Palaeointensity Estimates Research Articles

Thermal rock magnetic cycling (TRMC): a method to track thermal alteration details for palaeointensity interpretations

SUMMARY Accurate absolute palaeointensity is essential for understanding dynamo processes on the Earth and other planetary bodies. Although great efforts have been made to propose techniques to obtain magnetic field strength from rock samples, such as Thellier-series methods, the amount of high-fidelity palaeointensities remains limited. One primary reason for this is the thermal alteration of samples that pervasively occurred during palaeointensity experiments. In this study, we developed a comprehensive rock magnetic experiment, termed thermal rock magnetic cycling (TRMC), that can utilize measurements of critical rock magnetic properties at elevated temperatures during multiple heating-cooling cycles to track thermal changes in bulk samples and individual magnetic components with different Curie temperatures in samples for palaeointensity interpretations. We demonstrate this method on a Galapagos lava sample, GA 84.6. The results for this specimen revealed that GA 84.6v underwent thermophysical alteration throughout the TRMC experiment, resulting in changes in its remanence carrying capacity. These findings were then used to interpret the palaeointensity results of specimen GA 84.6c, which revealed that the two-slope Arai plot yielded two linear segments with distinct palaeointensity values that were both biased by thermophysical alteration. To further test the TRMC method, we selected another historical lava sample (HS 2) from Mt Lassen, detecting slight thermal-physical changes after heating the specimen HS 2–8C to a target temperature of 400 °C. We also isolated a stable magnetic component with a Curie temperature below 400 °C using the TRMC method, which may provide a more reliable palaeointensity estimate of 51 μT. By providing a method for tracking thermal alteration independent of palaeointensity experiments, the TRMC method can explore subtle, unrecognizable thermal alteration processes in less detailed palaeointensity measurements, which can help to assess the thermal stability of the measured samples and interpret the changes in the TRM unblocking spectrum and palaeointensity estimates, facilitating the acquisition of more reliable records for constrain the formation of the inner core and the evolution of Earth's magnetic field.

Palaeomagnetic field intensity measurements from the 2.6 Ga Yandinilling dyke swarm (Western Australia)

SUMMARY Precambrian palaeointensity measurements provide fundamental constraints on the evolution of the deep Earth. Core evolution models predict trends in dipole moment on billion-year timescales that can be tested by palaeomagnetic records. Here, we report new palaeointensity results from the recently identified ∼2.62 Ga Yandinilling dyke swarm of the Yilgarn Craton, Western Australia, and consider them alongside published measurements spanning 500 Myr across the late Archaean to earliest Proterozoic. Rock magnetic and scanning electron microscopy analysis confirm that the magnetic mineralogy is fine-grained magnetite, appearing mostly as exsolved lamellae with ilmenite. Six sites produced acceptable palaeointensity estimates from thermal and microwave IZZI protocol Thellier experiments and from double-heating technique Shaw experiments. These site mean values of 9–26 µT translate to virtual dipole moments of 11–44 ZAm2 that are considerably lower than today's dipole moment of ∼80 ZAm2 and the value predicted for this time period by some thermal evolution models. Their average (median = 41 ZAm2) is, however, similar to the long-term average during both of the intervals 2300–2800 Ma (median = 44 ZAm2; N = 103) and 10–500 Ma (median 41 ZAm2; N = 997). While there is little evidence for a substantial net change in average dipole moment between the late Archaean and Phanerozoic, there is preliminary evidence that its variance has increased between the two intervals. This lower variance more than two billion years ago supports the idea that the geodynamo, even while not producing a stronger magnetic field, was more stable on average at the Archaean–Proterozoic transition than it is today.

Mesoproterozoic geomagnetic field strength from Nova Guarita mafic dykes (Amazon Craton)

Palaeointensity data from the Precambrian are key to understanding the timing of the Earth’s Inner Core Nucleation (ICN). Due to the scarcity of data, the ICN timing is still poorly constrained and is thought to have occurred between 2500 to 500 Ma. Numerical dynamo simulation models predict an increase in entropy, a stronger driving force for convection that could affect the field strength and show an anomaly in the palaeointensity record at ICN. We present new estimates of the geomagnetic field intensity (palaeointensity) from the Mid-Mesoproterozoic (1406 ± 1424 Ma) Nova Guarita dyke swarm, in the northern Mato Grosso State (SW Amazon Craton, Brazil). To obtain palaeointensity estimates, we used the non-heating Preisach method, with palaeointensity criteria at the specimen, and site level. Five sites provided accepted palaeointensity results, yielding virtual dipole moment (VDM) estimate of 65 ± 12 ZAm2 at 1416 ± 13 Ma, 53 ± 4 ZAm2 at 1418 ± 3 Ma, 12 ± 2 and 8 ± 2 ZAm2 at 1418 ± 5 Ma, and 71 ± 16 ZAm2 at 1424 ± 16 Ma, thus an average estimate of 43 ± 30 ZAm2 for ∼1410 Ma. The estimate is similar to the average VDM data (∼50 ZAm2), calculated for the period from 1600 to 1000 Ma. This average represents only a snapshot of the Earth’s magnetic field strength. While the new data are too limited in time to contribute directly to the question of ICN, they nevertheless contribute to constraints useful for assessing numerical simulations of the Mesoproterozoic geodynamo.

New palaeodirections and palaeointensity data from extensive profiles through the Ediacaran section of the Volyn Basalt Province (NW Ukraine)

SUMMARY Changes in the geomagnetic field over geologic timescales can be used to study the evolution and processes of its sources in Earth's deep interior. As a stark contrast to the geomagnetic field behaviour of the recent past, the field in the late Neoproterozoic is defined by ambiguous polar wander paths, ultralow field strengths and a period of extreme reversal hyperactivity. Palaeointensity data from this time are still scarce and conclusions made from these data suffer from large uncertainties brought by a low sampling density. In this study, we present new palaeomagnetic and palaeointensity data from quarry outcrops and five deep drill cores, covering the longest and most complete profiles through the Ediacaran units in the Volyn Basalt Province (NW Ukraine) to date. Palaeodirectional analysis of thermal and alternating field demagnetization experiments reveals at least six reversals in the profile as well as four palaeopoles that agree with previously published poles. Whole-rock palaeointensity estimates are derived from a multimethod approach that utilizes thermal and microwave Thellier experiments performed using Coe and IZZI protocols as well as Wilson and double-heating Shaw experiments. These produce instantaneous site-mean palaeointensity estimates in the range of 2.1 ± 0.4 to 11.1 ± 3.5 µT and virtual dipole moments in the range of 0.31 ± 0.06 × 1022 to 1.67 ± 0.53 × 1022 A m2. Small-scale variations of palaeointensities throughout the profile not only show extremely weak field strengths around polarity changes but could also suggest that the field strength never fully recovered between reversals. These new results increase the density of intensity data coverage for Baltica in the Late Ediacaran and, in combination with previously published Ediacaran dipole moments, support an extremely weak field that seems to increase in strength towards the Ediacaran–Cambrian transition.

Experimental test of the cooling rate effect on blocking temperatures in stepwise thermal demagnetization

SUMMARYUpon cooling, most rocks acquire a thermoremanent magnetization (TRM); the cooling rate at which this happens not only affects palaeointensity estimates, but also their unblocking temperatures in stepwise thermal demagnetization experiments, which is important, for example, to estimate volcanic emplacement temperatures. Traditional single-domain (SD) theory of magnetic remanence relates relaxation times to blocking temperatures— the blocking temperature is the temperature at which the relaxation time becomes shorter than the experimental timescale—and therefore strictly only applies to remanence acquisition mechanisms at constant temperatures (i.e. viscous remanent magnetizations, VRMs). A theoretical framework to relate (constant) blocking temperatures to (time-varying) cooling rates exists, but this theory has very limited experimental verification—partly due to the difficulty of accurately knowing the cooling rates of geological materials. Here we present an experimental test of this ‘cooling rate effect on blocking temperatures’ through a series of demagnetization experiments of laboratory-induced TRMs with controlled cooling rates. The tested cooling rates span about 1 order of magnitude and are made possible through (1) extremely accurate demagnetization experiments using a low-temperature magnetic properties measurement system (MPMS) and (2) the use of a ‘1-step-only’ stepwise thermal demagnetization protocol where the relaxation process is measured over time. In this way the relaxation time corresponding to the blocking temperature is measured, which can be done to much higher accuracy than measuring the blocking temperature directly as done in traditional stepwise thermal demagnetization experiments. Our experiments confirm that the cooling rate relationship holds to high accuracy for ideal magnetic recorders, as shown for a synthetic weakly interacting SD magnetoferritin sample. A SD-dominated low-Ti titanomagnetite Tiva Canyon Tuff sample, however, showed that natural samples are unlikely to be sufficiently ‘ideal’ to meet the theoretical predictions to high accuracy—the experimental data agrees only approximately with the theoretical predictions, which may potentially affect blocking temperature estimates in stepwise thermal demagnetization experiments. Moreover, we find a strongly enhanced cooling rate effect on palaeointensities for even marginally non-ideal samples (up to 43 per cent increase in pTRM for a halving of the cooling rate).

Redeposition experiments with natural sediments from the SE Black Sea in magnetic fields between about 2 and 114 µT

SUMMARY Diluted slurry with an initial density of 1.30 g cm−3 prepared from natural glacial Black Sea sediments was deposited under controlled field conditions between 1.72 and 114.21 µT. Compaction was achieved by evaporation of a portion of the pore water from the open sample boxes over 4–7 d until a density of about 1.44–1.55 g cm−3 was reached. Magnetizations M acquired in a magnetic field B follow a slightly non-linear function with M ∼ B0.82. Similar relationships were also obtained for palaeointensity estimates for the artificial samples by using normalization by low-field susceptibility, anhysteretic remanent magnetization and saturation magnetization. All samples are characterized by a magnetic fabric typical for natural sediments as checked by determining the anisotropy of magnetic susceptibility. The sediments’ anisotropy is in the range of 6–7 per cent and it is characterized by an oblate anisotropy ellipsoid with its small axis perpendicular to the deposition plane. According to scanning electron microscope imaging low-Ti magnetite particles present in the used sediments are in the range of 1–30 µm with the majority of particles 2–5 µm in size. Obtained results confirm the conclusion from other magnetostratigraphic studies that the Laschamps geomagnetic polarity excursion in the Black Sea area exhibited field values as low as 2.8 µT in its transitional phase between normal and reversed polarity.

A 4500 year record of palaeomagnetic secular variation and relative palaeointensity from the Tyrrhenian Sea

Abstract A marine sediment core from the western Mediterranean provides a new high-resolution 4500 year record of palaeomagnetic secular variation and relative palaeointensity. In 2013, the 7.1 m C5 core was recovered from the Tyrrhenian Sea as part of the NextData climate data project. The coring site, 15 km offshore from the Volturno river mouth, is well located to record combined marine and terrestrial palaeoclimatic influences, and the fine-grained, rapidly deposited sediments are effective palaeomagnetic recorders. We investigate the palaeomagnetic field direction and strength recorded in the core, which provide a valuable high-resolution record of Holocene geomagnetic variation in the area. Using rock magnetic techniques, we constrain the magnetic mineralogy of the studied sediments and confirm their suitability for palaeomagnetic analysis. Palaeomagnetic declination and inclination records were determined by stepwise alternating-field demagnetization, and relative palaeointensity estimates were obtained based on normalization to anhysterestic and isothermal remanent magnetization and to magnetic susceptibility. The age of the core is well constrained with a tephra and biostratigraphic age model, and its magnetic records are compared with relevant core and model data for the region, demonstrating that our record is compatible with previous results from the area. An automated curve matching approach is applied to assess the compatibility of our data with the existing secular variation path for the Mediterranean area.

Palaeo- and rock magnetic investigations of Late Quaternary sediments from the Upper Congo deep-sea fan: on the difficulty in obtaining palaeomagnetic secular variation records from low latitudes

We report here on results of palaeo- and rock magnetic investigations of two sediment cores from the Upper Congo deep-sea fan. The sediments have a high organic content and contain a heterogeneous Fe-mineral assemblage with biogenic magnetite and detrital (Ti-)magnetite as the main magnetic carrier minerals. Pyrite, hematite, and Fe-oxyhydroxides were identified by comparing high-temperature magnetic susceptibility curves with those from Fe-minerals of known composition. According to AMS 14C dates, the 6.8 m-long profile spans the last 37 kyr. Sediments older than 20 ka are affected by reductive diagenesis that has led to a loss of the fine-grained magnetic mineral fraction. Sediments younger than 20 ka have stable magnetizations. Characteristic remanent magnetization records of inclination and declination were obtained for each core. There is a little agreement between these records, modelled curves, and other sediment records from Equatorial Africa, so no composite record could be established. The cores are not ideal relative palaeointensity recorders and estimates using different normalizers did not yield consistent signals from both cores. Normalization methods used for relative palaeointensity estimation were not developed for sediments that contain large amounts of ultra-fine-grained biogenic magnetite; therefore, the relative palaeointensity estimates should be considered with caution. However, in view of the incoherent picture given by the scarce available palaeointensity information from the region off South-West Africa, the GeoB6517-2 record may provide a tentative relative palaeointensity record for comparison, at least for the past 10 kyr.

The multimethod palaeointensity approach applied to volcanics from Terceira: full-vector geomagnetic data for the past 50 kyr

The island of Terceira (38.5°N, 27.2°W) in the Azores Archipelago boasts a wealth of well-dated recent volcanic products; it is therefore the ideal location to improve the resolution of the full-vector geomagnetic field record for the Mid-Atlantic Ocean. We investigated 21 (sub-)sites sampled from 10 different cooling units; six of these cooling units are from the Holocene, the other four are approximately coeval with the age of known geomagnetic excursions. After successfully determining the palaeodirections from all but one site, we applied the multimethod palaeointensity approach to all sites. Here, we used three different palaeointensity methods: the IZZI-Thellier protocol, the Multispecimen technique, and the calibrated pseudo-Thellier method. For the Holocene sites four robust palaeointensity estimates were obtained, they mostly agree with data from the nearby island of Sao Miguel and data from the Iberian Peninsula and West Africa. Two older sites, dated at 25.7 and 46.3 ka, produced low palaeointensities: 24.4 and 24.1 μT, respectively. As both the IZZI-Thellier and pseudo-Thellier methods yielded technically acceptable results for these sites we used them to improve the pseudo-Thellier calibration relation and establish its applicability to lower palaeointensities. Sixty per cent of all sampled cooling units/ages in our study yielded a technically successful palaeointensity estimate; this unusually high success rate is attributed both to our sampling strategy and the implementation of the multimethod approach.

Anisotropy of magnetic susceptibility in alkali feldspar and plagioclase

Feldspars are the most abundant rock-forming minerals in the Earth’s crust, but their magnetic properties have not been rigorously studied. This work focuses on the intrinsic magnetic anisotropy of 31 feldspar samples with various chemical compositions. Because feldspar is often twinned or shows exsolution textures, measurements were performed on twinned and exsolved samples as well as single crystals. The anisotropy is controlled by the diamagnetic susceptibility and displays a consistent orientation of principal susceptibility axes; the most negative or minimum susceptibility is parallel to [010], and the maximum (least negative) is close to the crystallographic [001] axis. However, the magnetic anisotropy is weak when compared to other rock-forming minerals, 1.53 × 10−9 m3 kg−1 at maximum. Therefore, lower abundance minerals, such as augite, hornblende or biotite, often dominate the bulk paramagnetic anisotropy of a rock. Ferromagnetic anisotropy is not significant in most samples. In the few samples that do show ferromagnetic anisotropy, the principal susceptibility directions of the ferromagnetic subfabric do not display a systematic orientation with respect to the feldspar lattice. These results suggest that palaeointensity estimates of the geomagnetic field made on single crystals of feldspar will not be affected by a systematic orientation of the ferromagnetic inclusions within the feldspar lattice.

Palaeomagnetism of the upper volcanic supergroup, southern part of the Sierra Madre Occidental, Mexico

Ash flow tuffs, or ignimbrites have been recently proposed to be a good material for palaeointensity determination. In this paper, we present a multidisciplinary study, combining geochronology, petrology, rock magnetism and palaeomagnetism, carried out on Oligocene to Early Miocene ignimbrites and related flows from the southern part of the Sierra Madre Occidental. Two new 40Ar/39Ar ages were determined for ignimbrites; 20.4 ± 0.2 Ma (the youngest age obtained so far in this area) and 29.2 ± 0.5 Ma. Density measurements, as a proxy for welding, proved to be extremely useful to estimate the emplacement temperature and the origin of the magnetizations carried by the ignimbrites. After alternating field and thermal demagnetizations, the mean palaeomagnetic pole (Lat = 66.8°N; Long = 180.5°E; Kappa = 142; A95 = 6.3°), calculated for the period 28–31 Ma, is in close agreement with our only Miocene determination. Comparison with the North America Synthetic Apparent Polar Path indicates a net counter-clockwise vertical axis rotation of about 10 ± 4° compared to stable North America, which occurred likely during the last extensional episode in the Late Miocene (ca. 12–9 Ma). Palaeointensity estimates, obtained with the Thellier–Coe method, are mainly questionable and should not be used for global interpretation. Therefore, these ignimbrites are not a viable material for reliable palaeointensity determinations.

An evaluation of the influence of the experimental cooling rate along with other thermomagnetic effects to explain anomalously low palaeointensities obtained for historic lavas of Mt Etna (Italy)

Methodological aspects in obtaining reliable absolute palaeointensity estimates have attracted renewed attention in recent years. Obtaining a reliable palaeointensity from lavas, however, still is notoriously difficult: in many cases lavas have been shown to be a non-ideal recorder of the palaeofield. Here we assess the influence of various thermomagnetic effects on the outcome of palaeointensity experiments on a suite of recent lavas from Mt Etna. Several palaeointensity studies have targeted recent flows of Mt Etna before; often palaeointensity underestimates were found. We apply both Thellier- and multispecimen-style palaeointensity experiments and obtain estimates generally 20–35 per cent below the expected reference value—in line with previous studies. We explored subtle chemical alteration, viscous magnetic behaviour and differences between the natural and laboratory cooling rates used during the palaeointensity experiments as possible causes for the observed underestimates of the palaeofield. For the latter, a furnace with controlled, programmable cooling rates was constructed. It can cool samples very slowly, with rates that are in the same order of magnitude as natural cooling rates typical of Mt Etna lava flows (∼10 K/day). Both viscous magnetic behaviour and differences in cooling rate appear to influence the outcome of the palaeointensity experiments. However, the magnitude of the changes observed is not sufficient to explain the observed underestimates. We, therefore, hypothesize that the observed underestimates may be primarily explained by transdomain processes occurring during the heating steps of the palaeointensity experiments.

Assessing the reliability of Holocene relative palaeointensity estimates: a case study from Swedish varved lake sediments

We present palaeomagnetic secular variation and relative palaeointensity estimates from multiple sediment cores recovered from a varved lake sediment sequence in Kalksjon, a lake in central west Sweden. Alternating field demagnetization of discrete samples reveals natural remanent magnetizations dominated by a stable, single component. Magnetic grain size indicators-which include Day plots and first-order reversal curves-indicate that the main carrier of the natural remanence is single-domain magnetite throughout the varved sequence, which extends to similar to 9200 cal. yr BP. The millennial-scale features of the Kalksjon relative palaeointensity, inclination and declination data are well matched against the Fennoscandian master curves FENNORPIS and FENNOSTACK, and a global dipole moment reconstruction curve. There is evidence, however, of environmental bias in the relative palaeointensity estimates during specific time intervals. We find that bias is particularly pronounced during the early Holocene, specifically between similar to 9000 and 7800 cal. yr BP. During this period the bias is apparent as a series of relative palaeointensity cycles, with a wavelength of similar to 300 years. Some bias is also evident between 7800 and 3200 cal. yr BP. By comparing Kalksjon's smoothed relative palaeointensity data with the reconstructed geomagnetic field intensity from C-14 on timescales longer than 500 years-thereby removing the effect of relatively high frequency environmental bias in the early Holocene-we suggest that a distinct peak in intensity at centred on similar to 8700 cal. yr BP is a true feature of the geomagnetic field. A minimum at similar to 7400 cal. yr BP and a maximum at similar to 2500 cal. yr BP are also robust features. Our main conclusion is that sedimentary-based relative palaeointensity reconstructions must be carefully assessed if short-term, centennial variations are to be examined. (Less)

Evaluation of the domain-state corrected multiple-specimen absolute palaeointensity protocol: a test of historical lavas from Iceland

Palaeointensity estimates were made for a suite of five historical lava flows from Mt Askja, Iceland, with the purpose of testing the newly proposed domain-state corrections to the multispecimen parallel differential pTRM palaeointensity method (MSP), where pTRM stands for partial thermoremanent magnetization. Before beginning the experimental procedure, the chemical stability on heating to the determined pTRM induction temperature was assessed; some flows were found to be unaltered by heating, some displayed inconsistent repeatability, while one flow was severely affected by heating. To implement the domain-state corrections and interpret the data successfully, we had to implement two new approaches to the MSP method: (1) we used vector addition/subtraction in our calculations, and (2) we introduced a new selection/rejection criterion that improves the robustness of the estimates by preferentially removing outliers based on the error analyses of the linear regression. This rejection criterion typically rejected one or two points, and was found to significantly improve the estimates. With the exception of the one flow that displayed significant chemical alteration during the experiment, the palaeointensity estimates determined using the original MSP protocol yielded estimates within ±3 μT of the known field of 49.5 μT. The domain-state corrected MSP estimates were all within ±4 μT, with no clear relationship between the uncorrected and domain-state corrected palaeointensity estimates for this data set.

Multi-protocol palaeointensity determination from middle Brunhes Chron volcanics, Datong Volcanic Province, China

A full vector palaeomagnetic analysis of basalts from the Datong Volcanic Province, China, has been carried out to constrain the orientation and magnitude of the geomagnetic field during the mid-Brunhes Chron at approximately 0.41 Ma. Palaeomagnetic directions were determined using thermal demagnetization. Palaeointensity information was determined using a multi-protocol approach: (1) a Thellier-type protocol, (2) the multi-specimen parallel differential partial thermoremanence method, and (3) the Wilson method. The most consistent data set was obtained with the Wilson method, whilst similar average values, with wider data scatter, were obtained with the Thellier method. Higher values were obtained with the multi-specimen method. We suggest that consistency of results from the different protocols, which access different sections of the blocking/unblocking spectra, provide a useful test of the robustness of palaeointensity estimates. A mean virtual dipole moment of 5.4 ± 0.6 × 10 22 Am 2 was determined, which agrees well with coeval values from the PINT database and with the higher average palaeointensities for the mid-Brunhes Chron indicated by more continuous sedimentary relative palaeointensity records.

New palaeointensity data from Holocene Icelandic lavas

We present rock magnetic and palaeointensity data from Holocene Icelandic lava flows, including historical and pre-historical lavas. Susceptibility versus temperature measurements indicate that the magnetic mineralogy is dominated by primary titanomagnetite that have experienced variable degrees of high-temperature oxidisation. The analyses also demonstrate that virtually all studied samples are thermally stable up to their Curie temperatures. Thellier palaeointensity experiments were carried out on 89 samples from fourteen different lava flows (spanning the age of 167 to 9450 cal. years BP). Microwave Thellier experiments were used for one third of the samples, while conventional thermal Thellier was used for the remainder; the two methods produced similar palaeointensities. Altogether, reliable absolute palaeointensity estimates were obtained from all of the lava flows, with values ranging from 38.2 ± 5.0 μT to 117.9 ± 8.3 μT. The new results indicate that the Holocene geomagnetic field intensity was more dynamic than suggested by the current field models, possibly including brief intervals of anomalously high field strength.

Environmental influence on relative palaeointensity estimates from Holocene varved lake sediments in Finland

Relative palaeointensity records were reconstructed from two sediment cores obtained from two small lakes, Lake Lehmilampi and Lake Kortejärvi, in eastern Finland. The sediments are annually laminated, and they cover the last 5100 and 3700 years in Lake Lehmilampi and Lake Kortejärvi, respectively. Mineral magnetic investigations indicate uniform magnetic properties with magnetic mineralogy dominated by single and pseudo single domain magnetite and only small variations in the concentration of magnetic minerals, which makes them interesting targets for reconstructing relative palaeointensity. Three estimates of relative palaeointensity were obtained by normalising the natural remanent magnetisation after 30 mT alternating field (AF) demagnetisation (NRM 30mT) by (1) low-field magnetic susceptibility, (2) anhysteretic remanent magnetisation after 30 mT AF demagnetisation and (3) saturation isothermal remanent magnetisation imprinted at 1000 mT (SIRM 1000mT). Comparison of the palaeointensity estimates with relative mean X-ray density reflecting variations in sediment composition, suggested the normalised records were carrying an environmental imprint, which contaminated especially the centennial features. This interpretation was confirmed by coherence analyses, which also indicated that NRM 30mT/SIRM 1000mT from Lake Lehmilampi was the most efficiently normalised. Despite the inevitable environmental imprint, relative palaeointensity record NRM 30mT/SIRM 1000mT from Lake Lehmilampi shows similar millennial features in comparison with relative palaeointensity reconstructions from other lakes in the area, archaeomagnetic records and CALS7K.2 geomagnetic field model.

Are systematic differences between thermal and microwave Thellier-type palaeointensity estimates a consequence of multidomain bias in the thermal results?

A meta-analysis which directly compares 13 datasets of microwave and thermal Thellier palaeointensity measurements obtained from igneous rocks is reported. The results clearly indicate systematic differences in the measurements made using the two approaches: the thermal results tend to be higher (often by several tens of percent) and associated with slightly noisier Arai plots. Additionally, in nearly half of the cases considered, higher thermal palaeointensity estimates were reported to be calculated from the low-temperature portions of concave-up Arai plots while microwave measurements made on specimens from the same core sample or rock unit were derived from more linear Arai plots. The most plausible explanation for the bulk of these differences appears to be that non-ideal behaviour caused by multidomain, vortex state, and/or interacting ferrimagnetic grains is exaggerated in the results of the thermal experiments over their microwave counterparts. A detailed investigation of the individual studies concludes that such effects may be adversely affecting some of the measurements (thermal and/or microwave) in 10 of the 13 datasets considered. It is probable that these non-ideal effects are enhanced in the thermal results over their microwave counterparts, at least in part, because of the different Thellier-type protocols that are used in the two types of study. The findings of this study suggest that biasing of absolute palaeointensity estimates by multidomain effects may be much more prevalent than previously thought and that this effect might be responsible for certain discrepancies that have been observed in the GEOMAGIA50 database. They also clearly indicate that, in addition to taking the steps they already do to avoid biasing from secondary overprints and thermally-induced alteration, future absolute palaeointensity studies of any type must also take great care to avoid bias from these multidomain effects.

Revisiting a domain-state independent method of palaeointensity determination

The use of a domain-state independent method (the Wilson method) of acquiring absolute palaeointensities is tested using three sets of historical lavas from Mexico, Italy and Iceland. The Wilson method works by plotting continuous thermal demagnetisation curve data for the original natural remanent magnetisation against those for a laboratory-induced thermoremanence. By comparing the predicted intensities with the known fields, a new empirically derived rejection criterion is proposed that accounts for chemical alteration and improves the accuracy of the palaeointensity estimates. The Wilson palaeointensity estimates are compared with palaeointensity determinations made on sister samples using standard Thellier-type heating. Generally, the Wilson palaeointensity estimates compare favourably for samples that yielded technically correct Thellier-type determinations. However, at one locality the Wilson method did not yield any estimates due to severe chemical alteration, in contrast it was still possible to determine palaeointensity estimates from the lower temperature Thellier data. To assess the Wilson palaeointensity protocol's ability to return a correct estimate for a true multidomain system, a synthetic multidomain magnetite sample was imparted with a thermoremanent magnetisation. The Wilson protocol returned the correct palaeointensity within error, in contrast the Thellier-type protocol failed. As the Wilson method is significantly faster than Thellier-type methods, it is suggested that the Wilson method should be tried as a ‘first-approach’ in palaeointensity studies, or for samples dominated by multidomain material.

An assessment of the reliability of palaeointensity results obtained from the Cretaceous aged Suhongtu section, Inner Mongolia, China

Here we present microwave palaeointensity results from 89 sister samples from the study of Zhu et al. [Zhu, R., Pan, Y., He, H., Qin, H., Ren, S., 2008. Palaeomagnetism and 40Ar/ 39Ar age from a Cretaceous volcanic sequence, Inner Mongolia, China: implications for the field variation during the Cretaceous normal superchron. Phys. Earth Planet. Int., 169, 59–75] who carried out Thellier palaeointensity analysis as part of their integrated palaeomagnetic and 40Ar/ 39Ar dating study of Cretaceous lava from the Suhungtu section, Inner Mongolia, China. Additionally, a comprehensive rock magnetic investigation has been carried out in order to determine the mineralogy and hence the validity of assuming that the remanence is a thermal remanent magnetisation (TRM). The microwave results are of apparent high quality and give flow mean palaeointensity estimates ranging from 13 to 49 μT corresponding to virtual dipole moment (VDM) estimates ranging from 2.5 to 8.9 × 10 22 Am 2, and an overall mean VDM of 5.5 ± 1.9 × 10 22 Am 2 for the 24 flows (aged 110.6 ± 0.1 Ma). When the microwave results (using the perpendicular applied field method with partial microwave thermal remanence (pT MRM) and pT MRM tail checks) are compared to those obtained with the Thellier method (Coe version with pTRM but not tail checks, and heating in argon atmosphere) differences are seen at the sample, flow and palaeomagnetic unit level however, the overall means and spread in palaeointensity estimates are consistent. Some discrepancy is due to the differing sized sample sets and sample inhomogeneity but discrepancy is also interpreted to be due to the differing protocols, methodology, plus the subjectivity in interpretation. Considering only those results that are consistent to within 20% the spread in palaeointensity estimates remains. There is substantial rock magnetic evidence from progressive heating in air and argon experiments (both showing irreversible thermomagnetic behaviour) as well as looking at samples under the scanning electron microscope to suggest that maghaemite is present (albeit to varying degrees) in many of the samples. Alteration therefore occurred in nature and it is likely that the remanence will have been affected to differing degrees potentially causing underestimates in palaeointensity. Maghaemite is interpreted to be a remanence carrier where a component of remanence remains after heating to 580 °C and loss of pTRM acquisition capacity is found on heating. No correlation was found between the estimated palaeointensity (microwave or Thellier datasets) and amount of high temperature remanence or any rock magnetic parameter. This could suggest that the palaeointensity estimates are reliable or as seems likely more than one factor (such as methodology, protocol, interpretation, chemical remanent magnetisation (CRM) contamination and the geomagnetic field) are influencing the palaeointensity estimates. In an attempt to remove the influence of palaeointensity protocol and methodology the palaeointensity datasets were reduced to consist only of those results mutually consistent to 20% ( N = 22). No obvious correlation was again seen between the inferred maghaemite contribution and palaeointensity estimate. There is therefore no clear evidence for the biasing of results due to CRM contamination (assuming a simple relationship between biasing and maghaemite contribution) and it can thus be inferred that the geomagnetic field was a major influencing factor. Further studies are however needed to fully elucidate the extent and result of each possible influencing factor and ultimately the reliability of palaeointensity estimates from rocks of this type.