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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.

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.

Mitra, R, Tauxe L, McIntosh SK.  2013.  Two thousand years of archeointensity from West Africa. Earth and Planetary Science Letters. 364:123-133.   10.1016/j.epsl.2012.12.027   AbstractWebsite

This study presents 17 archeointensity estimates from Senegal and Mali, two neighboring countries in West Africa, for the period 1000 BCE to 1000 CE. The archeological artifacts used in this study were collected during the course of two separate projects, together spanning 22 years and across 8 separate excavations. A primary objective of this study was to get accurate dates, hence, only samples with independent age constraints from pottery style, detailed stratigraphy and C-14 dates were used. A total of 236 specimens from 63 samples were subjected to a double heating paleointensity experiment (IZZI method) from which 95 specimens were selected using a set of very strict selection criteria. The paleointensity results were corrected for differential cooling rate effects and remanence anisotropy. Additionally, we demonstrate the equivalence of using tensors derived from anhysteretic and thermal remanences for correcting remanent anisotropy of the specimens and use the form!

Cromwell, G, Tauxe L, Staudigel H, Constable CG, Koppers AAP, Pedersen RB.  2013.  In search of long-term hemispheric asymmetry in the geomagnetic field : Results from high northern latitudes. Geochemistry Geophysics Geosystems. 14:3234-3249.   10.1002/ggge.20174   AbstractWebsite

Investigations of the behavior of the geomagnetic field on geological timescales rely on globally distributed data sets from dated lava flows. We present the first suitable data from the Arctic region, comprising 37 paleomagnetic directions from Jan Mayen (71 degrees N, 0.2-461 ka) and Spitsbergen (79 degrees N, 1-9.2 Ma) and five paleointensity results. Dispersion of the Arctic virtual geomagnetic poles over the last 2 Ma (27.34.0 degrees) is significantly lower than that from published Antarctic data sets (32.15.0 degrees). Arctic average virtual axial dipole moment (76.824.3 ZAm(2)) is high in comparison to Antarctica over the same time interval (34.88.2 ZAm(2)), although the data are still too sparse in the Arctic to be definitive. These data support a long-lived hemispheric asymmetry of the magnetic field, contrasting higher, more stable fields in the north with lower average strength and more variable field directions in the south. Such features require significant non-axial-dipole contributions over 10(5)-10(6) years.

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.

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.

2008
Ben-Yosef, E, Tauxe L, Ron H, Agnon A, Avner U, Najjar M, Levy TE.  2008.  A new approach for geomagnetic archaeointensity research: insights on ancient metallurgy in the Southern Levant. Journal of Archaeological Science. 35:2863-2879.   10.1016/j.jas.2008.05.016   AbstractWebsite

We present results from an archaeointensity investigation based on a relatively unexploited recording medium, copper slag deposits. Together with a recently improved experimental design for the archaeointensity experiment, we demonstrate the applicability of this medium, as well as other archaeometallurgical artifacts, for the study of the ancient geomagnetic field intensity. In addition to archaeointensity data from well-dated archaeological contexts, we obtained reliable archaeointensity results from poorly dated or contentious archaeometallurgical sites in the Southern Levant. These results shed new light on the dating of these sites, among them the copper smelting installation of Timna 39b a site that has important implications for the beginning of metallurgy during the fifth millennium BCE. The paper also aims to introduce archaeointensity research to the archaeologist scholar, and to encourage further collaboration between the disciplines in future research. (C) 2008 Elsevier Ltd. All rights reserved.

Ben-Yosef, E, Ron H, Tauxe L, Agnon A, Genevey A, Levy TE, Avner U, Najjar M.  2008.  Application of copper slag in geomagnetic archaeointensity research. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005235   AbstractWebsite

Paleointensity and archaeointensity studies since the 1950s have produced numerous geomagnetic intensity data for the last seven millennia. As a consequence of different experiments and materials, there is a complex and internally inconsistent picture of the geomagnetic field behavior. In this study we present data using a recently developed experimental design on a heretofore unexploited recording medium: copper slag deposits. Our results, based on hundreds of specimens from various archaeometallurgical sites of the Southern Levant, demonstrate the applicability of copper slag material for archaeointensity studies. In addition to frequently exhibiting good experimental behavior, slag has further advantages such as dense multilayer deposits and in cases embedded charcoals, which open the door to data sets with excellent age control and resolution. The data presented here augment the high quality database from the Middle East and support previously observed periods of rapid change of the intensity of the geomagnetic field.

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.

Granot, R, Tauxe L, Gee JS, Ron H.  2007.  A view into the Cretaceous geomagnetic field from analysis of gabbros and submarine glasses. Earth and Planetary Science Letters. 256:1-11.   10.1016/j.epsl.2006.12.028   AbstractWebsite

The nature of the geomagnetic field during the Cretaceous normal polarity superchron (CNS) has been a matter of debate for several decades. Numerical geodynamo simulations predict higher intensities, but comparable variability, during times of few reversals than times with frequent reversals. Published geomagnetic paleointensity data from the CNS are highly scattered suggesting that additional studies are required. Here we present new paleointensity results from 18 sites collected from the lower oceanic crust of the Troodos ophiolite, Cyprus (92.1 Ma old). Together with recently published data from the Troodos upper crust we obtain three independent palcointensity time-series. These sequences reveal quasi-cyclic variations of intensities about a mean value of 54 +/- 20 Z Am(2), providing insight into the fluctuating nature of the Cretaceous magnetic field. Our data suggest the CNS field was both weaker and more variable than predicted by geodynamo simulations. The large amplitudes of these variations may explain the wide range of dipole moments previously determined from the CNS. (c) 2007 Elsevier B.V. All rights reserved.

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, Staudigel H.  2004.  Strength of the geomagnetic field in the Cretaceous Normal Superchron: New data from submarine basaltic glass of the Troodos Ophiolite. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000635   AbstractWebsite

[1] We present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the Troodos Ophiolite ( similar to 92 Ma), formed during the Cretaceous Normal Superchron (CNS), a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all. Monte Carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength. Our data suggest a dipole strength equivalent to the present field or nearly twice the post-CNS average. The mean and standard deviation of the dipole moment (81 +/- 43 ZAm(2); Z = 10(21)) from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from DSDP Site 522. The new data set for the CNS suggests a picture of a strong and stable field during the period of time when it stopped reversing. Moreover, the similarity of the CNS data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability.

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.

1997
Tauxe, L, Hartl P.  1997.  11 million years of Oligocene geomagnetic field behaviour. Geophysical Journal International. 128:217-229.   10.1111/j.1365-246X.1997.tb04082.x   AbstractWebsite

An II million year long record of the Oligocene geomagnetic field has been obtained from pelagic sediments of DSDP Hole 522. An average sample spacing of 4 cm yielded approximately one specimen per 4 to 8 kyr. The rock magnetics are remarkably consistent across the entire interval. Previous work demonstrated a magnetic mineralogy dominated by magnetically stable magnetite. The natural remanent magnetism (NRM) carries an Oligocene polarity timescale that is in excellent agreement with the Oligocene reversal record as determined from marine magnetic anomalies (MMAs), including many of the so-called 'crypto-chrons'. Normalized NRM intensities from the undisturbed portions of the record yield a time series of variations with features consistent with a number of other palaeointensity time series derived from both sedimentary and lava sequences. These features include consistent, major decreases in palaeointensity (DIPs) at reversal boundaries, and occasional DIPs between reversal boundaries that could correspond to lineated 'tiny wiggles' in the MMA records. The data set suggests that the overall field strength was 40 per cent higher in the first half of the Oligocene when the average reversal frequency was 1.6 Myr(-1) than in the second half when the reversal frequency was 4 Myr(-1). There is also a weak dependence of average field strength on length of polarity interval. Finally, in the three cores suited to spectral analysis (of coherent polarity and relative intensity independent of lithological contamination), there is a persistent ca. 30-50 ka periodicity in the variations of the relative intensity, suggesting that the geomagnetic held 'pulses' at about this frequency, not only during the Brunhes (as demonstrated by Tauxe & Shackleton 1994), but in the Oligocene as well.

1996
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.

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.

Tauxe, L.  1993.  Sedimentary Records of Relative Paleointensity of the Geomagnetic-Field - Theory and Practice. Reviews of Geophysics. 31:319-354.   10.1029/93rg01771   AbstractWebsite

Sediments have proved irresistible targets for attempts at determining the relative variations in the Earth's magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution. The assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited. Early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short, no dependence on magnetic field was expected. Nonetheless, a number of experiments showed that sedimentary magnetizations varied in accordance with the field, albeit not always in a simple, linear fashion. Experiments in which the sediments were stirred in the presence of a field (to simulate bioturbation) showed a reasonably linear relationship with the applied field, and these results spurred the hope that variations in the Earth's magnetic field might indeed be recoverable from appropriate sedimentary sequences. Examination of existing paleointensity data sets allows a few general conclusions to be drawn. It appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the Earth's magnetic field. The dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences. Moreover, when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences, there is considerable agreement among the various records. When viewed over timescales of hundreds to thousands of years, relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another, suggesting that they are dominated by nondipole field behavior. When viewed over timescales of a few tens of thousands to hundreds of thousands of years, however, the records show coherence over large distances (at least thousands of kilometers) and may reflect changes in the dipole field. On the basis of a sequence spanning the Brunhes and Matuyama chrons, the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years, but these oscillations are not clear in the record prior to about 300 ka; thus they are probably not an inherent feature in the geomagnetic field, and the correspondence of the period of oscillation to that of obliquity is probably coincidence.

1992
Tric, E, Valet JP, Tucholka P, Paterne M, Labeyrie L, Guichard F, Tauxe L, Fontugne M.  1992.  Paleointensity of the Geomagnetic-Field During the Last 80,000 Years. Journal of Geophysical Research-Solid Earth. 97:9337-9351.   10.1029/91jb01620   AbstractWebsite

High-resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores. Three duplicate records were used to estimate the regional coherency of the data within a single area (Tyrrhrenian Sea) while the two others document the field variations in the eastern Mediterranean and the southern Indian Ocean. Careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes. The time-depth control was provided by oxygen isotopes, and small-scale variations in the deposition rates were constrained by means of tephrachronology. The synthetic curve calculated from the Mediterranean records provides a continuous record of the intensity variations during the last 80,000 years (80 kyr). which correlates well with the sparse volcanic data available for the period 0-40 kyr. The fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the Pacific Ocean establishes the truly dipolar character of these variations. The dipole field moment is characterized by large-scale changes as shown by the existence of pronounced drops (at 39 and 60 kyr) alternating with periods of higher intensity. The record suggests a periodic nature for these intensity variations; however, the period studied is not sufficiently long to state this conclusively. These results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time.