Lisa Tauxe

Sedimentary records of relative paleointensity of the geomagnetic field: Theory and practice

Sediments have proved irresistible targets for attempts at determining the relative variations in the Earths 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 Earths 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 Earths 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.

Potbellies, wasp-waists, and superparamagnetism in magnetic hysteresis

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-domain/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 an 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.

Long-term variations in palaeointensity

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 × 1022 A m2) is dramatically higher than the average dipole moment over the preceding 300 Myr (ca.5 × 1022 A m2). 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.

A simplified statistical model for the geomagnetic field and the detection of shallow bias in paleomagnetic inclinations: Was the ancient magnetic field dipolar?

The assumption that the time-averaged geomagnetic field closely approximates that of a geocentric axial dipole (GAD) is valid for at least the last 5 million years and most paleomagnetic studies make this implicit assumption. Inclination anomalies observed in several recent studies have called the essential GAD nature of the ancient geomagnetic field into question, calling on large (up to 20%) contributions of the axial octupolar term to the geocentric axial dipole in the spherical harmonic expansion to explain shallow inclinations for even the Miocene. In this paper, we develop a simplified statistical model for paleosecular variation (PSV) of the geomagnetic field that can be used to predict paleomagnetic observables. The model predicts that virtual geomagnetic pole (VGP) distributions are circularly symmetric, implying that the associated directions are not, particularly at lower latitudes. Elongation of directions is North-South and varies smoothly as a function of latitude (and inclination). We use the model to characterize distributions expected from PSV to distinguish between directional anomalies resulting from sedimentary inclination error and from non-zero non-dipole terms, in particular a persistent axial octupole term. We develop methodologies to correct the shallow bias resulting from sedimentary inclination error. Application to a study of Oligo-Miocene redbeds in central Asia confirms that the reported discrepancies from a GAD field in this region are most probably due to sedimentary inclination error rather than a non-GAD field geometry or undetected crustal shortening. Although non-GAD fields can be imagined that explain the data equally well, the principle of least astonishment requires us to consider plausible mechanisms such as sedimentary inclination error as the cause of persistent shallow bias before resorting to the very expensive option of throwing out the GAD hypothesis.

Strength of the geomagnetic field in the Cretaceous Normal Superchron: New data from submarine basaltic glass of the Troodos Ophiolite

[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.

Paleointensity of the geomagnetic field during the last 80,000 years

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.

Paleomagnetic chronology, fluvial processes and tectonic implications of the Siwalik deposits near Chinji village, Pakistan.

A 2800-m section of Siwalik strata containing the stratotypes for both the Chinji and Nagri formations has been dated by magnetic polarity stratigraphy, and the observed polarity zonation has securely been correlated with the Chron 17-7 segment of the time scale. The base of the section is the base of the Kamlial formation, which occurs near the top of Chron 17 (18.3 m.y.). The Kamlial-Chinji formation boundary occurs in the middle of Chron 15 (14.3 m.y.), the Chinji-Nagri boundary near the bottom of Chron 10 (10.8 m.y.), and the Nagri-Dhok Pathan boundary at the Chron 8-9 boundary (8.5 m.y.). The Siwalik deposits near Chinji Village consist of four distinct classes of fluvial cycles, each with a characteristic periodicity: a first order of

Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows

10^{7} years, a second order of 10^{6} years, a third order of 10^{4}-10^{5} years