Verhalten des Erdmagnetfeldes während Superchrons: Vergleichsuntersuchung der in den Lavaströmen des Okhotsk-Chukotka-Gürtels (NO Russland) und in den Sedimenten des Yenisey-Rückens (SW Sibirien) gespeicherten Kreide und Meso-Neoproterosoikum Maya Ereignisse.
Zusammenfassung der Projektergebnisse
The geomagnetic field, generated by a self-sustaining dynamo in the Earth’s fluid outer core, is modelled in first approximation by a geocentric axial dipole, yet experiencing fluctuations on centennial-to-millennial timescales - termed palaeosecular variation (PSV) - and being able to stochastically reverse its polarity. Over geological timescales, the geomagnetic field can also produce prolonged (> 20 Myr) periods with stable polarity-termed superchrons. In this project, we aimed at better characterising the geomagnetic behaviour, both in terms of PSV and absolute palaeointensity (API) changes, during the Cretaceous Normal Superchron (CNS, 83–121 Ma) and the Proterozoic Maya Superchron (PMS, ~1,000 Ma). For the geomagnetic behaviour during the CNS, we conducted two field campaigns in summers 2019 and 2020 to extensively explore the lava flows from the Okhotsk-Chukotka Volcanic Belt (OCVB, NE Russia). We mainly focussed on the lava flows emplaced in the Kupol area, dated at 84–90 Ma in this project. From a collection of ~1000 samples out of 80 lava flows, our palaeodirectional analysis indicates that (i) the mean direction is consistent with an emplacement of the flows at high latitude (~81°N) during a stable period of the geodynamo, and (ii) the VGP scatter S = 22.5° (20.2–25.5°; N=74) is consistent with previous estimates at high-latitude. API experiments, conducted on the most promising eight lava flows, yielded two instantaneous virtual dipole moments of 48 ± 3 ZAm2 (N=7) and 91 ± 9 ZAm2 (N=8). More generally, we scrutinised the API for the Cretaceous period. We first confirmed that determinations based on nonglassy whole rocks, submarine basaltic glasses, and single crystals are mutually inconsistent, suggesting that a separate analysis of their distributions is more appropriate. Restricting the analysis to nonglassy whole rocks, we showed that the distribution of dipole moments is slightly bimodal with a leptokurtic dominant mode, i.e. more inclined to produce outliers and suggesting distinct geomagnetic field behaviour during the CNS. For the geomagnetic behaviour during the PMS, we conducted two field campaigns in summers 2019 and 2020 to densely sample the carbonate Kartochka formation in SW Siberia. We collected oriented palaeomagnetic samples for ~120 stratigraphic layers for a section covering the equivalent of 1–8 Myr. Demagnetisation experiments yielded unambiguous determinations of the characteristic remanent magnetisation whereas pseudo-Thellier was used to provide estimates of the relative changes in palaeointensity. From a theoretical viewpoint, the fundamental assumption behind API experiments is the existence of a linear relationship between the remanent magnetisation and the ambient magnetic field. In this project, we derived exact analytical solutions of the kinetic equations for the acquisition of a pure thermoremanent magnetisation (TRM) as well as various kinds of thermochemical remanent magnetisations (TCRM). In particular, we showed that TCRMs formed at fixed grain volume by increase of the Curie point TC and pure TRMs are of comparable amplitude, with the consequence that Thellier-type protocols yield reliable API determinations in both cases. In contrast, TCRMs produced by increase of grain volume are significantly less intense than pure TRMs and produce convex Arai-Nagata diagrams indistinguishable from those produced by multi-domain (MD) remanence carriers.
Projektbezogene Publikationen (Auswahl)
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On the Reliability of Absolute Palaeointensity Determinations on Basaltic Rocks Bearing a Thermochemical Remanence. Journal of Geophysical Research: Solid Earth, 124(8), 7616-7632.
Shcherbakov, V. P.; Gribov, S. K.; Lhuillier, F.; Aphinogenova, N. A. & Tsel'movich, V. A.
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Paleomagnetism of Paleocene‐Maastrichtian (60–70 Ma) Lava Flows From Tian Shan (Central Asia): Directional Analysis and Paleointensities. Journal of Geophysical Research: Solid Earth, 125(9).
Meng, Jun; Lhuillier, Florian; Wang, Chengshan; Liu, Hao; Eid, Baha & Li, Yalin
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Exact Analytical Solutions for Kinetic Equations Describing Thermochemical Remanence Acquisition for Single‐Domain Grains: Implications for Absolute Paleointensity Determinations. Journal of Geophysical Research: Solid Earth, 126(5).
Shcherbakov, V. P.; Lhuillier, F. & Sycheva, N. K.
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Stratigraphy of the Okhotsk–Chukotka Belt in the Headwaters of the Malyi Anyui River (the Vicinity of Kupol Deposit): U–Pb and 40Ar/39Ar Age Data. Doklady Earth Sciences, 501(2), 1059-1064.
Tikhomirov, P. L.; Lebedev, I. E.; Lhuillier, F. J. L. & Pavlov, V. E.
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Do changes in geomagnetic secular variation, dipole moment and polarity reversal frequency correlate over the past 155 Myr?. Geophysical Journal International, 230(2), 1132-1146.
Eid, B.; Lhuillier, F.; Shcherbakov, V. P. & Shcherbakova, V. V.
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High‐Latitude Paleointensities During the Cretaceous Normal Superchron From the Okhotsk–Chukotka Volcanic Belt. Journal of Geophysical Research: Solid Earth, 127(2).
Bobrovnikova, E. M.; Lhuillier, F.; Shcherbakov, V. P.; Shcherbakova, V. V.; Zhidkov, G. V.; Lebedev, I. E.; Eid, B. & Pavlov, V. E.
