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Shackleton, NJ, Hall MA, Raffi I, Tauxe L, Zachos J.  2000.  Astronomical calibration age for the Oligocene-Miocene boundary. Geology. 28:447-450.   10.1130/0091-7613(2000)28<447:acafto>;2   AbstractWebsite

The stratotype section for the base of the Miocene is at a reversed (below) to normal (above) magnetic transition that is claimed to represent magnetic chron C6Cn.2n (o). Deep Sea Drilling Project (DSDP) Site 522 is the only location we are aware of that unambiguously records the three normal events of C6Cn. We have quantitatively determined the range of the short-lived nannofossil Sphenolithus delphix and the tower limit of S. disbelemnos in DSDP Holes 522 and 522A in order to calibrate their precise relationship to the magnetostratigraphy and to confirm the completeness of the record at this site. Astronomical tuning of Ocean Drilling Program (ODP) Sites 926, 928, and 929 shows that S. disbelemnos appears at 22.67 Ma and that the entire range of S. delphix is from about 22.98 Ma to 23.24 Ma. Using these ages, linear interpolation in DSDP Site 522 suggests that the age of C6Cn.2n (o) and of the Oligocene-Miocene boundary is 22.92 +/- 0.04 Ma. Our value, conservatively expressed as 22.9 +/- 0.1 Ma, is 0.9 m.y. younger than the currently accepted age of the Oligocene-Miocene boundary and of C6Cn.2n (o), which was assigned an age of 23.8 Ma, based on an estimate of 23.8 +/- 1 Ma for the Oligocene-Miocene boundary. The bulk-sediment carbon isotope data from DSDP Site 522 is correlated to the record from benthic foraminifera at ODP Site 929 to refine the calibration of magnetic reversals from C6Cn.1n (o) to C7n.2n (o) at DSDP Site 522 on the astronomical time scale.

Selkin, PA, Tauxe L.  2000.  Long-term variations in palaeointensity. Philosophical Transactions of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences. 358:1065-1088. AbstractWebsite

We compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass. The basaltic glass data more than double the number of reliable (Thellier method with pTRM checks) palaeointensity estimates available. Although the new data dramatically improve both spatial and temporal coverage, there is still a strong bias toward the most recent past. The last 0.3 Ma claim over half of the data in our combined database. We therefore divide the data into two groups, the densely sampled last 0.3 Myr and the more sparsely sampled period of time comprising roughly half of the data from 0.3 to 300 Ma. Separating them in this way, it is clear that the dipole moment of the Earth over the past 0.3 Myr (ca. 8 x 10(22) A m(2)) is dramatically higher than the average dipole moment over the preceding 300 Myr (ca. 5 x 10(22) A m(2)). Inclusion of poor-quality results leads to an overestimate of the average dipole moment. Interestingly, no other significant changes in the distribution of dipole moments are evident over the 300 million year span of the data.

Juarez, MT, Tauxe L.  2000.  The intensity of the time-averaged geomagnetic field: the last 5 Myr. Earth and Planetary Science Letters. 175:169-180.   10.1016/s0012-821x(99)00306-4   AbstractWebsite

The existing database for paleointensity estimates of the ancient geomagnetic field contains more than 1500 data points collected through decades of effort. Despite the huge amount of work put into obtaining these data, there remains a strong bias in the age and global distribution of the data reward very young results from a few locations. Also, few of the data meet strict criteria for reliability and most are of unknown quality. In order to improve the age and spatial distribution of the paleointensity database, we have carried out paleointensity experiments on submarine basaltic glasses from a number of DSDP sites. Of particular interest are the sites that provide paleointensity data spanning the time period 0.3-5 Ma, a time of relatively few high quality published data points. Our new data are concordant with contemporaneous data from the published literature that meet minimum acceptance criteria, and the combined data set yields an average dipole moment of 5.49 +/- 2.36 x 10(22) Am-2. This average value is comparable to the average paleofield for the period 5-160 Ma (4.2 +/- 2.3 x 10(22) Am-2) [T. Juarez, L. Tauxe, J.S. Gee and T. Pick (1998) Nature 394, 878-881] and is substantially less than the value of approximately 8 x 10(22) Am-2 often quoted for the last 5 Myr (e.g. [McFadden and McElhinny (1982) J. Geomagn. Geoelectr. 34, 163-189; A.T. Goguitchaichvili, M. Prevot and P. Camps (1999) Earth Planet. Sci. Lett. 167, 15-34]). (C) 2000 Elsevier Science B.V. All rights reserved.

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.

Kok, YS, Tauxe L.  1999.  A relative geomagnetic paleointensity stack from Ontong-Java Plateau sediments for the Matuyama. Journal of Geophysical Research-Solid Earth. 104:25401-25413.   10.1029/1999jb900186   AbstractWebsite

We present a stack of geomagnetic paleointensity estimates for the Matuyama Chron (0.778-2.582 Ma). Sedimentary magnetizations from 4 Ocean Drilling Program (ODP) cores from the Ontong-Java Plateau (OJP) are analyzed with thermal techniques. Thellier-Thellier experiments are used to determine magnetic properties and indicate that a thermal treatment to 250 degrees C can be used for paleointensity estimates. The correlated records form a coherent OJP stack, which is dated at reversal boundaries. A coherent feature with two other data sets spanning the Matuyama Chron is the occurrence of a 150-kyr cyclicity. However, the time control of the OJP stack is not accurate, except for the 10 tie points at the reversals. This could, at least in part, explain why the three Matuyama paleointensity records show no other coherent frequencies.

Hicks, JF, Obradovich JD, Tauxe L.  1999.  Magnetostratigraphy, isotopic age calibration and intercontinental correlation of the Red Bird section of the Pierre Shale, Niobrara County, Wyoming, USA. Cretaceous Research. 20:1-27.   10.1006/cres.1998.0133   AbstractWebsite

The Red Bird section of the Pierre Shale in eastern Wyoming contains a relatively complete sequence of fine-grained marine clastics that were deposited between 81 and 69 million years ago in the Late Cretaceous epicontinental seaway of the US Western Interior. These units not only contain a well-studied, high-resolution ammonite biostratigraphic sequence, by which the far-hung exposures of the seaway sediments are correlated across this region, but they are also isotopically well-dated due to the presence of numerous sanidine-bearing volcanic ash layers. The magnetostratigraphy of the Red Bird section consists of three geomagnetic reversals which can be independently calibrated by seven Ar-40/Ar-39 isotopic ages in an interval that spans 12 million years of the Campanian and Maastrichtian stages. The magnetostratigraphic section can be confidently correlated to that part of the geomagnetic polarity rime scale (GPTS) that ranges from the base of subchron C33n to the base of C31n. Linear interpolation and extrapolation from the isotopic ages gives the following age estimates for these reversal boundaries: C32n/C31r, 70.44 +/- 0.7 Ma; C31r/C31n, 69.01 +/- 0.5 Ma. The C33n/C32r reversal boundary cannot be identified with complete confidence but it is certainly younger than the 74.62 +/- 1.2 Ma age interpolated for the reversal found at the top of C33n. These age estimates make a significant contribution to the calibration of the GPTS for the Cretaceous Period, which has previously relied heavily on interpolation between three or fewer calibration points that are widely spaced in age. In addition, the recognition of the chrons C33 through C31 in this section enables us to correlate the high resolution ammonite zonation of the US Western Interior directly to the time-equivalent European pelagic microfossil zonation based on the magnetostratigraphic reference section at Gubbio in north-central Italy. (C) 1999 Academic Press.

Staudigel, H, Tauxe L, Gee JS, Bogaard P, Haspels J, Kale G, Leenders A, Meijer P, Swaak B, Tuin M, Van Soest MC, Verdurmen EAT, Zevenhuizen A.  1999.  Geochemistry and Intrusive Directions In Sheeted Dikes in the Troodos Ophiolite: Implications for Mid-Ocean Ridge Spreading Centers. Geochemistry Geophysics Geosystems. 1 AbstractWebsite

Sheeted dikes at mid-ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity. As such, they are crucial for the interpretation of many observations at mid-ocean ridges or other volcanoes with pronounced rift zones, including topography, hydrothermal systems, petrology, and geochemistry. We carried out a structural, magnetic, and chemical investigation of a 4 x 10 km sheeted dike section in the Troodos ophiolite, Cyprus. On the basis of major and trace element geochemistry, we distinguish dikes that may be correlated with the basal high-Ti series (HTS) lavas from those of the overlying low-Ti series (LTS) lavas. All dikes studied are nearly parallel to each other, with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center. Anisotropy of magnetic susceptibility measurements suggests that the HTS and LTS dikes intrude in fundamentally different ways. HTS dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber. LTS dikes show a bimodal, orthogonal set of intrusive directions, one shallow and one near vertical. Near-lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system, and near-vertical dike propagation directions are probably associated with feeder dikes to down-rift surface flows. Our study suggests that the types of dike intrusive behavior in the Troodos ophiolite may also be typical for "normal" mid-ocean ridges or other major shield volcanoes with well-developed rift zones.

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.

Varga, RJ, Gee JS, Staudigel H, Tauxe L.  1998.  Dike surface lineations as magma flow indicators within the sheeted dike complex of the Troodos Ophiolite, Cyprus. Journal of Geophysical Research-Solid Earth. 103:5241-5256.   10.1029/97jb02717   AbstractWebsite

Mesoscopic flow lineations and anisotropy of magnetic susceptibility (AMS) have been measured for dikes within the Cretaceous-age Troodos ophiolite with the goal of comparing the direction of initial magma now through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth. Dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite peudostratigraphy and ridge-and-groove structures termed hot slickenlines found throughout the complex. A unique now direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin. Significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or backflow of magma during the waning stages of intrusion. Surface lineations generally lie subparallel to the direction of flow inferred from AMS determinations on cores within 5 cm of dike margins. Surface lineations also lie subparallel to the long axis (epsilon(1)) of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from AMS cores. Correlation of surface lineations with interior indicators of flow (AMS, plagioclase trachytic texture) indicate that the surface features are good proxies for grain-scale magma flow directions during dike propagation in Troodos dikes. Orientations of surface flow features in the dikes of the Troodos ophiolite indicate an approximately equal mix of subhorizontal to near-vertical magma flow, contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers. Our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest.

Constable, CG, Tauxe L, Parker RL.  1998.  Analysis of 11 Myr of geomagnetic intensity variation. Journal of Geophysical Research-Solid Earth. 103:17735-17748.   10.1029/98jb01519   AbstractWebsite

We have conducted a detailed exploratory analysis of an II million year long almost continuous record of relative geomagnetic paleointensity from a sediment core acquired on Deep Sea Drilling Project Leg 73, at Site 522 in the South Atlantic. We assess the quality of the paleointensity record using spectral methods and conclude that the relative intensity record is minimally influenced by climate variations. Isothermal remanence is shown to be the most effective normalizer for these data, although both susceptibility and anhysteretic remanence are also adequate. Statistical analysis shows that the paleointensity variations follow a gamma distribution, and are compatible with predictions from modified paleosecular variation models and global absolute paleointensity data. When subdivided by polarity interval, the variability in paleointensity is proportional to the average, and further, the average is weakly correlated with interval length. Spectral estimates for times from 28.77 until 22.74 Ma, when the reversal rate is about 4 Myr(-1), are compatible with a Poisson model in which the spectrum of intensity variations is dominated by the reversal process in the frequency range 1-50 Mgr(-1) In contrast, between 34.7 and 29.4 Ma, when the reversal rate is about 1.6 Myr(-1), the spectra indicate a different secular variation regime. The magnetic field is stronger, and more variable, and a strong peak in the spectrum occurs at about 8 Myr(-1). This peak magi be a reflection of the same signal as recorded by the small variations known as tiny wiggles seen in marine magnetic anomaly profiles.

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.

Tauxe, L, Gee JS, Staudigel H.  1998.  Flow directions in dikes from anisotropy of magnetic susceptibility data: The bootstrap way. Journal of Geophysical Research-Solid Earth. 103:17775-17790.   10.1029/98jb01077   AbstractWebsite

One of the first applications of anisotropy of magnetic susceptibility (AMS) was an attempt to determine flow directions from mafic dikes [Khan, 1962]. Since the seminal work of Knight and Walker [1988] defining the expected behavior of AMS in response to magma flow, there has been increasing interest in using AMS for this purpose. Here we present a quantitative method for interpretation of AMS data from dikes, using a parametric bootstrap. First, dikes must be sampled with at least five land preferrably more) samples from within 10 cm of the dike margin. The distributions of the eigenvalues and eigenvectors of the AMS tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets. We generate paradata sets by first selecting a sample at random, then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site. The bounds containing 95% of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest. Classification of dikes proceeds as follows: Sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable. In addition, sites with principal eigenvectors with angles > 45 degrees away from the dike margin (inverse) or with markedly different directions on either side of the dike (scissored) are excluded. The remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors. If neither side has principal eigenvectors distinct from the dike plane, the dikes are classified as having lineation information only. A study comprising 251 dikes from the Troodos ophiolite has 151 sites with directional data, 38 sites with lineations only, 7 inverse sites, 5 scissored sites, and 55 sites not fitting into any other category. The flow directions interpreted from the data were generally southerly, toward a fossil transform zone.

Juarez, MT, Tauxe L, Gee JS, Pick T.  1998.  The intensity of the Earth's magnetic field over the past 160 million years. Nature. 394:878-881. AbstractWebsite

In contrast to our detailed knowledge of the directional behaviour of the Earth's magnetic field during geological and historical times(1,2), data constraining the past intensity of the field remain relatively scarce. This is mainly due to the difficulty in obtaining reliable palaeointensity measurements, a problem that is intrinsic to the geological materials which record the Earth's magnetic field. Although the palaeointensity database has grown modestly over recent years(3-5), these data are restricted to a few geographical locations and more than one-third of the data record the field over only the past 5 Myr-the most recent database(5) covering the time interval from 5 to 160 Myr contains only about 100 palaeointensity measurements. Here we present 21 new data points from the interval 5-160 Myr obtained from submarine basalt glasses collected from locations throughout the world's oceans. Whereas previous estimates for the average dipole moment were comparable to that of the Earth's present field(6), the new data suggest an average dipole moment of (4.2 +/- 2.3) x 10(22) A m(2), or approximately half the present magnetic-field intensity. This lower average value should provide an important constraint for future efforts to model the convective processes in the Earth's core which have been responsible for generating the magnetic field.

Flynn, JJ, Tauxe L.  1998.  Magnetostratigraphy of the Upper Paleocene-Lower Eocene marine and terrestrial sequences . Late Paleocene-Early Eocene events. ( Aubry MP, S.G L, Berggren WA, Eds.).:67-90.: Columbia University Press Abstract
Tauxe, L.  1998.  Paleomagnetic principles and practice. Modern approaches in geophysics. :xi,299p.., Dordrecht ; Boston: Kluwer Academic Publishers Abstract
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.

Barton, C, Kono M, Tauxe L, Wei QY.  1997.  Untitled - Preface. Journal of Geomagnetism and Geoelectricity. 49:471-471. AbstractWebsite
Constable, C, Tauxe L.  1996.  Towards absolute calibration of sedimentary paleointensity records. Earth and Planetary Science Letters. 143:269-274.   10.1016/0012-821x(96)00128-8   AbstractWebsite

Using relative paleointensity estimates derived from twelve globally distributed pelagic sediment cores, we assess whether they record a signal consistent with that expected from a dominant geocentric axial dipole, The cores span the Matuyama-Brunhes boundary and we normalize the observations by supposing that at the time the direction reverses the intensity low reflects only the non-axial-dipole contribution to the field. We further assume that this non-axial-dipole contribution to the field is invariant with geographic location. From absolute paleointensity compilations we estimate its size to be about 7.5 mu T; this supplies the calibration for the axial dipole signal away from the extreme low in intensity, The data predict the dipole field variation with latitude with similar accuracy to that observed in absolute paleointensity records, and show similar behavior when transformed to virtual axial dipole moments.

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.

Tauxe, L, Mullender TAT, Pick T.  1996.  Potbellies, wasp-waists, and superparamagnetism in magnetic hysteresis. Journal of Geophysical Research-Solid Earth. 101:571-583.   10.1029/95jb03041   AbstractWebsite

Because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material, much can be learned by monitoring that response through what is known as a ''magnetic hysteresis loop''. The measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research. Yet the interpretation of hysteresis loops is not simple. It has become apparent that although classic ''single-domain'', ''pseudo-single-domain'' and ''multidomain'' loops described in textbooks occur in natural samples, loops are frequently distorted, having constricted middles (wasp-waisted loops) or spreading middles and slouching shoulders (potbellies). Such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions. The physics of the problem have been understood for nearly half a century, yet numerical simulations appropriate to geological materials are almost unavailable. In this paper we discuss results of numerical simulations using the simplest of systems, the single-damain/superparamagnetic (SD/SP) system. Examination of the synthetic hysteresis loops leads to the following observations: (1) Wasp-waisting and potbellies can easily be generated from populations of SD and SP grains. (2) Wasp-waisting requires an SP contribution that saturates quickly resulting in a steep initial slope, and potbellies require low initial slopes (the SP contribution approaching saturation at higher fields). The approach to saturation is dependent on volume hence the cube of grain diameter. Therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size. (3) We were unable to generate potbellies using a SP/SD threshold size as large as 30 nm, and wasp waists cannot be generated using a threshold size as small as 8 nm. The occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm (+/- 5). (4) Simulations using a threshold size of 15-20 nm with populations dominated by SP grain sizes, that is with a small number of SD grains, produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass. (5) Simulations and measurements using two populations with distinct coercivity spectra can also generate wasp-waisted loops. A relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp-waisting resulting from SP/SD behavior.

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.

Harris, JD, Johnson KR, Hicks J, Tauxe L.  1996.  Four-toed theropod footprints and a paleomagnetic age from the Whetstone Falls Member of the Harebell Formation (Upper cretaceous: Maastrichtian), northwestern Wyoming. Cretaceous Research. 17:381-401.   10.1006/cres.1996.0024   AbstractWebsite

The Harebell Formation is a syntectonic sequence of conglomeratic sediments deposited in a narrow, rapidly subsiding trough that formed in the latest Cretaceous along the eastern margin of the ancestral uplift of what are today the Teton and Gros Ventre Mountains of northwestern Wyoming. On at least two occasions subsidence temporarily exceeded the rate of sediment supply and the area was flooded by a brackish or marine incursion from the Western Interior Seaway that lay to the east. The age of the Harebell Formation is Maastrichtian, corroborated by K-40/Ar-40 isotropic ages, vertebrate and palynomorph biostratigraphy, and a preliminary magnetostratigraphic analysis which correlates it to the geomagnetic reversal time scale from the upper part of C31R to the base of C30N. Sandstone slabs collected from the lower Whetstone Falls Member contain nine partial and complete footprints attributable to a theropod (Dinosauria: Saurischia). The footprints were formed as surface tracks in the tabular-bedded sandstone by dinosaurs that roamed the burrowed and leaf-littered sand flats and shallow waters along the margins of a low-energy, brackish-water embayment. Eight of the nine footprints represent a hitherto unknown ichnogenus, representing a four-toed pedal morphology for a theropod dinosaur which is unprecedented in the Late Cretaceous. The theropod nature of the tracks is implied by the length and narrowness of the digits and the sharp claw impressions. The tracks have clearly defined impressions of four toes, none of which appears to be a hallux in the traditional theropod sense of a small, retroverted hallux. The metapodial impression is also unlike that of other known theropod tracks: greater in relief than the digits but quite small in area. The tracks represent at least two individuals, although no clear trackways are available. Exallopus lovei, gen. et sp. nov., represents a type of theropod not currently recognized from body fossils. (C) 1996 Academic Press Limited

Kok, YS, Tauxe L.  1996.  Saw-toothed pattern of relative paleointensity records and cumulative viscous remanence. Earth and Planetary Science Letters. 137:101-108.   10.1016/0012-821X(95)00210-4  
Behrensmeyer, AK, Potts R, Plummer T, Tauxe L, Opdyke N, Jorstad T.  1995.  The Pleistocene Locality of Kanjera, Western Kenya - Stratigraphy, Chronology and Paleoenvironments. Journal of Human Evolution. 29:247-274.   10.1006/jhev.1995.1059   AbstractWebsite

Kanjera is well known as the source of controversial hominid fossils collected by L. S. B. Leakey in the 1930s. Since 1935, the context of fossils and artifacts from the locality has been in doubt, due to a claim that sediment slumping had commingled materials from stratigraphic units of different ages. A careful re-examination of the geology demonstrates that the Kanjera deposits consist of approximately 37 m of volcaniclastic, fluvial, mudflat and lacustrine sediments that we assign to three major units: the Kanjera Formation, the Apoko Formation, and the Black Cotton Soil, Outcrops cover approximately 2 km(2) in two adjacent areas, the Northern and Southern Exposures. Fossils and artifacts are found in primary contexts through much of the stratigraphic column; and extensive trenching failed to reveal any sediment slumping that would have disturbed these contexts. Faulting, rapid lateral facies changes, and an erosional unconformity between the Kanjera and Apoko Formations result in complex geological relationships. Magnetostratigraphic and faunal determinations indicate that the Kanjera Formation is approximately 1.5-0.5 Ma, the Apoko Formation younger than 0.5 Ma, and the Black Cotton Soil latest Pleistocene to Holocene. The hominid sample is derived from the Black Cotton Soil except for Leakey's Hominid 3, which probably was an intrusive burial into Kanjera Formation Bed KN-2. The Theropithecus oswaldi type specimen originated from KN-2a and is dated between 1.1 and at most 1.76 Ma. The Kanjera Formation provides the youngest known records of Metridiochoerus andrewsi and Deinotherium bozasi at about 1.0 Ma. (C) 1995 Academic Press Limited

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.

Hicks, JF, Obradovich JD, Tauxe L.  1995.  A New Calibration Point for the Late Cretaceous Time-Scale - the Ar-40/Ar-39 Isotopic Age of the C33r/C33n Geomagnetic Reversal from the Judith River Formation (Upper Cretaceous), Elk Basin, Wyoming, USA. Journal of Geology. 103:243-256. AbstractWebsite

We present a new calibration point for the latest Cretaceous time scale, an interval that at present contains no well-dated polarity reversals. The magnetostratigraphy of Campanian-aged sediments in the northern Bighorn basin of Wyoming documents a geomagnetic reversal in the upper part of the Judith River Formation that occurs in direct association with ash fall layers dated by the Ar-40/Ar-39 method. Extrapolation based on three new Ar-40/Ar-39 laser fusion dates from sanidines extracted from the ash fall layers dates the reversal at 79.34 Ma; thus the geomagnetic reversal in the Judith River formation can be confidently correlated to the C33r/C33n boundary of the Geomagnetic Reversal Time Scale. This age estimate is in good agreement with recent time scales that date this polarity interval by interpolation between few and widely spaced calibration points. The Judith River Formation is nonmarine and cannot be correlated regionally by traditional ammonite biostratigraphic methods. This study shows that in northern Wyoming the formation was deposited from 80 to 79 Ma, and can be correlated both magnetostratigraphically and by isotopic age to the Campanian-aged ammonite zones of the marine record that range from the top of Baculites obtusus to the lower part of B. perplexus (early form). The Judith River is underlain by the Claggett Shale, which contains one of the ash fall levels of the Ardmore bentonite. This ash fall is an important datum level found throughout the Western Interior U.S. in the range zone of B. obtusus, one we have dated at 80.71 +/- 0.55 Ma.

Hartl, P, Tauxe L, Herbert T.  1995.  Earliest Oligocene Increase in South-Atlantic Productivity as Interpreted from Rock Magnetics at Deep-Sea Drilling Project Site-522. Paleoceanography. 10:311-325.   10.1029/94pa03150   AbstractWebsite

The magnetic properties of the sediments (''rock magnetics'') at DSDP Site 522 in the South Atlantic exhibit clear differences between the latest Eocene and earliest Oligocene. Based on low temperature behavior of saturation remanence and hysteresis loops, we attribute the difference to a slightly greater proportion of the finest grained, so-called ''superparamagnetic'' magnetite in the Eocene sediments. We believe that the lower proportion of very fine-grained magnetite in the Oligocene sediments is a result of incipient reduction diagenesis caused by increased productivity and hence increased labile organic carbon transport to the sediments due to an early Oligocene increase in thermohaline circulation. The Eocene-to-Oligocene transition at Site 522 is also expressed by changes in microfossil assemblages, increased carbonate content, decreased insoluble residue, and decreased foraminiferal shell fragmentation. The increase in carbonate is synchronous with and parallels a change in the ratio of two of the rock magnetic parameters, a ratio that tracks the decrease in the very fine-grained magnetite component. Also parallel to these is a trend toward heavier delta(13)C values in foraminiferal tests. The increase in organic carbon transport to the sediments led to chemical dissolution of the finest grain-size fraction of magnetite in the Oligocene sediments, hence a reduction in the superparamagnetic component and the change in the rock magnetic ratio. In this way, rock magnetics can be sensitive indicators of environmental changes, such as fluctuations in organic carbon transport, which may leave little other trace in the sedimentary record.

Pick, T, Tauxe L.  1994.  Characteristics of Magnetite in Submarine Basaltic Glass. Geophysical Journal International. 119:116-128.   10.1111/j.1365-246X.1994.tb00917.x   AbstractWebsite

We have compared the rock-magnetic characteristics of Cretaceous submarine basaltic glass (SBG) from DSDP/ODP Holes 417D, 418A, 807C and 543A to data from Holocene glasses from the East Pacific Rise. Both groups are very similar in that they contain single-domain, low-Ti magnetite as the carrier of a strong and stable remanent magnetization. The behaviour of magnetic hysteresis in SBG is, however, dominated by superparamagnetic grain-size populations. Transmission electron microscopy revealed spherically shaped grains of 10-20 nm diameter with crystal lattice properties consistent with magnetite. These grains could be responsible for the magnetic behaviour of SBG.

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.

Tauxe, L, Gee J, Gallet Y, Pick T, Bown T.  1994.  Magnetostratigraphy of the Willwood Formation, Bighorn Basin, Wyoming - New Constraints on the Location of Paleocene Eocene Boundary. Earth and Planetary Science Letters. 125:159-172.   10.1016/0012-821x(94)90213-5   AbstractWebsite

The lower Eocene Willwood Formation in the Bighorn Basin of Wyoming preserves a rich and diverse mammalian and floral record. The paleomagnetic behavior of the sequence of floodplain paleosols of varying degrees of maturation ranges from excellent to poor. We present a magnetostratigraphic section for a composite section near Worland, Wyoming, by using a set of strict criteria for interpreting the step-wise alternating field and thermal demagnetization data of 266 samples from 90 sites throughout the composite section. Correlation to the geomagnetic reversal time scale was achieved by combining magnetostratigraphic and biostratigraphic data from this section, from a section in the Clark's Fork Basin in northern Wyoming, and from DSDP Site 550, with the isotopic date determined on a tuff near the top of our section. Our correlation suggests that the Bighorn Basin composite section in the Worland area spans from within Chron C24r to near the top of Chron C24n, or from approximately 55 to 52 Ma. This correlation places the Paleocene/Eocene boundary within the vicinity of the base of the section. Cryptochron C24r.6 of Cande and Kent is tentatively identified some 100 m above the base of the section. The temporal framework provided here enables correlation of the mammalian biostratigraphy of the Bighorn Basin to other continental sequences as well as to marine records. It also provides independent chronological information for the calculation of sediment accumulation rates to constrain soil maturation rates. We exclude an age as young as 53 Ma for the Paleocene/Eocene boundary and support older ages, as recommended in recent time scales. The location of a tuff dated at 52.8 +/- 0.3 Ma at the older boundary C24n.1 is consistent with the, age of 52.5 Ma estimated by Cande and Kent and inconsistent with that of 53.7 Ma, from Harland et al.

Tauxe, L, Watson GS.  1994.  The Fold Test - an Eigen Analysis Approach. Earth and Planetary Science Letters. 122:331-341.   10.1016/0012-821x(94)90006-x   AbstractWebsite

We combine eigen analysis and parameter estimation techniques for a newly constituted, more versatile fold test. The method is automatic, requiring no assumptions about the polarity or distribution of data, and gives confidence limits on the degree of unfolding required to produce the tightest grouping of data. We illustrate the method using several published data sets that show the tightest data groupings before, after and during correction for bedding tilt. The latter case is usually ascribed to acquisition of remanence during folding, but we show that this behavior can also arise from undetected multiple rotations. In our simulation, the beds undergo rotation about a vertical axis as well as a horizontal one, a case likely to occur in nature. These data, when rotated back to horizontal around what would be the observed strike, exhibit a peak in concentration at about 60% unfolding, very like the behavior of many published data sets. Thus, the origin of remanence in many such cases may not be syn-folding at all, but the behavior may purely be the result of an artifact of structural complications.

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.

Hartl, P, Tauxe L, Constable C.  1993.  Early Oligocene Geomatnetic-Field Behavior From Deep-Sea Drilling Project Site-522. Journal of Geophysical Research-Solid Earth. 98:19649-19665.   10.1029/93jb02019   AbstractWebsite

Hydraulic piston coring operations at Deep Sea Drillng Project site 522 in the South Atlantic retrieved an unusually continuous section of late Eocene to late Oligocene pelagic sediments, which we sampled at 3-4 cm intervals (approximately 3-5 kyr). Natural remanent magnetization demagnetization studies indicate a well-behaved remanence. Various rock magnetic procedures strongly suggest the magnetic carrier is dominated by pseudo-single domain magnetite appropriate for recording relative intensity variations of the paleomagnetic field. Nine zones of unusually low relative paleointensity were identified within the 2 my Chron C12R interval. Seven can be typified by a approximately 20-40 kyr interval of low field intensity accompanied by apparently random, low-amplitude, short-duration directional fluctuations. The other two are of approximately equal duration and intensity but exhibit an orderly progression of directional changes that result in well-defined virtual geomagnetic pole (VGP) paths confined along a preferred meridian of approximately 70-90-degrees-W longitude. We propose that both styles occur when the main dipole term diminishes significantly but that the former result when undimished ''normal'' secular variation is continuous during the period of low axial dipole moment. We propose that the other two lows in relative paleointensity, along with one reversal record, reflect a field structure of low axial dipole moment dominated by a low-degree nonzonal spherical harmonic term. Alternatively, the confined VGP paths could be an artifact of heavy remanence smoothing between nonantipodal, semistable transitional geomagnetic pole positions. Geographical control of VGP paths, particularly along approximately 70-90-degrees-W longitude, has recently been noted for much younger reversals. The site 522 record may indicate that the underlying cause of this phenomenon was present at 32 Ma. We compare our C12R record of paleointensity lows with C12R marine magnetic anomaly ''tiny wiggles''. These data appear to indicate that C12R tiny wiggles resulted from periods of low geomagnetic field intensity that were sometimes accompanied by directional excursions.

Gee, J, Staudigel H, Tauxe L, Pick T, Gallet Y.  1993.  Magnetization of the La Palma Seamount Series - Implications For Seamount Paleoples. Journal of Geophysical Research-Solid Earth. 98:11743-11767.   10.1029/93jb00932   AbstractWebsite

Paleopoles determined from seamount magnetic anomalies constitute the major data source for the Pacific apparent polar wander path, but relatively little is known about the processes of remanence acquisition in seamounts. Since magnetic anomalies reflect both natural remanence (NRM) and the induced field, it is important first to assess whether the NRM is likely to represent an original field direction and second to constrain the magnitude of the induced component. To this end, we present paleomagnetic data from an uplifted, subaerially exposed section through a seamount on La Palma, Canary Islands. The Pliocene Seamount Series of La Palma comprises a >6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts. The structural tilt of the Seamount Series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting (e.g., viscous remanence). The NRM provides a poor indication of the original magnetization direction, although the characteristic magnetization of many La Palma samples is compatible with the original pretilt direction. Hydrothermal alteration has resulted in the production of Ti-poor magnetite and an increasing contribution of hematite with increasing degree of alteration. More importantly, well-defined magnetization directions which deviate from any reasonable geomagnetic direction at La Palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization. Based on a comparison of the magnitude of low-stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization, we conclude that viscous and induced magnetization probably account for 15-25% of the total magnetization of seamounts. The resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4-degrees to 16-degrees for Cretaceous seamounts in the Pacific.

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.

Pick, T, Tauxe L.  1993.  Geomagnetic palaeointensities during the Cretaceous normal superchron. Nature. 366:238-242.   10.1038/366238a0   Abstract

High-quality palaeointensity data have been obtained from Thellier-Thellier experiments on recent and Cretaceous submarine basaltic glasses. Whereas the recent samples faithfully yield today's geomagnetic intensity at the site, the palaeointensities for the beginning and end of the Cretaceous normal superchron are only 45% and 25%, respectively, of today's value. The data thus extend the 'Mesozoic dipole low' into the Cretaceous superchron, and confirm that submarine basaltic glass is an excellent material for palaeointensity studies.

Gallet, Y, Gee J, Tauxe L, Tarduno JA.  1993.  Paleomagnetic analyses of short normal polarity magnetic anomalies in the Matuyama Chron. Proceedings of the Ocean Drilling Program, Scientific Results. 130:547-559.   10.2973/   Abstract

We document three short normal intervals in the natural remanent magnetization of sediments within the Matuyama Chron. These three anomalous zones of magnetization between the Jaramillo and Olduvai subchrons were identified from continuous measurements of archive halves from Hole 803 A using the pass-through 2G cryogenic magnetometer at Scripps. The U-channel samples were taken from the three intervals, analyzed using the pass-through system, and then cut into discrete 1 -cm-thick samples. Measurements on discrete samples confirmed the presence of the upper normal polarity zone. Based on sedimentation rate calculations, this zone is confidently correlated with the Cobb Mountain Subchron. For the two other anomalous zones, complete thermal demagnetization revealed a high-stability component (250°-575°C) of reversed polarity. The intensity of the low-stability normal polarity component, normalized by susceptibility, remains roughly constant throughout the entire interval sampled, whereas the intensity of the high-stability reversed component is much lower within the normal zone than outside. We interpret these two normal zones, then, as periods of low (reversed polarity) geomagnetic field intensity resulting in low magnetization of the sediments; the periods of these low magnetization reversed polarity zones are completely masked by the component acquired viscously in a normal polarity field.

Staudigel, H, Gee J, Tauxe L, Varga RJ.  1992.  Shallow Intrusive Directions of Sheeted Dikes in the Troodos Ophiolite - Anisotropy of Magnetic-Susceptibility and Structural Data. Geology. 20:841-844.   10.1130/0091-7613(1992)020<0841:sidosd>;2   AbstractWebsite

Sheeted dikes play a central role in the formation of oceanic crust. It is commonly assumed that sheeted dikes intrude vertically upward, from elongated mid-ocean ridge (MOR) magma chambers, but there are no direct observational data bearing on this hypothesis. This assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes. We have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust, the Troodos ophiolite. Structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow (<20-degrees) to nearly vertical, upward magma-transport directions. These data suggest that horizontal emplacement has to be considered for sheeted dikes at MORs, implying more centralized MOR plumbing systems than previously thought. Such plumbing systems provide ample opportunity for complex mixing, fractionation, and contamination of MOR lavas in magma chambers and tabular magma-storage volumes. Whether the MOR magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems.

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.

Friedman, R, Gee J, Tauxe L, Downing K, Lindsay E.  1992.  The Magnetostratigraphy of the Chitarwata and Lower Vihowa Formations of the Dera-Ghazi-Khan Area, Pakistan. Sedimentary Geology. 81:253-268.   10.1016/0037-0738(92)90074-2   AbstractWebsite

Three sections of the Chitarwata and lower Vihowa formations were sampled along the Dalana River on the southeastern flank of the Zinda Pir Dome in the southern Sulaiman Range near Dera Ghazi Khan, Pakistan. Together they provide nearly 800 m of a continuous sedimentary record of the Miocene derived from the uplifted Himalayan highlands. Previous studies have examined the Middle and Upper Miocene sediments, the Siwalik Group, of the Potwar Plateau to the north. However, detailed investigations of earlier periods are impossible in that area due to the absence of Oligocene and Lower Miocene sediments caused by continued overthrusting associated with the Himalayan orogeny. Fortunately, the Sulaiman basin to the south, which was further removed from the tectonic activity, provides a record of the Early Miocene in the form of the Chitarwata Formation. The Dalana A, B, and C sections (DGA, DGB, and DGC) were examined and sampled for this study. A magnetostratigraphic analysis was carried out to correlate and date the Chitarwata and Vihowa formations exposed in this area. The samples were subjected to step-wise demagnetization to resolve the primary and secondary remanent magnetization components. Thermal demagnetization trajectories indicate that the majority of samples have sufficient internal consistency that their calculated polarities are reliable. Likewise, the majority of the 126 measured sites showed statistically significant agreement between the three measured samples per site. However, the bimodal data set does not pass the reversal test and so is deemed unsuitable for tectonic interpretations. The individual sections were initially correlated using lithologic and stratigraphic methods, and the relationship was reinforced by the magnetostratigraphy. The composite DG section was tentatively correlated with the standard magnetic polarity time scale placing the disconformable base of the Chitarwata in the DGA section at just older than 22 Ma, the Chitarwata/Vihowa contact at 18.6 Ma, and the top of the Vihowa in the DGC section at about 16 Ma. The assignment of these dates to the Chitarwata will aid in future biostratigraphic and lithostratigraphic correlation of Early Miocene sediments, effectively extending the well-established Siwalik faunal sequence back by four million years.

Tauxe, L, Deino AD, Behrensmeyer AK, Potts R.  1992.  Pinning Down the Brunhes Matuyama and Upper Jaramillo Boundaries - a Reconciliation of Orbital and Isotopic Time Scales. Earth and Planetary Science Letters. 109:561-572.   10.1016/0012-821x(92)90114-b   AbstractWebsite

Until recently, the temporal calibration of the Plio/Pleistocene portion of the geomagnetic reversal time scale (GRTS) was based exclusively on K-Ar isotopic ages of volcanic rocks. The so-called "chronogram technique" estimates the age that minimizes discrepancies between a given age assignment for a reversal boundary and the population of dates of known polarity. The most widely quoted date, 0.73 Ma, for the Brunhes/Matuyama boundary was derived in such a manner [1]. Radioactive decay is not the only source of temporal information in the geologic record, however. Variations in several of the orbital parameters of the Earth (e.g. precession, obliquity, eccentricity) can be calculated for the last several million years and the climatic response approximated. A recent analysis of the so-called "astronomical technique" on oxygen isotopic data from deep-sea sediments suggested an age of 0.78 Ma for the Brunhes/Matuyama boundary [2], some 50,000 years older than the chronogram estimate. We reconcile the two estimates for the age of the last reversal by first presenting new magnetostratigraphic data tied to high-quality Ar-40/Ar-39 dates. Our date for the upper Jaramillo boundary is 0.992 +/- 0.039 Ma, considerably older than the estimate of 0.91 Ma in the standard (chronogram-based) time scale [3]. Furthermore, an age of 0.746 +/- 0.009 Ma was obtained for sediments immediately overlying the Brunhes/Matuyama boundary. These data are consistent with the astronomical estimates of 0.78 Ma and 0.99 Ma for the Brunhes/Matuyama and upper Jaramillo boundaries, respectively. Finally, we present a new method for estimating the uncertainties of ages calculated using the chronogram technique that employs a simple bootstrap resampling scheme. The 95% confidence region for the chronogram estimate of the Brunhes/Matuyama boundary, using an updated compilation of date/polarity pairs, spans from 0.73 to 0.78 Ma. We find, therefore, that the discrepancies between the isotopic data set and the astronomical calibration of the Brunhes/Matuyama boundary disappear when appropriate uncertainties are applied for the most recent compilation of isotopic data.

Tauxe, L.  1992.  Chemical Remanent Magnetization. Encyclopedia of the Earth System Science. 1:447-453. Abstract
Tauxe, L, Gallet Y.  1991.  A Jackknife for Magnetostratigraphy. Geophysical Research Letters. 18:1783-1786.   10.1029/91gl01223   AbstractWebsite

We investigate the problem of evaluating the reliability of a magnetostratigraphic section using a jackknife resampling scheme. We define a parameter, J, as the slope relating the percentage of polarity zones of a sampled section remaining after random deletion of sites increasing in proportion to a total of 20% of the initial collection. So defined, J quantifies the dependence of a magnetostratigraphic record on the distribution of sampling sites. Based on simulated magnetostratigraphic sections, J was found to be an indicator of the approximate percentage of polarity zones present in the original reference sections that were recovered in the sampled section. J will perhaps be most helpful as a guide to investigators as to whether further sampling would provide useful information as well as to the non-paleomagnetist user of black and white interpretive polarity logs as an estimate of the reliability of a given section.

Glass, BP, Kent DV, Schneider DA, Tauxe L.  1991.  Ivory-Coast Microtektite Strewn Field - Description and Relation to the Jaramillo Geomagnetic Event. Earth and Planetary Science Letters. 107:182-196.   10.1016/0012-821x(91)90054-l   AbstractWebsite

During the present study the Ivory Coast microtektite layer was found in cores from five equatorial Atlantic sites, bringing the total number of Ivory Coast microtektite-bearing cores to eleven. The strewn field appears to be restricted to between 9-degrees-N and 12-degrees-S latitude. There is a general increase in the concentration of microtektites towards the Bosumtwi crater, which is generally thought to be the source of the Ivory Coast tektites. The relationship between the onset of the Jaramillo subchron and the Ivory Coast microtektite layer has been investigated in six cores. A plot of the difference in depth between the base of the Jaramillo subchron and the microtektite layer versus sediment accumulation rate was used to determine the average post-depositional remanent magnetization (PDRM) acquisition depth and the age difference between the onset of the Jaramillo subchron and the deposition of the microtektites. Assuming that the PDRM acquisition depth does not vary with sediment accumulation rate, we find that the average PDRM acquisition depth is 7 cm and that the microtektites were deposited approximately 8 ky after the onset of the Jaramillo subchron. This indicates that the impact responsible for the Ivory Coast tektites and microtektites could not be causally related to the geomagnetic reversal at the base of the Jaramillo subchron.

Pick, T, Tauxe L.  1991.  Chemical Remanent Magnetization in Synthetic Magnetite. Journal of Geophysical Research-Solid Earth and Planets. 96:9925-9936.   10.1029/91jb00706   AbstractWebsite

As a magnetic grain produced by a chemical process below its blocking temperature grows through a critical volume in the presence of a magnetic field, its moment becomes blocked and it acquires a chemical remanent magnetization, CRM. Despite its importance to paleomagnetism, the properties of CRM and the controls on its behavior are still poorly understood. We have modified techniques developed by Taylor et al. [1987] to synthesize magnetite at room temperature and atmospheric pressure, whereby magnetite is produced by alteration of "green rust" under reducing conditions. The period of synthesis is twenty-four hours and a viscous component acquired over this amount of time can be demagnetized with alternating fields of 10 mT. The average direction of the CRM parallels the direction of the applied field and the intensity increases with increasing intensity of the applied field. The intensity is linearly related to the applied field up to approximately 1 mT, consistent with existing CRM theory developed by analogy to the acquisition of thermal remanent magnetization (TRM). Above about 1 mT, however, the CRM intensity approaches saturation at a rate substantially lower than that predicted by the TRM analog theory. Furthermore, the ratio of the CRM to an anhysteretic remanence acquired in the same field strongly depends on the intensity of the growth field. Analogy to TRM predicts this ratio to be independent of growth field. Both, the principal eigenvector of the anisotropy of isothermal remanence (AIR) matrix becomes more aligned with growth field and the percent anisotropy increases as the intensity of the growth field increases. The above observations suggest that the model of CRM acquisition based on analogy to TRM acquisition is incomplete. We therefore propose a revised model which incorporates the increased alignment of easy axes with growth field. Finally, the alignment of magnetic anisotropy ellipsoids of our synthetic samples with the growth field suggests a means to distinguish CRM from TRM in natural remanence.

Tauxe, L, Kylstra N, Constable C.  1991.  Bootstrap Statistics for Paleomagnetic Data. Journal of Geophysical Research-Solid Earth and Planets. 96:11723-11740.   10.1029/91jb00572   AbstractWebsite

The power and utility of paleomagnetic analyses stem largely from the ability to quantify such parameters as the degree of rotation of a rock body, or the orientation of an anisotropy axis. Until recently, estimates for uncertainty in these paleomagnetically determined parameters derived from assumptions concerning the underlying parametric distribution functions of the data. In many geologically important situations, the commonly used parametric distribution functions fail to model the data adequately and the uncertainty estimates so obtained are unreliable. Such essentials as the test for common mean require data sets consistent with a spherically symmetric underlying distribution; their application in inappropriate circumstances can result in flawed interpretations. Moreover, the almost universally used approximation for a cone of 95% confidence for the mean of a sample drawn from a Fisher distribution is quite biased even for moderate dispersions (kappa = 25). The availability of inexpensive, powerful computers makes possible the empirical estimation of confidence regions by means of data resampling techniques such as the bootstrap. These resampling schemes replace analytical solutions with repeated simple calculations. We describe a bootstrap approach for the calculation of uncertainties for means or principal directions of paleomagnetic data. The method is tested on means of simulated Fisher distributions with known parameters and is found to be reliable for data sets with more than about 25 elements. Because a Fisher distribution is not assumed, the approach is applicable to a wide range of paleomagnetic data and can be used equally well on directions or associated virtual poles. We also illustrate bootstrap techniques for the discrimination of directions and for the fold test which enable the use of these powerful tests on the wider range of data sets commonly obtained in paleomagnetic investigations.

Gee, JS, Tauxe L, Barge E.  1991.  Lower Jaramillo polarity transition records from the equatorial Atlantic and Indian oceans. Proceedings of the Ocean Drilling Program Scientific Results. 121:377-394.   10.2973/   Abstract

Two records of the geomagnetic polarity transition at the beginning of the Jaramillo Subchron (0.97 Ma) have been obtainedfrom sediments in the equatorial Atlantic (Leg 108, Site 665; 2.95°N, 340.33°E) and Indian (Leg 121, Site 758; 5.38°N, 90.35°E)oceans. Both cores yielded high-quality magnetostratigraphic results; however, the relatively low sedimentation rates, the weakmagnetizations, and complex demagnetization behavior of some transitional samples suggest that the record of the transitional fieldbehavior may be less reliable. In addition, variations in grain size preclude reliable paleointensity determinations although theremanence in both cores is apparently dominated by magnetite. Despite these possible complications, the two cores yield transitionalpaths that are neither far-sided nor near-sided. Together with published records that meet minimum reliability standards, the twoequatorial records presented here suggest that the lower Jaramillo transitional field morphology was significantly nonaxisymmetric.The mean normal and reversed inclinations from both cores deviate from the inclination expected from a geocentric axial dipole, asnoted in virtually all marine sediment cores. The observed inclinations provide further support for a polarity-dependent nondipolecontribution to the time-averaged field.

Constable, C, Tauxe L.  1990.  The Bootstrap for Magnetic Susceptibility Tensors. Journal of Geophysical Research-Solid Earth and Planets. 95:8383-8395.   10.1029/JB095iB06p08383   AbstractWebsite

In studies of the anisotropy of susceptibility or remanence of paleomagnetic samples it is conventional to specify the anisotropy in terms of the parameters of the anisotropy ellipsoids, namely the directions of the principal axes of the ellipsoid and their associated eigenvalues. Confidence intervals for these parameters have in the past often been estimated by using a linearization scheme to propagate the effect of small changes through the eigenvalue decomposition. The validity of these approximations is explored using a Monte-Carlo simulation from measurements that are presumed normally distributed, showing that there are circumstances in which the linearization scheme gives confidence intervals that are much too small. Q-Q plots indicate that the common assumption that the noise in the measurements is Gaussian does not always hold. Because of these shortcomings in the conventional technique we propose using a bootstrap resampling scheme to find empirically the distribution of uncertainties in the results. Confidence intervals for the eigenvalues are found directly from their empirical distributions. For the principal axes, approximate elliptical regions of confidence on the unit sphere are parameterized in terms of the Kent or FB5 distribution. The number of modes observed in the distribution of eigenvalues obtained by bootstrapping is used to classify the shape of the susceptibility ellipsoid as spherical, oblate, prolate or triaxial. The empirical nature of the bootstrap technique allows the extension of the analysis of uncertainties to parameters derived from the principal susceptibilities, such as percentage anisotropy or shape factor.