Absolute Palaeointensity Research Articles

A reliable absolute palaeointensity determination obtained from a non-ideal recorder

Reliable measurements of the intensity of the palaeomagnetic field are notoriously difficult to obtain. The approach generally taken is to produce multiple estimates per rock unit and assume that those which meet certain minimum standards of technical quality will combine to produce an accurate mean. However, here, using results produced from a package of 20th century basaltic lava flows from Mount Etna in Sicily, we demonstrate that this approach can fail. In this case, the application of typical sets of selection criteria actually introduces bias into the mean determination that we measure. We demonstrate that this is caused by two types of non-ideal behaviour acting in combination. The first is a result of the multidomain grains that the samples contain and the second is caused by differences in the natural and laboratory cooling rates. We discuss means of avoiding these sources of error in future palaeointensity studies performed on ancient rocks. We also develop a new, more general reliability criterion which is effective here and which we argue should be applied wherever possible in future palaeointensity studies in conjunction with standard criteria. It requires two distinct types of materials which both produce some good-quality palaeointensity measurements and uses their range of overlap to constrain the true palaeointensity. Both the application of this criterion and general reliability considerations require that future palaeointensity studies should measure many samples per cooling unit and that these should be as diverse in terms of their rock magnetic properties and cooling histories as possible.

A comparison of a quasi-perpendicular method of absolute palaeointensity determination with other thermal and microwave techniques

We describe, test, and explain the relative effectiveness of six thermal and microwave techniques of absolute palaeointensity determination applied to lava samples from Mount Etna (Sicily) that acquired their thermoremanence (TRM) in a known geomagnetic field. The application of a single alternating field demagnetisation treatment prior to each measurement in a standard Thellier experiment is demonstrated to be beneficial: increasing the quality of the results and making the analysis simpler. A new quasi-perpendicular (QP) method dramatically improves the linearity of Arai plots by reducing or altogether eliminating non-ideal effects due to multidomain (MD) grains. Two broad types of sample were encountered by this study and the QP method is the only one capable of producing accurate measurements from them both. Its ease of use, speed, and increased reliability should allow it to replace standard double heating methods in studies such as these where laboratory alteration can be detected by other means than pTRM checks. The results of experiments performed using a microwave palaeointensity system provide further proof that the microwave (de)magnetisation process is equivalent to conventional thermal excitation. They also demonstrate that microwave experiments are subject to the same risks as conventional experiments when performed on samples containing MD grains. The behaviour of the samples in both the thermal and microwave experiments is in good agreement with a recently proposed model of MD TRM which is based on first-order symmetry of the remagnetisation process.

Absolute palaeointensity variation during a precursor to the Matuyama-Brunhes transition recorded in Chilean lavas

Following a detailed radioisotropic dating and palaeomagnetic investigation of transitional polarity lava flows of the Tatara-San Pedro volcanic complex (36°S, 71°W), in the Quebrada Turbia valley, Central Chile ( Brown et al., 2004), palaeointensity analysis has been carried out to complete a characterisation of the full geomagnetic field vector during a period of transitional geomagnetic field behaviour at 791.7 ± 3.0 ka, 16 ky before the full Matuyama-Brunhes transition (MBT). Seventy-one samples from 30 lava flows were analysed in total, with 34 found suitable for palaeointensity experiments. After conducting microwave palaeointensity experiments, 16 lava flows from two sections (2 km apart) yielded 31 successful individual palaeointensity determinations which record consistently weak field intensity values during the transitional directions (mean value of 5 ± 1.1 μT from 11 flows), excluding a flow immediately above a reversed polarity flow). Palaeointensity values are nearly an order of magnitude lower than the overlying normal polarity lavas (ranging from 30 to 47 μT), and half the values recorded in the underlying reversed polarity lavas (7 and 13 μT). These post-transitional field high palaeointensity values and comparisons with relative palaeointensity records from sedimentary rocks suggest that the Quebrada Turbia lavas may document a partial record of the MBT precursor and the latter stages of the MBT itself.

FENNOSTACK and FENNORPIS: Varve dated Holocene palaeomagnetic secular variation and relative palaeointensity stacks for Fennoscandia

A Holocene palaeomagnetic secular variation master curve (FENNOSTACK) and a relative palaeointensity curve (FENNORPIS) for Fennoscandia are presented. These curves were produced by stacking palaeomagnetic data obtained from six annually laminated (varved) lake sediment sequences and one non-laminated sediment sequence. The six independent varve chronologies were combined to form a timescale, which extends between the present year and 10,200 Cal yr BP. Smoothed inclination and declination curves show trends that are similar to a palaeomagnetic secular variation curve for the United Kingdom, except that the ages of most of the statistically significant features are a few hundred years younger in the Fennoscandian data set. These differences are most likely due to excessively old ages of the United Kingdom features produced by the radiocarbon dating of bulk sediments. The standardized relative palaeointensity stack shows that the intensity of the geomagnetic field in Fennoscandia was highest at 2300 Cal yr BP and lowest at 7000 Cal yr BP. Three palaeointensity peaks (PIPs) and three palaeointensity lows (PILs) are identified in FENNORPIS and indicate that the intensity of geomagnetic field varied significantly on millennial scales in this region during the Holocene, but there is much scatter in the reconstruction prior to 8000 Cal yr BP. Higher frequency palaeointensity features present in individual records are not significant in the stacked data set, which implies that these are a result of local environmental bias. Significant discrepancies exist between the data and the output of the CALS7K.2 geomagnetic field model for the last 7000 yr. In particular, the largest declination swing observed in the data at 2670 Cal yr BP (feature “f”) is not produced by the model, which is most likely due to the influence of an inaccurately dated Icelandic secular variation curve on the model result. Short term trends in standardized relative palaeointensity do differ from the model output, but it is difficult to make a comparison because of the relative nature of the sediment record. Better absolute palaeointensity data are needed to reduce uncertainties in reconstructions of past geomagnetic field strength.

Low geomagnetic field intensity during the Kiaman superchron: Thellier and microwave results from the Great Whin Sill intrusive complex, northern United Kingdom

Coe‐Modified Thellier (CMT) and Microwave (MW) paleointensity experiments were conducted on samples from the multi‐phase late Carboniferous (ca. 301 Ma) to early Permian (ca. 270 Ma) Great Whin Sill complex. Statistically comparable results were obtained with both techniques from high quality data. Forty three samples (25 with CMT and 18 with MW) yielded mean paleointensity values of 12.0 ± 5.8 μT and 16.5 ± 8.2 μT for the early and late intrusive phases respectively, corresponding to VDMs of (1.6 ± 0.8) × 1022 and (2.1 ± 1.1) × 1022 Am2. When combined with previous data, the results suggest a generally low dipole moment during the KRS, reducing from ∼75% of the present value at 301 Ma to 20–27% of the present value later in the superchron. These data imply either a decoupling between processes controlling reversal rate and field strength, or a more complex, intriguing time‐lag relationship. More absolute palaeointensity results are needed to further evaluate these alternatives.

Magnetic interaction analysis of basaltic samples and pre-selection for absolute palaeointensity measurements

For a successful palaeointensity experiment a sample should only consists of non-interacting single domain (SD) particles. As this cannot be completely guarantied for natural samples we applied two different interaction methods (ΔM plots and FORC-diagrams) on basaltic samples from Fernando de Noronha, Brazil, in order to determine the degree of interaction and estimate their use as a preselection tool for palaeointensity experiments. An independent classification of the samples in terms of SD/MD contribution and the occurrence of partial self-reversals is obtained by rockmagnetic means, Thellier-like experiments and continuous thermal demagnetization. The samples show a variable amount of mean interaction fields, which tends to be higher for samples with partial self-reversal. Nevertheless, high mean interaction fields can also be present in samples of successful palaeointensity experiments with high mean coercivities. This suggests, that the recording potential of a sample depends on the interplay of magnetic interaction and its mean coercivity, i.e. samples of higher mean coercivity resist higher mean interaction fields. As this is difficult to assess by normal rockmagnetic means, interaction measurements, especially the ΔM plot derived parameters ΔM 1 and 2, provide a fast tool for preselection.

An absolute palaeointensity record from SOH1 lava core, Hawaii using the microwave technique

Absolute palaeointensity results were obtained from 392 samples (166 of 241 flows) from the upper 779 m of the Scientific Observation Hole 1 (SOH1) basalt core, Kilauea volcano, Hawaii, using the microwave palaeointensity technique. Based on K–Ar dates and a previously published age model, this covers the last 45 ka thus giving a high-resolution record of geomagnetic intensity in Hawaii. Comparisons with other Hawaiian absolute palaeointensity data show similar trend of higher intensity than present day for the last 3 ka and then a general decrease in intensity over the last 45 ka, though this study reports consistently lower palaeointensity values. Direct comparison with previous Thellier data from the SOH1 core shows some discrepancies between the two methods, with the Thellier data yielding generally higher palaeointensity estimates than the microwave data. In view of these discrepancies between the two methods, the introduction of raw palaeointensity data into the public domain is suggested.

Further evidence for low intensity of the geomagnetic field during the early Cretaceous time: using the modified Shaw method and microwave technique

SUMMARY We report new absolute palaeointensity estimates using basalts from northeastern China (K/Ar age, 125‐120 Ma) using the modified Shaw method in conjunction with the microwave technique. Samples for the palaeointensity experiments were selected mainly based on their good reversibility of thermomagnetic curves and single primary magnetization characteristics. Using the modified Shaw method, 28 out of 45 measured samples from 10 cooling units give a virtual dipole moment of (3.1 ± 1.0) × 10 22 Am 2 , and the microwave technique using 14 acceptable determinations (out of 20 measured) give an average value of (2.9 ± 0.9) × 10 22 Am 2 . Results using both the modified Shaw method and the microwave technique demonstrate that the geomagnetic field strength recorded by these lavas was low. This is in agreement with previous results of the same time interval obtained by the Thellier method with partial thermal remanence (p-TRM) checks. The fact that different techniques give qualitatively compatible low palaeointensity results provides greater confidence that the weak field features seen just prior to the Cretaceous normal superchron (CNS) are the result of the actual field recorded by the basalts as opposed to artefacts of the method/analysis. This study also demonstrates that the microwave technique can be used for very old basalts.

A detailed palaeointensity and inclination record from drill core SOH1 on Hawaii

A new record of absolute palaeointensity was obtained from drill core Scientific Observation Hole 1 (SOH1) on Kilauea volcano, Hawaii. Kilauea’s high eruption rate resulted in a relatively continuous record and stratigraphic constraints preserved the chronological order. Three hundred and sixty samples were studied with the Thellier–Thellier technique, which gave 195 successful palaeointensity and 271 successful inclination determinations. Three geomagnetic excursions were observed, which exhibited intensity reductions of about 50%. Initial age control from K–Ar and Ar/Ar dating only constrained the total age between 20 and 120 ka. The final age model was obtained by stretching the SOH1 record relative to other Hawaiian palaeomagnetic data. This gave an age range of 0–45 ka for the flows and identified the excursions as the Hilina Pali, Mono Lake and Laschamp events. The SOH1 record of the Hilina Pali event is the most detailed ever, incorporating data from around 40 flows. This age model suggests that Kilauea had a burst of activity at the SOH1 site around 20 ka. All available data was combined to form a composite record of palaeointensity and inclination on Hawaii for 0–45 ka.

Detecting multidomain magnetic grains in Thellier palaeointensity experiments

The Thellier technique is the most commonly used method for absolute palaeointensity studies. It is based on Thellier’s laws stating that partial thermoremanent magnetisations (pTRMs) are independent and additive, which however only are true for non-interacting single domain (SD) grains. For multidomain (MD) grains, individual pTRMs are not independent and Thellier palaeointensity experiments on rocks with a significant proportion of MD grains may therefore lead to erroneous palaeofield estimates. By introducing a “pTRM-tail check” in the Coe version of the Thellier technique [J. Geophys. Res. 72 (1967) 3247], we present a simple method to detect and discard such rocks. The usefulness of the pTRM-tail check is exemplified by Thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy. Miocene baked sediments from French Massif Central are shown to be ideal for Thellier studies with the remanent magnetisation residing in thermally stable SD grains. For Palaeocene–Eocene basaltic lavas from Faeroe Islands, 64% of otherwise acceptable palaeointensity estimates are discarded due to the failure of pTRM-tail checks indicating the presence of significant concentrations of MD grains.

Investigating the ancient Martian magnetic field using microwaves

The new microwave palaeointensity technique has been used to investigate samples from the Martian meteorite Nakhla. This technique is a promising new way to obtain absolute palaeointensity information regarding the ancient Martian magnetic field as recorded by the Martian meteorites. Assuming that a part of the magnetic remanence is of thermal origin and originating on Mars the two samples studied yield estimates of 4 μT for the Martian magnetic field at 1.35 Ga.

Geomagnetic intensity variations over the past 780 kyr obtained from near-seafloor magnetic anomalies.

Knowledge of past variations in the intensity of the Earth's magnetic field provides an important constraint on models of the geodynamo. A record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials, and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences. But a long-term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust Here we show that near-seafloor magnetic anomalies recorded over the southern East Pacific Rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr. Moreover, the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well-documented dipole intensity pattern for the past 50 kyr. A comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region. The record of geomagnetic intensity preserved in the oceanic crust should provide a higher-time-resolution record of crustal accretion processes at mid-ocean ridges than has previously been obtainable.

Absolute palaeointensity results from the Trans-Mexican Volcanic Belt: implications for the late Miocene geomagnetic field strength

From a large collection (238 orientated cores) of Central Mexico Miocene (8–11 Ma) volcanic units sampled for magnetostratigraphy and tectonics, 36 samples were selected for Thellier palaeointensity experiments. Only 14 samples, coming from five individual lava flows, yielded reliable palaeointensity estimates, with the flow‐mean virtual dipole moments (VDMs) ranging from 5.6 to 8.5 × 1022 A m2. Our results, although not numerous, are of high technical quality and are comparable with other palaeointensity data recently obtained on younger lava flows. The NRM fractions used for palaeointensity determination range from 31 to 72 per cent, and the quality factors vary between 4.8 and 17.5, usually being greater than 6. The VDM values obtained are significantly higher than those recently reported from Miocene submarine basaltic glasses but are very similar to the mean reported from the Steens Mountain continental basaltic flows. More data are needed in order to better understand the behaviour of the Miocene geomagnetic field strength and to constrain the transition mode between the Mesozoic low and Neogene high field.

Palaeointensity of West Greenland Palaeocene basalts: asymmetric intensity around the C27n–C26r transition

Palaeomagnetic sampling of the lower part of the Palaeocene flood basalts of West Greenland, covering a composite succession of 1.6 km and close to 350 individual lava flows, revealed two polarity zones and a 200-m zone of intermediate directions. 40 Ar/ 39 Ar ages indicate that the transition is C27n–C26r. Following palaeodirectional and rock magnetic analysis, 102 samples from 64 flows from the pre-transitional normal polarity zone, the zone of intermediate directions, and the post-transitional reverse polarity zone were chosen for Thellier palaeointensity experiments. Altogether, 83 samples yielded reliable palaeointensity estimates. The mean intensity of the pre-transitional C27n field is 16.1±6.9 μT (17 flows), which is only twice the mean transitional field strength, 9.9±3.7 μT (10 flows). The post-transitional C26r field intensity is generally high with a mean 43.3±15.0 μT (21 flows). The asymmetric palaeointensity around the C27n–C26r transition with a high post-transitional field resembles the only other detailed absolute palaeointensity studies of geomagnetic transitions recorded in volcanic sequences in Hawaii (∼4 Ma) and Steens Mountain (∼15.5 Ma). These new data, therefore, suggest that a strong post-transitional field might be a systematic feature of geomagnetic reversals, probably indicating a prolonged period of an unusual state of the geodynamo following polarity transitions.

A new quality check for absolute palaeointensities of the Earth magnetic field

Absolute palaeointensity determinations of recent lavas from Mt. Etna have been carried out using the Koenigsberger‐Thellier‐Thellier (KTT) method but with two different data presentations. The first, which is the standard method, compares the natural remanent magnetization (NRM) demagnetized and the thermoremanent magnetization (TRM) acquired in successive temperature intervals cumulatively. The second compares the NRM and TRM in the successive intervals in an incremental manner. The latter method appears to distinguish aberrant behaviour better than the classical method. Such aberrant behaviour can be related to changes in hysteresis ratio parameters, saturation remanent magnetization to saturation magnetization and remanent coercive force to coercive force, which are reflected by displacements in the Day et al. (1977) plot. These changes in hysteresis parameters are caused by irreversible changes in domain state or transformations of magnetic phases brought about by the thermal cycling. The changes in hysteresis parameters serve as a useful quality factor for absolute palaeointensity determinations.