Remanence Acquisition Research Articles

A preliminary magnetic study on lacustrine sediments from Zoigê Basin, eastern Tibetan Plateau, China: Magnetostratigraphy and environmental implications

A preliminary magnetic study was conducted on Core RM (310 m long), drilled in the Zoigê Basin (33 °57′N, 102 °21′E), on the eastern Tibetan Plateau. Greigite in SD state exists as a main magnetic contributor along the whole core. In addition, magnetite and haematite are detected between 160-45 m. NRM directions show a complex record with many apparent changes in inclination, for which greigite or the remanence acquisition process may be responsible. Nevertheless, the Matuyama/Brunhes boundary and, possibly, the Blake Event are identified. Between 310 and 160 m (as well as between 45 and 4.5 m), predominant anoxic conditions favour the formation of greigite, leading to higher susceptibility (χ) and remanence peaks. However, higher carbonate content during dry periods in these zones, may dilute the concentration of magnetic minerals, resulting in lower values of magnetic parameters. Between 160 and 45 m, weak oxic conditions preserve detrital magnetic minerals. The increasing χ background in this zone may indicate changes in the concentration of magnetite and haematite related to the uplift of the Tibetan Plateau, while higher χ peaks may indicate fluctuations to anoxic conditions, which enhance greigite concentration.

Geomagnetic field intensity from 71 to 12 ka as recorded in deep‐sea sediments of the Blake Outer Ridge, North Atlantic Ocean

In this paper we estimate relative geomagnetic field paleointensity between 71 and 12 kyrs BP as recorded in three deep sea sediment cores from the Blake Outer Ridge, western North Atlantic Ocean. Paleointensities were estimated by normalizing sediment natural remanant magnetization separately to (1) magnetic susceptibility, (2) anhysteretic remanant magnetization and (3) saturation isothermal remanent magnetization (SIRM). In one core, paleointensities were estimated within short time windows which display uniform sediment magnetic characteristics, and offsets between windows were removed to minimize environmental biases. We find that most features of our records are preserved regardless of normalizer choice, but we chose SIRM as the best normalizer for the final paleointensity estimates. Our three records preserve and agree upon a number of short‐duration (∼103 years) paleointensity features even though the cores are separated by almost 250 km. We conclude that these are real geomagnetic field signals of at least local extent. We also identify a number of differences between our records which must be artifacts of sediment remanence acquisition or paleointensity normalization. Such artifacts occur as either (1) baseline shifts between time intervals with slightly different sediment magnetic characteristics or (2) differences in amplitude of short duration events. In spite of these environmental biases, the number and ages of relative paleointensity highs and lows are preserved. Thus sediment paleointensity estimates may be used locally for high‐resolution chronostratigraphic correlation. Correlation of our paleointensity records with other records from the same extended region (North Atlantic Ocean‐western Europe) indicates that major paleointensity features appear in all regional records. However, differences in the ages and disagreement in magnitude and number of individual features call into doubt the use of these relative paleointensity records for high‐resolution chronostratigraphic correlation on a broader regional scale or for quantitative estimation of past geomagnetic field variability.

Late Cretaceous magmatism in Madagascar: palaeomagnetic evidence for a stationary Marion hotspot

Late Cretaceous basaltic volcanics in the Morondava Basin (SW Madagascar) possess high-quality and pre-fold palaeomagnetic data (declination 353.5°, inclination −54.8°, α 95 = 2.4°). The palaeomagnetic data are all of normal magnetic polarity, and remanence acquisition is linked to the terminal stages of the Cretaceous Normal Superchron (≥83 Ma). This is sustained by an 40Ar/ 39Ar age of 83.6±1.6 Ma from one of the tested basaltic flows. A precise U/Pb zircon–baddeleyite age from northeast Madagascar demonstrates magmatism at least back to 91.6±0.3 Ma; thus reliable isotope ages for the Madagascar Cretaceous igneous province span a range of 8 million years. Late Cretaceous palaeomagnetic data obtained from volcanics and dolerites all over Madagascar are directionally concordant, and the combined palaeomagnetic pole (latitude 68.5°N, longitude 230.3°E, A 95 = 5.5°, N=8 studies; sampling age range ca. 84–90 Ma) represents one of the best Late Cretaceous poles for the former Gondwanan elements. The collective palaeomagnetic data yield a palaeolatitude of 45.3°S +5.3 −4.7 for the proposed focal point of the Marion plume (Volcan de l'Androy, southeast Madagascar) during the Late Cretaceous. This is in perfect agreement with hotspot-controlled reconstructions that place the Marion hotspot (46.0°S) beneath southeast Madagascar during the Late Cretaceous. Setting true polar wander aside, hotspot movements in the Indian Ocean do not appear to exceed ca. 0.75 cm/yr, and the Marion hotspot appears stationary within the resolution power of palaeomagnetic data.

Paleomagnetic data from Triassic strata, Zuni uplift, New Mexico: Further evidence of large‐magnitude Triassic apparent polar wander of North America

Upper Carnian (∼225 Ma) strata of the Bluewater Creek Formation of the Chinle Group of the Colorado Plateau, western New Mexico, contain predominantly reverse polarity characteristic magnetizations of high coercivity and distributed unblocking temperature up to 695°C. Data from a syndepositional structure suggest acquisition of a stable remanence during or soon after deposition. The mean paleomagnetic pole for 13 virtual geomagnetic poles (VGP) accepted (of 17 sites collected) is located at 55.2°N, 87.5°E (A95 = 6.7°, K = 39.7). The angular distance between this pole and the mean of recently published middle and late Norian (∼210 Ma) poles from overlying strata is 12.5°, supporting the hypothesis that significant apparent polar wander occurred during Late Triassic time. Furthermore, the Bluewater Creek pole lies to the west (6.1°, angular distance) of the cratonic reference pole for Carnian time supporting the hypothesis of a small Colorado Plateau rotation since the early Mesozoic. Anisian strata of the Moenkopi Formation contain dual polarity magnetizations of high coercivity and distributed unblocking temperatures up to 690°C, The mean VGP of six sites yields is located at 53.1°N, 96.3°E (A95 = 6.4°, K = 109.2), which is similar to previously determined Middle Triassic poles. Triassic paleomagnetic poles from southwest North America from strata in continuous stratigraphic succession, both on and off the Colorado Plateau, are not consistent with the hypothesis of a Late Triassic stand still of the geomagnetic pole.

Remanence acquisition and its alteration on sediments

Previous palaeomagnetic work shows that the mechanism of remanence acquisition on sediments is very complicated. Different magnetizing processes under different conditions may produce some anomalous magnetic signals which have nothing to do with the changes of the geomagnetic field and are difficult to explain. The depositional remanent magnetization (DRM) may produce an inclination error and a bedding error, while postdepositional remanent magnetization (PDRM) may result in a time delay between the magnetic and sedimentary ages. When there are some different magnetic phases with different lock-in depths in the sediments, it is difficult to judge the depth of the reversal or excursion, or even the artificial magnetic signal may be produced. Even if there is only one magnetic phase in the sediment, the recorded magnetic signals may also be different if the lock-in depth is changed comparative to the lasting period of one reversal or excursion. Demagnetization may also change the primary magnetic records and the changes of water content may result in the displacement of magnetic boundary. The so-called “unstable core”, representing subtle changes in mineralogy into and out of a self-reversal region or a viscous magnetization region, gives a complicated palaeornagnetic record with many apparent changes in the palaeomagnetic direction. Moreover, some physical disturbances, such as biodisturbance, deformation, drilling processes and drying effects, may result in the remanence alteration. Some palaeomagnetic investigations demonstrate that good magnetic signals are invariably achieved on homogenous mud or clay sediments, but the worse signals on inhomogeneous profiles, especially on sandy layers.

Magnetic properties of polycrystalline diamonds

The remanent magnetism and bulk magnetic properties of polycrystalline diamonds have been investigated. The diamonds are of two distinct types, those occurring together with gem-grade diamonds in kimberlite pipes in S Africa, and carbonados, found in placer deposits notably in Brazil and the Central African Republic. Both types generally possess measurable remanent magnetization, the former stronger than the latter, and stability tests indicate the presence of primary and secondary components. Magnetite is the dominant carrier of NRM in the kimberlite diamonds, but the very small content of magnetic mineral in the carbonados makes identification of the carrier difficult. Possible contributors are tetrataenite, native iron and cohenite. Anomalous acquisition of isothermal remanence occurs in some carbonados, saturation being unachievable in applied fields in excess of 1 T. The ratio of saturation remanence to initial NRM is anomalously low in some of the kimberlite diamonds, and the possibility of magnetic contamination is investigated.

Gyromagnetic remanence acquired by greigite (Fe3S4) during static three-axis alternating field demagnetization

SUMMARY A magnetic study was carried out on lacustrine sediments from the Zoigebasin, Tibetan Plateau, in order to obtain a better understanding of palaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisition is unexpectedly observed during static three-axis alternating field (AF) demagnetization in about 20 per cent of a large number of samples. X-ray diVraction (XRD) analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier. Scanning electron microscopy (SEM) reveals that the greigite particles are in the grain size range of 200-300 nm, possibly in the single-domain state. Greigite clumps of about 3 mm size are sealed by silicates. Fitting of XRD peaks yields a crystalline coherence length of about 15 nm, indicating that the particles seen in the SEM are polycrystalline. GRM intensities of most samples are of the same order as the NRM, while others show much stronger GRM although their magnetic properties are similar. Variation of the demagnetization sequence confirms that GRM is mainly produced perpendicular to the AF direction. The anisotropy direction can be derived from GRM, but more systematic studies are needed for detailed conclusions. An attempt to correct for GRM failed due to high GRM intensities and because smaller GRM acquisition was also found along the demagnetization axis. Behaviours of acquisition and AF demagnetiz- ation of GRM are comparable with those of NRM, ARM, IRM, indicating fine grain sizes of remanence carriers.

Some new aspects for the modelling of isothermal remanent magnetization acquisition curves by cumulative log Gaussian functions

Rock samples containing (ferri)magnetic minerals acquire an isothermal remanent magnetization (IRM) after exposure to a direct magnetic field. Measuring IRM(B) allows to produce IRM acquisition curves which were modelled by cumulative log Gaussian (CLG) functions (Robertson and France, 1994) to discriminate the different magnetic phases. An approach of how to find the best model has been added to this method. From the investigation of a magnetite sample it is inferred that with the method the remanence acquisition coercive force for each magnetic component in a sample can be deduced. The method allows to detect minute concentrations of magnetically hard material in a predominant magnetically soft sample and vice versa. It furthermore allows to extrapolate the IRM behaviour of samples that do not reach saturation IRM in the experiment. The study of a natural hematite renders it possible that even well‐defined hematite may be contaminated by (ferri)magnetic impurities biasing the remanence acquisition coercive force of the sample towards lower values.

The origin of magnetization and geochemical alteration in a fault zone, Kilve, England

The origin of an apparently syndeformational chemical remanent magnetization (CRM) and geochemical alteration in a fault zone in the Bristol Channel Basin, southwest England, was investigated. Deformation in the fault zone occurs in Jurassic aged, organic-rich limestones and consists of numerous normal and oblique-slip faults and associated folds. Migration of basinal, radiogenic fluids is indicated by elevated 87Sr/86Sr values for calcite veins that occur throughout the fault zone. Some of the calcite veins contain hydrocarbons sourced from deeper strata. Elevated 87Sr/86Sr values in the host Jurassic limestones indicate that they were also extensively altered by radiogenic fluids that migrated through microfractures in addition to major fault and fracture planes. Folded and tilted host limestones contain a pervasive secondary CRM residing in magnetite that was acquired during deformation in the Tertiary. The association between this pervasive CRM and the pervasive geochemical alteration is consistent with a genetic connection between the orogenic fluids and the CRM although the timing of CRM acquisition (Tertiary) is not consistent with structural interpretations for the timing of most veining. An alternative remagnetization mechanism which is not triggered by externally derived fluids, such as diagenesis of hydrocarbons, might account for the CRM. Hydrocarbon-bearing veins also contain a CRM that resides in magnetite, although the time for remanence acquisition is not well constrained by field tests. © 1998 John Wiley & Sons, Ltd.

Imaging distribution patterns of magnetic minerals by a novel high-Tc-SQUID-based field distribution measuring system: application to Permian sediments

SUMMARY A newly developed field distribution measuring system based on a high-T c SQUID has been employed in the study of magnetic mineral distribution in several Permian sedimentary rocks. The instrument consists of a small, 1.4◊1.4 mm sized YBaCu-oxide SQUID magnetic field sensor that is operated in a thin-walled dewar, so that the sample’s surface, at room temperature, can be scanned at a distance of only ~1.5 mm. The samples were subjected to a saturation remanence perpendicular to the surface and the scanning measurements in zero field reveal that the magnetization might be carried by only a small part of a sample, in one case associated with secondary oxide phases. High-resolution magnetic scans can aid in the interpretation of the magnetic remanence acquisition process.

Characterizing stone tools by rock-magnetic methods

Of the many petrophysical properties such as acoustic velocity, dielectric constant, electrical resistivity, etc., the widest variation is shown by magnetic responses. Susceptibilities commonly range over 4 orders of magnitude between different lithic samples. Thus, magnetic properties show great potential for the characterization, provenancedetermination, thermal history, and treatment of stone tools and masonry. In order of increasing complexity and decreasing value to the archaeologist, we recommend lowfield susceptibility, saturation remanence, hysteresis ratios, isothermal remanence acquisition, and alternating field demagnetization of a saturation remanence. The natural remanence (NRM) is rarely of any value in characterizing artifacts. The physical basis for this discrimination is the nature of the magnetic minerals, their abundance and distribution through the sample, and, for remanence-bearing minerals, their domain structure, crystallographic perfection, and grain-size. These characteristics may be changed in distinctive ways by cultural effects such as percussion or fire, and surface conditions such as frost action, forest fires, or subsequent weathering. © 1998 John

Magnetic petrology of arc xenoliths from Japan and Aleutian Islands

Xenoliths from Japan and the Aleutian Islands were studied for the purpose of identifying source rocks with sufficient magnetization to account for positive subduction zone Magsat anomalies. Two types of magnetic lithologies are identified: (1) amphibolites and hornblende gabbros containing up to 8.6% titanomagnetite, mostly as large, unoxidized, multidomain crystals, and (2) pyroxene gabbros and metagabbros with up to 20.3% titanomagnetite, usually in association with primary ilmenite. The first type is represented by xenoliths from Ichinomegata, Japan and Adak Island in the Aleutians. The second, restricted to Iki Island, Japan and Kanaga Island, Aleutians, has abundant coarse‐grained titanomagnetite which in most samples has been oxidized, producing secondary, trellis‐type ilmenite lamellae; a few Iki Island xenoliths show more advanced oxidation. Arc xenoliths show 3 orders of magnitude range in natural remanent magnetization and saturation remanence and about 2 orders of magnitude range in magnetic susceptibility. Nearly all Iki Island rocks and many from Ichinomegata and the Aleutians possess induced magnetization 2 A m−1 and greater, with the expectation of additional magnetization due to viscous remanence at elevated temperature. The TiO2 content of titanomagnetite, together with the presence of appreciable Al2O3 and MgO, ensure that most xenoliths have Curie points between 400° and 550°C. Remanence acquisition curves exhibit a range reflecting varying degrees of magnetic hardening due to oxidation. Correspondingly, each xenolith site groups distinctly on a plot of magnetic hysteresis ratios. This paper indicates that lithologies with total magnetizations of many A m−1 exist in lower crust arc settings. Their volumetric distribution is uncertain based on the limited xenolith population reported herein.

Geomagnetic fluctuations during a polarity transition

The extensive Roza Member of the Columbia River Basalt Group (Washington State) has intermediate paleomagnetic directions, bracketed by underlying normal and overlying reverse polarity flows. A consistent paleomagnetic direction was measured at 11 widely distributed outcrops; the average direction has a declination of 189° and an inclination of −5°, with greater variation in the inclination [Rietman, 1966]. In this study the Roza Member was sampled in two Pasco Basin drillcores, where it is a single cooling unit and its thickness exceeds 50 m. Excellent core recovery allowed uniform and dense sampling of the drillcores. During its protracted cooling, the Roza flow in the drillcores recorded part of a 15.5 Ma geomagnetic polarity transition. The inclination has symmetric, quasicyclic intraflow variation, while the declination is nearly constant, consistent with the results from the outcrops. Thermal models of the cooling flow provide the timing for remanence acquisition. The inclination is inferred to have progressed from 0° to −15° and back to −3°over a period of 15 to 60 years, at rates of 1.6° to 0.5°/yr. Because the geomagnetic intensity was probably weak during the transition, these apparently high rates of change are not significantly different from present‐day secular variation. These results agree with the hypothesis that normal secular variation persists through geomagnetic transitions. The Iow‐amplitude quasicyclical fluctuations of the field over tens of years, recorded by Roza, suggest that the geomagnetic field reverses in discrete steps, and that more than 15–60 years were required to complete this reversal.

Synfolding magnetization: detection, testing and geological applications

A correct method for determining the direction of a synfolding remanence is described, and a new test, which in a number of cases can discriminate between a true synfolding remanence and a remanence that is the sum of pre- and post-folding components, is suggested. It is shown that it is possible to reveal the folding evolution of a local object under an assumption that the acquisition of the synfolding remanence was synchronous over this object.

Palaeomagnetism of the Borrowdale and Eycott volcanic groups, English Lake District: primary and secondary magnetization during a single late Ordovician polarity chron

Late Ordovician volcanic rocks of the English Lake District typically have a magnetic remanence dominated by a single characteristic component. Previous investigations have interpreted this remanence as both of primary (pre-folding) and secondary origin. Palaeomagnetic field tests have been conducted on (a) andesite blocks from an autobrecciated lava top, (b) andesite blocks in mass-flow breccias, and (c) fault-blocks tilted during Ordovician caldera collapse to establish the time of remanence acquisition. All three tests show that the lavas retain a magnetization acquired during initial cooling: magnetizations of the breccias are coherent within clasts and random between clasts, whilst magnetizations of the tilted fault blocks converge with better that 95% confidence when corrected for the effects of caldera collapse. In contrast, the volcaniclastic sedimentary and pyroclastic rocks possess an Ordovician secondary remanence acquired after strata had been tilted by volcano-tectonic subsidence. A distributed sample of 65 andesite and basalt sheets through the Borrowdale Volcanic Group has a mean remanence direction D/I=341.9/−48.9° (α95=4.0°) yielding a positive fold test and a palaeomagnetic pole at 12.7°E, 4.3°S (dp/dm=3.5/5.3°). A progressive steepening of the palaeofield direction is recorded during emplacement of the Borrowdale Volcanic Group (∼I=−39° to I=−51°) which continued into the interval of volcanotectonic overprinting (I=−62°); the equivalent motion of Eastern Avalonia is ∼20° into higher southerly latitudes.Both the Eycott and Borrowdale volcanic groups exhibit uniform normal polarity throughout. Correlation with the geomagnetic time scale for the Ordovician restores the broad correlation between the two groups by constraining their emplacement and partial overprinting to a single long normal polarity chron occupying the Nemagraptus gracilis and earlier part of the Diplograptus multidens biozones (late Llandeilo and early Caradoc). All the volcanism, therefore, occurred within a period of no more than ∼5 Ma. The palaeomagnetic evidence confirms that the Borrowdale Volcanic Group was affected by both syn-volcanic deformation (caldera collapse) and regional compressive deformation prior to deposition of the (late Ordovician–Silurian) Windermere Supergroup. The succession of primary and secondary Ordovician palaeomagnetic poles from the Lake District inlier defines an anticlockwise apparent polar wander (APW) loop with the apex correlating with ‘soft’ closure of the Iapetus Ocean and late Ordovician deformation. The APW paths from Avalonia and Baltica converge at this point as subduction ceased and the arc subsided beneath the sea after mid-Caradoc times.

Absolute paleointensity between 60 and 400 ka from the Kohala Mountain (Hawaii)

Magnetic experiments including thermal demagnetization of the natural remanent magnetization (NRM), mineralogical studies and paleointensity measurements have been conducted on ten lava flows, with ages between 60 ka and 400 ka, from the Kohala Mountain, Hawaii. Of the samples, 62 were subjected to double heating paleointensity experiments under vacuum. In total, 35% of the specimens did not exhibit significant magnetomineralogical changes during heating and met all the criteria for successful determinations of absolute paleointensity. A technique of corrections [1] was attempted for samples that exhibited changes in their ability to acquire partial thermoremanent magnetization (pTRM) during heating but did not show acquisition of chemical remanence (CRM). This procedure doubled the rate of success, with consistent results between the uncorrected and the corrected data from within the same flows. The successful paleointensity estimates obtained for 8 lava flows are found to be in good agreement with previous absolute paleointensities obtained from other areas. The results are also consistent with the synthetic curve (Sint-200) of relative paleointensity obtained for the past 200,000 years from deep-sea sediment cores [2]. There is thus no reason to infer the presence of large non-dipole fields in the vicinity of Hawaii. Overall, the geomagnetic intensity appears to have the same variability for at least the past 200 ka.

A detailed record of normal-reversed-polarity transition obtained from a thick loess sequence at Jiuzhoutai, near Lanzhou, China

SUMMARY A record of normal-reversed-polarity transition has been obtained from a 4 m thickness of loess exposed at a section near Lanzhou, China. Magnetostratigraphic studies suggest it may represent a reversal bounding the onset of a reversed-polarity zone within the Jaramillo Normal Subchron. The natural remanent magnetization consists of two components: a low-coercivity (I 20 mT), low-unblocking-temperature (I 300 C) component of viscous origin and a high-coercivity (> 20 mT), high-unblockingtemperature (25G700 C) component carrying the characteristic remanence. Mineral magnetic analyses confirmed the presence of magnetite, its low-temperature oxidation products and haematite, each contributing to the remanence properties. Grain size and concentration showed limited variations and there was little evidence for the presence of the ultrafine magnetic phase commonly associated with palaeosol formation. Pedogenic processes appeared negligible and their effects unimportant, with detrital processes dominating the mineralogy and most probably the acquisition of the characteristic remanence. The reversal record was characterized by the decay and recovery of the geocentric axial dipole term with large directional swings occurring during periods of reduced relative palaeofield intensity. The virtual geomagnetic poles traced a complex path exhibiting no particular geographical confinement. Relative palaeofield intensity determinations were insensitive to the choice of normalization parameter and showed a distinctive asymmetry. Striking similarities were observed with the Matuyama-Jaramillo reversal record, obtained from the same section (Rolph 1993), and the Steens Mountain reversal record (PrCvot et al. 1985), lending further support for the existence of unusually high post-transitional field intensities

Absolute paleointensity and magnetomineralogical changes

Determinations of absolute paleointensity are often hampered by magnetomineralogical changes produced within samples of lava flows during successive heatings. Such changes which directly affect the capacity for a sample to acquire a partial thermoremanent magnetization (PTRM) are reflected by negative pTRM checks, a deviation between the original pTRM and a repeated measurement after heating at a higher temperature. Because the deviations indicated by the PTRM checks can provide a direct estimate of the effects to the pTRMs, we suggest that they can be used to correct pTRMs. The correction method we present requires additional measurements, in particular pTRM checks performed after each pTRM and specific diagrams to detect acquisition of chemical remanence. Performing multiple successive pTRM checks can also provide information on the blocking temperatures of the alteration product. These experiments conducted on lava flows from different localities show that the magnetomineralogical changes mainly involved grains with blocking temperatures lower than the last heating step. The corrections have been tested on recent and historic lava flows from several localities. We have also compared results on samples from the same lava flows that were heated in air and in vacuum. The mean paleointensity values were found to lie within less than 10% from the expected field intensity at each site. The corrections resulted in the recovery of reliable paleointensity determinations for about twice as many specimens heated in vacuum and 65% additional data for specimens heated in air. Successive heatings at the same temperature in air are often accompanied by a time‐dependent oxidation of the natural remanent magnetization which is responsible for paleointensity values lower than the expected field, a problem that is not related to the corrections. We conclude that corrections can be used with fairly high confidence to the pTRMs performed in vacuum and in air.

Magnetobiostratigraphy of the Spathian to Anisian (Lower to Middle Triassic) Kçira section, Albania

SUMMARY Magnetobiostratigraphic data are presented from three Early/Middle Triassic HanBulog Limestone successions from Kqira, northern Albania. A total of 206 standard palaeomagnetic samples were obtained for thermal demagnetization and statistical analysis from the 42, 10 and 5m thick sections. The reversal-bearing characteristic component, carried by haematite and magnetite, defines a composite sequence of six main polarity intervals (Kqln to Kq3r) in which are embedded four short polarity intervals, one at the base of Kqln and three towards the top of Kqlr. The early acquisition of the characteristic remanence is supported by the lateral correlation of magnetozones between sections. The Early/Middle Triassic boundary, approximated by the first occurrence of the conodont Chiosella timorensis, falls close to the Kqlr/Kq2n polarity transition. This is in good agreement with recently published magnetobiostratigraphic data from the coeval Chios (Greece) sections. The palaeomagnetic pole calculated from the Kqira characteristic directions lies close to the Triassic portion of the apparent polar wander path for Laurussia (in European coordinates). However, a 40-45 clockwise rotation of the external zone of the Albano-Hellenic Belt to the south of the Scutari-Pec Line is thought to have occurred since the Early-Middle Miocene. The Kqira pole acquires a West Gondwana affinity when restored for the Neogene clockwise rotation. If the clockwise rotation was entirely related to Neogene tectonics, the Kqira area was evidently associated with West Gondwana and located at 12-16N of the western Tethys margin.

Non-steady state magnetic mineral reduction, chemical lock-in, and delayed remanence acquisition in pelagic sediments

Magnetic hysteresis data collected from pelagic sediment sites in the western equatorial Pacific Ocean define a progressive deepening of modern redox boundaries with decreased organic carbon input. Paleomagnetic data from the site with the highest organic carbon shows marked changes in the natural remanent magnetization at the modern Fe-redox boundary, suggesting a type of chemical lock-in process in which some magnetic grains form in situ above the boundary and freely orient in the field. The depth of chemical lock-in suggests paleomagnetic data will be smoothed on time scales ranging from less than 40 kyr to more than 425 kyr. The higher values are compatible with models calling for a delayed remanence acquisition to account for the ‘saw-tooth’ pattern observed in some sedimentary paleointensity records.