Paleointensity Variations Research Articles

Geomagnetic paleointensity variations in the northern South China Sea since the Late Pleistocene

The geomagnetic field plays multiple roles in the investigation of the geodynamic processes of Earth's internal fluids and dating of deposition sequences. However, discrepancies in geomagnetic field intensity on suborbital and millennial time scales across different regions have hindered discussions on these topics. In this study, we constructed a master relative intensity curve (SPIS-150) covering the past ∼150 kyr by stacking the data from a new core (PC27) with published intensity curves for the northern South China Sea based on the chronology established through the oxygen isotope stratigraphy and relative paleointensity (RPI) correlation. Rock magnetic experiment results reveal that the sediments effectively record geomagnetic field information, although the magnetic minerals are primarily composed of terrestrial and biogenic magnetite. The stacked relative paleointensity (SPIS-150) exhibits a consistent pattern with the global stacked master curve on the orbital time scale, reflecting dipole-dominated changes in the geomagnetic field. However, specific features, such as the profound intensity lows between 75 and 85 kyr and a relatively low peak during 110–120 kyr, display potential differences with the global stacked virtual axial dipole moment (VADM) curves. Additionally, the disagreement in RPI across different regions have emerged significantly since about 25 kyr. Our record appears to align with the western tropical Pacific Ocean and the southern hemisphere but, to some extent, contradicts the stacked VADM from the northern hemisphere. This pattern may indicate the possible influence of reverse flux patches along the magnetic equator and normal flux patches in the north hemisphere.

Geomagnetic Field Paleointensity Variations Recorded in the Archeological Ceramics of the Late Bronze and Early Iron Ages in the Central Russian Plain

Geomagnetic Field Paleointensity Variations Recorded in the Archeological Ceramics of the Late Bronze and Early Iron Ages in the Central Russian Plain

Spectrum of Quaziperiodic Variations in Paleomagnetic Activity in the Phanerozoic

Abstract —Detection of common quasiperiodicities in the paleointensity behavior and lengths of polarity intervals of the Earth’s magnetic field was carried out. The paleointensity data were analyzed in the 170 Ma–present day interval. Behavior of the lengths of geomagnetic polarity intervals was investigated within the interval spanning the entire Phanerozoic (540 Ma–present age). It was found that the spectrum of the main paleointensity variations and polarity interval lengths is discrete and includes quasiperiodic variations with characteristic times of 15, 8, 5, and 3 Ma. The characteristic times of these quasiperiodic variations in the geomagnetic field at the beginning and end of the Phanerozoic differed not more than 10%. The spectral density of quasiperiodic changes in the geomagnetic field changed cyclically over geological time. The connection between the behavior of the amplitudes of paleointensity variations, the lengths of geomagnetic polarity intervals, and their spectral density is shown. The spectral density of quasiperiodic paleointensity variations (geomagnetic activity) was relatively high in the 150–40 Ma interval (Cretaceous–early Paleogene). At this time, the amplitudes of paleointensity variations and the lengths of geomagnetic polarity intervals increased. Within the intervals spanning 170–150 Ma and 30 Ma–present age, the quasiperiodic variations of paleointensity were barely expressed against its background noise variations, while the amplitudes of paleointensity variations and the lengths of polarity intervals were decreasing. Alternations of the time intervals in which paleointensity variations acquired either a quasiperiodic or noise character took place during the evolution of the geomagnetic field.

A tephra-based approach to calibrating relative geomagnetic paleointensity stacks to absolute values

Relative paleointensity (RPI) stacks provide a relative, global paleointensity variation, which reflects variations in the geocentric axial dipole moment. Previous calibrations of an RPI stack rely on adjusting a mean RPI to a mean of virtual axial dipole moments (VADMs), which are selected from absolute paleointensity (API) databases. To achieve more quantitative calibration of an RPI stack into absolute values, we propose a novel approach that compares (1) tephra-derived APIs and (2) the same tephras' horizons, in the oxygen isotope stratigraphy, tied RPIs (hereafter termed the “TA-TOR” approach). For the TA-TOR approach, APIs were newly determined for welded tuffs of 15 pyroclastic flow deposits in Japan. Eight of the 15 new APIs and 2 previously reported APIs have tephra horizons in the oxygen isotope stratigraphy. Based on the 10 data of the TA-TOR approach, combined with 4 data of the transitional field, VADMs calculated from the APIs (VADMAPI) were compared with isochronous RPIs from two RPI stacks, PISO-1500 and Sint-2000. The 14 data points in RPI vs. VADMAPI diagrams show a linear relationship with correlation coefficients of 0.83–0.89. Using the observed linear relationships, the two RPI stacks were reliably calibrated to VADM values (VADMRPI) for the period since 1.1 Ma. Compared with the VADMs calibrated by the previous studies, the present calibration provides a 12–13% smaller median/mean and a larger dispersion as represented by a 15–53% larger standard deviation for the two RPI stacks. The similarities between the cumulative distributions of VADMRPI from the two stacks and those of VADMs selected from the API database indicate the reliability of the present calibration. These results show that the present calibration, which relies on the TA-TOR approach, accurately converts a RPI stack into VADMRPI values and will contribute to studies on time variations in the geocentric axial dipole moment, sedimentary RPI, and cosmogenic radionuclides. Variations in VADMRPI for the past 1.1 Myr, obtained from PISO-1500 and Sint-2000 via the present calibration, can be used as standard geocentric axial dipole variations for this period.

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.

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.

Paleointensity Bursts in the Geomagnetic Field History

Regularities in the distribution of extreme (burst) increases in paleointensities from 3000 Ma to the present have been studied. Paleointensity bursts that exceed several times its average values Havg have been found to occur over the entire geomagnetic history. The data obtained from thermomagnetized rocks have made it possible to identify variations with characteristic times of 200 Ma in the spectrum of paleointensity variations. Wavelet analysis of sedimentary rock data revealed paleointensity variations with characteristic times of 5 Ma, 400 000 years, and 100 000 years. Until 40 Ma, the most significant burst increases in paleointensities with amplitudes of 8–9Havg occurred with characteristic times of 5 Ma. Regardless of the burst increases in paleointensity, their durations did not exceed 100 000 years. After 40 Ma, there was a decrease in the amplitudes and characteristic times of paleointensity variations. Cyclic changes in paleointensity have the same length as the cycles of lithospheric and space–planetary processes.

Synchronization of Variations in Geomagnetic Field Intensity with Eustatic Cycles and Bursts of Magmatism Intensification in the Cretaceous–Early Paleogene

The relationship between cyclic processes in the core and lithosphere in the Middle Jurassic–Paleogene (167–22 Ma) was investigated. The dynamics of cyclic processes was associated with paleointensity behavior and sea level. It is shown that variations in paleointensity and ocean level are two broadband oscillatory processes. The characteristic times of variations in geomagnetic and eustatic processes changed in different geologic ages. We discovered synchronization of variations in geomagnetic and eustatic processes that occurred in the Cretaceous–Early Paleogene (125–40 Ma) based on a spectral analysis of paleointensity and sea level data. The characteristic times of coherent variations in paleointensity and global ocean level ranged from 1 to 16 Ma. Possible reasons for the synchronization of geomagnetic and lithospheric processes have been identified. Features of geomagnetic and lithospheric processes in the interval of 125–40 Ma, expressed as sharp increases in the amplitudes of paleointensity variations and activation of magmatic effusive processes, are considered.

One Hundred Thousand Years of Geomagnetic Field Evolution

AbstractPaleomagnetic records from sediments, archeological artifacts, and lava flows provide the foundation for studying geomagnetic field changes over 0–100 ka. Late Quaternary time‐varying spherical harmonic models for 0–100 ka produce a global view used to evaluate new data records, study the paleomagnetic secular variation on centennial to multimillennial timescales, and investigate extreme regional or global events such as the Laschamp geomagnetic excursion. Recent modeling results (GGF100k and LSMOD.2) are compared to previous studies based on regional or global stacks and averages of relative geomagnetic paleointensity variations. Time‐averaged field structure is similar on Holocene, 100 ky, and million‐year timescales. Paleosecular variation activity varies greatly over 0–100 ka, with large changes in field strength and significant morphological changes that are especially evident when field strength is low. GGF100k exhibits a factor of 4 variation in geomagnetic axial dipole moment, and higher‐resolution models suggest that much larger changes are likely during global excursions. There is some suggestion of recurrent field states resembling the present‐day South Atlantic Anomaly, but these are not linked to initiation or evolution of excursions. Several properties used to characterize numerical dynamo simulations as “Earth‐like” are evaluated and, in future, improved models may yet reveal systematic changes linked to the onset of geomagnetic excursions. Modeling results are useful in applications ranging from ground truth and data assimilation in geodynamo simulations to providing geochronological constraints and modeling the influence of geomagnetic variations on cosmogenic isotope production rates.

Variations in the Geomagnetic Field Intensity with Characteristic Times of Five and One Million Years

Variations in the paleointensity of the geomagnetic field over the Middle Jurassic–Paleogene (167–20 Ma) interval has been studied. It has been found that the amplitudes of paleointensity variations changed cyclically. The characteristic times of these changes are estimated to be approximately five and one million years. The cycles found in the evolution of paleointensity are compared with the cycles of changes in the ocean level. It is shown that characteristic times of cyclical changes in the amplitudes of paleointensity variations (∼5 and 1 Ma) coincide with the mean duration of transgression-regression cycles (T-R cycles) of the second and third order of the Geologic Time Scale 2008 (GTS 08). Increases in the amplitudes of paleointensity variations occur generally during regressions of the World Ocean. The equal characteristic times and existence of the correlation between the transgression–regression cycles and variations in paleointensity provide evidence that they reflect the same planetary process.

Low absolute paleointensity during Late Miocene Noma excursion of the Earth’s magnetic field

Paleointensity variation is investigated for a sequence of andesitic lavas which records the Noma geomagnetic excursion at 6.66 ± 0.45 Ma. Sixty-six samples are collected from 11 lava flows at the Kamegaoka Mountain (31° 21′N, 130° 13′E) on the Noma Peninsula, Kyushu Island, Japan. Rock magnetic experiments confirm the presence of Ti-poor titanomagnetite grains with pseudo-single-domain or a mixture of single and multi-domains as the carrier of the remanent magnetization. The application of the Tsunakawa-Shaw method yields reliable geomagnetic paleointensity data set of 29 samples from 9 lavas that cover the Noma excursion. The mean paleointensities of lava flows vary from 1.9 to 10.3 µT. Paleointensity variation reveals a ‘V -shape’ of high–low–high paleointensities during the excursion. The highest value of 10.3 ± 3.5 µT appears prior to the excursion, the paleointensity decreases down to 1.9 ± 0.3 µT as the excursion progresses, and the paleointensity finally regenerates to 5.1 ± 3.0 µT at the end of the excursion. The virtual dipole moment (VDM) during the Noma excursion (0.3–1.1 × 1022 Am2) is less than 16% of the mean VDM during the period between 4 and 8 Ma in Miocene, suggesting that this weak geomagnetic field intensity is ascribed to the nature of excursions rather than the Miocene geomagnetic field properties. The weakest VDM (0.3 × 1022 Am2) in the Noma excursion is detected at the pole position that is located at fairly high latitude (58.9°N) in the opposite hemisphere. The Noma excursion is characterized by a large shift in virtual geomagnetic poles of more than 140° from the geographic pole and by a weak VDM of 1.0 × 1022 Am2 or less.

Paleointensity Study on the Holocene Surface Lavas on the Island of Hawaii Using the Tsunakawa–Shaw Method

Investigating volcanic paleointensity during the Holocene is important for linking archeointensity and sedimentary paleointensity. Across the globe, the island of Hawaii is one of the most studied subaerial locations. Many published data from Hawaii are accessible in the paleointensity databases, but it is necessary to reassess these data because they were determined with experimental protocols not incorporating a modern test for multi-domain particles and with relatively loose selection criteria. To obtain a new paleointensity dataset based on coercivity spectra rather than blocking temperature spectra, we applied the Tsunakawa–Shaw method to Holocene surface lavas collected from 34 sites on the island of Hawaii. In total, 135 successful results were obtained after applying the specimen-level selection criteria, yielding 22 site-mean Tsunakawa–Shaw paleointensities (TS dataset) that fulfilled the site-level selection criteria. We compared the TS dataset with the IZZI dataset, which includes recently reported blocking-temperature-based paleointensities determined by the IZZI Thellier method with the stringent criteria CCRIT. The cumulative distribution function (CDF) curve of the TS dataset, except for three sites, almost overlaps that of the IZZI dataset, and the medians coincide with each other, 42.9 µT (N = 19) for the TS dataset and 43.5 µT (N = 28) for the IZZI dataset. The coincidence of the CDF curves suggests equivalent reliability of the Tsunakawa–Shaw method and the IZZI Thellier method. The Holocene paleointensity variation in Hawaii is thought to be reliably characterized by both the TS dataset and the IZZI dataset: overall, the paleointensity throughout the Holocene is suggested to be higher than the present-day field. It is also inferred that there are possible decadal- and centennial-timescale large-intensity variations between ~1,800 and ~2,000 cal BP, and between ~3,000 and ~3,500 cal BP. The latter variation might be inferred as a global-scale dipolar phenomenon, as consistent paleointensity results are reported from archaeological sources in the Levant by the IZZI Thellier method.

Paleointensity Behavior and Intervals Between Geomagnetic Reversals in the Last 167 Ma

The results of comparative analysis of the behavior of paleointensity and polarity (intervals between reversals) of the geomagnetic field for the last 167 Ma are presented. Similarities and differences in the behavior of these characteristics of the geomagnetic field are discussed. It is shown that bursts of paleointensity and long intervals between reversals occurred at high mean values of paleointensity in the Cretaceous and Paleogene. However, there are differences between the paleointensity behavior and the reversal regime: (1) the characteristic times of paleointensity variations are less than the characteristic times of the frequency of geomagnetic reversals, (2) the achievement of maximum values of paleointensity at the Cretaceous–Paleogene boundary and the termination of paleointensity bursts after the boundary of 45–40 Ma are not marked by explicit features in the geomagnetic polarity behavior.

Remanence acquisition efficiency in biogenic and detrital magnetite and recording of geomagnetic paleointensity

AbstractRelative paleointensity (RPI) variations of Earth's magnetic field are widely used to understand geomagnetic field behavior and to develop age models for sedimentary sequences. RPI estimation is based on a series of assumptions. One key assumption that is rarely considered is that all magnetic particles in the sediment acquired a magnetization in an identical manner. In this paper, we test this assumption for sediments from the eastern equatorial Pacific Ocean that record well‐documented global RPI variations over the last ∼780 kyr. The magnetization is carried by two stable single domain magnetic components, which we identify as magnetite magnetofossils and titanomagnetite nanoparticle inclusions within larger silicate particles. By analyzing signals carried by the two components separately, we determine for the first time that magnetic nanoparticle inclusions can cause their host particles to record reliable but inefficient sedimentary paleomagnetic signals. The magnetization carried by biogenic magnetite is acquired more efficiently than that carried by the nanoparticle inclusions. Variations in the concentration of both components are modulated climatically so that they record nearly identical RPI signals. In many sediment types, there is no correlation between the concentrations of different magnetic components so that variable remanence acquisition efficiency will complicate RPI recording. Our work demonstrates that detailed assessment of paleomagnetic recording by each constituent magnetic component needs to become a routine part of sedimentary RPI analysis.

Paleomagnetic direction and paleointensity variations during the Matuyama\u2013Brunhes polarity transition from a marine succession in the Chiba composite section of the Boso Peninsula, central Japan

The youngest geomagnetic polarity reversal, the Matuyama–Brunhes (M–B) boundary, provides an important plane of data for sediments, ice cores, and lavas. The geomagnetic field intensity and directional changes that occurred during the reversal also provide important information for understanding the dynamics of the Earth’s outer core, which generates the magnetic field. However, the reversal process is relatively rapid in terms of the geological timescale; therefore, adequate temporal resolution of the geomagnetic field record is essential for addressing these topics. Here, we report a new high-resolution paleomagnetic record from a continuous marine succession in the Chiba composite section of the Kokumoto Formation of the Kazusa Group, Japan, that reveals detailed behaviors of the virtual geomagnetic poles (VGPs) and relative paleointensity changes during the M–B polarity transition. The resultant relative paleointensity and VGP records show a significant paleointensity minimum near the M–B boundary, which is accompanied by a clear “polarity switch.” A newly obtained high-resolution oxygen isotope chronology for the Chiba composite section indicates that the M–B boundary is located in the middle of marine isotope stage (MIS) 19 and yields an age of 771.7 ka for the boundary. This age is consistent with those based on the latest astronomically tuned marine and ice core records and with the recalculated age of 770.9 ± 7.3 ka deduced from the U–Pb zircon age of the Byk-E tephra. To the best of our knowledge, our new paleomagnetic data represent one of the most detailed records on this geomagnetic field reversal that has thus far been obtained from marine sediments and will therefore be key for understanding the dynamics of the geomagnetic dynamo and for calibrating the geological timescale.Graphical abstract.

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.

Behavior of paleointensity during the Paleogene

Behavior of the intensity of geomagnetic field in the Paleogene was studied by the analysis of the sedimentary rocks of the Russian Plate. It is revealed that in the beginning of the Paleogene the variations of little-amplitude paleointensity (about 0.5H0, where H0 is the value of modern geomagnetic field taken as 40μT) were alternating with bursts of large amplitude (up to 5H0). By the end of the Paleogene only the paleointensity variations with a little amplitude were revealed.The relation between behavior of the paleointensity and the polarity regime is analyzed. On the basis of the behavior of the reversal frequency and asymmetry of the polarity and according to Molostovskii et al. (2007) the Paleogene was divided into two hyperchrons. In the first hyperchron (66–40)Ma the polarity of the geomagnetic field was predominantly reverse. In the second hyperchron (40–22)Ma the intervals of normal and reverse polarity alternated. The bursts of the paleointensity were revealed only in the beginning of the first hyperchron. In addition, during the first hyperchron both the average values (for the geological age) and the amplitude of the paleointensity variations decreased; within the interval of (40–22)Ma both the amplitude and its mean values were changing inconsiderably.

Estimating the variations in paleointensity from the Siberian traps of Maymecha-Kotui and Norilsk regions

The results of determining the paleointensity of the Siberian traps sampled from the Kotui sequence, Truba ravine, Onkuchak Suite, and dated to Permian-Triassic are presented. Most of the Hanc values for separate lava flows are significantly lower than the values of the present geomagnetic field at the observation point (approximately 50 μT). This is consistent with the known conclusion that the intensity of the geomagnetic field at the Permian-Triassic boundary was lower by a factor of two to three lower than its present value. We carried out the comparative analysis of the behavior of Han and VDM for two trap sequences (Kotui (the Onkuchak Suite) and Norilsk) from the standpoint of the eruptive pulse hypothesis. In both sections, the average VDM values and their dispersion are similar. For comparison, similar analysis of the VDM behavior is conducted for the Brunhes epoch and Miocene in the interval of 5–10 Ma. It is hypothesized that during the three considered periods, namely, the eruption of the Siberian traps, in the Brunhes epoch, and in Miocene (in the interval of 5–10 Ma), the time behavior of the geomagnetic field was close to a stationary stochastic process.

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.