Palaeointensity Estimates Research Articles

Evaluation of the multispecimen parallel differential pTRM method: a test on historical lavas from Iceland and Mexico

SUMMARY Obtaining reliable geomagnetic palaeointensity (PI) estimates is still a difficult objective. Most common techniques in PI studies are based on various modifications of the Thellier method, but high failure rates and large uncertainties are common. Furthermore, only rocks which obey stringent criteria are assumed to yield faithful PI estimates. In general, magnetic particles must be of single domain size, the magnetization must be a thermoremanent magnetization and chemical alteration of magnetic carriers must be avoided during the laboratory experiment. Recently, a new method for obtaining PIs was proposed, which, on the grounds of results from two historic lavas and three simulated experiments, is assumed to be independent of magnetic domain state. This method uses multiple samples subjected to only one heating step at moderate temperatures. Here we present a test of the multispecimen parallel differential pTRM method on 11 historical lavas from Iceland and Mexico, from which the actual field intensity is known, either from magnetic observatory data, or from magnetic field models. Our results show, that the majority of PI estimates after the multispecimen parallel differential pTRM method yielded results that are very close or indistinguishable within the range of error from the expected intensity and thus largely confirm the findings from the initial study, however, there is in general a small overestimate, which we show is associated with multidomain material. When compared to a Thellier-type PI experiment, carried out on sister samples from five Icelandic lava flows, the average multispecimen PI is closer to the actual field intensity. Samples from one lava, characterized by low Curie temperatures, failed the Thellier approach due to laboratory-induced alteration, but here the multispecimen method returned a reasonable PI. Even though there is evidence that the method is not entirely independent of magnetic domain state, as previously proposed, we regard this method as a valuable alternative in future PI studies.

Microwave palaeointensities from Holocene age Hawaiian lavas: Investigation of magnetic properties and comparison with thermal palaeointensities

Sixteen C 14 -dated Hawaiian surface lava flows spanning 0–4.5 ka have been investigated using the microwave perpendicular applied field palaeointensity technique and classical thermal Thellier–Thellier palaeointensity technique in parallel. The overall microwave experimental success rate is 63% compared to 59% in the thermal experiments. Nineteen percent of the microwave results are deemed to be first class compared to 52% of the thermal results. High palaeointensities and a large amount of within-flow variation are seen in the microwave results with reliable palaeointensity estimates ranging from 25.10 to 82.94 μ T. These variations do not appear to be systematically related to variations in rock magnetic properties such as mineralogy, grain size, texture or oxidation state. The flow-means from the microwave and thermal studies agree for 9 of the 13 flows that can be directly compared and the average difference between the two techniques is not significantly different from zero. Neither experimental technique produces mean palaeointensities systematically lower or more precise than the other, suggesting that the differences seen are a reflection of natural within-flow variation rather than experimental technique.

High-resolution data of the Iceland Basin geomagnetic excursion from ODP sites 1063 and 983: Existence of intense flux patches during the excursion?

Based on discrete samples, we report new high-resolution records of the ∼ 185 kyr Iceland Basin (IB) geomagnetic excursion from Ocean Drilling Project (ODP) Site 1063 on the Bermuda Rise (sedimentation rate 32 cm kyr − 1 ) and from ODP Site 983 in the far North Atlantic (sedimentation rate 18 cm kyr − 1 ). Two records from Holes 1063A and 1063B are very consistent, and provide the highest resolution of the detailed field behaviour during the IB excursion obtained so far. Inclination records from Holes 983B and 983C in the far North Atlantic are also very consistent, whereas declination anomalies deviate more notably. The pseudo-Thellier (PT) technique was applied along with more conventional palaeointensity proxies (NRM/ARM and NRM/ κ) to recover relative palaeointensity (RPI) estimates from Hole 1063A and Hole 983B. As expected, these proxies indicate that the field intensity generally dropped at both sites during the IB excursion, but also that the history of RPI from the two sites is different. VGPs from Site 1063 indicate that the field at this location experienced some stop-and-go behaviour between patches of intense vertical flux over North America and the tip of South America, areas which coincide fairly well with patches of preferred transitional VGP clustering from reversals and zones of high seismic velocity in the lower mantle. Changes in RPI at this location were generally gradual, possibly due to the proximity of these flux patches, and the first period of VGP-clustering over North America was accompanied by a conspicuous increase in RPI. VGPs from Site 983 track along a different path, and the associated RPI changes are very abrupt and completely synchronous with the onset and termination of the excursion. The differing VGP paths from Sites 1063 and 983 indicate that the global field structure during the IB excursion was not dominated by a single dipole.

Determining palaeointensity from the Gilbert Gauss Reversal recorded in the Pu’u Heleakala lava section, Wai’anae Volcano, Oahu, Hawaii

The ability of the Earth's magnetic field to reverse is one of its most intriguing characteristics. A full vector (directions and intensity) description during reversals is necessary to fully understand the magnetic field generation process in the Earth's liquid outer core. The reverse to normal Gilbert Gauss reversal (3.58 Ma) is recorded in a 470 m high sequence of lava flows at Pu’u Heleakala, Waianae Volcano, Oahu, Hawaii. 133 samples from 34 flows (sister samples from the directional study of Herrero-Bervera and Coe (J. Geophys. Res., 104 (1999) 29157–29173)) were subjected to microwave demagnetisation experiments. Unfortunately many samples have been overprinted with a lightning induced remanence so that only 65 samples (49%) were deemed suitable for microwave palaeointensity analysis. Both the 8.2 and 14 GHz systems were used with the perpendicular applied field and Coe analogue methods. Individually acceptable results were obtained for 63% of the studied samples however not all flows gave internally consistent results. In total, 10 flows yielded microwave palaeointensity results from 2 or more samples that met the 25% self-consistency criteria with flow mean palaeointensities ranging from 16 to 39 μT. It is pertinent to ascertain if there is any systematic behaviour seen between different reversals. From the sparse data set obtained in this study the behaviour of the field appears to be different to that of the nearby Pu’u Paheehee section that records the Lower Mammoth (3.33 Ma) transition. However, the two sections have different temporal resolution and the uncertainties in the palaeointensity estimates found in this study may account for some of the discrepancies.

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.

Geomagnetic field intensity changes in Sweden between 9000 and 450 cal BP: extending the record of “archaeomagnetic jerks” by means of lake sediments and the pseudo-Thellier technique

The pseudo-Thellier technique was applied to recover relative palaeointensity (PT-RPI) estimates from four piston cores retrieved from a Holocene lake sediment sequence in the province of Småland, central southern Sweden. A time–depth model between 11,000 and 450 cal BP was based on a combination of calibrated radiocarbon ages and the ages of established palaeomagnetic secular variation (PSV) features that were transferred from a varve-dated regional master curve. In Fennoscandia, significant peaks in geomagnetic field intensity occurred at ∼8400, ∼6400, 3900 and 2800 cal BP. The maximum field intensity at 2800 cal BP was associated with the most rapid change in the direction of the geomagnetic vector. While the high-resolution sedimentary data show that significant century scale increases and decreases in relative field intensity between 4000 and 2000 cal BP were associated with abrupt changes (jerks) in the direction of the geomagnetic vector, archaeomagnetic jerks younger than 2000 years were not reproduced in this multiple sediment core study. On the other hand, the PT-RPI reconstructions display century to millennial scale trends between 5000 and 450 cal BP that are coherent with archaeomagnetic data sets from western Europe and Central Asia. This agreement indicates that palaeointensity may be used as a relative dating technique across this region. Fennoscandian PSV and RPI records for the earlier Holocene do not show the same rapid style of vector movement, which suggests that northern Europe has been affected by a more turbulent geodynamo since ∼4000 cal BP.

Rock magnetic studies and a comparison between microwave and Thellier palaeointensities for Canadian Precambrian dykes

A selection of Precambrian dolerite dykes from the Canadian Shield has been studied to ascertain the magnetic field strength at the time of formation. Chilled margins samples from dyke swarms with U–Pb ages between 2450 and 2104 Ma were analysed using the microwave and Thellier palaeointensity techniques. Rock magnetic analyses have been carried out in order to evaluate magnetic mineralogy and grain size. Thermomagnetic analyses show single distinct Curie temperatures at 585 °C indicating pure magnetite. Hysteresis and backfield IRM measurements suggest grains within the pseudo-single domain (PSD) grain size range. Lowrie–Fuller tests suggest that single domain grains carry the remanence. High quality palaeointensity estimates were obtained, showing markedly low palaeointensities throughout the Proterozoic. Corresponding virtual dipole moments (VDMs) were calculated, typically being less than 4 × 10 22 Am 2, less than half the value of the present field. These data are similar to recently published palaeointensities from the same dyke swarms, determined using the conventional Thellier procedure.

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.

Palaeointensities determined from the middle Cretaceous basalt in Liaoning Province, northeastern China

A palaeomagnetic and geochronologic investigation has been conducted on a basalt lava sequence at Jianguo in Liaoning Province, northeastern China. Radiometric 40 Ar/ 39 Ar dating experiments indicate the volcanism occurred in relatively short time intervals between 105.5 and 102.2 Ma. Thermomagnetic experiments and electron microprobe show that main primary carrier of remanence to be titanomagnetite with Curie temperature of ∼300 °C. A modified version of the Thellier–Thellier technique with systematic partial thermoremanent magnetization (pTRM)-checks was used for palaeointensity determinations. Fourteen out of 61 samples from the studied lavas yield reliable palaeointensity estimates with a mean virtual dipole moment (VDM*) of (4.07±0.2)×10 22 Am 2, about half the strength of the modern-day field.

New palaeointensity results from Cretaceous basalt of Inner Mongolia, China

We present new Thellier–Thellier palaeointensity results from three cooling units (32 samples) of Inner Mongolia lava flows (91.7 Ma) emplaced during the Cretaceous Normal Superchron (CNS). Based on rock-magnetic and microscopy observations the magneto-mineralogy of all samples is determined to be primary and unaltered high-Ti titanomagnetite. Accepted palaeointensity determinations, obtained in the 80–200 °C temperature interval, are of good technical quality with positive standard partial thermoremanent magnetisation (pTRM) checks and pTRM-tail checks. Obtained palaeointensity estimates range from 14.7 to 28.0 μT, with virtual axial dipole moments (VADM) of 2.4 to 4.6 (× 10 22 Am 2). The data agree well with recently published results from the same region and, combining the two datasets, we obtain independent estimates from six different cooling units yielding a time-averaged VADM of 3.2 ± 1.6 (× 10 22 Am 2). These data suggest a relatively low dipole moment towards the end of the Cretaceous Normal Superchron.

Directions and intensities of the Earth’s magnetic field during a reversal: results from the Permo-Triassic Siberian trap basalts, Russia

An extensive palaeomagnetic study was carried out on an approximately 250 Ma old reversed to normal transition of the Earth’s magnetic field (EMF) recorded in 86 volcanic lava flows of the Siberian trap basalts, North Siberia, Russia. In addition to the investigation of the directional behaviour of the field (≈700 specimens) a total of 298 specimens was subjected to Thellier-type palaeointensity determinations. Adding several modifications to the original Thellier experiment, such as tests for MD tails and the additivity of partial thermoremanent magnetisation yields highly reliable palaeointensity estimates of the Late Permian/Early Triassic EMF. Transitional directions of the EMF were obtained from 20 flows. During the reversal a clustering of the virtual geomagnetic poles (VGPs) is observed (15 flows). Palaeointensity estimates suggest that this feature is not an artifact due to rapid flow emplacement since the directional cluster is associated with a well-defined increase in palaeointensity from 6 to 13 μT. Subsequently, the next VGPs move towards the pole position of normal polarity. Departing in a sudden movement from normal polarity the VGPs form a second directional cluster comprising the results of 14 flows. This feature is interpreted as a post-transitional excursion but lacks the characteristic intensity variation recorded during the first transitional cluster. The rest of the section (41 flows) is of normal polarity. The characteristic features of this reversal, low intensities and directional clustering during the reversal and an excursion shortly after the reversal, were also observed in records of polarity transitions of younger age. This suggests that the underlying reversal processes were similar. The mean virtual dipole moment calculated for the stable normal part of the studied section yields a rather low value of 2.2±0.9×10 22 Am 2. These findings confirm that the Mesozoic Dipole Low extends at least down to the Late Permian/Early Triassic. The geomagnetic VGP dispersion was calculated for the stable normal part of the section and yields values similar to those for the last 5 Ma. Considering that the intensity of the EMF was considerably higher in this time interval than in the Early Triassic, a direct relationship between intensity and secular variation seems unlikely.

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±9.5 μT, within errors of that expected from global data for the time period. The dispersion is high but could be reduced to 5.5 μT 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±9.8 μT (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±7.1 μT, 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.

Absolute palaeointensity of Oligocene (28-30 Ma) lava flows from the Kerguelen Archipelago (southern Indian Ocean)

We report palaeointensity estimates obtained from three Oligocene volcanic sections from the Kerguelen Archipelago (Mont des Ruches, Mont des Tempetes, and Mont Rabouillere). Of 402 available samples, 102 were suitable for a palaeofield strength determination after a preliminary selection, among which 49 provide a reliable estimate. Application of strict a posteriori criteria make us confident about the quality of the 12 new mean-flow determinations, which are the first reliable data available for the Kerguelen Archipelago. The Virtual Dipole Moments (VDM) calculated for these flows vary from 2.78 to 9.47 10e22 Am2 with an arithmetic mean value of 6.15+-2.1 10e22 Am2. Compilation of these results with a selection of the 2002 updated IAGA palaeointensity database lead to a higher (5.4+-2.3 10e22 Am2) Oligocene mean VDM than previously reported, identical to the 5.5+-2.4 10e22 Am2 mean VDM obtained for the 0.3-5 Ma time window. However, these Kerguelen palaeointensity estimates represent half of the reliable Oligocene determinations and thus a bias toward higher values. Nonetheless, the new estimates reported here strengthen the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0.3 Ma.

Analysis of long-term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics

SUMMARY The 1167 published cooling unit (CU) palaeointensity estimates contained in the 400‐10 Ma portion of the PINT global database were rigorously filtered according to accurate age determinations, palaeodirectional reliability, recognition of polarity and the method of palaeointensity acquisition. The remaining 865 estimates (group 1) were further filtered to ensure self-consistency, reducing the data set to 425 estimates (group 2). Group 1 and 2 data were clustered into temporally and/or spatially distinct rock suites (RS) enabling each part of the record to be assessed for potential biasing by overrepresentation of palaeosecular variation (PSV). The record was segmented according to the distribution of the data, rather than using arbitrary time windows, to ensure quasi-consistent behaviour within each segment. Differences between these segments clearly indicate that a significant long-timescale (10 7 and 10 8 yr) variation of the mean geomagnetic poloidal field intensity (GPFI) occurred during the 400‐10 Ma period and hence that changing lowermost mantle conditions affect the capacity of the geodynamo to generate a poloidal field. Both the mean dipole moment and its standard deviation appear to be a function of the range of values each CU may adopt at one particular time. This range is itself controlled by the variation of the maximum limit of dipole moment, while the value of the minimum limit remains relatively constant. Tentative support is provided for the recent suggestion that PSV may have been reduced during the Cretaceous normal superchron (CNS), though more data are needed in the range 120‐60 Ma to confirm this. No conclusive evidence was found to support the suggestion that the GPFI record may be biased towards low or high values by palaeointensity determinations obtained using methods that do not adopt pTRM checks. Indeed, offsets caused by unreliable data in well-represented parts of the record are likely to be random and cancel one another out. When GPFI variation is analysed at a sufficiently high resolution to allow comparisons with the geomagnetic polarity reversal frequency (RF), it is not possible to confirm whether the two parameters are anticorrelated, decoupled or related in some more complex way. However, it is clear that GPFI and RF are definitely not positively correlated as has been previously suggested. The present database documents sharp increases in GPFI around the onset times of the two recognized superchrons, itself implying an anticorrelation. The implications, for geodynamo and mantle modelling, of both an anticorrelation and a decoupling of the geomagnetic parameters are discussed briefly. A generic geodynamic model is proposed to explain the relationship between observed longterm changes in GPFI and global geodynamic processes. The model predicts that changes in GPFI result from a chain of geodynamic processes extending from crust to core, beginning with plate reorganizations at the surface and culminating in increases in the vigour of outer core convection. Supercontinents are transient surface expressions of such geodynamic processes and provide the potential to test the generic model. Four time stages are proposed to describe the major long-term changes in GPFI since the Early Devonian: 400‐350, 350‐250, 250‐175

Experimental procedure to detect multidomain remanence during Thellier–Thellier experiments

Reliability checks in Thellier-type experiments usually focus on the detection of chemical alteration of the magnetic mineral component or the formation of new magnetic phases during the heating process. However, a major problem in Thellier-type experiments is the presence of multidomain (MD) particles which can lead either to complete failure of palaeointensity determinations or to serious misinterpretation. We present a modification of the Thellier–Thellier experiment that detects the presence of MD particles by verifying the law of additivity of pTRMs. The law of additivity is valid for regular pTRMs (i.e. the upper temperature of pTRM acquisition is reached by cooling from the Curie temperature) for both SD and MD particles, whereas it is not valid for pTRM * (the upper temperature of pTRM acquisition is reached by heating from room temperature) in the case of MD particles. As the partial thermoremanences imparted in Thellier–Thellier experiments are of the pTRM * type, additivity as a prerequisite for the validity of the obtained result is not given if the remanence is carried predominantly by MD particles. The proposed method is applied to seven synthetic samples covering a grain size range of 23 nm to 12.1 μm. The obtained palaeointensity estimates show a significant error for all samples with grain sizes >0.7 μm due to the failure of the law of additivity. Our experiment is able to detect this failure.

Thellier palaeointensity experiments on Faroes flood basalts: technical aspects and geomagnetic implications

Based on palaeomagnetic and rock magnetic results, 108 samples from 27 flows from Palaeogene flood basalts of Faroe Islands were chosen for whole-rock Thellier palaeointensity experiments. Altogether 90 samples were rejected due to either chemical alterations or typical multidomain (MD) behaviour evidenced by pTRM-tails. AF pre-treatment was used to reduce the effect of MD grains on Thellier experiments. Only five flows (18 samples) yielded acceptable palaeointensity estimates, with flow mean VDMs ranging from 3.5 to 7.4×10 22 A m 2. Modest selection criteria imposed on all published 5–160 Ma palaeointensity data left only 15 palaeomagnetic dipole moments: eight from whole-rock samples, six from submarine basaltic glass and one from single plagioclase crystals. More data are needed before the intriguing differences between results from different materials can be put into a geomagnetic context.

Palaeomagnetic investigation of Tertiary lava from Barrington Tops, NSW, Australia, using thermal and microwave techniques

Tertiary lava (53 Ma) from Barrington Tops, New South Wales, Australia has been studied using conventional thermal and microwave techniques. Twenty-seven flows in two sections were sampled and their rock magnetic characteristics determined. On heating the majority of samples exhibited a single Curie temperature at around 200°C, indicating a titanium rich titanomagnetite. Some samples exhibited two magnetic phases and a few a single low titanium titanomagnetite phase. Thermal demagnetisation yielded a mean direction of D=189.5, I=63.6, α 95=4.3, which corroborates the previous findings of Wellman et al. [Geophys. J. R. Astron. Soc. 18 (1969) 371–395]. A pilot conventional Thellier palaeointensity analysis was unsuccessful mainly due to the samples being highly susceptible to thermo-chemical alteration. Far greater success was achieved (58%) using the microwave palaeointensity technique, where heating of the bulk sample and hence alteration is vastly reduced. Palaeointensity estimates range from 3 to 28 μT (mean 11±5 μT), which, assuming that the remanence is a primary thermal remanent magnetisation, indicates a low field intensity (a fifth of the present day value) in the early Tertiary. This study demonstrates the applicability of the microwave palaeointensity technique to ancient lava.

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.

Impact of early diagenesis and bulk particle grain size distribution on estimates of relative geomagnetic palaeointensity variations in sediments from Lama Lake, northern Central Siberia

Summary High-resolution analyses of rock magnetic and sedimentological parameters were conducted on an 11 m long sediment core from Lama Lake, Northern Siberia, which encompasses the late Pleistocene and the Holocene epochs. The results reveal a strong link between the median grain size of the magnetic particles, identified as magnetite, and the oxidation state of the sediment. Reducing conditions associated with a relative high total organic carbon (TOC) content of the sediment characterize the upper 7 m of the core (≈ Holocene), and these have led to a partial dissolution of detrital magnetite grains, and a homogenization of grain-size-related rock magnetic parameters. The anoxic sediments are characterized by significantly larger median magnetic grain sizes, as indicated, for example, by lower median destructive fields of the natural remanent magnetization (MDFNRM) and lower ratios of saturation remanence to saturation magnetization (MSR/MS). Consequently, estimates of relative geomagnetic palaeointensity variations yielded large amplitude shifts associated with anoxic/oxic boundaries. Despite the partial reductive dissolution of magnetic particles within the anoxic section, and consequent minimal variations in magnetic concentration and grain size, palaeointensity estimates for this part of the core were still lithologically distorted by the effects of particle size (and subsidiary TOC) variations. Anomalously high values coincide with an interval of significantly more fine-grained sediment, which is also associated with a decrease in TOC content, which may thus imply a decreased level of magnetite dissolution in this interval. Calculation of relative palaeointensity estimates therefore seems to be compromised by a combined effect of shifts in the particle size distribution of the bulk sediment and by partial magnetite dissolution varying in association with the TOC content of the sediment.

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.