Relative Paleointensity Variations Research Articles

A full-vector paleomagnetic secular variation record from 55,000 to 33,000 cal. years BP from Río Valdéz glaciolacustrine outcrop (Tierra Del Fuego, Argentina)

High-resolution paleomagnetic investigations were performed on Río Valdez outcrop at the Fagnano Lake, central Isla Grande de Tierra del Fuego, southernmost South America. Our aims were to develop a full-vector paleomagnetic secular variation (PSV) record and establish paleomagnetic chronostratigraphy for the outcrop based on radiocarbon dating and the correlation of the PSV record with other dated PSV records in the region. We detected two distinct anomalous directional intervals at 35,400 and 33,800 cal. years BP and at 41,500 and 39,000 cal. years BP, which could be associated with Mono Lake and Laschamp excursions. Rock magnetic investigations revealed slight changes in concentration, mineralogy, and magnetic grain size along the sedimentary sequence, but the reconstruction of past direction and relative paleointensity variations in the geomagnetic field remained feasible. The record provides new insights into the behavior of the geomagnetic field at high latitudes in the Southern Hemisphere, about which very little is currently known.

Recording Fidelity of Relative Paleointensity Characteristics in the North Pacific Ocean

AbstractNorth Pacific Ocean sediments are important archives of geomagnetic field and paleoenvironmental evolutions, yet late Quaternary sedimentary sequences in the North Pacific Ocean are less explored because of generally low sedimentation rates and the challenge of dating sediments deposited below the calcite compensation depth. Core NP02 sediments from the North Pacific Ocean contain three visible tephra layers, which are identified as the To‐Of, Spfa‐1, and Kt‐3 tephra, respectively. An age model for core NP02 is established based on tephra correlation and one radiocarbon date, which gives a mean sedimentation rate of ∼11.6 cm/kyr. Magnetic analyses suggest the dominance of two remanence carriers; detrital vortex state and biogenic single domain magnetite. Variable depth offsets between the relative paleointensity (RPI) signals carried by the two remanence carriers adds a negligible age error (<0.5 kyr) to the core NP02 RPI record. Pronounced RPI amplitude variations are observed, and the Rockall and Laschamp excursions are identified in the core NP02 RPI record based on the age model. The fidelity of the core NP02 RPI record is further verified by comparison with other RPI stacks and records, and we demonstrate that the RPI variations can assist in chronology refinement.

Regional chronostratigraphy in the eastern Lesser Antilles quaternary fore-arc and accretionary wedge sediments: Relative paleointensity, oxygen isotopes and reversals

Paleomagnetism is a powerful tool for establishing an almost continuous chronostratigraphy for an entire region. When combined with other dating methods, absolute or relative, it can be used to develop a regional reference chronostratigraphic framework. During the summer of 2016, several piston cores were collected along the Atlantic side of the Lesser Antilles onboard the R/V Pourquoi pas? as part of the CASEIS Expedition. Core CAS16-24PC was devoted to chronostratigraphic analysis and allowed development of a quasi-continuous Quaternary record. In this study, after identifying and removing rapidly deposited layers such as turbidites and tephra layers, we reconstructed the relative paleointensity (RPI) variations by normalizing the natural remanent magnetization with the laboratory induced anhysteretic remanent magnetization. An age model was developed by comparing our RPI record with the PISO-1500 stack and paleomagnetic axial dipole moment model for the past 2 Myr (PADM-2M) and the planktic oxygen isotopic record (δ18O) for core CAS16-24PC with the LR04 benthic stack. By combining the δ18O stratigraphy with paleomagnetic analyses, we established an age model covering the Brunhes/Matuyama boundary and Jaramillo Subchron back to ~1.15 Ma with a mean sedimentation rate of 1.7 cm/kyr. This age model complements the paleomagnetic data from IODP campaigns and volcanic records, and offers almost complete inclination, declination and RPI records as a local reference.

On the Possibility of Obtaining a High Resolution Relative Paleointensity Record of the Pringle Falls Excursion at the Type Locality of Pringle Falls, Oregon, USA

In order to further understand the full vector excursional details of the geomagnetic field, a paleomagnetic and rock magnetic study of four sites has been conducted at the type locality of Pringle Falls, Oregon where 827 samples were drilled and spaced along a distance of 5 km, for their detailed directional and relative paleointensity studies. The profiles have registered a high-reso- lution (>10 cm/kyr) paleomagnetic record of the excursion (ca. 211+/13 ka) as recorded by diatomaceous lacustrine sediments. Remanence as well as induced magnetization experiments to investigate the reproducibility of the signal throughout the profiles have been conducted. In addition, low-field susceptibility vs. temperature analysis was performed indicating that the main magnetic carrier is pure magnetite (Curie point 575 ℃). The magnetic grain size also has indicated Single Domain-Multi-Domain (SD-MD) magnetite. The demagnetization was done by alternating field (a.f.) experiments, and the mean directions were determined by principal component analyses. In addition, induced magnetic tests were done, such as magnetic susceptibility (x) analyses, saturation IRM, anhysteretic remanent magnetization (ARM70) as well as the normalization of J17.5 mT/ARM70 to attempt to obtain relative paleointensity records of these sediments in question. The results of the induced rock magnetic tests such as the normalization studies indicate a direct correlation between the decrease of the relative paleointensity variations (i.e. lows) with respect to the directional changes. The detailed behavior of the paleosignal is highly consistent, since they are rapidly deposited sediments providing a detailed representation of the paleofield. The dissected VGP paths in 3 different phases are highly internally consistent and are defined by clockwise and anticlockwise loops traveling from the high northern latitudes over eastern North America and the North Atlantic to South America and then to high southern latitudes. They then return to the high northern latitudes through the Pacific and over to Kamchatka. This VGP behavior defines the geomagnetic signature of the Pringle Falls excursion as recorded at the type locality.

Nongeocentric axial dipole field behavior during the Mono Lake excursion

AbstractA new record of the Mono Lake excursion (MLE) is reported from the Summer Lake Basin of Oregon, USA. Sediment magnetic properties indicate magnetite as the magnetization carrier and imply suitability of the sediments as accurate recorders of the magnetic field including relative paleointensity (RPI) variations. The magnitudes and phases of the declination, inclination, and RPI components of the new record correlate well with other coeval but lower resolution records from western North America including records from the Wilson Creek Formation exposed around Mono Lake. The virtual geomagnetic pole (VGP) path of the new record is similar to that from another high‐resolution record of the MLE from Ocean Drilling Program (ODP) Site 919 in the Irminger Basin between Iceland and Greenland but different from the VGP path for the Laschamp excursion (LE), including that found lower in the ODP‐919 core. Thus, the prominent excursion recorded at Mono Lake, California, is not the LE but rather one that is several thousands of years younger. The MLE VGP path contains clusters, the locations of which coincide with nonaxial dipole features found in the Holocene geomagnetic field. The clusters are occupied in the same time progression by VGPs from Summer Lake and the Irminger Basin, but the phase of occupation is offset, a behavior that suggests time‐transgressive decay and return of the principal field components at the beginning and end of the MLE, respectively, leaving the nonaxial dipole features associated with the clusters dominant during the excursion.

Oligocene–Miocene relative (geomagnetic) paleointensity correlated from the equatorial Pacific (IODP Site U1334 and ODP Site 1218) to the South Atlantic (ODP Site 1090)

Late Oligocene to Early Miocene relative paleointensity (RPI) proxies can be correlated from the equatorial Pacific (IODP Site U1334 and ODP Site 1218) to the South Atlantic (ODP Site 1090). Age models are constrained by magnetic polarity stratigraphy through correlation to a common geomagnetic polarity timescale. The RPI records do not contain significant power at specific (orbital) frequencies, and hence there is no significant coherency between RPI proxies and the normalizers used to construct the proxies, although orbital power is present in some normalizers. There is no obvious control on RPI proxies from mean sedimentation rate within polarity chrons, magnetic grain size proxies or magnetic concentration parameters. The salient test is whether the equatorial Pacific records can be correlated one to another, and to the records from the South Atlantic. All records are dominated by RPI minima at polarity reversals, as expected, although the comparison within polarity chrons is compelling enough to conclude that the intensity of the Earthʼs axial dipole is being recorded. This is supported by the fit of RPI data from Sites U1334 and 1218 after correlation of the two sites using diverse core-scanning data, rather than polarity reversals alone. We do not see a consistent relationship between polarity-chron duration and mean RPI, and no consistent skewness (“saw-tooth” pattern) for RPI within polarity chrons. Stacks of RPI records for 17.5–26.5 Ma include long-term changes in RPI on Myr timescales that are superimposed on RPI minima associated with polarity reversals, and shorter-term variations in RPI with an apparent pacing of ∼50 kyr. The equatorial Pacific to South Atlantic correlations indicate that RPI can be used as a (global) stratigraphic tool in pre-Quaternary sediments with typical pelagic sedimentation rates of a few cm/kyr.

Rock‐magnetic artifacts on long‐term relative paleointensity variations in sediments

Long‐term changes of geomagnetic field intensity, including possible dependence on lengths of polarity intervals, provide fundamentally important information for understanding the geodynamo. A positive correlation between paleointensity and polarity interval length was previously suggested from an Oligocene (ca. 23–34 Ma) relative paleointensity record at Deep Sea Drilling Program Site 522 in the Atlantic Ocean, which is the only continuous paleointensity data set published so far for this age interval. We have conducted a paleomagnetic study of Eocene to Oligocene sediments at three sites in the eastern equatorial Pacific Ocean. Our objectives include revisiting the issue of the paleointensity‐polarity length correlation. Magnetic properties of the sediments meet the frequently used criteria for reliable relative paleointensity estimation. Although short‐wavelength normalized remanence intensity fluctuations associated with polarity boundaries and possible geomagnetic excursions agree among the three sites, long‐term changes are inconsistent. Apparent positive correlation between normalized intensity and polarity length was observed, but the normalized intensity has an obvious anti‐correlation with the ratio of anhysteretic remanent magnetization (ARM) to isothermal remanent magnetization (IRM), which is mainly controlled by the relative abundance of biogenic and terrigenous magnetic minerals. Furthermore, the normalized intensity correlates with sedimentation rate. These facts indicate a lithological contamination on the normalized intensity records. The dependence on ARM/IRM and sedimentation rate is also evident at Site 522. It is inferred that variations in sedimentation rate and the relative abundance of biogenic magnetite on depositional remanent magnetization acquisition efficiency may not be well compensated by the normalization. It is therefore premature to conclude that stronger geomagnetic fields were recorded during longer polarity intervals from currently available normalized intensity records.

Relative paleointensity and environmental magnetism since 1.2 Ma at IODP site U1305 (Eirik Drift, NW Atlantic)

Integrated Ocean Drilling Program (IODP) Expedition 303 to the North Atlantic in 2004 recovered rapidly deposited deep-sea sediments at IODP Site U1305 on Eirik Drift, located south of Greenland at 3460m water depth, along the path of the Western Boundary Under Current (WBUC). About 200m of sediment was sampled with u-channels from the composite section, providing a continuous record of paleomagnetic field directions and relative paleointensity (RPI) variations covering the past 1.2Myr. The age model, based on an oxygen isotope record, is consistent with the fit of the RPI record to a calibrated template, and indicates higher sedimentation rates during interglacials relative to glacial epochs. Magnetite grain-size and concentration proxies indicate higher concentrations of magnetite with larger grain sizes during interglacials. Enhanced interglacial deposition can be attributed to a combination of elevated entrainment of terrigenous detritus into the WBUC due to glacial retreat on continents flanking the upstream path of the WBUC (east Greenland and Iceland), and of increased bottom current (WBUC) vigor leading to elevated transport and deposition at the site during interglacials. This pattern is opposite to observations of flow of the Antarctic circum polar current (ACC) in the south Indian Ocean, which suggests an inter-hemispheric antiphase in marine circulation at the Milankovitch scale, with strong circulation in the deep North Atlantic when the ACC is weak, and vice versa.

Variations of the geomagnetic field during the Cretaceous

This study provides a compilation of the paleointensity records for the Cretaceous period derived from sediments of the Russian Plate and adjacent areas. The paleoinetensity values were calibrated using the laboratory redeposition experiments. Remarkable differences in the relative paleointensity variations were detected at the Berriasian–Early Barremian, Late Barremian–Santonian and Early–Late Maastrichtian boundaries. In the Berriasian–Early Barremian interval, the paleointensity varied stochastically, with the amplitude of about 1Ho and the mean value of 0.63Ho (Ho is the present-day geomagnetic field intensity assumed to be 40 μT). The records for the Barremian–Santonian give a picture of the geomagnetic field with alternating high- and low-amplitude features. The mean paleointensities remain constantly high (being on average 0.87Ho), and intervals of low-amplitude variation alternate with the pronounced bursts (3.5Ho). The Late Maastrichtian interval is characterized by high-amplitude paleointensity variations (4Ho) and a sharp drop towards the end of the interval. All records show remarkable similarities near the boundaries between geological time intervals, which are an increase in the amplitude and mean values of intensity at the end of intervals followed by a decrease towards the beginning of the subsequent interval.

From superchrons to secular variation: A broadband dynamo frequency spectrum for the geomagnetic dipole

We build a broadband frequency spectrum for geomagnetic dipole intensity variations originating in the core using results from numerical dynamos. Scaling dipole intensity spectra from numerical dynamos with diverse input parameters (Ekman, Rayleigh, and magnetic Prandtl numbers) and magnetic Reynolds numbers from 170 to 1985 yields a composite spectrum ranging from ultra-low frequencies (∼100Myr period) to very high frequencies (∼1yr period). The amplitude and shape of our composite spectrum compare favorably with estimates of the geomagnetic and paleomagnetic dipole moment spectra. Significant power is found in an ultra-low frequency band f<0.1Myr−1 that corresponds to geomagnetic superchrons and also in a low frequency band f=0.1−5Myr−1 that corresponds to the time scales of geomagnetic polarity chrons. In higher frequency bands our composite spectrum varies with frequency approximately like f−n. In a transitional frequency band from f=5Myr−1 to f=2kyr−1, characteristic of relative paleointensity variations in sediments, we find n≃1.8. In a high frequency band with f>2kyr−1, representing archaeomagnetic and historical geomagnetic secular variation, we find n≃4 on average, with spectral slope increasing at very high frequencies to near n≃6 at f=1yr−1. The secular variation and the secular acceleration spectra for the dipole field reach their peak values in the high frequency band, close to frequencies where spectra for the dynamo fluid velocity decrease sharply.

A Holocene paleosecular variation record from the northwestern Barents Sea continental margin

A high-resolution paleomagnetic and rock magnetic study has been carried out on sediment cores collected in glaciomarine silty-clay sequences from the continental shelf and slope of the southern Storfjorden trough-mouth fan, on the northwestern Barents Sea continental margin. The Storfjorden sedimentary system was investigated during the SVAIS and EGLACOM cruises, when 10 gravity cores, with a variable length from 1.03 m to 6.41 m, were retrieved. Accelerator mass spectrometry (AMS) 14C analyses on 24 samples indicate that the cores span a time interval that includes the Holocene, the last deglaciation phase and in some cores the last glacial maximum. The sediments carry a well-defined characteristic remanent magnetization and have a valuable potential to reconstruct the paleosecular variation (PSV) of the geomagnetic field, including relative paleointensity (RPI) variations. The paleomagnetic data allow reconstruction of past dynamics and amplitude of the geomagnetic field variations at high northern latitudes (75°–76° N). At the same time, the rock magnetic and paleomagnetic data allow a high-resolution correlation of the sedimentary sequences and a refinement of their preliminary age models. The Holocene PSV and RPI records appear particularly sound, since they are consistent between cores and they can be correlated to the closest regional stacking curves (UK PSV, FENNOSTACK and FENNORPIS) and global geomagnetic model for the last 7 ka (CALS7k.2). The computed amplitude of secular variation is lower than that outlined by some geomagnetic field models, suggesting that it has been almost independent from latitude during the Holocene.

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.

Geomagnetic relative paleointensity chronostratigraphy of sediment cores from the Okhotsk Sea

We conducted a paleomagnetic study of three piston cores from the central Okhotsk Sea to enable inter-core correlation and age estimation using relative paleointensity of the geomagnetic field, which can provide the basis for paleoceanographic studies in this area. The cores are composed of diatom-bearing dark olive gray silty clay. Magnetic overprints were generally removed by alternating-field demagnetization of up to 10 mT. Magnetic properties reveal no evidence of reductive diagenesis or magnetite dissolution. The anhysteretic remanent magnetization (ARM) was used as a normalizer for relative paleointensity estimations, because there is a smaller coherence between normalized intensity and the ARM than with saturation isothermal remanent magnetization (SIRM). Depths of the three cores were converted to ages by correlating relative paleointensity variations with those from Ocean Drilling Program Site 983, which was selected as a target curve because of its higher resolution than the widely-used Sint-800 paleointensity stack. The age at the bottom of the cores is estimated to be between 360 and 540 ka. Inter-core correlation using relative paleointensity variations reveals that changes in magnetic properties such as magnetic susceptibility and magnetic mineral grain size are not synchronous between the northernmost site and the southern two sites despite their close proximity, which indicates regional differences in paleoenvironmental variations associated with glacial–interglacial changes. Nevertheless, relative paleointensity variations among the three cores are mutually consistent, and agree with other published paleointensity curves, which indicates that our paleointensity records are not influenced significantly by lithological changes.

Relative geomagnetic paleointensity of the Brunhes Chron and the Matuyama–Brunhes precursor as recorded in sediment core from Wilkes Land Basin (Antarctica)

Paleomagnetic and rock magnetic investigation was performed on the 35-m long MD03-2595 CADO (Coring Adélie Diatom Oozes) piston core recovered on the continental rise of the Wilkes Land Basin (East Antarctica). Analysis of the characteristic remanent magnetization (ChRM) inclination record indicates a normal magnetic polarity for the uppermost 34 m of the sequence and a distinctive abrupt polarity change at the bottom of the core. This polarity change, which spans a 27 cm thick stratigraphic interval, represents a detailed record of the Matuyama–Brunhes (M–B) transition and it is preceded by a sharp oscillation in paleomagnetic directions that may correlate to the M–B precursor event. Paleomagnetic measurements enable reconstruction of geomagnetic relative paleointensity (RPI) variations, and a high-resolution age model was established by correlating the CADO RPI curve to the available global reference RPI stack, indicating that the studied sequence reaches back to ca. 800 ka with an average sedimentation rate of 4.4 cm/ka. Orbital periodicities (100 ka and 41 ka) were found in the ChRM inclination record, and a significant coherence of ChRM inclination and RPI record around 100 ka suggests that long-term geomagnetic secular variation in inclination is controlled by changes in the relative strength of the geocentric axial dipole and persistent non-dipole components. Moreover, even if the relatively homogeneous rock magnetic parameters and lithofacies throughout the recovered sequence indicates a substantial stability of the East Antarctic Ice Sheet during the middle and late Pleistocene, influence of the 100 ka and 41 ka orbital periodicities has been detected in some rock magnetic parameters, indicating subtle variations in the concentration and grain-size of the magnetic minerals linked to orbital forcing of the global climate.

Age calibrated relative paleointensity for the last 1.5 Myr at IODP Site U1308 (North Atlantic)

Integrated Ocean Drilling Program (IODP) Site U1308 (central North Atlantic) records paleomagnetic directional and relative paleointensity (RPI) variations for the last 1.5 Myr, in 110 m of the sediment sequence at a mean sedimentation rate of 7.3 cm/kyr. A detailed benthic oxygen isotope record was combined with RPI to produce an integrated, high-resolution magneto-isotopic stratigraphy for Site U1308. Apart from the well-known polarity reversals in this interval, the Punaruu excursion is recorded at 1092 ka and the Cobb Mountain Subchron in the 1182–1208 ka interval. The paleointensity proxies are determined as slopes of NRM versus ARM and NRM versus ARMAQ (ARM acquisition) with linear correlation coefficients to monitor the quality of the linear fit. The RPI record for Site U1308 is compared with the three other paleointensity records (one from the Western Equatorial Pacific and two from the North Atlantic) that cover the same time interval and have accompanying oxygen isotope records. The Match protocol of Lisiecki and Lisiecki (2002) is used to optimize the correlation of paleointensity records. Beginning with the original (published) age models for each record, the Match routine is used to optimize the RPI correlations to Site U1308, with checks to ensure compatibility with oxygen isotope records. Squared wavelet coherence (WTC) indicates significant improvement in RPI (and oxygen isotope) correlations after matching each RPI record to Site U1308, particularly for periods > 10 kyr. The level of coherence for the Atlantic RPI records and the lower resolution Pacific record implies synchronous global variability (at scales > 10 kyr) that can be attributed to the axial dipole geomagnetic field.

Rock magnetic response to climatic changes in west Philippine Sea for the last 780 ka: Discussion based on relative paleointensity assisted chronology

We conducted rock magnetic and paleomagnetic research on two deep-sea sediment cores from the west Philippine Sea, located to the east of Benham Rise with the length of 4 m and water depth of over 5000 m. At the bottom of core 146 occurs a reversal of inclination and deflection of relative declination, which is recognized as Brunhes-Matuyama Polarity Boundary (MBPB). No reversal occurs in core 89, which implies a younger bottom age than that of core 146. Rock magnetic results reveal magnetic uniformities in mineralogy, concentration and grain size along the two cores, thus relative paleointensity variations are acquired. The three normalizers-anhysteresis remanent magnetization (ARM), magnetic susceptibility (k) and saturation isothermal remanent magnetization (SIRM) are used for normalization to obtain relative paleointensities. The three normalization results are averaged to indicate the paleoitensity of the cores and are further stacked together to get a synthetic curve for west Philippine Sea (named asWPS800 in this paper). Based on the magnetic correlation between cores and paleointensity to Sint800, we transfer the changes of rock magnetic parameters from depth to time. Furthermore, the astronomically tuned oxygen isotope from ODP site 1143 in the south China Sea is used for the glacial and interglacial indicator. Three concentration proxies (ARM, k and SIRM) and grain size indicators (kARM/SIRM, kARM/k) are examined according to the paleointensity-assisted chronology. The grain size changes in the two cores display a consistent pattern with the climatic changes embodied by oxygen isotope. The magnetic sizes are usually coarser in glacial periods and finer in interglacial times, which may reflect the influence of chemical erosion rather than fining from sea level rising on the source sediment. Furthermore, the sub-peaks and sub-troughs in interglaciations almost correspond with that of oxygen isotope records, which means sedimentation can reflect the subtle changes in interglaciations. This kind of revelation of climatic fluctuation by magnetic size is also found in the South China Sea, which shows a common pattern of magnetic signals to climate at least within East Asia. The concentration of ARM (representing more about fine grain) also shows similar response to glacial and interglacial cycles, that is, high in interglacial cycle and low in glacial cycle; but k and SIRM (reflecting more about coarse grain) lack the response to the climatic cycles. At the same time, S-ratio lacks the correlation with aeolian dust record and rhythmic changes, indicating the dominant source of main magnetic carrier (low coercivity magnetite) is the suspended matter instead of dust. The decreasing trend of sedimentation rate from west to east also reveals that the sediments are mainly from west Luzon and adjacent land. Grain sizes first became coarse and then stable around 400 ka B.P., and at the same time all the magnetic contents lowered and amplitude of magnetic mineral changes increased. The magnetic transition around 400 ka B.P. is simultaneous with the decreases of carbonate content, reflecting a global carbonate dissolution event, i.e. mid Brunhes event. The synchronization of magnetic content and grain size with climatic cycles of glacials and interglacials imply the validity of paleointesnityassisted chronology. Also, the response of rock magnetic signals to stable oxygen isotope changes and carbonate variation reveals that rock magnetismmethod can be an effective tool for paleoclimatic and paleoceanographic research.

Evidence for multiple paleomagnetic intensity lows between 30 and 50 ka BP from a western Equatorial Pacific sedimentary sequence

A paleomagnetic study was carried out on the radiocarbon dated MD97-2134 core located in the western Equatorial Pacific (Southern Papua New Guinea margin). Rock magnetic investigations revealed changes of the magnetic mineralogy along the hemi-pelagic sedimentary sequence but the reconstruction of past direction and relative paleointensity variations of the geomagnetic field remained feasible. Four successive paleointensity drops are recorded between 30 and 50 ka BP. The largest one is associated with an abrupt swing of declination and inclination interpreted as a smoothed signature of the Laschamp excursion (∼41 ka BP). The succession of four events of weak intensity between 30 and 50 ka BP introduces a complex behaviour of the geomagnetic field in the time interval spanning over the Laschamp and the Mono Lake excursions.

Geomagnetic moment instability between 0.6 and 1.3 Ma from cosmonuclide evidence

The reliability of paleomagnetic records as proxies of the geomagnetic field intensity is still a matter of controversy since volcanic materials hardly provide continuous records, and marine sediments are suspected to carry a remanence biased by post‐depositional realignments and/or by overprints. Such long standing debate emphasizes the need for the development of methods independent from paleomagnetism to decipher geomagnetic intensity variations. High resolution measurements of authigenic 10Be/9Be along with a detailed sedimentary record of directional and relative paleointensity variations evidence, over the 0.6–1.3 Ma time interval, frequent and recurrent excursions or short events in the late Matuyama and the early Brunhes epochs, among which two Brunhes‐Matuyama reversal precursors and an intra‐Jaramillo excursion. The results of this study confirm the idea of a highly unstable geomagnetic field as suggested by paleomagnetic evidences.

A 2.14‐Myr astronomically tuned record of relative geomagnetic paleointensity from the western Philippine Sea

We present a 2.14‐Myr astronomically tuned relative geomagnetic paleointensity record from the western Philippine Sea. Pseudosingle‐domain titanomagnetite is the only magnetic mineral identified and variations in titanomagnetite concentration fall well within the accepted limits for relative paleointensity variations. No significant temporally persistent periodicities are observed in wavelet analyses of the paleointensity time series or in the rock magnetic parameters used for relative paleointensity normalization. This suggests that our paleointensity record is largely free of rock magnetic or lithological artefacts and that it represents a reliable record of geomagnetic behavior with no evidence for modulation of the field at Earth orbital periods. The paleointensity record is highly coherent with the Sint‐800 global paleointensity stack for the last 800 kyr and with a coeval record from the West Caroline Basin. Our record confirms that it is normal for the geomagnetic field to undergo dynamic changes within polarity intervals, with relatively frequent collapses of the field to low paleointensities and concomitant deviations away from the stable field direction. We do not observe an asymmetrical sawtooth form in our paleointensity record, which might suggest that previously observed asymmetrical sawtooth paleointensities result from rock magnetic artefacts. Also, we do not observe a persistent 100‐kyr inclination periodicity, in contrast to the suggestion that geomagnetic field directions are modulated by orbital eccentricity. Good agreement between our paleointensity record and the coeval West Caroline Basin record provides the beginning of a detailed view of geomagnetic field behavior between 0.8 and 2.14 Ma.

Long-term secular variation in geomagnetic field inclination during Brunhes Chron recorded in sediment cores from Ontong-Java Plateau

A paleomagnetic study has been conducted on three gravity cores obtained from the Ontong-Java Plateau (OJP) in the western equatorial Pacific in order to investigate long-term secular variations during the Brunhes Chron. The cores consist of nanofossil foraminiferal ooze, and covers the last 500,000–800,000 years with average sedimentation rates of 6–9 m/m.y. Downcore changes of magnetic concentration represented by magnetic susceptibility are a factor of six or less, and variations in magnetic grain size and mineralogy are estimated to be small from the hysteresis parameters, ratio of anhysteretic remanent magnetization (ARM) to saturation isothermal remanent magnetization (SIRM), and S ratio. These results indicate that the sediments are rock-magnetically homogeneous in general, and are hence suitable for relative paleointensity estimation. I chose SIRM as a normalizer for relative paleointensity. The three cores could be precisely correlated with each other using relative paleointensity variations. Depth-to-age conversion was carried out by tying to the Sint-800 paleointensity stack (Guyodo and Valet, 1999). The ages are consistent with the oxygen-isotope stratigraphy available for one of the three cores. Inclination variations were obtained after alternating-field demagnetization. Large inclination anomaly, about −7° in average, was observed as has been suggested to occur in the western equatorial Pacific by some studies. The inclination records of the three cores were integrated into a single record based on the correlation by the relative paleointensity. Long-term inclination variations of the order of 10,000–100,000 years were recognized. The inclinations tend to deflect toward negative when relative paleointensity is low. The variations are similar in general to the record from the West Caroline Basin (Yamazaki and Oda, 2002), which supports their model that long-term inclination variation is controlled by relative strength of the geocentric axial dipole (GAD) and persistent non-dipole components.