. The test has two purposes: 1) to describe the characteristic non-SD patterns occurring in Arai plots, and 2) to identify the optimal approach in interpreting non-SD behavior. We carried out paleointensity experiments on 40 specimens from 4 synthetic re-melted slag samples with identical magnetic properties (mineralogy, texture, and non-SD state) produced under different field intensities. We ran three batches of IZZI experiments using different conditions that allow for a detailed characterization of the non-SD behavior. We find that the curvature of the Arai plot is systematically dependent on the angle and the proportion between the field used in the paleointensity experiment (B(TRM)) and the field in which the NRM was acquired (B(NRM)). Straight-line Arai plot occur when the two fields are parallel and equal, and seems to always give the 'true' slope. Convex curves occur when B(TRM) is parallel and significantly stronger than B(NRM). Concave curves occur in all the other cases and yield two end-case slopes that are always different than the 'true' slope. In addition, zigzagged patterns increase with the angle the proportion between B(TRM) and B(NRM). We test the accuracy of the 'best fitting' line approach and conclude that 'best fitting' line in curved plots cannot provide robust paleointensity estimates. Yet, the two 'end-case' slopes in concave curves provide adequate constraints for the true value. We introduce a new procedure to calculate a 95% confidence interval of the paleointensity from curved plots using bootstrap statistics. We substantiate the new procedure by conducting two independent tests. The first uses synthetic re-melted slag produced under known field intensities - 3 SD samples and 4 non-SD samples. The second compares paleointensity determinations from archeological slag samples of the same age - 34 SD samples and 10 non-SD samples. The two tests demonstrate that the bootstrap technique may be the optimal approach for non-ideal dataset. (C) 2011 Elsevier BM. All rights reserved.

2010
Dallanave, E, Tauxe L, Muttoni G, Rio D.  2010.  Silicate weathering machine at work: Rock magnetic data from the late Paleocene-early Eocene Cicogna section, Italy. Geochemistry Geophysics Geosystems. 11   10.1029/2010gc003142   AbstractWebsite

We describe a scenario of climate forcing on sedimentation recorded in the late Paleocene-early Eocene Cicogna marine section from the Belluno Basin ( NE Italy). Previously published magneto-biostratigraphic data revealed that the similar to 81 m Cicogna section extends from Chron C25r to Chron C23r spanning the NP7/NP8-NP12 nannofossil zones (similar to 52.2-56.6 Ma). Using previously published rock magnetic data, augmented by data from this study, we describe and thoroughly discuss a pronounced increase of hematite ( relative to maghemite or magnetite) between similar to 54.9 and 54.6 Ma immediately above the Paleocene-Eocene boundary, followed by a second, long-term increasing trend from similar to 54 Ma up to similar to 52.2 Ma in the early Eocene. This hematite is essentially of detrital origin, insofar as it is associated with a strong shallow bias of paleomagnetic inclinations, and is interpreted to have formed on land by the weathering of Fe-bearing silicates and other primary minerals. We speculate that the warm and humid climate typical of the Paleocene-Eocene thermal maximum (PETM, similar to 54.9 Ma) as well as of the warming trend leading to the early Eocene climatic optimum (EECO; similar to 52-50 Ma) enhanced continental weathering of silicate rocks with the consequent production, transport, and sedimentation of detrital hematite grains. This hypothesis is confirmed by a statistical correlation between the rock magnetic properties and global climate as revealed by a standard benthic oxygen isotope record from the literature. Our temporal coupling between oxidation state of sedimentary magnetic phases and global climate is therefore consistent with the existence in the Paleocene-Eocene of the silicate weathering negative feedback mechanism for the long-term stabilization of the Earth's surface temperature.

2009
Mitra, R, Tauxe L.  2009.  Full vector model for magnetization in sediments. Earth and Planetary Science Letters. 286:535-545.   10.1016/j.epsl.2009.07.019   AbstractWebsite

Sediments provide a continuous record of past geomagnetic field variations. Although it is theoretically possible to get both the direction and intensity of the geomagnetic field from sediment records the, mechanism is not fully understood. Previous workers have postulated that flocculation plays an important role in detrital remanent magnetism (DRM). Flocs are porous, loose and highly fragile aggregates of microscopic clay particles and their behavior in a viscous medium is likely to be different than single particles of magnetic minerals. In order to understand the role of flocculation in sediment magnetization, we carried out a set of redeposition experiments at different field intensities and a quasi-constant field inclination of 45 degrees. We present here a simple numerical model of flocculation, incorporating both magnetic and hydrodynamic torques to explain the experimental data. At small floc sizes DRM acquisition is likely to be non-linear in field strengths comparable to the Earth's, but the sediments may be able to record the directions accurately. With increasing floc sizes sediments may retain a record of the intensity that is linearly related to the applied field or a direction parallel to the applied field, but are unlikely to do both at the same time. Also, the majority of the magnetic particles in the sediments may not be contributing significantly towards the net DRM and any bulk normalizing parameter may be unsuitable if the depositional environment has changed over the depositional period. (C) 2009 Elsevier B.V. All rights reserved.

Tauxe, L, Kodama KP.  2009.  Paleosecular variation models for ancient times: Clues from Keweenawan lava flows. Physics of the Earth and Planetary Interiors. 177:31-45.   10.1016/j.pepi.2009.07.006   AbstractWebsite

Statistical paleosecular variation models predict distributions of paleomagnetic vectors as a function of geographic position. Such models have been used in a variety of applications that test whether a given data set fairly represents the variability and average properties of the geomagnetic field. The simple relationship between inclination of the geo magnetic field and latitude predicted by geocentric axial dipole (GAD) models has been a cornerstone for plate reconstructions for decades, yet many data sets exhibit a tendency to be shallower than expected for a dominantly axial geocentric magnetic field. Too shallow inclinations have variously been interpreted as plate motion, permanent non-dipole field components or bias in inclination from sedimentary processes. Statistical PSV models could in principle be used to resolve the cause of inclination anomalies because there is a simple relationship between the elongation of the distribution of directions in the vertical plane and the average inclination. Shallowing of inclinations from sedimentary processes results in a progressive transformation of the elongation direction in the vertical plane containing the average direction into a pronounced elongation in the plane perpendicular to that. However, the applicability of statistical models based on the last 5 million years for more ancient times is an open question. Here we present new data from the Keeweenawan North Shore Volcanics (similar to 1.1 Ga). These data are consistent with statistical PSV model predictions and are less well fit by models that include a 20% axial octupole component. We also find evidence for a pervasive overprinting by hematite in a shallower direction and find support for the contention that the asymmetric reversal(s) observed in Keweenawan aged rocks along the North shore of Lake Superior can be explained as an age progression, with the reverse directions being older than the normal directions. Finally, we re-consider implications from an analysis of inclinations from the Global Paleomagnetic Database for the Paleozoic and Pre-Cambrian. We find that the data are inconsistent with a random sampling of any simple geomagnetic field model and conclude that the data set under-samples the field in a spatial sense. (C) 2009 Elsevier B.V. All rights reserved.

2007
Selkin, PA, Gee JS, Tauxe L.  2007.  Nonlinear thermoremanence acquisition and implications for paleointensity data. Earth and Planetary Science Letters. 256:81-89.   10.1016/j.epsl.2007.01.017   AbstractWebsite

In paleointensity studies, thermoremanence is generally regarded as a linear function of ambient inagnetic field at low fields comparable to that of the present-day Earth. We find pronounced nonlinearity at low fields for a class of materials with silicate-hosted magnetite that otherwise perforin well in paleointensity experiments. We model this nonlinearity with narrow size ranges of large, acicular single domain grains, which are most likely in a vortex state (i.e. nonuniformly magnetized, sometimes labeled pseudosingle domain). Simple TRM theory predicts that even certain single domain particles will also exhibit a nonlinear response, saturating in fields as low as the Earth's. Such behavior, although likely to be rare, may bias some paleointensity estimates. The bias is especially pronounced when the laboratory field is higher than the ancient field. Fortunately, the fundamental assumption that thermoremanence is proportional to applied field can (and should) be routinely checked at the end of successful paleointensity experiments by adding two extra heating steps. (c) 2007 Elsevier B.V. All rights reserved.

2006
Tauxe, L.  2006.  Long-term trends in paleointensity: The contribution of DSDP/ODP submarine basaltic glass collections. Physics of the Earth and Planetary Interiors. 156:223-241.   10.1016/j.pepi.2005.03.022   AbstractWebsite

The Deep Sea Drilling Project and the Ocean Drilling Program have been collecting fresh appearing submarine basaltic glass from the world's oceans for over three decades. This glass has proved nearly ideal for estimating paleointensity variations of the Earth's magnetic field. We compile here data for 726 paleointensity experiments from six publications on paleointensity using DSDP/ODP glass. We also include new data for an additional 225 specimens. These were obtained through the so-called "IZZI" paleointensity experiment of [Tauxe, L., Staudigel, H., 2004. Strength of the geomagnetic field in the cretaceous normal superchron: new data from submarine basaltic glass of the troodos ophiolite. Geochem. Geophys. Geosyst. 5 (2), Q02H06, doi: 10.1029/2003GCO00635] whereby infield-zerofield steps are alternated with the zerofield-infield steps to enhance quality assessment of the resulting data. The entire collection of data from 951 experiments was prepared for uploading to the MagIC data base (http://earthref.org), including original measurements, interpretations, and useful metadata. Excellent results were obtained throughout the depth (> 1400 mbsf) and age (0-160 Ma) range sampled. DSDP/ODP glass data are compared with published paleointensity data meeting minimal acceptance criteria from the time interval 1-160 Ma. Paleolatitudes were estimated for all cooling units in a self-consistent manner for use in calculating virtual axial dipole moments. We conclude: (1) There is about a 20% difference in mean values between the SBG and the lava flow data (48 +/- 36 and 57 +/- 29 ZAm(2) respectively). The difference is caused by the fact that there are more higher values in the lava flow data than in the SBG data set rather than a difference in the minimum values. Lava flows cooling over a periods of days to months can account for the discrepancy. (2) The positive relationship between polarity interval length and average paleofield intensity first hypothesized by [Cox, A.V, 1968. Lengths of geomagnetic polarity intervals. J. Geophys. Res. 73, 3247-3260] is supported by data compiled here. The Brunhes data (for which we have only a minimum estimate for polarity interval length) are consistent with a long polarity interval, suggesting that instead of racing toward reversal [Hulot, G., Eymin, C., Langlais, B., Mandea, M., Olsen, N., 2002. Small-scale structure of the geodynamo inferred from oersted and magsat satellite data. Nature 416, 620-623], we could instead be in the midst of a long stable polarity interval. (3) Because the average value appears to be a function of polarity interval length, it is probably not useful to calculate a mean value. Nonetheless, it appears that most of the time (apart from the Brunhes and the longest polarity intervals), the average dipole moment is substantially less than the present day value as suggested by [Juarez, T., Tauxe, L., Gee, J.S., Pick, T., 1998. The intensity of the earth's magnetic field over the past 160 million years. Nature 394, 878-881]. (c) 2006 Published by Elsevier B.V.

2005
Kent, DV, Tauxe L.  2005.  Corrected Late Triassic latitudes for continents adjacent to the North Atlantic. Science. 307:240-244.   10.1126/science.1105826   AbstractWebsite

We use a method based on a statistical geomagnetic field model to recognize and correct for inclination error in sedimentary rocks from early Mesozoic rift basins in North America, Greenland, and Europe. The congruence of the corrected sedimentary results and independent data from igneous rocks on a regional scale indicates that a geocentric axial dipole field operated in the Late Triassic. The corrected paleolatitudes indicate a faster poleward drift of similar to0.6 degrees per million years for this part of Pangea and suggest that the equatorial humid belt in the Late Triassic was about as wide as it is today.

Yan, MD, Van der Voo R, Tauxe L, Fang XM, Pares JM.  2005.  Shallow bias in Neogene palaeomagnetic directions from the Guide Basin, NE Tibet, caused by inclination error. Geophysical Journal International. 163:944-948.   10.1111/j.1365-246X.2005.02802.x   AbstractWebsite

Too-shallow inclinations have frequently been observed in Cenozoic sedimentary strata in central Asia, and new palaeomagnetic results obtained by us from the Guide Basin in NE Tibet are no exception. We use a statistical analysis technique developed by Tauxe and Kent (TK03.GAD), which is based on a geomagnetic field model that predicts distributions of palaeomagnetic directions, and show that the too-shallow Neogene mean inclination (44 degrees) from 627 sites can be corrected to a value of 58 degrees, which closely matches the inclination predicted for the area. We conclude that syn- to post-depositional flattening is the most likely cause for the widely observed inclination bias in central Asia.

2004
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.

2002
Bowles, J, Gee J, Hildebrand J, Tauxe L.  2002.  Archaeomagnetic intensity results from California and Ecuador: evaluation of regional data. Earth and Planetary Science Letters. 203:967-981.   10.1016/s0012-821x(02)00927-5   AbstractWebsite

We present new archaeointensity data for southeastern California (similar to33degreesN, similar to115degreesW, 50-1500 yr BP) and northwestern South America (Ecuador, 2.4degreesS, 80.7degreesW, 4000-5000 yr BP). These results represent the only data from California, as well as the oldest archaeointensity data now available in northwestern South America. In comparing our results to previously published data for the southwestern United States and northwestern South America, we note that significant scatter in the existing data makes comparisons and interpretations difficult. We undertake an analysis of the sources of data scatter (including age uncertainty, experimental errors, cooling rate differences, magnetic anisotropy, and field distortion) and evaluate the effects of scatter and error on the smoothed archaeointensity record. By making corrections where possible and eliminating questionable data, scatter is significantly reduced, especially in South America, but is far from eliminated. However, we believe the long-period fluctuations in intensity can be resolved, and differences between the Southwestern and South American records can be identified. The Southwest data are distinguished from the South American data by much higher virtual axial dipole moment values from similar to 0-600 yr BP and by a broad low between similar to 1000-1500 yr BP. Comparisons to global paleofield models reveal disagreements between the models and the archaeointensity data in these two regions, underscoring the need for additional intensity data to constrain the models in much of the world. (C) 2002 Elsevier Science B.V. All rights reserved.

2001
Cronin, M, Tauxe L, Constable C, Selkin P, Pick T.  2001.  Noise in the quiet zone. Earth and Planetary Science Letters. 190:13-30.   10.1016/s0012-821x(01)00354-5   AbstractWebsite

We have carried out a detailed paleomagnetic investigation of two stratigraphically overlapping sections from the Scaglia Bianca Formation (similar to 85-89.5 Ma) in the Umbria-Marche area in central Italy. Sampling was conducted over 32 in and 7 in intervals at La Roccaccia and Furlo respectively. After AF cleaning the majority of specimens show the expected normal magnetic field orientation, however a number of specimens are directionally anomalous. Some of these deviant specimens are accompanied by apparent spikes or dips in normalized intensity. A detailed investigation of rock magnetics shows that most of these deviations are not a sign of excursionary geomagnetic field behavior, but rather correspond to specimens with distinct rock magnetic characteristics and are therefore rock magnetic 'noise'. Such specimens should not be interpreted as records of the geomagnetic field. Our experience suggests that detailed rock magnetic and magnetic fabric analysis should be done on all anomalous directions prior to interpreting them as geomagnetic field behavior. After elimination of rock magnetic noise in the Scaglia Bianca data sets, there is a high degree of agreement in direction and to a lesser extent relative intensity between correlative portions of the two sections. We therefore offer this data set as a robust record of geomagnetic field behavior during the 4.5 Myr interval represented by the La Roccaccia section. A statistical analysis of the relative intensity observations suggests that this period of the Cretaceous Normal Superchron is characterized by a normalized variability in paleointensity (standard deviation about 28% of the mean value) that is significantly lower than seen during the Oligocene over intervals in which reversals or tiny wiggles occur (typically about 50%). The directional stability results in virtual geomagnetic pole dispersion compatible with that found in volcanic rocks from around the same latitude and ranging in age from 80 to 110 Ma. (C) 2001 Elsevier Science B.V. All rights reserved.

2000
Selkin, PA, Gee JS, Tauxe L, Meurer WP, Newell AJ.  2000.  The effect of remanence anisotropy on paleointensity estimates: a case study from the Archean Stillwater Complex. Earth and Planetary Science Letters. 183:403-416.   10.1016/s0012-821x(00)00292-2   AbstractWebsite

Paleomagnetism of Archean rocks potentially provides information about the early development of the Earth and of the geodynamo. Precambrian layered intrusive rocks are good candidates for paleomagnetic studies: such complexes are commonly relatively unaltered and may contain some single-domain magnetite 'armored' by silicate mineral grains. However, layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy. Magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected. Here we examine the magnetic anisotropy of an anorthosite sample with a well-developed magmatic foliation. The effect of the sample's remanence fabric on paleointensity determinations is significant: paleointensities estimated by the method of Thellier and Thellier range from 17 to 55 muT for specimens magnetized in a field of 25 muT. We describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the Stillwater Complex (MT, USA) of similar to 32 muT (adjusted for the effects of slow cooling). (C) 2000 Elsevier Science B.V. All rights reserved.

1999
Kok, YS, Tauxe L.  1999.  Long-tau VRM and relative paleointensity estimates in sediments. Earth and Planetary Science Letters. 168:145-158.   10.1016/s0012-821x(99)00041-2   AbstractWebsite

Geomagnetic paleointensity measurements from sedimentary records can be severely affected by viscous remanent magnetization (VRM). We present a method for determining varying amounts of long-term VRM acquired during the present polarity interval, using the typically non-linear relationship between acquisition of artificial magnetization and demagnetization of NRM. The non-linear parts are to be avoided for paleointensity determinations, but here we focus on their use for indicators of long-relaxation time VRM. The method, which does not require determining paleointensity values, suggests correlations with paleoclimate curves and age-dependent growth of VRM. Furthermore, it appears that the long-tau VRM acquired during the Pleistocene is accompanied by short-tau effects detected in the laboratory environment. (C) 1999 Elsevier Science B.V. All rights reserved.

1998
Johnson, CL, Wijbrans JR, Constable CG, Gee J, Staudigel H, Tauxe L, Forjaz VH, Salgueiro M.  1998.  Ar-40/Ar-39 ages and paleomagnetism of Sao Miguel lavas, Azores. Earth and Planetary Science Letters. 160:637-649.   10.1016/s0012-821x(98)00117-4   AbstractWebsite

We present new Ar-40/Ar-39 ages and paleomagnetic data for Sao Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. Ar-40/Ar-39 age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K-Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across Sao Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The Ar-40/Ar-39 ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0-0.1 Ma) and up to 0.78 Myr (0-0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78-0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction. (C) 1998 Elsevier Science B.V. All rights reserved.

1996
Kok, YS, Tauxe L.  1996.  Saw-toothed pattern of sedimentary paleointensity records explained by cumulative viscous remanence. Earth and Planetary Science Letters. 144:E9-E14.   10.1016/s0012-821x(96)00175-6   AbstractWebsite

The relative paleointensity of the earth's magnetic field from ODP Site 851 has been characterized by progressive decay towards polarity reversals, followed by sharp recovery of pre-reversal values [1]. We resampled the Gilbert-Gaup reversal boundary of this deep-sea core, and show that during demagnetization this 'saw-toothed' pattern disappears. Further, the recently published Cumulative Viscous Remanence model [2] using the herewith obtained paleointensity record and constraints from thermal treatment replicates the saw-tooth of [1], implying that it is of non-geomagnetic origin.

Tauxe, L, Herbert T, Shackleton NJ, Kok YS.  1996.  Astronomical calibration of the Matuyama-Brunhes boundary: Consequences for magnetic remanence acquisition in marine carbonates and the Asian loess sequences. Earth and Planetary Science Letters. 140:133-146.   10.1016/0012-821x(96)00030-1   AbstractWebsite

We have compiled 19 records from marine carbonate cores in which the Matuyama-Brunhes boundary (MBB) has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes. By correlation of the delta(18)O data to a timescale based on astronomical forcing, we estimate astronomical ages for each of the MBB horizons. In all but one record the MBB occurs within Stage 19. Most magnetostratigraphic sections in Asian Loess place the MBB within a loess interval. Since loess deposition is presumed to be associated with glacial intervals, loess horizons should correspond to even-numbered oxygen isotope stages. A glacial age for the MBB is at odds with the results presented here, which firmly place the MBB within interglacial Stage 19. Inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed. The mean of the Stage 19 age estimates for the MBB is 777.9 +/- 1.8 (N = 18). Inclusion of the single Stage 20 age results in a mean of 778.8 +/- 2.5 (N = 19). The astronomical age estimate of the MBB compares favorably with an (unweighted) mean of 778.2 +/- 3.5 (N = 10) from a compilation of Ar-40/Ar-39 results of transitional lava flows. Combining the two independent data sets yields a grand mean of 778.0 +/- 1.7 (N = 28). The new compilation shows virtually no trend in placement of the MBB within isotope Stage 19 as a function of sediment accumulation rate. We interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment-water interface. Separating the cores into two geographic regions (an Indo-Pacific-Caribbean [IPC] Group and an Atlantic Group) results in a significant difference in the position of the mid-point of the reversal with respect to the astronomical time scale. The data presented here suggest a difference of several thousand years between the two regions. This observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest, systematic differences in remanence acquisition, or by genuine differences in the timing of the directional changes between the two regions.

Hartl, P, Tauxe L.  1996.  A precursor to the Matuyama/Brunhes transition-field instability as recorded in pelagic sediments. Earth and Planetary Science Letters. 138:121-135.   10.1016/0012-821x(95)00231-z   AbstractWebsite

The period some 20-25 kyr just prior to the most recent generally recognized geomagnetic field polarity transition, the Matuyama-to-Brunhes reversal, appears to be marked by significant geomagnetic variability, manifested as pronounced oscillations in intensity. We compare several previously published paleomagnetic records with new high resolution paleomagnetic data obtained from five pelagic marine sites: North Atlantic DSDP Hole 609B; equatorial Atlantic ODP Hole 665A; and western equatorial Pacific ODP Holes 803B, 804C, and 805B. Using standard rock magnetic normalization for all of the samples, as well as a Thellier/Thellier method on the sediments of Hole 804C, we consistently find a decrease in paleointensity (DIP of [1]) approximately 15 kyr prior to the Matuyama-to-Brunhes transition in the five new records, as well as in the previously published records. Despite sedimentation accumulation rates (SAR) that range from 11 cm/kyr to 1 cm/kyr, these sequences yield paleointensity curves that are broadly similar in form, even at the lowest SARs. The intensity of the pre-reversal tow (DIP1) appears to be of the same magnitude as that of the transition itself (DIP2 of [1]). In some of the records, a directional excursion to nearly full normal polarity accompanies DIP1 and remains after alternating field (AF) and/or thermal demagnetization, whereas in other records the directional changes vanish with demagnetization and appear to be caused by overprinting. A viscous remanent magnetization (VRM) contribution to NRM was identified in two of the records and, until removed by AF or thermal demagnetization, was found to blur the 'double-DIP' nature of the paleointensity profiles into an apparent single-DIP, and also resulted in an apparent, but erroneous, 'sawtooth'-like post-transitional sudden increase in paleointensity. After appropriate normalization, the magnitude of the post-transitional recovery was much reduced. The magnetic directions of three of the new records after 'cleaning' and adjusting the stable declinations to either 0 (normal) or 180 (reverse), map to VGP positions lying in the Pacific; the directional variations, however, are far less consistent than the intensity variations. The confirmed global existence of this DIP so closely preceding a major reversal invites questions about its relation to the reversal itself. The apparent normal character of this interval can also present problems for magnetostratigraphical interpretations based on coarse or incomplete sampling by mimicking the Brunhes/Matuyama reversal some 15 kyr earlier in the section than its true location.

1995
Tauxe, L, Pick T, Kok YS.  1995.  Relative Paleointensity in Sediments - a Pseudo-Thellier Approach. Geophysical Research Letters. 22:2885-2888.   10.1029/95gl03166   AbstractWebsite

We present a method for normalizing sedimentary records for estimating relative paleointensity of the geomagnetic field, similar to that successfully used to obtain absolute paleointensity from thermally blocked remanences. It has the advantages that it is more effective in removing unwanted viscous remanence, thereby improving agreement among various records and that it allows the estimation of the uncertainty in the relative paleointensity calculated.

1994
Tauxe, L, Shackleton NJ.  1994.  Relative Paleointensity Records from the Ontong-Java Plateau. Geophysical Journal International. 117:769-782.   10.1111/j.1365-246X.1994.tb02469.x   AbstractWebsite

Rock magnetic, palaeomagnetic and oxygen isotopic results are presented from core RNDB 75p, which was recovered from the Ontong-Java Plateau (OJP). A high degree of uniformity in magnetic properties characterized by relatively small changes in concentration and grain size in the upper 4 m of the core, combined with a lack of coherence between the normalized remanence and rock magnetic data suggests that the natural remanence normalized by saturation remanence reflects variations in relative palaeointensity of the geomagnetic field. The record from RNDB 75p replicates other Ontong-Java records spanning the last 400 Ka and extends the record back to some 700 Ka. Spectral analysis of the Ontong-Java record suggests periodic behaviour in the relative palaeointensity record with a dominant period of between 30 and 40 Ka, which appears not be be an artefact of lithologic variability. This dominant period lies between functions describing climatic precession and obliquity changes in the Earth's orbit. Comparison of the normalized remanence record with astronomical precession (26 Ka period), however, is much more favorable. None the less, 'tuning' the palaeointensity record to that of astronomical precession appears inconsistent with existing isotopic age constraints derived from the SPECMAP time-scale. Based on these data, we must choose between assuming that the Earth's orbit controlled ice volume (inherent in the SPECMAP time-scale) and assuming that the Earth's magnetic field is driven by astronomical precession. The former assumption has a substantial theoretical and observational base and we prefer to interpret the data presented here as suggesting that the Earth's orbit has not played a detectable role in the modulation of the magnetic field. Plots of saturation remanence and magnetic susceptibility are very sensitive to quite subtle changes in magnetic grain size. A slight shift within the pseudo-single-domain grain-size range toward the multidomain (or superparamagnetic) field was detected at about 4 m in RNDB 75p. This change in grain size may reflect a diagenetic alteration of the magnetite (such as dissolution) and may be related to the phenomenon responsible for the loss of magnetic remanence at depth detected in other cores from the region.

1993
Pick, T, Tauxe L.  1993.  Holocene Paleointensities - Thellier Experiments on Submarine Basaltic Glass from the East Pacific Rise. Journal of Geophysical Research-Solid Earth. 98:17949-17964.   10.1029/93jb01160   AbstractWebsite

A complete description of the geomagnetic field requires both paleodirectional and paleointensity data. Although the paleointensity data base has grown steadily over the last three decades, it remains limited in time and space (the majority of data are of Holocene age and come from Europe). Furthermore, it has been difficult to assess the reliability of the paleointensity determinations. Here we present, paleointensity determinations on precisely dated Holocene (0 to 3500 years old) submarine basaltic glass from the East Pacific Rise (15-degrees-S to 22-degrees-S). Although hysteresis measurements and low-temperature isothermal remanent magnetization (IRM) acquisition experiments document a significant contribution of superparamagnetic grains, high blocking temperatures (above 400-degrees-C) and Curie temperatures between 490-degrees-C and 550-degrees-C indicate a single-domain low-Ti magnetite as the carrier of the remanent magnetization. This notion is further supported by the fact that saturation of remanence is achieved in moderate fields of about 200-300 mT. Submarine basaltic glass proves to be nearly ideal for paleointensity determinations in that it produces a high success rate for Thellier experiments. Twenty-six out of 30 samples resulted in acceptable paleointensity determinations. Multiple experiments on splits from the same sample show good reproducibility. The paleointensities for zero age glasses correspond precisely with the present field intensity at the site of recovery. The results of the remaining samples range from 16.7 to 53.9 muT with corresponding virtual axial dipole moments (VADM) of 3.61 X 10(22) to 11.9 X 10(22) A m2. The intensities vary rapidly with time excluding a westward drifting nondipole component as the source for these fluctuations. Basaltic glass is frequently recovered in both dredged and drilled material froin the ocean floor. The availability of submarine basaltic glass throughout the world oceans therefore holds great potential for a better distribution of paleointensity data through time and space.