Recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the Earth's past magnetic field at numerous field sites. The compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the Earth's geodynamo itself. A previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1, with the geomagnetic origin of the signal supported by an independent analysis of 10Be production made on different cores2. The persistence of long-term features in the Earth's magnetic intensity or the existence of long-term periodic changes cannot, however, be resolved in this relatively short time span. Here we present the integration of 33 records of relative palaeointensity3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19 into a composite curve spanning the past 800 kyr. We find that the intensity of the Earth's dipole field has experienced large-amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction, reflecting the emergence of non-dipole components. No stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the Earth's orbital parameters have a direct and strong influence on the geodynamo.

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Global changes in intensity of the Earth's magnetic field during the past 800kyr

Although it has been known for 25 years that some Paleozoic sedimentary rock units in Europe and North America were remagnetized during Pennsylvanian or Permian time, it is only very recently that the complex and widespread nature of the late Paleozoic remagnetization phenomenon has been generally acknowledged. It is now recognized that many Paleozoic paleomagnetic poles for North America that were once considered reliable are in fact the result of remagnetization, and as a consequence the paleomagnetic data base for the Paleozoic is undergoing rapid and drastic revision. The causes of late Paleozoic remagnetization in North America are currently the focus of much interest and active research. The remagnetization can reside in either hematite or magnetite, and different remagnetization mechanisms have been important in different settings. Chemical remagnetization processes, some related to specific diagenetic events, are dominant in hematite-bearing sandstones and carbonates. In magnetite-bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important, but it is difficult to determine which process is the dominant one in some settings. Some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain-building events that affected much of the continent during the late Paleozoic. Paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids.

The occurrence and origin of Late Paleozoic remagnetization in the sedimentary rocks of North America

Sedimentary magnetism, environmental magnetism, and magnetostratigraphy

[1] After many years spent by paleomagnetists studying the directional behavior of the Earth’s magnetic field at all possible timescales, detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components. A significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr. A second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field. A third step was the juxtaposition of these signals with those derived from cosmogenic isotopes. Contemporaneous with the acquisition of records, new techniques have been developed to constrain the geomagnetic origin of the signals. Much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials, proper selection of samples, and investigations of complex changes in magnetization during laboratory experiments. Altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 Myr. On longer timescales the field variability and its average behavior is relatively well documented for the past 400 Myr. Section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past. In each case, current limits and potential promises are discussed. The section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records. Preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features. Special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and/or materials.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001RG000104

Time variations in geomagnetic intensity

The top of Earth's liquid outer core is nearly 2900 km beneath Earth's surface, so we will never be able to observe it directly. This hot, dense, molten iron-rich body is continuously in motion and is the source of Earth's magnetic field. One of the most dynamic manifestations at Earth's surface of this fluid body is, perhaps, a reversal of the geomagnetic field. Unfortunately, the most recent polarity transition occurred at about 780 ka, so we have never observed a transition directly. It seems that a polarity transition spans many human lifetimes, so no human will ever witness the phenomenon in its entirety. Thus we are left with the tantalizing prospect that paleomagnetic records of polarity transitions may betray some of the secrets of the deep Earth. Certainly, if there are systematics in the reversal process and they can be documented, then this will reveal substantial information about the nature of the lowermost mantle and of the outer core. Despite their slowness on a human timescale, polarity transitions occur almost instantaneously on a geological timescale. This rapidity, together with limitations in the paleomagnetic recording process, prohibits a comprehensive description of any reversal transition both now and into the foreseeable future, which limits the questions that may at this stage be sensibly asked. The natural model for the geomagnetic field is a set of spherical harmonic components, and we are not able to obtain a reliable model for even the first few harmonic terms during a transition. Nevertheless, it is possible, in principle, to make statements about the harmonic character of a geomagnetic polarity transition without having a rigorous spherical harmonic description of one. For example, harmonic descriptions of recent geomagnetic polarity transitions that are purely zonal can be ruled out (a zonal harmonic does not change along a line of latitude). Gleaning information about transitions has proven to be difficult, but it does seem reasonable to draw the following conclusions with varying degrees of confidence. There appears to be a substantial decrease in the mean intensity of the dipole field during a transition to ∼25% of its usual value. The duration of an average geomagnetic polarity transition is not well known but probably lies between 1000 and 8000 years. Values outside these bounds have been reported, but we give reasons as to why such outliers are likely to be artifacts. The reversal process is probably longer than the manifestation of the reversal at Earth's surface as recorded in paleomagnetic directional data. Convection hiatus during a geomagnetic polarity transition seems unlikely, and free-decay models for reversals appear to be generally incompatible with the data. This implies that certain theorems in dynamo theory, such as Cowling's theorem, should not be invoked to explain the origin of reversals. Unfortunately, the detailed description of directional changes during transitions remains controversial. Contrary to common belief, certain low-degree nondipole fields can produce significant longitudinal confinement of virtual geomagnetic poles (VGP) during a transition. The data are currently inadequate to refute or verify claims of longitudinal dipole confinement, VGP clustering, or other systematics during polarity transitions.

Geomagnetic polarity transitions

A link between geomagnetic reversals and events and glaciations

Domain size, closure domains, and the importance of magnetostriction in magnetite

Black magnetic microspherules, commonly found in Cretaceous-Tertiary (K-T) boundary sediments and believed to be of an impact melt origin, were found to be responsible for a strong magnetic anomaly at the K-T boundary in Petriccio, Italy, and in almost all Deep Sea Drilling Project (DSDP) sediment cores investigated in which the K-T boundary was present. The enriched concentrations of magnetic minerals have been detected by measurements of the magnetic susceptibility which is in a good approximation proportional to the magnetic microspherule content. A rock magnetic characterization of extracted microspherules by hysteresis, Isothermal Remanent Magnetization acquisition, demagnetization and thermomagnetic measurements describes the magnetic minerals in the microspherules as ferrites with a near magnetite composition and ‘soft’, i.e. multidomain, magnetic behavior, in agreement with earlier nonmagnetic studies. On the contrary, volcanic ash from Hawaii has distinctively ‘harder’ magnetic properties, an important finding with regard to the proposed alternative, a volcanic origin of the well-known Ir-anomaly at the K-T boundary. It is suggested here that from magnetic susceptibility scanning of numerous cross-sections of the K-T boundary a distinct worldwide pattern might evolve that can potentially reveal the impact/volcano site because of a presumed higher microspherule concentration near the source. The advantage of mapping the microspherule distribution over the previous unsuccessful attempt using Ir-concentrations lies in the large size (hundreds of microns) of the microspherules with relatively short airborn life times while the iridium might have adhered to small dust particles that circled the globe several times before settling.

Rock magnetic signature of the Cretaceous‐Tertiary boundary

Experimental studies have demonstrated that demagnetization temperatures for removing thermoviscous magnetizations (TVRMs) in rocks are often higher than predicted by Pullaiah et al.'s time-temperature (t-T) nomogram based on Neel's single domain (SD) theory. Better agreement has been found for t-T relationships based on Walton's calculations invoking volume distributions. It has been pointed out, however, that Walton's formula applies for t-T conditions for acquisition of magnetization only, not to demagnetization. We applied numerical methods for assessing the true influence of distributions in SD volume. As it turns out, for reasonable volume distributions, the differences between our results and Pullaiah et al.'s simpler approach are marginal; the differences in demagnetization temperature are significant only for moderate-term low-temperature acquisition. Discrepancies between experiments and theory appear because rocks contain predominantly pseudo-single and multi-domain grains to which SD theory does not apply.

Theoretical time‐temperature relationships of magnetization for distributions of single domain magnetite grains

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/GL016i006p00595

Horst-Ulrich Worm

Papers

A link between geomagnetic reversals and events and glaciations

Domain size, closure domains, and the importance of magnetostriction in magnetite

Rock magnetic signature of the Cretaceous‐Tertiary boundary

Theoretical time‐temperature relationships of magnetization for distributions of single domain magnetite grains

Comment on “Can remanent magnetization in the deep crust contribute to long wavelength magnetic anomalies?” by Peter N. Shive