Paleomagnetic Signal Research Articles

Deconvolution of continuous paleomagnetic data from pass-through magnetometer: A new algorithm to restore geomagnetic and environmental information based on realistic optimization

The development of pass-through superconducting rock magnetometers (SRM) has greatly promoted collection of paleomagnetic data from continuous long-core samples. The output of pass-through measurement is smoothed and distorted due to convolution of magnetization with the magnetometer sensor response. Although several studies could restore high-resolution paleomagnetic signal through deconvolution of pass-through measurement, difficulties in accurately measuring the magnetometer sensor response have hindered the application of deconvolution. We acquired reliable sensor response of an SRM at the Oregon State University based on repeated measurements of a precisely fabricated magnetic point source. In addition, we present an improved deconvolution algorithm based on Akaike's Bayesian Information Criterion (ABIC) minimization, incorporating new parameters to account for errors in sample measurement position and length. The new algorithm was tested using synthetic data constructed by convolving “true” paleomagnetic signal containing an “excursion” with the sensor response. Realistic noise was added to the synthetic measurement using Monte Carlo method based on measurement noise distribution acquired from 200 repeated measurements of a u-channel sample. Deconvolution of 1000 synthetic measurements with realistic noise closely resembles the “true” magnetization, and successfully restored fine-scale magnetization variations including the “excursion.” Our analyses show that inaccuracy in sample measurement position and length significantly affects deconvolution estimation, and can be resolved using the new deconvolution algorithm. Optimized deconvolution of 20 repeated measurements of a u-channel sample yielded highly consistent deconvolution results and estimates of error in sample measurement position and length, demonstrating the reliability of the new deconvolution algorithm for real pass-through measurements.

Variable remanence acquisition efficiency in sediments containing biogenic and detrital magnetites: Implications for relative paleointensity signal recording

AbstractWidespread geological preservation of biogenic magnetite makes it important to assess how such particles contribute to sedimentary paleomagnetic signals. We studied a sediment core from the South China Sea that passes the strict empirical criteria for magnetic “uniformity” used in relative paleointensity studies. Such assessments are based routinely on bulk magnetic parameters that often fail to enable identification of mixed magnetic mineral assemblages. Using techniques that enable component‐specific magnetic mineral identification, we find that biogenic and detrital magnetites occur in approximately equal concentrations within the studied sediments. We analyzed normalized remanence signals associated with the two magnetite components to assess whether co‐occurring biogenic and detrital magnetites record geomagnetic information in the same way and with the same efficiency. Paleomagnetic directions for the two components have no phase lag, which suggests that the biogenic and detrital magnetites acquired their magnetizations at equivalent times. However, we find that the biogenic magnetite is generally 2–4 times more efficient as the detrital magnetite in contributing to the natural remanent magnetization (NRM) despite their approximately equal magnetic contributions. Variations in the concentration and efficiency of remanence acquisition of the two components suggest that a significant part of the NRM is controlled by nongeomagnetic factors that will affect relative paleointensity recording. We recommend that methods suited to the detection of variable recording efficiency associated with biogenic and detrital magnetites should be used on a routine basis in relative paleointensity studies.

Paleomagnetic and paleoenvironmental implications of magnetofossil occurrences in late Miocene marine sediments from the Guadalquivir Basin, SW Spain

Although recent studies have revealed more widespread occurrences of magnetofossils in pre-Quaternary sediments than have been previously reported, their significance for paleomagnetic and paleoenvironmental studies is not fully understood. We present a paleo- and rock-magnetic study of late Miocene marine sediments recovered from the Guadalquivir Basin (SW Spain). Well-defined paleomagnetic directions provide a robust magnetostratigraphic chronology for the two studied sediment cores. Rock magnetic results indicate the dominance of intact magnetosome chains throughout the studied sediments. These results provide a link between the highest-quality paleomagnetic directions and higher magnetofossil abundances. We interpret that bacterial magnetite formed in the surface sediment mixed layer and that these magnetic particles gave rise to a paleomagnetic signal in the same way as detrital grains. They, therefore, carry a magnetization that is essentially identical to a post-depositional remanent magnetization, which we term a bio-depositional remanent magnetization. Some studied polarity reversals record paleomagnetic directions with an apparent 60–70 kyr recording delay. Magnetofossils in these cases are interpreted to carry a biogeochemical remanent magnetization that is locked in at greater depth in the sediment column. A sharp decrease in magnetofossil abundance toward the middle of the studied boreholes coincides broadly with a major rise in sediment accumulation rates near the onset of the Messinian salinity crisis (MSC), an event caused by interruption of the connection between the Mediterranean Sea and the Atlantic Ocean. This correlation appears to have resulted from dilution of magnetofossils by enhanced terrigenous inputs that were driven, in turn, by sedimentary changes triggered in the basin at the onset of the MSC. Our results highlight the importance of magnetofossils as carriers of high-quality paleomagnetic and paleoenvironmental signals even in dominantly terrigenous sediments.

The vast proto-Tibetan Plateau: New constraints from Paleogene Hoh Xil Basin

The surface uplift of the Tibetan Plateau is the key boundary condition in many Cenozoic geological events ranging from global cooling to changes of Asian environments during Cenozoic. However, poorly constrained timing for the uplift of Tibetan Plateau makes these interpretations highly debatable. Here we report results from sedimentology, detrital zircon U–Pb and Lu–Hf isotopic compositions, and paleomagnetic signatures from both the eastern and western Hoh Xil basins of north-central Tibetan Plateau. Sedimentary lithofacies and facies associations analyzed in the western Hoh Xil basin indicate they were deposited in a braided fluvial system and alluvial fan, similar with the Fenghuoshan Group, eastern Hoh Xil basin. Provenance analyses from conglomerate clast compositions, paleocurrent orientations, and detrital zircon U–Pb and Lu–Hf isotopic compositions document sediments in both western and eastern basins were derived from the Qiangtang and Lhasa blocks. These observations, in combination with comparative paleomagnetic results, imply that Hoh Xil basin was a single, wide basin during Paleogene. The period of Hoh Xil basin deposition was coeval with significant period of the early Cenozoic uplift and erosion of the Qiangtang and Lhasa blocks. These observations not only reinforce the suggestion that the Qiangtang and Lhasa blocks were uplifted during Eocene to form a proto-Tibetan Plateau, but also imply that the proto-Tibet Plateau is vast in areal extent. The large dimension and high elevation of the proto-Tibetan Plateau probably contributed to the global cooling during the early Eocene.

Post-depositional remanent magnetization lock-in for marine sediments deduced from 10Be and paleomagnetic records through the Matuyama–Brunhes boundary

Geomagnetic field intensity records from marine sediments have contributed to improved understanding of variations in the Earth's magnetic field, and have helped to establish age models for marine sediments. However, lock-in of the geomagnetic signal below the sediment–water interface in marine sediments through acquisition of a post-depositional remanent magnetization (PDRM) adds uncertainty to synchronization of marine sedimentary records. Although quantitative models enable assessment of delays in remanence acquisition associated with PDRM processes, the nature of the filter function and the PDRM lock-in zone thickness remain topics of debate. We performed both forward numerical simulations and inverse parameter estimation to assess the best-fit filter function and PDRM lock-in zone thickness in marine sediments based on a comparison of 10Be flux and relative paleointensity records. Our simulations reveal that the rate of PDRM lock-in increases in the middle of the lock-in zone and that a Gaussian function with a 17 cm lock-in zone thickness provides a good approximation to the PDRM lock-in within the studied core. With this function, PDRM lock-in is delayed, but with relatively little distortion of the geomagnetic signal. Our results also imply that a PDRM is not simply locked due to progressive marine sediment consolidation and dewatering, and that the arbitrary functions (linear, cubic, and exponential) that are often used to model PDRM lock-in starting from the base of the surface mixed layer cannot explain fully the observed paleomagnetic signal.

Magnetism and mineralogy of Almahata Sitta polymict ureilite (= asteroid 2008 TC3): Implications for the ureilite parent body magnetic field

Abstract– The Almahata Sitta meteorite is the first case of recovered extraterrestrial material originating from an asteroid that was detected in near Earth space shortly before entering and exploding in the high atmosphere. The aims of our project within the 2008 TC3 consortium were investigating Almahata Sitta’s (AS) magnetic signature, phase composition and mineralogy, focussing on the opaque minerals, and gaining new insights into the magnetism of the ureilite parent body (UPB). We report on the general magnetic properties and behavior of Almahata Sitta and try to place the results in context with the existing data set on ureilites and ureilite parent body models. The magnetic signature of AS is dominated by a set of low‐Ni kamacites with large grain sizes. Additional contributions come from micron‐sized kamacites, suessite, (Cr) troilite, and daubreelite, mainly found in the olivine grains adjacent to carbon‐rich veins. Our results show that the paleomagnetic signal is of extraterrestrial origin as can be seen by comparing with laboratory produced magnetic records (IRM). Four types of kamacite (I–IV) have been recognized in the sample. The elemental composition of the ureilite vein metal Kamacite I (particularly Co) clearly differs from the other kamacites (II‐IV), which are considered to be indigenous. Element ratios of kamacite I indicate that it was introduced into the UPB by an impactor, supporting the conclusions of Gabriel and Pack (2009).

Paleomagnetism of the Todos Santos and La Silla Formations, Chiapas: Implications for the opening of the Gulf of Mexico

We report paleomagnetic data for the Lower to Middle Jurassic La Silla and Todos Santos formations of southern Mexico, in west-central Chiapas and the Tehuantepec Isthmus region. Volcanic rocks and red beds of these formations were deposited prior to or during the early stages of Gulf of Mexico opening. Dual-polarity characteristic magnetizations reside primarily in hematite and pass intraformational conglomerate, regional tilt, and reversal tests; they are thus interpreted as primary magnetizations. Our sampling sites are concentrated in three localities; around La Angostura Lake, 17 accepted sites yield a tilt corrected mean of declination (Dec) = 325°, inclination (Inc) = 4.6° ( k = 11.9, α 95 = 10.8°); in the Matias Romero region, the mean is Dec = 312.9°, Inc = 3.2° (based on only seven sites); and in the Custepec area, Jurassic andesitic dikes intruding rocks of the Permian Chiapas Massif yield a corrected mean of Dec = 335.0°, Inc = 5.0° (six sites). The mean directions are discordant with respect to expected North America reference directions, and indicate a counterclockwise rotation of 35° to 40°. Inclinations indicate deposition or emplacement at near equatorial paleolatitudes (2.1°N ± 3.4°). This paleolatitude estimate is statistically indistinguishable from those previously observed in the La Boca Formation of the Huizachal Group in northeast Mexico. The localities we sampled in southern Mexico are separated by ∼150 km, suggesting that the paleomagnetic signature of these rocks reflects regional-scale rather than local deformation. These Jurassic paleomagnetic directions support a rotational origin for the Gulf of Mexico. The data are also consistent with an Early to Middle Jurassic reconstruction that places the Chiapas Massif offshore the Tamaulipas state in the western Gulf of Mexico, and an Euler rotation pole for the Maya Block for this time period in the eastern gulf. The apparent polar wander path defined by paleomagnetic poles for the Chiapas Massif and Jurassic rocks reported here suggests that relative motion between North America and the Maya Block occurred between Late Permian and Early Jurassic time, during a protracted rifting phase, and then in the Late Jurassic in association with seafloor formation in the Gulf.

Eocene dinoflagellate cyst biostratigraphy of research borehole 011-BP, Omsk Region, southwestern Siberia

A shallow marine Eocene section recovered in the cored borehole 011-BP, southwestern Siberia, was analyzed palynologically. Age-diagnostic dinoflagellate cyst events including the first occurrences of Charlesdowniea coleothrypta, Dracodinium politum, Ochetodinium romanum, Samlandia chlamydophora, Areosphaeridium diktyoplokum, Hystrichosphaeropsis costae, Wetzeliella eocaenica and Duosphaeridium nudum are recognized at successive levels in the Upper Lulinvor Formation (498.0–459.0 m), indicating a Middle-latest Ypresian age. A hiatus spanning the Early Lutetian is present at an unconformity at 459.0 m which underlies sediments of the uppermost Lulinvor Formation (459.0–456.0 m), which are referred to the Middle Lutetian based on the presence of Costacysta bucina, Cordosphaeridium cantharellus and Wilsonidium echinosuturatum. The overlying interval from 456.0 m to 265.0 m (Tavda Formation) is referred to the latest Lutetian to Priabonian primarily based on a combination of published paleomagnetic signals and the presence of Rhombodinium draco, Membranosphaeridium aspinatum, Svalbardella sp., Thalassiphora fenestrata, Thalassiphora reticulata, Rhombodinium perforatum and Rhombodinium longimanum. According to the combined dinoflagellate cyst data and normal magnetic polarity in the uppermost part of the Tavda Formation, marine sedimentation was interrupted in southwestern Siberia during the Late Priabonian (∼34.8 Ma). The dinoflagellate cyst assemblages are illustrated, the new species Thalassiphora dominiquei is formally described and a neotype for Wetzeliella coronata (Vozzhennikova 1967) Lentin & Williams 1976 is designated. The morphology and taxonomy of several taxa are discussed.

10Be evidence for delayed acquisition of remanent magnetization in marine sediments: Implication for a new age for the Matuyama–Brunhes boundary

Fluxes of the meteoric cosmogenic radionuclide 10Be vary with changes in the incoming cosmic rays modulated by geomagnetic field intensity variations. The variability in the 10Be flux can be used to synchronize ice cores, as well as marine sediments, by comparison with the relative paleointensity variations of the geomagnetic field. However, lock-in of the paleomagnetic signal at some depth below the sediment–water interface in marine sediments through acquisition of a post-depositional remanent magnetization (PDRM) adds uncertainty to synchronization. Despite the long history of such studies, the magnitude of the PDRM lock-in depth remains controversial. In this article, we present clear evidence for a downward offset of the paleointensity minimum relative to the 10Be flux anomaly at the Matuyama–Brunhes (M–B) geomagnetic polarity boundary, which we interpret to result from a ∼ 15 cm PDRM lock in depth. This lock-in depth indicates that up to several tens of thousands years of age offset probably occurs when a paleomagnetic record is used for dating marine sediments, and the age of the M–B boundary should be revised to ca. 10 kyr younger, which is consistent with a younger ice core derived age of 770 ± 6 ka (2 σ). This cosmogenic age tuning strategy will contribute to refining paleomagnetic-based age models for marine sediments and identifying of lead-lag relationships for global abrupt environmental changes.

Clastic Sediments in the Butler Cave – Sinking Creek System, Virginia, USA

The Butler Cave - Sinking Creek System in Bath County, Virginia, consists of a master trunk passage along the axis of a syncline with a trellis arrangement of dip-oriented side caves. The western set of dip passages contain a sequence of massively and chaotically bedded sand and cobble sediments. Massive cobble fills also occur in the strike-oriented trunk passage. Cave passages on the eastern side of the syncline contain mostly sand and silt. The light fraction of the sediments consists predominantly of quartz and rock fragments. The sediments contain several percent heavy minerals composed of iron oxides, zircon, rutile, tourmaline and other minerals. Measurement of the visible and near infrared diffuse reflectance spectra shows at least three populations of sediments to be present: an iron-rich, clay-poor group; a clay-rich group; and a gypsiferous sediment. The iron minerals provided a paleomagnetic signal. Sediments from the trunk passage, deposited by recent underground drainage, contained a normal pole direction. Sediments from the dip passages were paleomagnetically reversed, showing the deposition dates from prior to 780,000 years. In one instance reversed polarity deposits overlie normal polarity, implying a minimum age of 990,000 years for the reversed sediments.

Kulanaokuaiki Tephra (ca. A.D. 400-1000): Newly recognized evidence for highly explosive eruptions at Kilauea Volcano, Hawai'i

Kīlauea may be one of the world9s most intensively monitored volcanoes, but its eruptive history over the past several thousand years remains rather poorly known. Our study has revealed the vestiges of thin basaltic tephra deposits, overlooked by previous workers, that originally blanketed wide, near-summit areas and extended more than 17 km to the south coast of Hawai‘i. These deposits, correlative with parts of tephra units at the summit and at sites farther north and northwest, show that Kīlauea, commonly regarded as a gentle volcano, was the site of energetic pyroclastic eruptions and indicate the volcano is significantly more hazardous than previously realized. Seventeen new calibrated accelerator mass spectrometry (AMS) radiocarbon ages suggest these deposits, here named the Kulanaokuaiki Tephra, were emplaced ca. A.D. 400–1000, a time of no previously known pyroclastic activity at the volcano. Tephra correlations are based chiefly on a marker unit that contains unusually high values of TiO 2 and K 2 O and on paleomagnetic signatures of associated lava flows, which show that the Kulanaokuaiki deposits are the time-stratigraphic equivalent of the upper part of a newly exhumed section of the Uwēkahuna Ash in the volcano9s northwest caldera wall. This section, thought to have been permanently buried by rockfalls in 1983, is thicker and more complete than the previously accepted type Uwēkahuna at the base of the caldera wall. Collectively, these findings justify the elevation of the Uwēkahuna Ash to formation status; the newly recognized Kulanaokuaiki Tephra to the south, the chief focus of this study, is defined as a member of the Uwēkahuna Ash. The Kulanaokuaiki Tephra is the product of energetic pyroclastic falls; no surge- or pyroclastic-flow deposits were identified with certainty, despite recent interpretations that Uwēkahuna surges extended 10–20 km from Kīlauea9s summit.

Reductive diagenesis, magnetite dissolution, greigite growth and paleomagnetic smoothing in marine sediments: A new view

In many anoxic sedimentary environments, the onset of sulfate reduction, and pyritization of detrital iron-bearing minerals, leads to a precipitous decline in magnetic mineral concentration during early diagenesis. The usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution. Detailed rock magnetic and electron microscope analyses of rapidly deposited (~ 7 cm/kyr) latest Pleistocene–Holocene sediments from the continental margins of Oman (22°22.4′N, 60°08.0′E) and northern California (38°24.8′N, 123°58.2′W) demonstrate that pyritization during early diagenesis also leads to the progressive down-core growth of the ferrimagnetic iron sulfide greigite. Greigite growth begins with nucleation of large concentrations of superparamagnetic (SP) nanoparticles at the inferred position of the sulfate–methane transition, which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of SP particles. Looping of hysteresis parameters on a “Day” plot records the dissolution of single domain (SD) (titano-)magnetite and the formation of SP greigite, which then slowly and progressively grows through its SD blocking volume and acquires a stable paleomagnetic signal. This looping trend is also evident in data from several published records (Oregon margin, Korea Strait, Japan Sea, Niger Fan, Argentine margin, and the Ontong–Java Plateau), indicating that these processes may be widespread in reducing environments. Our observations have profound implications for paleomagnetic records from sulfate-reducing environments. The paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10's of kyr, and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10's to 100's of kyr. We therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis.

Post-depositional remanent magnetization lock-in and the location of the Matuyama–Brunhes geomagnetic reversal boundary in marine and Chinese loess sequences

Bioturbation disturbs detrital magnetic particles after deposition, so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post-depositional remanent magnetization (PDRM) whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle. The relationship between the Matuyama–Brunhes boundary (MBB) and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked-in. However, such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location. It is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis. When the global data set is subjected to such a rigorous analysis, few reliable data remain for the MBB. Using two complementary approaches, we estimate that the MBB is, on average, shifted ≤ 20 cm below its true position in marine sediments. This offset is the sum of the thickness of the bioturbated surface mixed layer, which is possibly dominant, and the PDRM lock-in depth. There is also controversy concerning observed differences in the position of the MBB relative to paleoclimatic proxies in marine sediments and Chinese loess deposits. For the Chinese loess, quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the MBB with respect to paleoclimatic boundaries. We conclude that the MBB occurs late in marine oxygen isotope stage 19, and in the upper part of Chinese paleosol unit S8, rather than at the mid or lower part of loess unit L8. Our results require adjustment of the generally accepted positions for the MBB, and resolve a longstanding chronological conundrum for marine and Chinese loess sequences.

On the possibility of recovering paleo-diurnal magnetic variations in transitional lava flows: I. Constraints from thermoremanence modelling for an experimental protocol

One of the tenets in paleomagnetism is that perturbations of the ground magnetic field due to magnetospheric or ionospheric current systems are too small to leave a detectable paleomagnetic signature in lava flows. As suggested by recent work in paleomagnetosphere modelling, however, external-field perturbations may be significantly enhanced during periods of transitional field behavior, particularly when the dipole-field axis is strongly tilted towards the equator, which then leads to an extremely dynamic magnetosphere on the diurnal time scale even for quiet solar wind conditions. We here demonstrate that thin (rapidly cooled) lava flows ( ∼ 50 cm thick) with high magnetic blocking temperatures (within ∼ 50 –100 ° C below the Curie temperature) indeed have the potential to record such diurnal perturbations. Further, an experimental protocol is suggested to paleomagnetically extract these perturbations. Our proof-of-concept is based on numerical modelling of thermoremanence (TRM) acquisition and simulation of thermal demagnetization surfaces for discrete temperature steps in function of vertical position in the flow. The TRM direction recovered at a given thermal demagnetization step varies with vertical position in the flow and thereby reveals the wave form of the external-field variation. Characteristically, the vertical position of a captured signal changes systematically with unblocking temperature, which reflects the oblique orientation of cooling isochrons, along which the signals are blocked. The signals have their largest amplitudes at the maximum unblocking temperatures, but decay away at lower temperatures. It is by these systmatic trends that external-field perturbations, if trapped, can be paleomagnetically identified and distinguished from a secondary overprint. The experimental procedure requires a sample spacing of 1 cm (with 1 cm drill cores) and small thermal demagnetization steps (15 ° C) at elevated temperatures.

On the possibility of recovering palaeo-diurnal magnetic variations in transitional lava flows: 2. An experimental case study

Geomagnetic field variations of external origin may be enhanced during periods of transitional field behaviour, particularly when the dipole moment is low, in which case they are likely to leave a paleomagnetic signature in rapidly cooled lava flows. To test this proposition, we have resampled en bloc and studied in fine detail a thin transitional Aa flow from a mid-Miocene lava sequence on Gran Canaria which was paleomagnetically investigated previously (Leonhardt, R., Soffel, H.-C., 2002. A reversal of the Earth's magnetic field recorded in mid-Miocene lava flows of Gran Canaria, Paleointensities. Journal of Geophysical Research 107, 2299. doi:10.1029/2001JB000949). The flow is characterised by high-unblocking temperatures, an equatorial VGP position and a very low absolute palaeointensity of ∼2 μT. Two slabs were cut out of the flow and sampled at 1 cm intervals, along four vertical profiles running parallel to each other. Thermal demagnetisation was performed on two profiles using heating steps as small as 15 °C at elevated temperatures. The high-temperature part of the unblocking spectrum was found to be remarkably constant across the flow, as was the Curie temperature of 540 °C, and the negligible anisotropy of magnetic susceptibility. The exsolution lamallae observed under the microscope point to deuteric (high temperature) oxidation having occurred prior to the acquisition of the primary thermoremanent magnetisation. While the absolute palaeointensity values vary only little with vertical position, the magnetisation directions recovered by thermal demagnetisation vary considerably (on average, by some 20° at 500 °C). These large variations can be attributed to an overprint by secondary minerals, formed by fluid diffusion around vesicles and low-temperature oxidation. Since the secondary magnetisation recorded transitional directions as well, the overprint must have occurred soon after emplacement. The directional variations typically decrease in amplitude with increasing blocking temperature, which is contrary to what would be expected if pronounced diurnal external field variations were trapped in the flow.

Morphology of the Iceland Basin Excursion from a spherical harmonics analysis and an iterative Bayesian inversion procedure of sedimentary records

Based on 5 published marine high-resolution sedimentary records of the Iceland Basin Excursion [IBE; Channell, J.E.T., Hodell, D.A., Lehman, B., 1997. Relative geomagnetic paleointensity and ∂ 18O at ODP Site 983/Gardar Drift, North Atlantic since 350 ka. Earth Planet. Sci. Lett. 153, 103–118; Laj, C., Kissel, C., Roberts, A., 2006. Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: a simple transitional field geometry? Geochem. Geophys. Geosystems. 7, Q03004, doi:10.1029/2005GC001122] dated around 186–190 kyr, we present models of the excursional geomagnetic field at the Earth's surface using two different approaches. First a spherical harmonics analysis is performed after synchronization of the records using their paleointensity profiles. Second, we have used an iterative Bayesian inversion procedure, calibrated using the single volcanic data available so far. Both modeling approaches suffer from imperfections of the paleomagnetic signals and mostly from the still poor geographical distribution of detailed records, presently available only from the North Atlantic and the West Pacific. For these reasons, our modeling results should only be regarded as preliminary models of the geomagnetic field during the IBE, susceptible to improvements when including results from future paleomagnetic studies. Nevertheless, both approaches show distinct similarities and are stable against moderate variations of modeling parameters. The general picture is that of a dipole field undergoing a strong reduction, but remaining higher than the non-dipole field all through the excursional process, except for a very short interval of time corresponding to the dipole minimum at the center of the excursion. On the other hand, some differences exist between the results of the two models with each other and with the real data when the virtual geomagnetic pole (VGP) paths are considered. The non-dipole field does not appear to undergo very significant changes during the excursion except for a slight increase just at the dipole minimum. The width of mid-height of the dipole minimum, which can be considered as an approximate measure of the duration of the IBE is of the order of 3 kyr according to the SPECMAP age model, consistent with a suggestion by Gubbins [Gubbins, D., 1999. The distinction between geomagnetic excursions and reversals. Geophys. J. Int. 137, F1–F3]. These results are compared to those obtained for the Matuyama-Brunhes geomagnetic reversal [Leonhardt, R., Fabian, K., 2007. Paleomagnetic reconstruction of the global geomagnetic field evolution during the Matuyama/Brunhes transition: iterative Bayesian inversion and independent verification. Earth Planet. Sci. Lett. 253, 172–195].

Paleoearthquake deformations recorded by magnetic variables

Faults and related deformational structures give evidence of earthquakes in the past. The ground shaking at high-magnitude seismic events usually generates liquefaction; the higher seismic magnitude, the larger area of liquefaction. We can now demonstrate that it is possible to discriminate between plastic and liquefied deformations with magnetic methods. Besides, it is possible to record internal crypto-deformations not visibly observed. This offers new means of recording past earthquakes. Fine particles carrying the magnetic remanence are shown to move, rotate and re-align with respect to the prevailing geomagnetic field direction or in response to the applied shaking forces. We call this seismomagnetization. This implies that paleoseismics may generate excursion-like paleomagnetic signals despite the absence of visible deformation structures.

Paleomagnetism of the Central Metasedimentary Belt, Grenville Province, Ontario: Differential uplift or collision of terranes?

A paleomagnetic traverse across the Central Metasedimentary Belt of the Precambrian Grenville Province in southern Ontario, linked to an earlier study of the adjoining Central Gneiss Belt, sampled 54 sites in the Frontenac, Sharbot Lake, Mazinaw, Elsevir and Bancroft terranes. 40Ar/ 39Ar thermochronometry indicates differential unroofing of these terranes, with slow cooling (1.0–1.5 °C/Ma) along parallel temperature–time paths. Presently exposed rocks cooled through ≈ 500 °C around 1100 Ma (Frontenac), 1010 Ma (Elsevir), 960 Ma (Central Metasedimentary Belt boundary zone) and 930 Ma (Mazinaw) [Cosca, M.A., Sutter, J.F., Essene, E.J., 1991. Cooling and inferred uplift/erosion history of the Grenville orogen , Ontario: constraints from 40Ar/ 39Ar thermochronology, Tectonics 10 (1991) 959–977]. 1100–930 Ma was a time of rapid paleolatitude shift but the various terranes do not have distinct paleomagnetic signatures. Frontenac, Sharbot Lake and Mazinaw terranes have similar reverse-polarity paleopoles in the central part of the Grenville polar wander track. Frontenac and Sharbot Lake normal poles are typical ≈ 1000 Ma Grenville A poles, while Mazinaw normal poles seem to be somewhat older. Frontenac poles bear no resemblance to 1100 Ma Keweenawan poles. The Elsevir pole and some Frontenac poles resemble Grenville B poles (600–800 Ma). Although 40Ar/ 39Ar hornblende ages were reset thermally between 1100 and 930 Ma during differential uplift of terranes, the terranes were not remagnetized at these same times. Frontenac, Sharbot Lake and Mazinaw terranes (like much of the boundary zone and the Central Gneiss Belt) seemingly were all remagnetized between ≈ 1000 and ≈ 900 Ma, perhaps by a widespread thermochemical event. A 600–800 Ma event affected Elsevir and part of the Frontenac, at the western and eastern extremes of the traverse, but not areas between. The lack of correlation between 40Ar/ 39Ar ages and paleomagnetic results for the Frontenac, Sharbot Lake, Mazinaw and Elsevir casts doubt on the thermochronometric age calibration of the Grenville apparent polar wander path, which is based on the assumption that the magnetic and isotopic systems respond in similar ways to slow cooling during uplift so that their blocking temperatures can be directly compared. The most discordant results – those of the Frontenac – could be reconciled if the Frontenac terrane acquired its A magnetization around 1100 Ma before it docked with Laurentia. This is geologically plausible but the match between the Frontenac poles and 900–1000 Ma Grenvillian poles seems too close to be coincidental.

Integrated stratigraphy and 40Ar/ 39Ar chronology of early Middle Miocene sediments from DSDP Leg 42A, Site 372 (Western Mediterranean)

An integrated magneto-biostratigraphic framework is presented for Middle Miocene sediments of DSDP Site 372 located in the Western Mediterranean. Detailed biostratigraphic analysis shows a nearly complete sequence of early Middle Miocene calcareous plankton bioevents in the Mediterranean, including the LCO (Last Common Occurrence) of the nannofossil Sphenolithus heteromorphus which has been astronomically dated in the Ras il Pellegrin (RIP) section on Malta Island [Abels, H.A., Hilgen, F.J., Krijgsman, W., Kruk, R.W., Raffi, I., Turco, E., Zachariasse, W.J., 2005. Long-period orbital control on middle Miocene global cooling: integrated stratigraphy and astronomical tuning of the Blue Clay Formation on Malta, Paleoceanography, 20, PA4012. doi: 10.1029/2004PA001129. 11 pp]. Thermal demagnetization of discrete samples revealed a characteristic low-temperature component with dual polarities despite a weak paleomagnetic signal. The resultant magnetostratigraphic record, combined with the calcareous plankton biostratigraphy, is straightforwardly correlated to the geomagnetic polarity time scale (CK95) of Cande and Kent [Cande, S.C., Kent, D.V., 1995. Revised calibration of the Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, J. Geophys. Res., 100, 6093–6095] and the Astronomical Tuned Neogene Time Scale (ATNTS04) of Lourens et al. [Lourens, L.J., Hilgen, F.J., Laskar, J., Shackleton, N.J., Wilson, D., 2004. The Neogene Period. In: Gradstein F.M., Ogg J.G., Smith A.G. (Eds.), A Geologic Time Scale, Cambridge Univ. Press, pp. 409–440]. The subchrons recorded in Site 372 succession range from C5Br up to C5AAr. To confirm the magnetostratigraphic calibration, 40Ar/ 39Ar dating was performed on feldspar of two volcanic ash layers. The radio-isotopic dating indicates a younger age for these two ash layers compared to the magnetostratigraphic calibrated ages according to the CK95 and ATNTS04 age models. However, if the astronomically calibrated age of 28.21 ± 0.04 Ma is used for the Fish Canyon standard (FCs), the age for the older ash layer exactly matches its ATNTS04 age. Ages for bioevents were calculated assuming constant sedimentation rates between magnetostratigraphic age-tie points. The S. heteromorphus LCO has an age of 13.54 Ma and 13.63 Ma according to CK95 and ATNTS04, respectively, which is consistent with the astronomical tuned age of 13.65 Ma determined at RIP section [Abels, H.A., Hilgen, F.J., Krijgsman, W., Kruk, R.W., Raffi, I., Turco, E., Zachariasse, W.J., 2005. Long-period orbital control on middle Miocene global cooling: integrated stratigraphy and astronomical tuning of the Blue Clay Formation on Malta, Paleoceanography, 20, PA4012. doi:10.1029/2004PA001129. 11 pp]. We therefore conclude that the magnetostratigraphic calibration of DSDP Site 372 is correct and that, for the time being, this site can be considered as a reference section for the early Middle Miocene of the Mediterranean region.

The contact zone between the ALCAPA and Tisza-Dacia mega-tectonic units of Northern Romania in the light of new paleomagnetic data

Paleomagnetic analyses were carried out on samples from 19 localities within two different mega-tectonic units in Northern Romania: Tisza-Dacia (11 localities) and ALCAPA (8 localities). The samples cover a range of different lithologies: (1) Late Cretaceous red-coloured marl to marly limestone, (2) Eo-Oligocene flysch sediments, and (3) mid-Miocene (Langhian) tuffite (Dej tuff and related sediments). The Late Cretaceous and mid-Miocene specimens carry secondary paleomagnetic signals exhibiting a counter clockwise deflection of the paleo-declinations by some 30°, while the Eo-Oligocene localities indicate an overall clockwise deflected (between some 45° and >90°) paleodeclination with respect to present-day north. Clockwise rotation postdates the age of sedimentation (Lower Oligocene), as well as (at least partially) thrusting of the Pienides onto the Tisza-Dacia mega-tectonic unit, which occurred between 20.5 and 18.5 Ma. Clockwise rotation predates post-12 Ma counter clockwise rotations inferred for the mid-Miocene localities. Surprisingly, the clockwise rotations of the first rotational stage not only affected the (par-) autochthonous sedimentary cover of the Tisza-Dacia megatectonic unit, but also the allochthonous flysch nappes of the Pienides, i.e. the eastern tip of the ALCAPA mega-tectonic unit. Well-documented opposed rotation of the remainder of ALCAPA necessitates a detachment of this eastern tip of ALCAPA after 18.5 Ma. The most likely location for this detachment zone is along the margins of the Transcarpathian depression. During a second (post-12 Ma) stage, counter clockwise rotations of up to 30° affected the entire working area. Regarding timing and magnitude, these second stage rotations are similar to rotations documented for the East Slovak basin, but different from those reported from the South Apuseni Mountains and the Central and Inner West Carpathians located west of the East Slovak basin.