Relative Paleointensity Research Articles

Beryllium ten production and relative paleointensity for the past 1.2 million years

Composite curves of relative paleointensity document the evolution of the geomagnetic field intensity during the past 1.2 Million years. Several records of production of 10Be cosmogenic isotope from the 10Be/9Be ratio (referred to as Be-ratio) covering this period have also been acquired. We add here new 10Be records to the database and produce a first composite curve of beryllium production (Be-1200) for the past 1.2 Myr. We compare Be-1200 with Sint-2000 (Valet et al., 2005) and PISO-1500 (Channell et al., 2009) paleointensity curves. The three curves show similar patterns, but frequently different amplitudes. The variations in dipole moment values derived from the Sint-2000 and PISO-1500 relative paleointensity composite curves were then confronted with the geomagnetic dipole moment record derived from the Be-1200 record. The predicted amplitudes of dipole moment loss during geomagnetic excursions reconstructed from the measured in the Be-1200 record are generally compatible with those deduced from SINT-2000 and PISO-1500. More specifically, the three datasets indicate very low field intensities during the Laschamp, that are consistent with the existence of reverse directions, and to a lesser extent during other events (Iceland basin, Calabrian Ridge/West Eifel events). The two paleointensity curves show a decrease in field intensity of at least 80–90% during the reversals. The Be- derived dipole moment decreases meet the predictions for the Brunhes/Matuyama transition, but are significantly smaller than expected for the two Jaramillo transitions. Such limited variability may result from the impact of environmental factors on the Be ratios, as suggested in a previous study of the last reversal (Savranskaia et al., 2021). This comparative study of 10Be records and relative paleointensity records reveals that, in particular cases, Be ratio records can underestimate the amplitude of virtual geomagnetic dipole variation. With their respective strengths and weakness, these two approaches provide similar first-order patterns relevant to the common control by the geomagnetic moment variation.

Paleomagnetic Secular Variations in North Greenland Around 81°N Over the Last 6,000 Years

AbstractWe investigate full vector paleomagnetic changes recorded in high‐resolution sediments of Petermann Fjord, North Greenland, deposited over the last 6 kyr, in the context of the recent rapid changes in the geomagnetic field. A Paleomagnetic Secular Variation (PSV) stack (inclination, declination, and relative paleointensity) was reconstructed using four marine sediment cores with an independent age model constrained by seven radiocarbon ages. Magnetic investigations demonstrate that the paleomagnetic signal is carried by low coercivity ferrimagnetic minerals and is well reproduced in all cores, attesting to the quality and reliability of the paleomagnetic recording of these sediments. This signal is broadly consistent in directional changes with distant records in North America and the northern North Atlantic at centennial and millennial timescales, and has millennial scale intensity variations that are consistent with model predictions. The offset between a magnetization age determined through comparison with a northern North Atlantic PSV reference curve, GREENICE, and the radiocarbon age model indicates either a reasonable lock‐in depth of magnetization (∼11 cm from the coretop) or centennial‐scale reservoir age variation through time in the fjord. Reconstructed virtual geomagnetic pole (VGP) migration for the last 6 kyr shows that the recent migration of the magnetic North Pole is consistent with secular paleomagnetic variations on geologic timescales. Our results suggest that magnetic field intensity variations (temporal and spatial) are linked to magnetic flux lobe dynamics and influence the VGP migration.

Paleomagnetic Field Variability and Revised Chronostratigraphy of Bering Sea (IODP Expedition 323) Deep‐Sea Sediments During MIS 6–7 (130–144 ka)

AbstractWe have developed replicate paleomagnetic secular variation (PSV) records for MIS 6–7 (130–244 ka) from IODP Ex. 3 23 Sites 1,343, 1,344, and 1,345 (Bering Sea). We can correlate the PSV at all three sites and identify 90 inclination features and 64 declination features. We have developed relative paleointensity records for all three sites by normalizing the demagnetized natural remanence to magnetic susceptibility. Paleointensity highs/lows can be tied to the PISO‐1500 global oxygen‐isotope‐dated paleointensity record. This provides a significant increase in chronostratigraphic resolution for these sites. Only one excursion is recorded in the MIS 6–7 sediments of the Bering Sea—the Iceland Basin Excursion (∼196 ± 3 ka). Replicate records of the inclinations and declinations both flip quickly to reversed polarity directions, stay there for several hundred years and then flip quickly back to normal polarity directions. However, the inclination flips occur without significant declinations changes and visa versa. A statistical PSV study was carried out by averaging the data in 3 ky and 9 ky windows. There is a distinctive bimodal pattern to the angular dispersions with most time spent with low angular dispersion values (∼10°–15°), but there is an interval of more than 30 ky (185–220 ka) with angular dispersions averaging two to three times higher amplitudes (∼25°–35°). This interval also has low paleointensities and the Iceland Basin Excursion. This same, coupled pattern of high angular dispersion, low paleointensity, and excursions is noted synchronously in the central North Atlantic Ocean and may indicate a global pattern to the geomagnetic field.

КОРРЕЛЯЦИЯ ТОРФЯНЫХ РАЗРЕЗОВ НИЖНЕГО ПРИАМУРЬЯ ПО РЕЗУЛЬТАТАМ РАСЧЕТА ОТНОСИТЕЛЬНОЙ ПАЛЕОНАПРЯЖЕННОСТИ

The paper considers the possibility of determining the age and correlating peat sections by solving the inverse problem of magnetostratigraphy based on the findings of comprehensive (palaeomagnetic, petromagnetic, microprobe, and radiocarbon) studies on peats of the «Tyapka» and «Chlya» peat sections in the Lower Amur region. For this purpose, we used the values of relative paleointensity calculated following the technique described by Bagina-Petrova. There is no difference in the rate of magnetization fixation among studied peat samples, and the age of magnetization is comparable to the age of the peat deposits itself. The solution of the inverse problem of magnetostratigraphy using the values of relative paleointensity can be used to correlate peat sections among themselves and to estimate the rate of peat accumulation in different periods of the Holocene, even in the absence of radiocarbon dating for one of the correlated sections.

On the relationship between paleomagnetic secular variation and excursions – Records from MIS 11 and 12 – ODP Leg 172, western North Atlantic Ocean

This study has developed paleomagnetic secular variation (PSV) records from Sites 1060, 1061, 1062, and 1063 (ODP Leg 172) from the western North Atlantic Ocean during MIS 11–12 (374–478 ka). We have identified 46 inclination features and 49 declination features that can be correlated among the records. We have also developed relative paleointensity records and identified 13 paleointensity features that can be correlated among them. These features can also be dated using the oxygen-isotope dated Global relative-paleointensity record PISO-1500 of Channell et al. (2009). There is one excursion located in these four records. We use the name Levantine/Bermuda Excursion developed elsewhere by Ryan (1972) and Channell et al. (2017). The Excursion occurred at 408 ± 4 ka. The excursion is recorded at Sites 1061, 1062, and 1063. The most detailed excursion records occur at Site 1062 where we have both shipboard PSV records and one u-channel PSV record of the excursion. The excursion has two intervals of excursional directions, one short interval (∼I ky) followed by a long interval (∼5 ky). Both intervals have counter-clockwise looping of the excursional directions and the excursion is considered to be a Class I Excursion (Lund et al., 2005). Statistical study of the PSV records after removal of the excursion directions has identified two characteristics of the ‘normal’ PSV. The first feature is a long-term (>104 yrs) deviation in inclination and declination averages from the overall site averages. These deviations suggest some type on long-term memory in the regional dynamo process. The second feature is the variation in PSV angular dispersion (a measure of directional variability). The angular dispersion is quite low (∼12°) for most of the 374–478 ka interval. But, there is a short (∼8 ky) interval with angular dispersion more than double the amplitude (∼25°-30°). This interval has a sharp (<3 ky) onset and termination, This interval contains the Levantine/Bermuda Excursion. Such high-angular dispersion intervals with associated excursions and low paleointensity appear to be regular distinctive features of the Brunhes Chron PSV.

Relative paleointensity of geomagnetic field during the last 9000 years estimated by the pseudo Thellier method from the bottom sediments of Lake Shira, Northern Khakassia

The results of rock magnetic studies and determination of relative paleointensity from sediments of Lake Shira, Khakassia, are presented. The NRMcarrier minerals were determined from the hysteresis parameters, thermomagnetic and X-ray diffraction (XRD) analyses. The age of the sediments was determined by radiocarbon dating. According to these measurements, the column spans about 9100 years. The qualitative determinations of relative paleointensity were obtained from linear segments of the pseudo-Arai–Nagata diagrams. The quality of the determinations was evaluated by the following criteria: number of points used to calculate the slope; quality criterion (q), fraction ofNRMdestroyed in the paleointensity determination interval, and relative paleointensity determination error (σ). According to rock magnetic studies and XRD analysis, the magnetization carriers are represented mainly by single-domain and pseudo-single-domain magnetite and hematite. The comparison of the obtained series of relative paleointensity data with both the model paleointensity values calculated for Shira coordinates from various models (CALS10K.1b [Korte et al., 2011], PFM9k.1 [Nilsson et al., 2014], HFM.OL1.AL1, CALS10k.2 ARCH10k.1 [Constable et al., 2016]) and with absolute paleointensity, as well as the aggregate results of the studies on sedimentary and igneous rocks and on archaeomagnetic objects has shown that these data are in good agreement with each other and have common trends. This provides a rationale for using this methodology to determine paleointensity from sediments of modern lakes using the pseudo-Thellier method.

Relative Paleointensity of Geomagnetic Field over the Past 9000 Years Estimated by the Pseudo-Thellier Method from the Bottom Sediments of Lake Shira, Northern Khakassia

Relative Paleointensity of Geomagnetic Field over the Past 9000 Years Estimated by the Pseudo-Thellier Method from the Bottom Sediments of Lake Shira, Northern Khakassia

High-resolution relative paleointensities over the last 40 ka from marine sediments in the western equatorial pacific: Magnetic events and interhemispheric correlation

Core MD01-2385 retrieved in the western equatorial Pacific (Northwest of Papua-New Guinea) has been sampled every 2 cm for paleomagnetic and radiocarbon measurements. The age model, well constrained by 47 14C ages (both new and former), indicates that our record spans approximately the last 38 ka. The average sedimentation rate is ∼30 cm/ka. Analysis of the magnetic mineralogy revealed that the natural remanent magnetization (NRM) is predominantly carried by magnetite. A relative paleointensity record (RPI) was derived, allowing us to estimate a lock-in depth of approximately 18 cm, corresponding to ∼600 years in our case, by comparing it with the CALS10k.2 model. Comparison with two other western Equatorial Pacific RPI, selected for their geographical proximity (∼1000 km) and high resolution, allowed us to compile a regional reference record (WEPEQ). Several intervals of lower intensities have been identified and may correspond to the known events of Mono Lake (∼34 ka), Rockall (∼27 ka), Hilina Pali (∼18 ka) and possibly Tianchi (∼5–8 ka). Overall, our equatorial curve exhibits strong similarities with RPI curves from mid to low latitudes, such as those from the Iberian Margin and Black Sea. A detailed investigation hints at the presence of non-dipolar structures in the Earth's magnetic field at the shorter time scale, especially during the low intensity intervals. In contrast, notable differences are observed when compared to high-latitude stacks, especially from the North Atlantic (NAPIS). This points to the potential climatic bias that may affect high-latitude records during the glacial maximum and glacial-interglacial transition.

Paleomagnetic field variability and revised chronostratigraphy of bering sea (IODP ex. 323) deep-sea sediments during MIS 8-10

This paper develops three composite paleomagnetic secular variation (PSV) records (directions and relative paleointensity) for MIS 8–10 (243–375 ka) from IODP Ex. 323 Sites 1343, 1344, and 1345 in the Bering Sea. There are 79 inclination features and 74 declination features in each of these records that can be correlated among them. Oxygen isotope records and correlation of our Bering Sea paleointensity records to the oxygen-isotope dated global PISO-1500 paleointensity record provide a detailed chronostratigraphy for our three sites with an uncertainty of ∼±2000 years. There are two excursions recorded reproducibly in our PSV records, the Portuguese Orphan Excursion (286 ± 2 ka) and the Calabrian Ridge Excursion (326 ± 2 ka). Both of these excursions have only short intervals (less than 500 years) of excursional directions. Both excursions have simple open looping of the directions. The Portuguese Orphan excursion has counter-clockwise (CC) looping, while the Calabrian Ridge Excursion has clockwise (C) looping. Statistical study of the PSV after removal of all excursional directions has been carried in overlapping 3-ky and 9-ky intervals. We have identified two distinctive features of the long-term secular variation. First, the ‘normal’ PSV has coherent long-term variability noted by several successive 9-ky inclination or declination averages distinct from overall averages. These persistent >104 ky variations indicate long-term memory in the regional pattern of dynamo activity. Such memory is not consistent with persistent millennial-scale drift of the Earth's magnetic field. Second, PSV angular dispersion (a measure of directional variability) is quite low (∼15°) for ∼75% of the MIS 8–10 time interval. But angular dispersion more than doubles (∼35°) in three short intervals around 265–272 ka, 283–295 ka, and 307–325 ka. The onset and termination of these three high-angular-dispersion intervals occurs fast (∼103 yrs). These same three high-angular-dispersion intervals also occur almost synchronously in the North Atlantic Ocean (Lund, 2022). These high-angular-dispersion intervals are also intervals in which excursions occur in both regions and are associated with relatively low paleointensity. We think this constitutes a distinctive bimodal pattern to overall pattern to Global secular variation in the Brunhes Chron.

Aeolian activity in the Yarlung Zangbo River Basin, southern Tibetan Plateau, began at 584 ka: implications for the glaciation of the Tibetan Plateau

Aeolian activity on the Tibetan Plateau (TP) has profound effect on the Asian and global climate. In the southern TP, loess deposition widely occurred associated with the massive glaciers and large rivers, recording the interplay history among the aeolian activity, atmosphere, and cryosphere. However, the initiation age of the loess and the factor controlling the loess formation are still controversial. Here, three drilling cores were retrieved from Linzhi area, in the eastern Yarlung Zangbo River Basin of the southern TP, which reached the bedrock and documented the loess development in the southern TP. Based on the optically stimulated luminescence, electron spin resonance, paleomagnetism dating and relative paleointensity correlation, the basal age of the drilling core was constrained at 584 ka revealing the onset of significant aeolian activity on the southern TP. Synthesize this study and previous work on the loess age across the TP, three stages were identified, namely 3600, 2800–2700, and 1200–600 ka, with the age trending younger from north to south. Extensive loess development on the TP occurred between 1200 and 600 ka, which was mainly ascribed to the global cooling during the mid-Pleistocene Transition. During this time, the ampler glaciers extent significantly expanded, westerly displaced southward and the wind intensity also intensified on the TP, which resulted in the enhancement of dust generation, arid and windy condition, leading to the loess preservation and development on the TP. Moreover, numerous evidences suggested that the glaciers on the TP may significantly developed since 800 ka, then enlarged at 500–600 ka, which may lead to the massive dust/loess input to the TP, Chinese Loess Plateau and north Pacific Ocean.

Magnetic Response to the Source‐To‐Sink Environmental Changes in the Bay of Bengal Since ∼60 ka

AbstractThe terrestrial magnetic minerals of marine sediments are utilized to track the climatic changes in the source area and the dynamic characteristics of sedimentation processes. However, due to the varied source‐to‐sink environments, the magnetic response to ambient climate cannot be generalized. Here, we conducted systematic environmental magnetic analyses on core CJ04‐50 from the Ninetyeast Ridge and investigated its magnetic response to source‐to‐sink environmental changes. Core CJ04‐50 covers the last 60 Kyr based on accelerator mass spectrometry (AMS) 14C dating and the relative paleointensity (RPI) record. Rare earth element (REE) results suggest that the terrestrial materials are fed by the Ganges‐Brahmaputra (G‐B) and Irrawaddy/Indo‐Burma Ranges. High/low magnetic mineral content corresponds to strong/weak terristrial input during the cold/warm period. This pattern differs from that in the East Asian marginal seas, which have a high magnetic mineral content in warm periods. It might be attributed to the heavier Indian summer monsoon (ISM) precipitation than that of East Asian summer monsoon. Excessive moisture (&gt;1,500 mm/year) would not favor the formation and preservation of magnetic minerals in the source area during interglacials. By contrast, the enhanced physical weathering during glacials results in more magnetic contributions. A significant local magnetite dissolution occurred at the layer of Middle MIS 3, which may be caused by the non‐steady state diagenesis following deposition.

High-resolution U-channel paleomagnetic secular variation and magnetic field excursions from ocean drilling program site 1233 (MIS 1–4; 0–71 ka)

This paper presents new u-channel paleomagnetic secular variation (PSV) data for ODP Site 1233 (Chile margin) for the last 70 ky. This is the highest-resolution, long-term PSV record ever recovered. The u-channel study has carried out detailed af demagnetization of the natural remanence (NRM) and developed a characteristic remanence (ChRM – final PSV direction) for each sampling horizon. The u-channel rock magnetic studies have measured magnetic susceptibility and anhysteretic remanence (ARM) with af demagnetization and used them to normalize the NRM and develop a new, detailed relative paleointensity record. The paleomagnetic record contains three excursions: the Mono Lake Excursion, the Laschamp Excursion, and the Greenland/Norwegian-Sea Excursion. These excursions have nominal sampling intervals of 5–8 years. Both the Mono Lake and Greenland/Norwegian-Sea Excursions involve short duration intervals of excursional directions (<300 years). The excursions are distinctly different from similar excursion records from other parts of the World. The Laschamp Excursion exhibits a full local reversal of the field with fast (<200 yr) movement to/from reversed directions and ∼ 500 years of full reversed polarity directions. This pattern is distinctly different from Laschamp excursion records from other parts of the World. We have carried out a statistical analysis of the Site 1233 u-channel PSV record. Notable specific features of that analysis include documentation that the field spends >1/3 of its time with anomalously large amplitude directional variability (high angular dispersion). These intervals occur at∼25–45 ka and 55–65 ka. These intervals have lower-frequency (more sluggish) directional variability and are associated with lower than average paleointensity.

Salient Changes of Earth's Magnetic Field Toward the End of Cretaceous Normal Superchron (CNS)

AbstractChanges in Earth's magnetic field during the Cretaceous Normal Superchron (CNS) spanning ∼121 Ma to ∼84 Ma hold important clues about the geodynamo evolution. Canonical models predict a persistently strong geomagnetic field with low variability during CNS, which, however, has not been observed in the available absolute paleointensity data and seafloor marine magnetic anomaly (MMA) records. The lack of relative paleointensity (RPI) data across CNS further impedes tests of model predictions. Here, we present a ∼9‐Myr (∼94–∼85 Ma) RPI record from a Turonian to Santonian hemipelagic succession from IODP Site U1512 offshore southern Australia. Detailed paleomagnetic and rock magnetic analyses demonstrate that the ratio of natural remanent magnetization (NRM) demagnetized at 20 mT over magnetic susceptibility (MS), that is, NRM20mT/MS, as a reliable proxy for the RPI of the Upper Cretaceous succession. The new RPI record shows marked changes in both intensity and variability at ∼90.8 Ma. Also, the 6 Myr‐long (∼94–∼88 Ma), near‐continuous, ∼1.2 kyr‐resolution RPI record exhibits a strong positive correlation between field intensity and variability. Assuming this correlation holds for the entire CNS, an extrapolated RPI curve for the entire CNS is obtained by integrating the positive correlation with field variability estimates from the MMA data. The extrapolated RPI curve shows a strong and highly variable field in the middle CNS but a weak and stable field at its beginning and ending. These features imply a much more dynamic geodynamo than previously thought, and provide crucial benchmarks for unraveling the geodynamo evolution during CNS.

Paleomagnetic secular variation and revised chronostratigraphy of Bering Sea (IODP expedition 323) deep-sea sediments (MIS 5)

This study presents new high-resolution, full-vector paleomagnetic secular variation (PSV) records from IODP Expedition 323 Sites U1339, U1343, U1344, and U1345 in the Bering Sea (51°N-60°N) during Marine Isotope Stage 5 (71–130 ka). The chronology of the records has been determined by oxygen isotope stratigraphy with an age uncertainty of ∼±2000 years. The data come from shipboard paleomagnetic measurements of deep-sea sediments with accumulation rates of 26–54 cm/ky. The sample interval in the composite records is 5 cm with a time resolution of ±100–200 years. The composite PSV records for each site were developed by correlation and analysis of 3–4 paleomagnetic records from individual holes at each site. The composite records are a combination of the best correlatable PSV data from each site. We have been able to significantly correlate the PSV records from all four sites. We have identified 56 inclination features (highs/lows) and 54 declination features (east/west extremes) in all the records. Relative paleointensity has been determined by normalizing the sediment natural remanence, after 20-mT af demagnetization, to magnetic susceptibility. This normalization process is complicated by environmental variability, thus our relative paleointensity records are limited to large-scale correlations among the four sites. We correlated our relative paleointensity records to the global PISO-1500 paleointensity record that is itself dated by oxygen isotope stratigraphy. We can identify 6 key intensity highs/lows in our composite records and the PISO-1500 record that provide additional dating isochrons for our records. There are three short magnetic field excursions recorded with replication at these four sites – The youngest excursion occurred at 100 ± 2 ka and is associated with the Fram Strait Excursion. Two older excursions occurred at 116 ± 2 ka and 119.5 ± 2 ka. We associate both of these with the Blake Event. All three of the excursions have very short time intervals (∼200–400 yrs) with somewhat out-of-phase inclination and declination variability. All three of these are Class I excursions. We have studied the statistical characteristics our composite records by calculating the 3ky and 9ky averaged inclinations, declinations, unit vectors, and relative paleointensity over 71–130 ka after removing all excursional directions. The averaged inclinations and declinations are consistent among the four sites. The averaged angular dispersion is bimodal in its overall pattern. Most of the time angular dispersion is low (∼±10°–12°) except for three shorter intervals that have significantly higher angular dispersion (∼±24–30°) and lower paleointensities. The high angular dispersion intervals are associated with the three short excursional intervals. The timing and pattern of interrelated high angular dispersion, low paleointensity, and excursions is synchronous with the PSV pattern recorded during MIS 5 in the central North Atlantic Ocean. We think this defines a distinctive low-intensity/low-energy state for the geomagnetic field.

Magnetostratigraphy and relative paleointensity study of a late Quaternary lacustrine sediment sequence from Southwest China and its chronological significance

Long and continuous lacustrine sedimentary sequences from southwest (SW) China is an excellent material for research on the evolution of the Indian summer monsoon (ISM). However, well-dated research spanning periods exceeding 50 ka in the late Quaternary remains scarce because of the limited dating range of the 14C method. Therefore, we collected a 33.9-m long lacustrine sedimentary core (PT2) from SW China for a chronology study in the late Quaternary using geomagnetic excursion (GE) and relative paleointensity (RPI) correlation. This study will improve our understanding of the dynamics and evolutionary rules of the ISM. Detailed rock magnetic analyses showed that single domain- and vortex-state magnetite (and/or maghemite) dominated the magnetic properties in moderate and strong magnetic intervals. Moderate variations in concentration-dependent (SIRM, ARM) and grain size-dependent (ARM/SIRM) parameters satisfied the criteria for RPI research. Consequently, we present the magnetostratigraphy and RPI record of the PT2 lacustrine sedimentary core based on stepwise alternating field demagnetization. Five GEs were identified by at least three continuous reversed inclinations. Using the radiocarbon dating results as constraints, we identified the Hilina Pali, Mono Lake, and Laschamp excursions. By proxy correlation with the adjacent sedimentary core, an age tie of 140.07 ka was obtained. Subsequently, four RPI lows matched the NRM/ARM troughs in the adjacent HQ2002 drill core according to their synchronous changes. Based on the age ties above, post-Blake and Blake excursions were identified. Accordingly, three RPI lows were correlated with troughs in the global target curve PISO-1500 between 60 and 100 ka. Using AMS 14C dates, proxy-matched ties, magnetic excursions, and RPI correlation ties as constraints, we established an age-depth model of the PT2 lacustrine sequence since ∼184 ka.

Geomagnetic relative paleointensity and direction during the last 40,000 years obtained from a sediment core in the Nankai Trough

Construction of regional geomagnetic secular variation curves for the last several tens of thousands of years is important for understanding the behavior of non-dipole fields and applications to geochronology. Around Japan, secular variation records of older than 10 ka was scarce, in particular for relative paleointensity (RPI). Here, we conducted a paleomagnetic study of a sediment core covering the last ~ 40 kyr taken from a small basin in the Nankai Trough. The core consists of homogenous hemipelagic sediments except for turbidites and volcanic ashes. The age model was constructed based on seven 14C datings and two volcanic ashes. Turbidites and volcanic ashes were excluded from the construction of secular variation curves because of geologically instantaneous deposition. It was revealed that the magnetization of this core is carried largely by detrital magnetic minerals, although magnetofossils are also contained. Bulk magnetic properties show some temporal changes in magnetic concentration and grain size, but still homogeneous enough for reliable RPI estimations except for turbidites and volcanic ashes. The resultant RPI shows no correlation with the normalizer, anhysteretic remanent magnetization, of the RPI estimations or with a proxy for a magnetic grain size and/or the proportion of magnetofossils to detrital magnetic minerals. The obtained RPI record shows a long-term increasing trend since ~ 40 ka, which coincides with global stack curves. On the other hand, there are some differences in shorter timescale variations, which may reflect non-dipole fields. This study demonstrated that hemipelagic sediments in the Nankai Trough have potential for recovering high-quality RPI records when turbidites and volcanic ashes were excluded and are useful for accumulating records to construct a regional master curve.Graphical abstract

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.

PEAT DEPOSITS AS A NEW SOURCE OF PALEOMAGNETIC RECORDS IN THE HOLOCENE AS EXEMPLIFIED BY THE ETRUSSIA EXCURSION

The paper presents the results of microprobe, paleomagnetic and rock magnetic studies on peats from the Tyapka peat section (Khabarovsk region) formed 4 thousand years. Alternating field demagnetization of peats yielded a horizon of 57–109 cm (1833–3083 cal. BP) with negative inclination values of the magnetization vector, which corresponds to the geomagnetic Etrussia excursion. Obtaining relative paleointensity estimates for peats was pioneered, which are consistent with the literature archeomagnetic data for the western part of Eurasia. It was found that the Etrussia excursion took place against the background of increased values of the Earth’s magnetic field, and its duration was ~ 1200 years.

Difference in Relative Paleointensity Recording Efficiency of Magnetic Mineral Constituents in a Sediment Core Off Chile

AbstractProgress of relative paleointensity (RPI) estimations using marine sediments has greatly contributed to better understanding of the behavior of the past geomagnetic field. To enhance further the reliability of RPI estimations, we must overcome the problem that climatically induced variations of magnetic‐mineral assemblages in sediments may influence RPI records. Two major constituents of magnetic‐mineral assemblages in marine sediments are magnetofossils and detrital magnetic minerals, and the latter consists of silicate‐hosted magnetic inclusions and unprotected magnetic minerals. It is necessary to understand different RPI recording efficiencies among those constituents to tackle the problem, but previous evaluations were inconclusive. We studied this issue using a sediment core taken from the southeast Pacific Ocean. Rock‐magnetic investigations revealed that the magnetic mineral assemblage of this core during the last ∼900 Kyr is a mixture of low‐coercivity magnetofossils and middle‐coercivity detrital unprotected partially oxidized magnetite. Natural remanent magnetization versus isothermal remanent magnetization demagnetization diagram showed strong convex curvature, and RPI signals carried by the two components could be separated by calculating RPI from the gradients of 20–40 and 70–160 mT segments. We confirmed that RPI recording efficiency of magnetofossils is lower than that of detrital unprotected magnetites/maghemites. Because of the marginal overlap between the coercivity ranges of the two components, changes in their relative abundance do not influence RPI estimations. This condition is ideal for RPI estimations, and the resulted RPI curve closely coincides with that of the PISO‐1500 stack despite changes in the relative abundance of the two components.

WEPAPIS 70 (Western Pacific paleointensity-stacking for the last 70 ka)

Earth's magnetic field comprises dipoles and non-dipole components. In some cases, the non-dipole magnetic fields can be detected only in certain areas. Some changes in the magnetic field might occur and be recorded only in certain regions, so it is necessary to make relative paleointensity stacking for regions to observe events that may not be recorded in other regions. To find out about these changes in the Western Pacific, a record called WEPAPIS-70 (Western Pacific Paleointensity Stacking for the Last 70 ka) was made by stacking the paleointensity record from twelve marine and lake cores. WEPAPIS-70 is comparable to other global and regional paleointensity models but shows features that are possibly specific to the Western Pacific region. WEPAPIS-70 identifies the Tianchi excursion that was not recorded in the western hemisphere. WEPAPIS-70 also identifies short-lived events, such as the Rockall excursion.