EMRP3.6

EDI
The short- and long-term variation of the Earth's magnetic field: new data and models

Constraining the past geomagnetic field variation is fundamental for several disciplines such as Geophysics, Stratigraphy, Volcanology, Palaeoclimatology, Human history, Archaeology etc. Despite the great effort made in recent years to improve both spatial and temporal coverages of palaeomagnetic data, fundamental properties of the field, such as the average strength and its spatial and temporal (short- and long-term) directional variations over time, remain topics of debate. The inherent difficulties in obtaining well distributed palaeomagnetic records and reliable palaeointensity data are reflected in the current palaeomagnetic databases. Available data are clearly biased toward the last 10 ka and mostly the northern latitudes. Even though, in recent years, our ability in selecting the most reliable data has improved, more data are still needed mostly from underrepresented geographical areas and temporal intervals.

This session welcomes abstracts presenting methodological advances, new directional and palaeointensity data, especially respecting the FAIR data management, as well as new palaeomagnetic reconstructions at local or global scale for a better understanding of the past behaviour of the Earth’s magnetic field. Contributions presenting relative palaeointensities, rock magnetic and micromagnetic investigation applied to address the palaeointensity and paleomagnetic determination issue are also welcome.

Convener: Anita Di ChiaraECSECS | Co-conveners: Julie Carlut, Miriam Gomez-Paccard MiriaGomez-Paccard, Evdokia Tema
vPICO presentations
| Wed, 28 Apr, 13:30–14:15 (CEST)

Session assets

Session materials

vPICO presentations: Wed, 28 Apr

Chairpersons: Anita Di Chiara, Miriam Gomez-Paccard MiriaGomez-Paccard, Evdokia Tema
13:30–13:35
13:35–13:37
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EGU21-1116
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ECS
Thomas Berndt, Chen Han, and Jose Devienne

A recent trend in paleomagnetism is the study of samples of ever decreasing sizes, going down to (sub)millimeter scales and even microscopic scales, including single-silicate-crystals and meteorites. Microscopic imaging has shown that some of these micro-scale samples appear to be much closer to ideal single-domain (SD) paleomagnetic recorders than bulk rocks. Small samples with large numbers of SD particles do, however, pose the problem of magnetic interactions affecting their paleomagnetic recording fidelity. We show that clusters of particles are common in micro-scale samples and that these interactions do affect thermoremanent magnetization (TRM) acquisition. We further show through numerical simulations that such interacting clusters may be difficult to detect in traditional experiments (such as FORC diagrams), but may nonetheless lead to over- or underestimates in paleointensities.

How to cite: Berndt, T., Han, C., and Devienne, J.: The Bigger, The Better? – Paleomagnetic recording fidelity of weakly interacting clusters of particles, and implications for micro-scale paleomagnetism, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1116, https://doi.org/10.5194/egusphere-egu21-1116, 2021.

13:37–13:39
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EGU21-1669
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ECS
Ievgen Poliachenko, Semyon Cherkes, and Dmytro Hlavatskyi

We present the results of palaeomagnetic study of Ediacaran terrigenous rocks from the SW part of East European Craton (EEC), Podolia (Ukraine). Samples are represented by red tufits of Grushkinska suite by Volhynian series, which is comparable to the upper part of the Ediacaran age by the international stratigraphic scale. Samples for paleomagnetic studies were taken at the reference section of the Grushkinsky suite of the Volhynia series in the village of Grushka (48.45°N 28°E). A total of 50 oriented core samples were selected. For the entire collection of samples, the standard procedure for paleomagnetic studies was applied. The samples underwent stepwise temperature demagnetization.  Demagnetization showed that all samples are completely demagnetized at a temperature close to 700°C. The results of demagnetization showed that additionally to the viscous components of the magnetization released up to 200°C, four more stable components of NRM are released: CLM-1–component, relatively low temperature, in the range of deblocking temperatures of 200–360°C. It is characterized by south-south-west declination and negative inclination (D/I = 197.9/-28.6); CLM-2–component, is allocated in the same temperature range as component CLM-1 (200-360°С), is characterized by south-south-west declination and positive inclination (D/I = 202.4/31); CMH–component, is strictly allocated in the range of unlocking temperatures of 590–630°C. It is characterized by northwestern declination and positive inclination (D/I = 311/18.9); CH –component, a bipolar high-temperature component, is released in the temperature range of 650–700°C. The middle direction of the forward and reverse polarity is characterized by north-north-west declination and positive inclination (D/I = 296.4/71.2). The directions of normal and reverse polarity of this component are closely antipodal and successfully pass the reversal test (γ/γc = 7.85/8.82), class “B” in accordance with [McFadden & McElhinny, 1990].

The coordinates of the virtual geomagnetic poles for the two low-temperature components, respectively, are located close to the Permian (Φ/Λ = -53.7/357.9) and Silurian part (Φ/Λ = -21.8/4.9) of the apparent polar wander path for the EEC [Torsvik et al., 2012]. The VGP, calculated from the middle-high-temperature component, is located in the Caribbean region (Φ/Λ = 33.8/271.4) and the VGP for the relatively high temperature component is located in the eastern part of the North Atlantic (Φ/Λ = -52.5/149.1) that close to the another paleomagnetic determinations with ages about 550 Ma and 570 Ma respectively for different parts of EEC.

New data demonstrate the palaeomagnetic information content of the studied rocks and the possibility of their more detailed study in order to analyze anomalous palaeomagnetic data in the ediacaran and study the evolution of the Earth's geomagnetic field.

The analysis of directions and poles indicates that the paleomagnetic results do not contradict the data on the extremely high variability of the geomagnetic field in the studied time interval.

The new paleomagnetic determinations correspond to the previous results obtained by other authors for different regions of the East European platform, thereby supplementing them.

How to cite: Poliachenko, I., Cherkes, S., and Hlavatskyi, D.: Paleomagnetism of terrigenous rocks of the Volynian series (Ediacaran) from Podolia (Ukraine), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1669, https://doi.org/10.5194/egusphere-egu21-1669, 2021.

13:39–13:44
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EGU21-9035
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ECS
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solicited
Annique van der Boon, Andy Biggin, Daniele Thallner, Mark Hounslow, Jerzy Nawrocki, Kristyan Wójcik, Mariusz Paszkowski, Peter Königshof, Tim de Backer, Pavel Kabanov, Sofie Gouwy, Richard Vandenberg, and Richard Bono

The Devonian has long been a problematic era for paleomagnetism. Devonian data are generally difficult to interpret and have complex partial or full overprints. These problems arise from paleomagnetic data obtained from both sedimentary and igneous rocks. As a result, the reconstruction of motions of tectonic plates is often troubling, as these rely on apparent polar wander paths constructed from Devonian paleomagnetic poles. Also the geomagnetic polarity time scale for this time period is poorly constrained. Paleointensity studies suggest that the field was much weaker than the field of today, and it has been hypothesised that this was accompanied by many polarity reversals (a hyperreversing field). We review studies on Devonian paleopoles, magnetostratigraphy and paleointensity. We tentatively suggest that the field during the Devonian might have been so weak and perhaps of a non-dipolar configuration, that obtaining reliable paleomagnetic data from Devonian rocks is extremely difficult.  In order to push forward the understanding of the Devonian field, we emphasise the need for studies to provide fully accessible data down to specimen level demagnetisation diagrams. Incorporating all data, no matter how complex or bad they might seem, is the only way to advance the understanding of the Devonian magnetic field. Recent paleointensity studies appear to suggest that the Devonian and Ediacaran were both extreme weak field intervals. For the Ediacaran, it has been hypothesised that the field had an impact on life on earth. A fundamentally weak and perhaps non-dipolar field during the Devonian might have had an influence on evolution and extinctions. As there is a large number of biological crises in the Devonian, we here pose the question whether the Earth’s magnetic field was a contributing factor to these crises. New independent evidence from the Devonian-Carboniferous boundary suggests that the Hangenberg event was caused by increased UV-B radiation, which is in line with a weak magnetic field.

How to cite: van der Boon, A., Biggin, A., Thallner, D., Hounslow, M., Nawrocki, J., Wójcik, K., Paszkowski, M., Königshof, P., de Backer, T., Kabanov, P., Gouwy, S., Vandenberg, R., and Bono, R.: Synthesis of palaeomagnetic datasets suggests that the geomagnetic field was persistently non-uniformitarian in the Devonian, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9035, https://doi.org/10.5194/egusphere-egu21-9035, 2021.

13:44–13:46
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EGU21-1737
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Highlight
Anita Di Chiara, Lisa Tauxe, Thomas Levy, Mohammand Najjar, Fabio Florindo, and Erez Ben-Yosef

Constraining the secular variations of the Earth’s magnetic field strength in the past is fundamental to understanding short term processes of the geodynamo. Such records constitute a powerful and independent dating tool for archaeological sites and geological formations. In this study, we present 10 new and robust archaeointensity results from Pre-Pottery to Pottery Neolithic and, for one of the first times, flint (burnt chert) from Jordan. Two of these results constitute the oldest archaeointensity data for the entire Levant, ancient Egypt and Mesopotamia extending the archaeomagnetic dating reference for the Holocene. Virtual Axial Dipole Moments (VADM)s show that the Earth’s magnetic field in the Southern Levant was weak (about half the present field) at around 7,600 years BCE, recovering its strength to greater than the present field around 7,100 BCE and gradually weakening again around 5,200 years BCE. In addition, successful results obtained from burnt flint demonstrate the potential of this rarely used material in archaeomagnetic research, in particular for prehistoric periods from the first use of fire to the invention of pottery.

How to cite: Di Chiara, A., Tauxe, L., Levy, T., Najjar, M., Florindo, F., and Ben-Yosef, E.: The strength of the Earth’s magnetic field from Pre-Pottery to Pottery Neolithic, Jordan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1737, https://doi.org/10.5194/egusphere-egu21-1737, 2021.

13:46–13:48
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EGU21-4232
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Highlight
Jerzy Nawrocki, Karol Standzikowski, Tomasz Werner, Maria Łanczont, Jan Gancarski, and Zdzisław Gil

The bricks can be one of the best material for archeomagnetic studies. Their backing technique (i.e. horizontal location in the furnace) allow to determine also the value of inclination of geomagnetic field.  However, reuse of older bricks for the construction of newer objects can limit the utility of this material in archeomagnetic studies. A set of the brick samples from 26 historical buildings in SE Poland was taken for archeomagnetic investigations. As a result of this study, the secular variations of palaeointensity and inclination of the geomagnetic field from 1200 to 1800 AD were defined for this part of Poland. The paleointensity of geomagnetic field  was determined using the IZZI-Thellier-Thellier protocol. The course of the new regional palaeosecular curves is approximately the same as so far obtained in other parts of Europe. Data obtained from four brick buildings, however, do not fit substantially to the reference European curves. The remarkable difference  is a rapid and deeper drop of inclination and significantly higher than expected values of  palaeointensity. These features indicate that bricks used for the construction of these buildings (dated on XVI – XVII centuries) were taken from older brick constructions, most probably from the Gothic time (XIII/XIV c.). We compared our data with the earlier data obtained from brick buildings in N Poland. The regional archeomagnetic curves calculated for these two regions of Poland are completely different in their segments as old as the first half of the 18th century. This fact could be explained by the reuse of medieval bricks during the construction of studied objects from N Poland (dated on the first half of the 18th century) and applied for the construction of reference curve or by later secondary heating of original bricks.

This research was supported by  the National Science Centre of Poland (project no: UMO-2016/23/B/ST10/0129).

How to cite: Nawrocki, J., Standzikowski, K., Werner, T., Łanczont, M., Gancarski, J., and Gil, Z.: Reuse of medieval bricks as important limitation for construction of geomagnetic secular variation curves based on archeomagnetic studies of brick buildings in Poland. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4232, https://doi.org/10.5194/egusphere-egu21-4232, 2021.

13:48–13:50
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EGU21-4385
Maria Kostadinova-Avramova, Petar Dimitrov, Andrei Kosterov, and Mary Kovacheva

Numerous historical sources and archaeological monuments attest the age of Antiquity in Bulgaria – from both the early Roman period (I – III c.) and Late Antiquity (IV – VI c.). Owing to systematic archaeological excavations, lasting more than 100 years, plenty of information has been accumulated concerning not only all aspects and manifestations of its material culture, but also their evolution and chronology.  This in turn allows for interdisciplinary fields such as archaeomagnetism to progress.

There are many archaeomagnetically studied archaeological structures from the Antiquity. The results included in the Bulgarian database form 74 reference points. However, only 20 of them are full-vector determinations because 70 % of the investigated materials are bricks. Hence, the secular variation of declination is poorly constrained within the considered period. Moreover, the reuse of bricks in the constructions occurred quite often (especially in the Late Antiquity) providing for possible errors in archaeological dating. In addition, stronger effects of magnetic anisotropy and cooling rate are usually expected for bricks than for hearths, domestic ovens, production kilns or burnt dwelling remains (there are no results from pottery in the Bulgarian dataset) and both factors are not evaluated for most of the older results. All this can explain the contradictions observed between some of the experimental results juxtaposed over the absolute time scale. In an attempt to clarify these contradictions 13 baked clay structures from eight archaeological sites were archaeomagnetically studied producing seven new directional and eight new intensity data. The samples collected possess variable magnetic properties suggesting differences in clay sources and/or firing conditions. Magnetically soft minerals prevail in seven structures but in the remaining six, abundant HCSLT phase is detected. The success rate of archaeointensity determination experiments vary from 49 to 100 %. It appears that samples containing HCSLT phase always produces good araeointensity results unlike those with the dominant presence of soft carriers.

The new reference points are compared with the present compilation of Bulgarian archaeomagnetic dataset and with the data from the neighboring countries.

 

This study is supported by the grant KP-06-Russia-10 from the Bulgarian National Science Fund and Russian Foundation of the Basic Research grant 19-55-18006.

How to cite: Kostadinova-Avramova, M., Dimitrov, P., Kosterov, A., and Kovacheva, M.: Secular geomagnetic field variations in Bulgarian lands during Classical Age and Late Antiquity – new archaeomagnetic data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4385, https://doi.org/10.5194/egusphere-egu21-4385, 2021.

13:50–13:52
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EGU21-13101
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ECS
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Highlight
Raquel Bonilla-Alba, Miriam Gómez-Paccard, Francisco Javier Pavón-Carrasco, Elisabet Beamud, Verónica Martínez-Ferreras, Josep Maria Gurt-Esparraguera, Enrique Ariño-Gil, Judit del Rio, Alicia Palencia-Ortas, Fátima Martín-Hernández, Annick Chauvin, and María Luisa Osete

Recent archeomagnetic studies performed in different regions of the world have revealed unusual periods of sharp changes in intensity during the first millennium. Here we focus on the study of intensity variations between 600 BCE and 600 CE in central Asia, where an important intensity decrease seems to be present during the second half of the 1st millennium BCE. For this purpose, we present a new paleosecular variation (PSV) curve obtained from 51 new archeointensities and the selected previous data located within a radius of 1000 km around Termez (Uzbekistan). The new curve shows an intensity maximum around 400 BCE followed by a rapid decrease. When the virtual axial dipole moment (VADM) values are compared with the Dipole Moment estimations derived from different global geomagnetic models key differences are observed, suggesting an important non-dipolar effect for this feature. Finally, in order to constrain the spatial behaviour of this phenomenon and its global implications, we investigate the PSV intensity and VADM trends from twelve regions distributed among Central America, Europe and Asia. A VADM maximum is observed in Western Europe (Iberia and Germany) around 450 BCE, associated to rates of change of about 9 µT/century. This feature is also observed eastwards, in the Caucasus and the Levant, but associated to lower rates of changes. In Central Asia (Uzbekistan) our new study suggests that maximum values of about 14 µT/century, between 400-300 BCE, were achieved. In other regions, as Eastern Asia and Central America, rapid variations of the intensity are not observed during the targeted period.

 

How to cite: Bonilla-Alba, R., Gómez-Paccard, M., Pavón-Carrasco, F. J., Beamud, E., Martínez-Ferreras, V., Gurt-Esparraguera, J. M., Ariño-Gil, E., del Rio, J., Palencia-Ortas, A., Martín-Hernández, F., Chauvin, A., and Osete, M. L.: Rapid intensity variations during the second half of the first millennium BCE in Central Asia and global implications., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13101, https://doi.org/10.5194/egusphere-egu21-13101, 2021.

13:52–13:54
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EGU21-3409
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ECS
Ahmed Nasser Mahgoub, Bernardo Ignacio García-Amador, and Luis Manuel Alva-Valdivia

We report 24 palaeomagnetic directions and 10 high-quality Thellier-derived palaeointensity (PI) values, obtained from 27 sites located in Baja California Peninsula, northwestern Mexico. Sampling was done in four rock units (magnesian andesites, calc-alkaline lavas, ignimbrites, adakites) belonging to San Borja and Jaraguay monogenetic volcanic fields. These units were erupted between ~ 15 and 2.6 Ma (previous K-Ar and 40Ar/39Ar data), hence results are presented in two consecutive periods: middle-late Miocene and Pliocene. Based on previous geological and geophysical records, the kinematic evolution of the region was carefully considered, allowing for the independent restoration of the palaeoposition of each sampled site. The identified main magnetic minerals are titanomagnetite, magnetite, and minor hematite, of variable grain size, present as intergrowths, which reflect varying oxidation/reduction conditions during emplacement of high-temperature magmas. We did not observe a clear relationship between the magnetic properties of the different sites and their success rate for PI experiments. This is with the exception of the FORC analysis which showed a fairly good correlation with PI success. Pliocene (Dec=359.2°; Inc= 47.4°; α95=7.6°; and k= 41.43) and Middle-late Miocene (Dec=353.9°; Inc= 38.5°; α95=9.2°; and k= 28.56) mean directions were calculated from 20 sites (10 sites per period), and PI mean values of 29.2 ± 9.1 μT and 23.2 ± 6.3 μT were determined for the two periods, respectively. Compiling global filtered PI data, together with our results, indicates that the strength of the geomagnetic field during middle-late Miocene was weak (virtual dipole moment = 5.0±2.2×1022 Am2) compared to Pliocene (6.4±2.8× 1022 Am2), and also relative to the present-day value (7.6 × 1022 Am2). This indicates the global nature of the low dipole moment during the middle-late Miocene, which is consistent with what was previously concluded that from the past 30 Ma to the present time the magnetic field strength has increased. However, issues related to the Spatio-temporal distribution of PI data still present an obstacle to validating these suggestions; therefore, more reliable data are still needed.

How to cite: Mahgoub, A. N., García-Amador, B. I., and Alva-Valdivia, L. M.: Comprehensive palaeomagnetic study of San Borja and Jaraguay monogenetic volcanic fields, Baja California (28–30°N): considerations on latitudinal corrections, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3409, https://doi.org/10.5194/egusphere-egu21-3409, 2021.

13:54–13:56
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EGU21-8159
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ECS
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Highlight
Yael Engbers, Andy Biggin, and J. Michael Grappone

A long-lived hypothesis is that, if averaged over sufficient time (ca 10 million years), the Earth’s magnetic field approximates a geocentric axial dipole (GAD). Despite this common assumption, the question of how significant the non-GAD features are in the time-averaged field is an important and unresolved one. In the present-day field, the South Atlantic Anomaly (SAA) is the biggest irregularity in the field. We know that this anomaly has not always been a part of the field, but in Engbers et al., 2020, it was shown that the magnetic field shows irregular behaviour in this region on a million-year timescale. The irregular behaviour was demonstrated through a substantially high VGP dispersion (21.9º) for lava flows from Saint Helena that are between 8 and 11 million years old. The island of Saint Helena is located at the margin of the present-day SAA and has declination -16.6º, inclination -57.5º relative to expected GAD values of 0.0º/-7.8º (Dec/Inc). We have now commenced the measurements of absolute palaeointensity data from this location. So far, we have performed thermal and microwave IZZI-Thellier experiments on 2 localities from Saint Helena. The site mean results show variable but generally very low field intensities, although further work is required to make these sufficiently robust. Our low field estimates suggest a field in the South Atlantic that is not only unstable, but mainly weaker than expected. This could mean that recurring reversed flux patches (RFP) are responsible for the irregularities and weaknesses in the field in this region, stretching back up to 11 million years ago.

How to cite: Engbers, Y., Biggin, A., and Grappone, J. M.: Recurring magnetic field anomalies in the South Atlantic and the first palaeointensities from Saint Helena, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8159, https://doi.org/10.5194/egusphere-egu21-8159, 2021.

13:56–13:58
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EGU21-13171
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ECS
Elizaveta Bobrovnikova and Ivan Lebedev

Studying of paleosecular variations (PSV) over geological time allows us to characterize not only the behavior and evolution of the geomagnetic field, but also to estimate the rate of formation of large igneous provinces (LIP). In order to use this paleomagnetic tool, the amplitude of paleosecular variations during the corresponding time interval has to be known, but for the end of the Cretaceous superchron, in particular for high latitudes, such the data sets are extremely small. Our study is aimed at obtaining a limit on the PSV amplitude for Late Cretaceous in order to use these data to estimate the rate of formation of the Okhotsk-Chukotka Volcanic Belt.

The formation of a paleomagnetic record in volcanic flows occurs by acquiring a thermal remanent magnetization (TRM) during their cooling below the Curie temperature of the magnetic minerals. Direction of this TRM can be used for calculation of the virtual geomagnetic pole (VGP), which characterizes the direction of the geomagnetic field at a given time and place. The angular dispersion of virtual geomagnetic poles (VGP scatter, Sb) is generally accepted as a measure of the paleosecular variations and uses to assess the duration of volcanic section formation. If the volcanic section was formed for a long time (more than 10 000 years), then the amplitude of the recorded geomagnetic variations will correspond to the expected dispersion for a given latitude. In the case of significantly higher eruption rates, the amplitude of the recorded PSV will be lower than it is predicted by the model for a given latitude.

During the 2019-2020 field seasons paleomagnetic studies were carried out on a number of Late Cretaceous volcanic sections of the Okhotsk-Chukotka Volcanic Belt located in the Bilibinsky District of the Chukotka Region. VGPs and their scatter were calculated for 79 flows of the Kupol object. Preliminary results show that the amplitude of PVS in the Cretaceous for high latitudes of the northern hemisphere was close to that for the last 5 million years (Sb=21.4, [19.0; 23.9]).

The work is supported by the Russian Science Foundation grant N 19-47-04110.

How to cite: Bobrovnikova, E. and Lebedev, I.: Amplitude of paleosecular variations during the Cretaceous superchron: Okhotsk-Chukotka Volcanic Belt, high latitudes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13171, https://doi.org/10.5194/egusphere-egu21-13171, 2021.

13:58–14:00
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EGU21-13406
Marcia Ernesto, Thamyris Britto, and George Caminha-Maciel

The existing relative paleointensity (RPI) database allowed the construction of reliable stacking curves for at least the last 1 Myr. Observed fluctuations in the RPI curves suggest both lithologic/climatic influence or geodynamo processes. Stacked power spectra for RPI data from ten North and South Atlantic cores revealed a spectral peak at ~5.3kyr for data covering the last 100 kyr. This signal exhibits a similar phase for most of the series. The observed spectral peak has no apparent correspondence in the benthic O18 spectra from the same cores, suggesting the RPI signal is free from the climatic influence. Therefore, it may be a real geodynamo feature.

How to cite: Ernesto, M., Britto, T., and Caminha-Maciel, G.: Harmonic fluctuations in the relative paleointensity data?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13406, https://doi.org/10.5194/egusphere-egu21-13406, 2021.

14:00–14:15