Evgeniy Kulakov

• Doubrovine, Pavel; Veikkolainen, Toni; Pesonen, Lauri; Piispa, Elisa; Ots, Siim; Smirnov, Aleksey; Kulakov, Evgeniy & Biggin, Andrew (2019). Latitude dependence of geomagnetic paleosecular variation and its 2 relation to the frequency of magnetic reversals: Observations from the Cretaceous and Jurassic. Geochemistry Geophysics Geosystems.  ISSN 1525-2027.  20 . doi: 10.1029/2018GC007863 Vis sammendrag

  Nearly three decades ago paleomagnetists suggested that there existed a clear link between latitude dependence of geomagnetic paleosecular variation (PSV) and reversal frequency. Here we compare the latitude behavior of PSV for the Cretaceous Normal Superchron (CNS, 84-126 Ma, stable normal polarity) and the preceding Early Cretaceous-Jurassic interval (pre-CNS, 126-198 Ma, average reversal rate of ~4.6 Myr-1). We find that the CNS was characterized by a strong increase in the angular dispersion of virtual geomagnetic poles (VGPs) with latitude, which is consistent with the results of earlier studies, whereas the VGP dispersion for the pre-CNS period was nearly invariant with latitude. However, the PSV behavior for the last 5 or 10 million years (average reversal frequency of ~4.4-4.8 Myr 1) shows that the latitude invariance of VGP scatter cannot be considered as a characteristic feature of a frequently-reversing field, and that a strong increase in VGP dispersion with latitude was not restricted to the long periods of stable polarity. We discuss models describing the latitude dependence of PSV and show that their parameters are not reliable proxies for reversal frequency and should not be used to make inferences about the geomagnetic field stability. During the pre-CNS interval, the geodynamo may have operated in a regime characterized by a high degree of equatorial symmetry. In contrast, more asymmetric geodynamos suggested for 0-10 Ma and the CNS were evidently capable of producing a very wide range of reversal frequencies.

• Gordienko, I.V.; Metelkin, D.; Vetluzhskikh, L.I.; Mikhaltsov, N.E. & Kulakov, Evgeniy (2018). New palaeomagnetic data from Argun terrane. Testing its association with Amuria and the Mongol-Okhotsk Ocean. Geophysical Journal International.  ISSN 0956-540X.  213(3), s 1463- 1477 . doi: 10.1093/gji/ggy057
• Slagstad, Trond; Roberts, Nick MW & Kulakov, Evgeniy (2017). Linking orogenesis across a supercontinent; the Grenvillian and Sveconorwegian margins on Rodinia. Gondwana Research.  ISSN 1342-937X.  44, s 109- 115 . doi: 10.1016/j.gr.2016.12.007
• Smirnov, Aleksey; Kulakov, Evgeniy; Foucher, Marine & Bristol, Katie (2017). Intrinsic paleointensity bias and the long-term history of the geodynamo. Science Advances.  ISSN 2375-2548.  3e1602306(2) . doi: 10.1126/sciadv.1602306 Fulltekst i vitenarkiv. Vis sammendrag

  Many geodynamo models predict an inverse relationship between geomagnetic reversal frequency and field strength. However, most of the absolute paleointensity data, obtained predominantly by the Thellier method from bulk volcanic rocks, fail to confirm this relationship. Although low paleointensities are commonly observed during periods of high reversal rate (notably, in the late Jurassic), higher than present-day intensity values are rare during periods of no or few reversals (superchrons). We have identified a fundamental mechanism that results in a pervasive and previously unrecognized low-field bias that affects most paleointensity data in the global database. Our results provide an explanation for the discordance between the experimental data and numerical models, and lend additional support to an inverse relationship between the reversal rate and field strength as a fundamental property of the geodynamo. We demonstrate that the accuracy of future paleointensity analyses can be improved by integration of the Thellier protocol with low-temperature demagnetizations.

• Smirnov, Aleksey; Tarduno, John A.; Kulakov, Evgeniy; McEnroe, Suzanne & Bono, Richard (2016). Palaeointensity, core thermal conductivity and the unknown age of the inner core. Geophysical Journal International.  ISSN 0956-540X.  205(2), s 1190- 1195 . doi: 10.1093/gji/ggw080
• Kulakov, Evgeniy (2014). Paleomagnetism of the ~1.1Ga Coldwell Complex (Ontario, Canada): Implications for Proterozoic geomagnetic field morphology and plate velocities. Journal of Geophysical Research - Solid Earth.  ISSN 2169-9313.  119, s 8633- 8654 . doi: 10.1002/2014JB011463
• Kulakov, Evgeniy (2013). Paleomagnetism of 1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan): new results and implications. Canadian journal of earth sciences (Print).  ISSN 0008-4077.  50, s 1085- 1096 . doi: 10.1139/cjes-2013-0003
• Kulakov, Evgeniy; Smirnov, Aleksey & Diehl, Jimmy (2013). Absolute geomagnetic paleointensity as recorded by ~1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan). Studia Geophysica et Geodaetica.  ISSN 0039-3169.  57, s 565- 584 . doi: 10.1007/s11200-013-0606-3

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• Kulakov, Evgeniy; Doubrovine, Pavel V.; Torsvik, T.H; Werner, Stephanie C. & Silkoset, Petter (2018). Paleomagnetism of Late Jurassic – Early Cretaceous coast-parallel dykes in SW Greenland.. Vis sammendrag

  It is a well-known and still unresolved problem of the Jurassic to Early Cretaceous apparent polar wander paths (APWP) discrepancy between those for North America (Laurentia) and Eurasia. Paleomagnetic poles derived from coeval rocks from the two continents differ significantly with North American poles characterized by shallower pole latitudes. The APWP mismatch cannot be resolved by correction for inclination shallowing or using different plate circuits. One of the possible explanations for the observed phenomenon is that paleomagnetic data from North America show the effects of younger remagnetizations and/or tectonic rotation. In an attempt to resolve this problem, paleomagnetic study of samples from ~35 Late Jurassic to Early Cretaceous coast-parallel mafic dykes exposed in SW Greenland (part of Laurentia) was carried out. Paleomagnetic samples have been subjected to a step-wise thermal and alternating field demagnetization techniques which yielded well-defined characteristic remanent components. A primary nature of the characteristic remanence is supported by positive baked-contact tests and the presence of paleomagnetic directions of both polarities that pass a reversal test. Our results indicate that the paleomagnetic pole for Greenland has higher latitude than the coeval North American poles and plots closer to the APW path for Eurasia. We will discuss the results and their implications for global Mesozoic apparent polar wander and paleogeography during Jurassic and early Cretaceous times.

• Kulakov, Evgeniy; Smirnov, Aleksey; Biggin, Andy; Hawkins, Louise; Sprain, Courtney; Paterson, Greig & Fairchild, Luke (2018). The long-term history of the Mesozoic geodynamo: A paleointensity perspective. Vis sammendrag

  The history of the Earth’s magnetic field, comprised of geomagnetic polarity reversals, secular variation and field strength, is crucial for understanding the geodynamo and planetary evolution. However, the question whether the long-term variations of these basic features of geomagnetic field have been intrinsically related, or they have varied independently throughout the geological history remains open. In particular, while a large class of geodynamo models predicts an inverse relationship between geomagnetic reversal frequency and paleointensity, the paleointensity data - dominated by results from bulk volcanic rocks – remain inconclusive. One reason for this is that the paleointensity record in rocks may be adversely affected by a plethora of physical processes, which must be taken into account when analyzing the paleointensity database. To address this problem, we have updated the global paleointensity database for 65-200 Ma using a set of paleointensity quality criteria (QPI) at the site-mean level. Selected paleointensity data were split into five time bins - the early Jurassic (200-171 Ma), the Jurassic hyperactivity period (JHAP, 171-155 Ma), the late Jurassic – early Cretaceous (155-126 Ma), the Cretaceous normal polarity superchron (CNS, 126-84 Ma), and the late Cretaceous (84-65 Ma) - representing the full range of reversal rate variations during the Mesozoic. For the QPI screened database, the non-parametric Kolmogorov-Smirnov (KS) test indicates that JHAP paleointensity values are significantly lower than the mean values for the other investigated periods (values are 40-60 per cent of the present-day intensity). The KS test also shows that, while high field values (those exceeding present day intensity) are rare, there is still a significant difference between the CNS period and the moderate reversal rate periods for relatively weak set of QPI’s. A stricter subset of QPI criteria substantially reduces the number of accepted pre-CNS data in each time bin to a statistically insignificant level making results of the statistical test rather ambiguous. When tested against all pre-superchron data, the CNS set also appears statistically different. In addition, inverse correlation between the reversal rate and geomagnetic field intensity is supported by the high-fidelity paleointensity data from single silicate crystals that consistently indicate high field strength during the superchron and low field strength during the period of very high reversal frequency during the JHAP. Overall, our analyses confirm an inverse relationship between geomagnetic reversal frequency and paleointensity, supporting the idea that reversal rate and paleointensity are governed by the same geodynamic processes rather than decoupled. We note that acquisition of new high-quality paleointensity data, particularly for the Jurassic, is crucial to resolve ambiguities in the paleointensity database. We will discuss our results in the context of current models of the long-term geomagnetic and thermal evolution of the Earth.

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