Andrew J. Biggin

The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

We firstly present the results of a detailed palaeointensity study performed on 54 samples from 9 volcanic units of late Archaean age (2724-2772 Ma) from the Pilbara Craton, Western Australia. These results were severely affected by magnetomineralogical alteration occurring during the laboratory heating process necessitating the application of a correction procedure. The correction allowed results from three lavas to pass strict selection criteria but we deem that only one of these exhibits sufficient internal consistency to be considered moderately reliable. It yields a virtual dipole moment of 47±6 ZAm2which is 60% of the present-day value. We combine this determination with a filtered dataset from the updated IAGA (International Association of Geomagnetism and Aeronomy) palaeointensity database, PINT08. Directional secular variation has recently been shown to have changed fundamentally since the Archaean, probably as a consequence of inner core growth since that time. However, here we argue that it is still unclear whether this evolution was accompanied by a single long timescale change in average poloidal field intensity. While the distribution of Precambrian palaeointensity determinations as a whole is significantly lower than that for the last 300 Myr, we show that this finding largely reflects data from the Proterozoic aeon. The distribution of more ancient measurements from the late Archaean-earliest Proterozoic is indistinguishable from that of the last 300 Myr which might suggest that a ‘Proterozoic dipole low’ period existed between two periods of higher field intensity. Were this pattern of long-term geomagnetic intensity variation to be supported by the addition of new data in the future, then it could indicate a related three-stage evolution in core dynamics, namely: vigorous thermal convection caused by high core-mantle heat flux early in the Earth’s history, weaker thermal convection later as the heat flux fell, and finally, strong compositional convection since the inner core nucleated.

On improving the selection of Thellier‐type paleointensity data

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

The application of acceptance criteria to results of Thellier palaeointensity experiments performed on samples with pseudo-single-domain-like characteristics

The results of a simulated Thellier palaeointensity experiment show that the low temperature portion of Arai plots produced by samples with pseudo-single-domain (PSD)-like hysteresis properties can produce significant inaccuracies despite passing conventional acceptance criteria. Furthermore, full Arai plots which fail these criteria can still produce reasonably accurate results. The evidence presented suggests that during a palaeointensity experiment: pTRM checks can fail despite no alteration occurring, pTRM tail checks are not reliable as indicators of MD-like behaviour, room temperature susceptibility monitoring is unreliable for detecting alteration, and that the minimum requirement for the f (fraction)-value should be increased from 0.15 to at least 0.5.

Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation

The Earth’s inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the Earth’s core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria. Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth.

Paleointensity Database Updated and Upgraded

The global absolute paleointensity (PINT) database, sponsored by the International Association of Geomagnetism and Aeronomy, recently was updated with new data and published alongside a new queryable interface at http://earth.liv.ac.uk/pint/. Absolute paleointensity determinations are spot readings of the strength of the geomagnetic field at the time and place at which a rock cooled down from elevated temperatures. The time generally can be estimated using an independent dating technique applied to the rock itself, and the place (or at least the paleolatitude) of the rock often can be estimated from the associated paleomagnetic direction.

Rapid regional perturbations to the recent global geomagnetic decay revealed by a new Hawaiian record

The dominant dipolar component of the Earth’s magnetic field has been steadily weakening for at least the last 170 years. Prior to these direct measurements, archaeomagnetic records show short periods of significantly elevated geomagnetic intensity. These striking phenomena are not captured by current field models and their relationship to the recent dipole decay is highly unclear. Here we apply a novel multi-method archaeomagnetic approach to produce a new high-quality record of geomagnetic intensity variations for Hawaii, a crucial locality in the central Pacific. It reveals a short period of high intensity occurring ~1,000 years ago, qualitatively similar to behaviour observed 200 years earlier in Europe and 500 years later in Mesoamerica. We combine these records with one from Japan to produce a coherent picture that includes the dipole decaying steadily over the last millennium. Strong, regional, short-term intensity perturbations are superimposed on this global trend; their asynchronicity necessitates a highly non-dipolar nature.

Microwave palaeointensities from a recent Mexican lava flow, baked sediments and reheated pottery.

Microwave palaeointensity (PI) estimates have been produced from samples from the 1670-yr old Xitle lava flow, Mexico. Seventeen out of 19 experiments were successful, producing high-quality estimates and a mean PI of 58.3 7 9.5 WT, within errors of that expected from global data for the time period. The dispersion is high but could be reduced to 5.5 WT by using stricter selection criteria. Previous data, obtained by the Thellier^Coe method using samples from the same lava flow, were less successful and of much lower quality, producing a higher mean of 67.9 7 9.8 WT (25 out of 65 samples). This difference is most probably caused by the presence of multi-domain particles producing concave-up Arai plots combined with alteration processes affecting the Thellier^Coe experiments at higher temperatures. PI calculations restricted to the initial part of an Arai plot therefore tend to overestimate the correct values. In addition to lava samples, microwave PIs were also determined from pottery fragments recovered from the contact zone between the lava flow and the underlying baked sediments, which were also studied. These materials also provided good-quality results, with a higher mean PI of 66.8 7 7.1 WT, which is still statistically indistinct from that produced by the microwave analysis of the lava samples. Nevertheless, these samples seem to be less suitable for PI determinations, because they are characterised by larger magnetic grains and apparently more prone to thermally induced alteration than the lava samples. We conclude that in the case of the Xitle lava the microwave PI method is superior to the conventional Thellier^Coe method in many respects and that the results produced by the latter method, even when satisfying strict acceptance criteria, may be unreliable. However, we also draw attention to the fact that microwave-produced PI determinations, though of high technical quality, may still be prone to inaccuracy when rock magnetic parameters indicate that the material is likely to be a poor PI recorder, as for the pottery, sediment, and some of the lava samples described here. = 2003 Elsevier B.V. All rights reserved.

Does the Mesozoic dipole low really exist

Documenting variations in the virtual dipole moment (VDM) of the geomagnetic field through geologic time has recently been the subject of considerable interest, not the least because a detailed and reliable VDM record would provide a key constraint for geodynamo modeling. Such a record would also inform the debate concerning issues related to Earths geodynamic evolution, such as how the geomagnetic field responds to stages in supercontinent cycling. The relationship between variations in geomagnetic polarity and field intensity with lowermost mantle (LMM) processes is fundamental, but the paucity of the VDM record has focused research on the links between polarity and LMM processes. Nonetheless, authors agree on the crucial need to produce a detailed VDM record in which we can place sufficient confidence to allow these major issues to be addressed [e.g., Biggin and Thomas, 2003; Heller et al., 2002; Riisager et al., 2002; Tarduno et al., 2001; Selkin and Tauxe, 2000].

A new set of qualitative reliability criteria to aid inferences on palaeomagnetic dipole moment variations through geological time

Records of reversal frequency support forcing of the geodynamo over geological timescales but obtaining these for earlier times (e.g. the Precambrian) is a major challenge. Changes in the measured virtual (axial) dipole moment of the Earth, averaged over several millions of years or longer, also have the potential to constrain core and mantle evolution through deep time. There have been a wealth of recent innovations in palaeointensity methods, but there is, as yet, no comprehensive means for assessing the reliability of new and existing dipole moment data. Here we present a new set of largely qualitative reliability criteria for palaeointensity results at the site mean level, which we term QPI in reference to the long-standing Q criteria used for assessing palaeomagnetic poles. These represent the first attempt to capture the range of biasing agents applicable to palaeointensity measurements and to recognise the various approaches employed to obviate them. A total of 8 criteria are proposed and applied to 312 dipole moment estimates recently incorporated into the PINT global database. The number of these criteria fulfilled by a single dipole moment estimate (the QPI value) varies between 1 and 6 in the examined dataset and has a median of 3. Success rates for each of the criteria are highly variable, but each criterion was met by at least a few results. The new criteria will be useful for future studies as a means of gauging the reliability of new and published dipole moment estimates.

A rapid multiple-sample approach to the determination of absolute paleointensity

[1] We present an alternative approach to absolute paleointensity determination, one which involves exactly five heatings, making possible rapid determinations without compromise to matters that insure reliability. While the Thellier-Thellier method involves a large number of temperature steps to validate a result for a particular specimen, the new approach makes use of the spatial variation in rock magnetic properties. The procedure involves the simultaneous thermal treatment of several subspecimens sampled from different regions throughout the igneous rock unit under investigation. For inclusion of data in a given determination, self-consistency criteria must be satisfied at the level of individual subsamples as well as at the stage of whole sample core consideration. The use of data taken en masse on a single Arai plot associated with samples from throughout a rock unit eliminates the need for further confirmation of an apparently successful result. The new method takes a balanced approach toward addressing the question of self-consistency, from intraspecimen to intersample, that we argue is preferable to the common practice of focusing attention primarily on the individual specimen with inadequate consideration paid to consistency throughout a rock unit.