研究者総覧

ABRAZHEVICH Aleksandra (アブラジエビツチ アレクサンドラ)

  • 大学院理工学研究科 数理物質科学専攻 講師
Last Updated :2020/09/16

研究者情報

J-Global ID

経歴

  • 2000年 - 2002年  The University of Hong Kong, ChinaResearch assistant

研究活動情報

論文

  • Alexandra Abrajevitch, Brad J. Pillans, Andrew P. Roberts, Kazuto Kodama
    Geochemistry, Geophysics, Geosystems 19 3 732 - 748 2018年03月 [査読有り]
     研究論文(学術雑誌) 
    Zebra Rock, a decorative stone remarkable for its unusual pattern of regularly spaced reddish bands and rods with white background, is found within the Neoproterozoic succession in East Kimberley, Western Australia. The unusual pigment distribution suggests that precipitation of hematite, or its precursor phase, occurred in a single episode. Magnetic properties of hematite pigment in Zebra Rock are distinctly different from those of the host shale, with a smaller median particle size and higher degree of structural perfection. The low thermal stability of the Zebra Rock pigment, with onset of thermal alteration at 300°C, suggests that the rocks have not undergone significant metamorphic heating. Stepwise thermal demagnetization reveals multiple magnetization components. Short-range variability in the relative contributions of the components to the total remanence is indicative of the stochastic nature of the hematite pigment growth process. In addition to seven magnetization components with shallow to intermediate inclinations that can be matched to the Paleozoic Australian apparent polar wander path, Zebra Rock samples contain a distinct steeply dipping magnetization that is not observed in the host shales. The steep magnetization appears to be primary, based on its unique association with the Zebra pattern, dissimilarity with younger directions, and evidence for low degree of thermal alteration of the rocks. The steep characteristic remanence contrasts with previous paleomagnetic indications of low Australian Neoproterozoic paleolatitudes. The characteristic Zebra Rock magnetization is the first Australian example of incompatible magnetization directions that have been reported previously from Ediacaran rocks in Laurentia, Baltica, and Africa.
  • Font Eric, Abrajevitch Alexandra, Florindo Fabio
    FRONTIERS IN EARTH SCIENCE 4 2016年11月 [査読有り]
  • Alexandra Abrajevitch, Lubov M. Kondratyeva, Evgeniya M. Golubeva, Kazuto Kodama, Rie S. Hori
    GEOPHYSICAL JOURNAL INTERNATIONAL 206 2 1340 - 1351 2016年08月 [査読有り]
     研究論文(学術雑誌) 
    Understanding the contribution of biogenic magnetic particles into sedimentary assemblages is a current challenge in palaeomagnetism. It has been demonstrated recently that magnetic particles produced through biologically controlled mineralization processes, such as magnetosomes from magnetotactic bacteria, contribute to the recording of natural remanent magnetization in marine and lacustrian sediments. Contributions from other, biologically induced, mineralization types, which are known from multiple laboratory experiments to include magnetic minerals, remain largely unknown. Here, we report magnetic properties of iron minerals formed by a community of iron- and manganese-reducing bacteria isolated from a natural groundwater deposit during a 2 yr long incubation experiment. The main iron phases of the biomineralized mass are lepidocrocite, goethite and magnetite, each of which has environmental significance. Unlike the majority of the previous studies that reported superparamagnetic grain size, and thus no remanence carrying capacity of biologically induced magnetite, hysteresis and first-order reversal curves measurements in our study have not detected significant superparamagnetic contribution. The biomineralized mass, instead, contains a mixture of single-domain to pseudo-single-domain and multidomain magnetite particles that are capable of carrying a stable chemical remanent magnetization. Isothermal remanent magnetization acquisition parameters and first-order reversal curves signatures of the biomineralized samples deviate from previously proposed criteria for the distinction of extracellular (biologically induced) magnetic particles in mixtures. Given its potential significance as a carrier of natural remanent magnetization, environmental requirements, distribution in nature and the efficiency in the geomagnetic field recording by biologically induced mineralization need comprehensive investigation.
  • Eric Font, Thierry Adatte, Gerta Keller, Alexandra Abrajevitch, Alcides Nobrega Sial, Luiz Drude de Lacerda, Jahnavi Punekar
    GEOLOGY 44 3 2016年03月 [査読有り]
  • Eric Font, Jorge Ponte, Thierry Adatte, Alicia Fantasia, Fabio Florindo, Alexandra Abrajevitch, Jose Mirao
    PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY 441 181 - 197 2016年01月 [査読有り]
     研究論文(学術雑誌) 
    The Deccan Volcanic Province (DVP) was built up by three major phases of eruptions; the most voluminous of which, the Deccan Phase 2, encompassed the Cretaceous-Palaeogene (KT) boundary. Deccan eruptions have been implicated as a contributor to the end-Cretaceous mass extinction, however, mechanism by which volcanic activity affected biota remains poorly understood. We applied a combination of rock magnetic techniques scanning electron microscopy to characterize mineral assemblages of three sections of intertrappean lacustrine sediments from the north-western Maharashtra Deccan Volcanic Provinces. Our results indicate that in sediments deposited during the early stages of the Deccan Phase 2, the Daiwal River and Dhapewada sequences, iron-bearing mineral association is dominated by detrital iron oxides (magnetite and hematite) sourced from the weathering of the surrounding basaltic bedrocks, with minor contribution form authigenic iron sulphides (framboidal pyrite, pyrrhotite and/or greigite). The sediments deposited during the final stages of Phase 2 (the Podgawan sequence) differ significantly in their characteristics. In particular, the Podgawan sediments have 1) very low magnetic susceptibility values, but higher terrigenous fraction (clays and shales) content; 2) more complex assemblage of magnetic minerals, 3) ubiquitous presence of Fe-Ca-Ce vanadates; and 4) unusual lithological variations in the middle part of the section (represented by a charcoal-rich level that is capped by a red clay layer containing fossilized bacterial colonies). We suggest that these unusual characteristics reflect increased acidity in the region during the deposition of the Podgawan sequence, likely due to cumulative effects of volcanic aerosols released during the Deccan Phase 2 eruptions. The combination of these features may be used to recognize episodes of increased acidity in the geological record. Our results also contribute to understanding of local vs. global effects of the Deccan volcanism. (C) 2015 Elsevier B.V. All rights reserved.
  • Alexandra Abrajevitch, Eric Font, Fabio Florindo, Andrew P. Roberts
    EARTH AND PLANETARY SCIENCE LETTERS 430 209 - 223 2015年11月 [査読有り]
     研究論文(学術雑誌) 
    The respective roles of an asteroid impact and Deccan Traps eruptions in biotic changes at the Cretaceous-Paleogene (K-Pg) boundary are still debated. In many shallow marine sediments from around the world, the K-Pg boundary is marked by a distinct clay layer that is often underlain by a several decimeter-thick low susceptibility zone. A previous study of the Gubbio section, Italy (Lowrie et al., 1990), attributed low magnetization intensity in this interval to post-depositional dissolution of ferrimagnetic minerals. Dissolution was thought to be a consequence of downward infiltration of reducing waters that resulted from rapid accumulation of organic matter produced by mass extinctions after the K-Pg event. We compare the magnetic properties of sediments from the Gubbio section with those of the Bidart section in southern France. The two sections are similar in their carbonate lithology and the presence of a boundary clay and low susceptibility zone. When compared to background Cretaceous sediments, the low susceptibility zone in both sections is marked by an absence of biogenic magnetite, a decrease in total ferrimagnetic mineral content, and a preferential loss of magnetite with respect to hematite features that are consistent with reductive dissolution. However, unlike the Gubbio section, where the low susceptibility zone starts immediately below the boundary clay, the low susceptibility zone and the clay layer at Bidart are separated by a similar to 4-cm carbonate interval that contains abundant biogenic magnetite. Such separation casts doubt on a causal link between the impact and sediment bleaching. More likely, the low susceptibility layer marks a different environmental event that preceded the impact. An episode of increased atmospheric and oceanic acidity associated with Deccan Traps volcanism that occurred well before the K-Pg impact is argued here to account for the distinct magnetic properties of the low susceptibility intervals. (C) 2015 Elsevier B.V. All rights reserved.
  • Ethan G. Hyland, Nathan D. Sheldon, Rob Van der Voo, Catherine Badgley, Alexandra Abrajevitch
    GEOLOGICAL SOCIETY OF AMERICA BULLETIN 127 7-8 975 - 981 2015年07月 [査読有り]
     研究論文(学術雑誌) 
    Description of precipitation patterns and changes in the hydrological cycle during periods of past global change is crucial for providing an understanding of terrestrial climate systems and for predicting impacts of future climate change such as shifting water availability. While a number of proxies and climofunctions exist for reconstructing paleoprecipitation using paleosols, all of the available tools for reconstructing paleoprecipitation are either limited to certain precipitation ranges (effective only for low-precipitation regimes; e.g., depth to Bk, chemical index of alteration [CIA-K]), or are relevant only to a limited range of paleosols (single-pedotype relationships; e.g., calcium-magnesium index [CALMAG]). Here, we measure the acquisition of isothermal remanent magnetization in B horizons of modern soils to quantify the ratio of pedogenic magnetic minerals goethite and hematite, and we use the relationship between these soil magnetic properties and measured climatic variables at each soil site to derive a new quantitative proxy for precipitation. By compiling both literature-derived and measured goethite-hematite (G/H) ratios and mean annual precipitation estimates for a global suite of modern soils (n = 70), we describe a strong linear relationship (R-2 = 0.96) between the G/H ratios of soil B horizons and mean annual precipitation that can be used to estimate paleoprecipitation values for a wide range of climatic regimes (100-3300 mm yr(-1)) and soil types (Inceptisols, Alfisols, Ultisols, Oxisols, Mollisols, Aridisols, Spodosols). We tested the new climofunction using paleosols from the early Eocene of Wyoming, which show that estimates based on G/H ratios compare favorably to and expand upon previously published estimates based on paleosol data.
  • Alexandra Abrajevitch, Brad J. Pillans, Andrew P. Roberts
    GEOPHYSICAL JOURNAL INTERNATIONAL 199 2 658 - 672 2014年11月 [査読有り]
     研究論文(学術雑誌) 
    Haematite pigment is a common constituent of sedimentary rocks, but its contribution to the natural remanent magnetization of rocks is poorly understood. Here, we describe magnetic properties of two distinct pigment types that produce a characteristic decorative 'print stone' found in the similar to 2.5 Ga Mount McRae Shale Formation, Hamersley Province, Western Australia. Distinct magnetic remanence directions observed in the Print Stone can be correlated to each pigment type. By comparison with the Australian apparent polar wander path, the remanence carried by uniformly distributed pigment can be dated to similar to 15-25 Ma, while two age options, the Mesoproterozoic (similar to 1.5 Ga) or the middle Carboniferous (similar to 320-310 Ma), are permissible for the remanence carried by the pigment responsible for the distinctive 'newsprint' pattern. Magnetic properties and demagnetization characteristics of the different pigment types overlap significantly, and thus are not predictive of the dominant remanence carrier. Magnetic characteristics of the uniformly distributed pigment vary significantly on short spatial scales. Strong local control on pigment formation raises the possibility that a primary remanent magnetization may survive locally in pockets within sedimentary red bed formations.
  • Alexandra Abrajevitch, Andrew P. Roberts, Kazuto Kodama
    PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY 410 1 - 13 2014年09月 [査読有り]
     研究論文(学術雑誌) 
    ODP Site 747, central Kerguelen Plateau, contains a nearly complete record of the Middle Miocene Climate Transition (MMCT). Its unusually well preserved and rich foraminiferal assemblages have resulted in the MMCT interval at Site 747 becoming a focus of high-resolution paleoclimatic studies that have established this sequence as a Southern Ocean reference. We combine X-ray fluorescence (XRF) elemental counts of Fe, Ti, and Al and concentration-dependent magnetic parameters to trace volcanic ash input to sediments at ODP Hole 747A. We observe systematic variations in Al/Ti and Fe/Ti ratios that are consistent with increased primary productivity at times of increased terrigenous input and, therefore, increased nutrient supply. A spike in terrigenous material, which we interpret as volcanic ash, coincides with major changes in species abundance and stable oxygen and carbon isotopic shifts within the MMCT interval. This correlation suggests that nutrient flux associated with volcanic iron fertilization played a significant role in controlling phytoplankton communities at Site 747. We demonstrate that rapidly obtained XRF and magnetic parameters can provide useful proxies for terrigenous input and, therefore, nutrient fluxes in pelagic marine settings that can help to resolve local versus global drivers of paleoenvironmental and paleoceanographic changes. (C) 2014 Elsevier B.V. All rights reserved.
  • Heslop David, Roberts Andrew P, Chang Liao, Davies Maureen, Abrajevitch Alexandra, De Deckker Patrick
    EARTH AND PLANETARY SCIENCE LETTERS 382 58 - 65 2013年11月 [査読有り]
  • Alexandra Abrajevitch, Rie S. Hori, Kazuto Kodama
    GEOLOGY 41 7 803 - 806 2013年07月 [査読有り]
     研究論文(学術雑誌) 
    The end-Triassic mass extinction event is regarded as one of the five largest extinction events of the Phanerozoic. The emerging consensus points to volcanic activity at the Central Atlantic Magmatic Province (CAMP) as the ultimate cause of the extinction, yet the underlying mechanisms and the nature of global environmental changes that accompanied the biotic turnover remain elusive. We present a rock magnetic study of the extinction interval found within a continuous chert sequence that provides an uninterrupted record of pelagic sedimentation in the Panthalassa Ocean. The variations in the relative abundances and characteristics of authigenic magnetic phases indicate that the Triassic-Jurassic transition progressed in two stages. The initial stage, characterized by a disappearance of the previously ubiquitous magnetofossils, started a few tens of thousands of years to 100 k.y. prior to the formal Triassic-Jurassic boundary as identified by the diagnostic radiolarian species. The second stage, defined by significant changes in optical and magnetic properties of hematite pigment, lasted a few tens of thousands of years. The stepwise change in magnetic properties is suggestive of the protracted environmental deterioration, likely prompted by the early episodes of the CAMP volcanism, which was followed by a sudden ocean acidification event, perhaps triggered by a catastrophic release of gas hydrates.
  • Mikhail L. Bazhenov, Natalia M. Levashova, Kirill E. Degtyarev, Rob Van der Voo, Alexandra V. Abrajevitch, Phil J. A. McCausland
    GONDWANA RESEARCH 22 3-4 974 - 991 2012年11月 [査読有り]
     研究論文(学術雑誌) 
    It is a common concept that different tectonic units in the western part of the Central Asian Orogenic Belt were united into the landmass of the Kazakhstania continent in the Paleozoic but many important details of its history remain enigmatic and controversial. Recently published paleomagnetic data from this region demonstrate that the similar to 2000 km long horseshoe-shaped Devonian Volcanic Belt was created by oroclinal bending of an originally rectilinear active margin of Kazakhstania. Still, the Silurian and Devonian paleomagnetic results which this interpretation is based upon are limited and unevenly spread along the belt, and additional middle Paleozoic data are highly desirable. Accordingly, we studied three mid-Paleozoic objects from different segments of this volcanic belt. Two of the three new objects yielded paleomagnetic directions that fit perfectly into the oroclinal scenario, whereas the third one provided no interpretable data. The earlier history of Kazakhstania, however, remains misty. We obtained three new Ordovician results in north-central Kazakhstan and found similar inclinations but widely dissimilar declinations. Previously published data show a large scatter of Ordovician declinations in South Kazakhstan and Kyrgyzstan as well. We analyzed all seven Middle-Late Ordovician paleolatitudes and came to the conclusion that a nearly E-W trending active margin of the Kazakhstania landmass had existed at low (similar to 10 degrees S) latitudes at that time. We hypothesize that this margin of the Kazakhstania landmass collided with another island arc, called Baydaulet-Akbastau, and with the Aktau-Junggar microcontinent by the Ordovician-Silurian boundary. As a result of this collision, subduction ceased, and regional deformation, magmatism, and rotations of crustal fragments took place in most of Kazakhstania. In Silurian time, Kazakhstania moved northward crossing the equator and rotating clockwise by similar to 45 degrees. This changed the orientation of the Kazakhstania to NW-SE, and thereby established the (rectilinear) predecessor of the modern curved Devonian Volcanic Belt. (C) 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
  • Alexandra Abrajevitch, Sergey Zyabrev, Alexei N. Didenko, Kazuto Kodama
    GEOPHYSICAL JOURNAL INTERNATIONAL 190 3 1439 - 1454 2012年09月 [査読有り]
     研究論文(学術雑誌) 
    A significant margin-parallel translation of terranes is postulated by all models for tectonic evolution of the East Asian continental margin, although the timing and magnitude of displacements of individual elements are poorly constrained as yet. The West Sakhalin Basina forearc basin associated with the Cretaceous volcanic arcsis one of the displaced tectonic elements. Our palaeomagnetic study of the basin rocks provides the first quantitative constraints on its Cretaceous history. We identified a characteristic magnetization in the BerriasianValanginian basalt representing the oceanic basement of the basin and four characteristic magnetizations in a continuous sequence of the Albian to Maastrichtian siliciclastic fill of the basin. A combination of the positive fold, conglomerate and reversals tests supports the primary origin of the remanence in the sediments. Palaeomagnetic data indicate a gradual shift of the West Sakhalin Basin from subequatorial latitudes during the Early Cretaceous to about 40oN by the Late Cretaceous. The main phase of the margin-parallel migration took place during the Early Cretaceous, and ended by the Cenomanian. The continuous sedimentary sequence records similar to 50 degrees of progressive clockwise rotation during the Late Cretaceous.
  • Ada R. Dominguez, Rob Van der Voo, Trond H. Torsvik, Bart W. H. Hendriks, Alexandra Abrajevitch, Mathew Domeier, Bjorn T. Larsen, Sonia Rousse
    GEOPHYSICAL JOURNAL INTERNATIONAL 186 2 529 - 550 2011年08月 [査読有り]
     研究論文(学術雑誌) 
    To better constrain Baltica's position within Pangea, we conducted a palaeomagnetic study of Permo-Triassic dykes from the Oslo Graben, as a follow-up to an initial, but rather limited, study by Torsvik and colleagues in 1998. The age of these so-called Lunner dykes had previously been determined as similar to 240 Ma in that study, but details in their analyses and new (40)Ar/(39)Ar ages reveal that there may have been some argon loss in the initially dated dyke minerals and that a combined (weighted mean) age of 271 +/- 2.7 (2 sigma) Myr for the dykes is preferable. We find two major components of magnetization in our samples: one carried by an Fe-sulphide (likely pyrrhotite) and the other carried by low-Ti magnetite; these magnetization components may be found together (superposed) in a given sample or they may occur apart. Micronmetre-sized crystals of Ti-Fe oxides, observed with a scanning electron microscope (SEM) show exsolution lamellae, formed upon cooling from intrusion temperatures. Assuming that the submicronmetre-sized (Ti)-magnetite grains that carry a stable remanence are of the same generation as the observed larger grains, we interpret the magnetite remanence in the dykes as of primary, thermoremanent origin. The sulphide remanence appears to be slightly younger, as seen by the SEM observations of pyrite framboids and a Fe-sulphide grain invading a Ti-magnetite grain. Moreover, the sulphide mineralization is likely of region-wide hydrothermal origin. The magnetizations carried by the pyrrhotite and magnetite have nearly identical directions and so, must be nearly of the same age. For this study, we sampled 56 sites including 39 dykes, 10 baked-contact rocks and 7 host rocks removed from the immediate dyke contacts. The dykes and the contact rocks have the same SW and up directions of magnetization, and contain the Fe-sulphide or the magnetite magnetization or both, as diagnosed by their relative unblocking temperatures. However, all the sampled carbonate and igneous host rocks far away from the dykes also have the same directions. Thus, all of the 10 originally planned contact tests are inconclusive. The new palaeopoles of this study are a few degrees apart; the magnetite pole (from dykes only, N = 25) is located at 51 degrees N, 164 degrees E, K = 69, A(95) = 3.5 degrees, whereas the pole calculated from iron sulphide magnetic directions (all rock types, N = 20) is at 54 degrees N, 166 degrees E, K = 112, A(95) = 3.1 degrees. All directions are of reversed polarity, suggesting that the magnetization was acquired during the Kiaman Reversed Superchron. The palaeomagnetic mean result from the magnetite-bearing sites implies a palaeolatitude of Oslo of 23 degrees N, whereas the palaeolatitude calculated from the pyrrhotite magnetizations is 25-27 degrees N, depending on choice of host lithologies. As noted in many previous publications, the palaeomagnetic poles for the late Palaeozoic and Early-Middle Triassic are in conflict with classical Pangea reconstructions. The poles with ages of 250 +/- 10 Ma, in particular, previously showed a discrepancy of some 25 degrees or more, when the Gondwana and Laurussia continents are restored to their juxtapositions in the Pangea-A fit, before the opening of the Atlantic Ocean. Proposed solutions to this conundrum have been controversial, involving doubts about (1) the geocentric coaxial dipole field model, (2) the reliability of the palaeomagnetic results or their ages, or (3) the validity of the Pangea-A reconstruction, leading to proposals of a Pangea B reconstruction in which Gondwana is displaced some 3500 km to the east with respect to Laurussia. The significance of our new result for this Pangea controversy resides in its improved age within an early Guadelupian (mid to late Permian) time interval where few results exist from well-dated igneous rocks in either Baltica or Laurentia. There are quite a few results from sedimentary rocks, but these may be suspected to suffer inclination shallowing, and are therefore less suitable to settle a palaeolatitudinal argument. Our new result of the magnetite magnetization, granted it is primary and acquired at about 270 Ma, combined with a new similar to 265 Ma result from Argentina and selected other poles from igneous rocks, leaves enough room for the north-south configuration of Pangea A at 270 Ma and avoids the overlap between Baltica and Gondwana that necessitated Pangea B, at least for the Late Permian.
  • Alexandra Abrajevitch, Kazuto Kodama
    GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS 12 2011年05月 [査読有り]
     研究論文(学術雑誌) 
    A rock magnetic study of the upper 40 m of the Ocean Drilling Program Site 133-820A recovered at the outer edge of the northeastern Australian continental margin shows that downcore variations in magnetic parameters are diagenetically driven and correlate with the changes in global sea level. We identified intervals enriched in single-domain (SD) magnetite in the studied section. Unlike previous studies that postulated a detrital source, we show that the bulk of the SD fraction is biologically produced and is likely to be authigenic. The abundance of SD magnetite thus cannot be used as an indicator for provenance or sediment transport mechanisms. The biogenic magnetite was preserved during the high sedimentation rate periods, likely due to a short residence time in the corrosive zone of active iron reduction. The presence of the biogenic magnetite thus can be used as an indicator for a low degree of reductive dissolution. More advanced dissolution during the periods of slow sedimentation, coincident with sea level highstands, resulted in significant changes in the composition of the detrital assemblage, particularly in relative abundances of different mineral phases. The overall stability of the detrital magnetic minerals toward dissolution varies in the studied section as hematite > magnetite > goethite. Due to the higher stability of hematite, reductive diagenesis in general will lead to the lowering of the goethite/hematite (G/H) ratio, which can be mistaken for an increased aridity in the sedimentary source areas in the conventional interpretation for this proxy. The sensitivity of the G/H proxy to diagenesis should be taken into account in paleoenvironmental studies.
  • Alexandra Abrajevitch, Rie S. Hori, Kazuto Kodama
    EARTH AND PLANETARY SCIENCE LETTERS 305 1-2 135 - 142 2011年05月 [査読有り]
     研究論文(学術雑誌) 
    Bedded chert is the only sediment type representative of the Paleozoic to early Mesozoic pelagic marine environment. Because of their association with ophiolites and island arc rocks, presence of datable microfossils and paleohorizontal reference provided by well-developed bedding surfaces, bedded chert sequences are often targeted for paleomagnetic tectonic studies. However, processes of magnetization acquisition in biosiliceous sediments, and consequently, the significance of their magnetic record, are not well understood. Our rock magnetic study of a Triassic-Jurassic radiolarian chert sequence, the Mino Terrane, Central Japan, shows that the ferrimagnetic assemblage of the gray chert units is of detrital origin, while the red chert's assemblage is dominated by authigenic phases - pigmentary hematite and biogenic magnetite - which contribute to the natural remanent magnetization. The presence of magnetofossils places red oxic chert in the category of prospective environmental archives. Magnetite-producing magnetotactic bacteria were apparently able to tolerate elevated concentrations of dissolved silica as well as a steep redox gradient in sedimentary pore-waters during the deposition of red chert layers. A strong uniaxial anisotropy due to chain-alignment of the biogenic magnetite grains likely contributes to the acquisition of anomalously stable partial thermoviscous magnetization by chert even at low metamorphic temperatures. (C) 2011 Elsevier B.V. All rights reserved.
  • Alexandra Abrajevitch, Rob Van der Voo
    EARTH AND PLANETARY SCIENCE LETTERS 293 1-2 164 - 170 2010年04月 [査読有り]
     研究論文(学術雑誌) 
    Paleomagnetic results obtained from rocks of Ediacaran age in several localities in Laurentia and Baltica persistently display co-existence of two magnetization components, one shallowly and the other steeply inclined. Both components pass criteria for a primary magnetization while geological considerations and radiometric age dating indicate that these magnetizations are surprisingly close in age. The conventional interpretation of these results, translating the inclination into paleolatitudes using the geocentric axial dipole hypothesis, would imply that rocks acquired magnetizations in positions switching back and forth between equatorial and near-polar latitudes. In a geographic reference frame, such large-scale and fast (>45 cm/yr) migrations of a continent have been rejected as dynamically implausible: neither plate tectonics nor True Polar Wander are thought to be able to attain the required velocities. A highly irregular behavior of the geomagnetic field during the Ediacaran, possibly an alternation of the geomagnetic dipole axis between a coaxial and an equatorial alignment, remains the only viable explanation for the paleomagnetic data. Such a behavior entails specific outer core conditions, which in turn impose strong constraints on the possible models for the thermal evolution of the Earth's interior. (C) 2010 Elsevier B.V. All rights reserved.
  • Alexandra Abrajevitch, Rob Van der Voo, David K. Rea
    MARINE GEOLOGY 267 3-4 191 - 206 2009年12月 [査読有り]
     研究論文(学術雑誌) 
    Relative abundances of goethite and hematite in marine sediments have been increasingly used in reflectance spectrometry studies to infer the precipitation regime at the source area of detrital sediments. This interpretation is contingent on the assumption that the ratio of these minerals is not modified by diagenetic processes. Our rock-magnetic study of the similar to 10.5-4.0 Ma interval of the Bengal Fan sedimentary sequence indicates that this is not always the case. We demonstrate that a statistical analysis of isothermal remanent magnetization acquisition curves provides an adequate estimate of the content of goethite and hematite in sediments. We also identified distinctive biogenic and "detrital" components (likely (titano-) magnetite or maghemite) in the studied samples. The sediments of the Bengal Fan contain a significant change in relative abundance of goethite and hematite at similar to 7 Ma. which is approximately coeval with a postulated climatic turnover in the sediment source region. However, systematic variations in the "detrital" and biogenic components, as well as an increase in the total organic carbon content and the appearance of pyrite concretions at the same stratigraphic level, suggest that this change in magnetic mineral content reflects a change in the degree of diagenetic alteration of the initial detrital assemblage rather than a climatic signal. When assigning climatic interpretations to changes in the relative abundance of goethite and hematite in marine sediments in future studies, the possibility of diagenetic modification should be evaluated. With rock-magnetic methods, alteration of ancient sediments can be detected and the severity of alteration can be estimated based on the presence and grain-size variation of the ferrimagnetic fraction (magnetite/maghemite), which is more sensitive to early reductive dissolution than high coercivity minerals (goethite and hematite). (C) 2009 Elsevier B.V. All rights reserved.
  • Alexandra Abrajevitch, Kazuto Kodama
    EARTH AND PLANETARY SCIENCE LETTERS 286 1-2 269 - 277 2009年08月 [査読有り]
     研究論文(学術雑誌) 
    An apparently complete carbonate-rich Cretaceous-Tertiary boundary interval in ODP section 119-738C-20R-5 from the southern Kerguelen Plateau provides a unique insight into processes of magnetization acquisition in marine carbonates. The boundary interval is characterized by a 1-m-thick clay-rich zone. Distinct depositional lamina are preserved within the basal 15 cm of this zone: the upper part is bioturbated. Previous studies have demonstrated that the bulk of the detrital fraction in the laminated and bioturbated carbonates has the same local source, and hence, the two intervals likely had similar initial detrital assemblages. Magnetic properties of these rocks, however, differ significantly. The laminated sediments have a higher content of non-silicate-bound iron, yet approximately an order of magnitude lower intensity of the natural remanent magnetization compared to the bioturbated rocks. Our detailed rock magnetic study indicates that in the bioturbated interval the dominant iron-bearing phase is single-domain magnetite, likely of biogenic origin. In the laminated interval, apart from a small ferromagnetic fraction with multi-domain-like behavior, non-silicate-bound iron is mainly sequestered in paramagnetic phases, likely poorly-crystalline oxyhydroxides. It appears that a shut-down of biological productivity after the K-T event allowed preservation of the initial detrital/early authigenic iron phases that are dominated by reactive iron oxyhydroxides. With recovery of the normal biological activity as evidenced by resumption of bioturbation, the oxyhydroxides had been replaced with biogenic magnetite. Thus produced biochemical magnetization led to a several-fold increase in the remanence. Our results suggest that in areas where bioavailable iron constitutes a significant part of the detrital input, such as in pelagic marine environments distant from clastic sources, the biochemical remanent magnetization may be the dominant process of magnetization acquisition. (C) 2009 Elsevier B.V. All rights reserved.
  • Natalia M. Levashova, Rob Van der Voo, Alexandra V. Abrajevitch, Mikhail L. Bazhenov
    GEOLOGICAL SOCIETY OF AMERICA BULLETIN 121 3-4 555 - 573 2009年03月 [査読有り]
     研究論文(学術雑誌) 
    The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is key to the formation of the Eurasian composite continent during Paleozoic time, but the views on this complicated process remain disparate and sometimes controversial. A study of three volcanic formations of mid-Silurian, Lower to Middle Devonian, and Middle Devonian age from the southwestern boundary of the Chingiz Range (NE Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A single-polarity characteristic component in mid-Silurian andesites yields a positive intraformational conglomerate test, whereas dual-polarity prefolding components are isolated from the two Devonian collections. Post-folding, reversed-polarity overprint directions have also been isolated and are likely of Permo-Triassic age. These new data can be evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confidence the polarity of each result, and hence to determine the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward, albeit with variable velocity, crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz Range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Middle Permian time. A comparison of the Chingiz paleolatitudes with those of Siberia shows, insofar as the sparse data allow, similarities between the northward motion of the Chingiz unit and that of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt.
  • Alexandra Abrajevitch, Rob Van der Voo, Mikhail L. Bazhenov, Natalia M. Levashova, Phil J. A. McCausland
    TECTONOPHYSICS 455 1-4 61 - 76 2008年07月 [査読有り]
     研究論文(学術雑誌) 
    The Kazakhstan orocline, a horseshoe-shaped belt with volcanic arcs of Devonian (external) and late Paleozoic (internal) age, is thought to have formed as a result of convergence between the cratons of Siberia, Baltica and Tarim leading to the amalgamation of Eurasia. Paleomagnetic and geologic data indicate that in the Middle Devonian the arc, which is now strongly curved, was nearly straight near the northwest-southeast trending volcanic margin of a Kazakhstania continent. To constrain the age of oroclinal bending we conducted a paleomagnetic study of Late Carboniferous to Late Permian subduction-related volcanics from the middle (NW) and north-eastern (NE) limbs of the orocline. Our new results indicate that the rotation of the middle arm of the orocline was essentially completed by the earliest Permian, while the NE arm probably was still similar to 50 degrees short of its final orientation with respect to Baltica. The rotation of, or rotation within, the NE arm was completed by the Late Permian. The paleomagnetic data constraining the timing and rotation patterns lead us to propose the following scenario for the bending of the Kazakhstan orocline. The orogenic deformation scenario began in the Late Devonian when an initial collision with Tarim pinned Kazakhstania's southern corner, while a dextral shear motion and a considerable clockwise rotation of Siberia dragged its northern end. Relative convergence between Siberia and Tarim caused initial buckling of the Kazakhstania continental element trapped between them, subdividing the belt into three (SW, NW, NE) segments. Continued subduction under the established limbs of the orocline with an estimated outward-directed subduction velocity of well less than 1 cm/yr gradually led to closure of the intervening Junggar-Balkhash oceanic basin and tightening of the orocline. (C) 2008 Elsevier B.V. All rights reserved.
  • Alexandra Abrajevitch, Rob Van der Voo, Natalia M. Levashova, Mikhail L. Bazhenov
    TECTONOPHYSICS 441 1-4 67 - 84 2007年08月 [査読有り]
     研究論文(学術雑誌) 
    A prominent feature of the central part of the Ural-Mongol orogenic belt is a series of concentric horse-shoe shaped volcanic arcs, with the youngest arc on the inside. This structure was long-suspected to be an orocline, but unequivocal evidence for this was lacking, mainly because paleomagnetic results of suitable age from this area remained sparse, but also because their interpretation was not straightforward due to a long history of deformations associated with the protracted late Paleozoic assembly of Asia. Our paleomagnetic study of Middle Devonian basaltic and andesitic flows in southeastern Kazakhstan revealed two main components of magnetization. The primary nature of a high temperature magnetization (tilt corrected Dec=286.5, Inc=46.4, alpha(95) = 7.8, k = 29.2, N = 13 sites) is supported by the presence of antipodal directions and a baked-contact test. We also isolated a post-folding overprint with an in situ mean direction Dec= 134.9, Inc=-43.0 (alpha(95) = 4.9, k =71.6, N = 13 sites). The age of this overprint can be estimated as Early Permian with a high degree of confidence. The declination of the overprint is seen to be deflected counter-clockwise by 100 +/- 6 degrees relative to the 290-Ma reference direction, indicating that the studied locality, similar to many other localities in the region, was affected by late-orogenic rotations. We use the overprint's deflection to correct the declination of the primary Devonian magnetization for these late-orogenic block-rotations. Declinations from other Silurian and Devonian paleomagnetic results in the subduction-related Devonian volcanic arc of Kazakhstan have been corrected for such rotations wherever the latter are reasonably well documented. Using corrected declinations as passive markers we restored the trend of the volcanic belt to its Devonian configuration. Our analysis indicates that the presently curved belt was nearly straight and NW-SE trending. This similar to 1500 km long volcanic belt characterized the northeastern margin of a landmass in today's central Kazakhstan where subduction occurred towards the southwest. Oroclinal bending of this arc took place in the interval between the Middle Devonian and the Late Permian. (C) 2007 Elsevier B.V. All rights reserved.

講演・口頭発表等

  • Magnetic and mercury anomalies at the Paleocene-Eocene Thermal Maximum (PETM): environmental acidification and the role of volcanism  [通常講演]
    アレクサンドラ アブラジェヴィチ
    EGU General Assembly Conference 2018年04月 ポスター発表

MISC

  • AV Abrajevitch, Ali, JR, JC Aitchison, Badengzhu, AM Davis, JB Liu, SV Ziabrev EARTH AND PLANETARY SCIENCE LETTERS 233 (1-2) 87 -102 2005年04月 
    Since 1999, independently derived geophysical and geological models have been published arguing for an intra-oceanic subduction system along essentially the entire width of the India-Eurasia collision belt. This idea conflicts with earlier proposals, where in the eastern part of the convergence zone Neotethyan mid-ocean ridge-generated lithosphere directly north of the Indian craton was consumed beneath Eurasia in Tibet in an Andean-type plate configuration. New palaeomagnetic data are reported from three Baffernian-Aptian (similar to 120 Ma) sequences of chert, siliceous mudstones and volcaniclastic rocks. These rocks form the uppermost part of the Dazhuqu supra-subduction zone ophiolite terrane, which crop out along substantial portions of the India-Eurasia (=Yarlung Tsangpo) suture zone in southern Tibet. The declination data provide little regionally-useful tectonic information; they are dominated by local rotations, presumably related to the Dazhuqu terrane's initial obduction onto the India plate in the Palaeocene and subsequent movement(s) as India later collided and indented into Eurasia. The inclination data are, however, useful because they yield consistent sub-equatorial formation sites (2.7 degrees S +/- 3.0 degrees, 7.9 degrees N +/- 2.7 degrees, 1.4 degrees N +/- 5.7 degrees), which correspond with the location of the Neotethyan intra-oceanic subduction system inferred from. the seismic tomographic data. At the time these Dazhuqu terrane rocks formed, the intra-Tethyan subduction zone would have been about 2500 km south of Eurasia. (c) 2005 Elsevier B.V. All rights reserved.
  • Jonathan C. Aitchison, Aileen M. Davis, Alexandra V. Abrajevitch, Jason R. Ali, Badengzhu, Jianbing Liu, Hui Luo, Isabella R.C. McDermid, Sergey V. Ziabrev Geological Society Special Publication 218 147 -164 2004年06月 
    Ophiolitic rocks distributed along the Yarlung Tsangpo suture zone in southern Tibet are the few remaining fragmentary remnants of many thousands of kilometres of the ocean space that formerly existed between India and Eurasia. Portions of mid-Jurassic and mid-Cretaceous intra-oceanic island arcs can be recognized amongst those rocks that have been studied in detail. Complete suprasubduction zone ophiolite successions are preserved in the Dazhuqu terrane, which crops out both east and west of Xigaze. Radiolarians in inter-pillow cherts and immediately overlying sedimentary rocks indicate a Barremian ophiolite generation event. Palaeomagnetic data show that this ophiolite formed at equatorial latitudes south of the Lhasa terrane before its south-directed emplacement onto the northern margin of India. Highly refractory ultramafic rocks in the Luobusa ophiolite appear to be of Mid-Jurassic age and are potentially related to intra-oceanic island arc remnants in the nearby Zedong terrane. Ophiolitic massifs along the suture in western Tibet are thrust southwards onto northern India and record Late Jurassic ocean-floor development. Miocene north-directed back-thrusting associated with India-Asia collision has further complicated interpretation of regional geology. The ophiolitic rocks of the Yarlung Tsangpo suture zone provide evidence for the former existence of multiple oceanic island arc segments within Neotethys and suggest that consumption of the oceanic space between India and Asia was more complicated than has been predicted by existing models. © The Geological Society of London 2003.
  • SV Ziabrev, JC Aitchison, AV Abrajevitch, Badengzhu, AM Davis, H Luo JOURNAL OF THE GEOLOGICAL SOCIETY 161 523 -538 2004年05月 
    The Bainang terrane, an intra-oceanic island are subduction complex into which Tethyan oceanic rocks were accreted during the Cretaceous, is preserved within the Yarlung-Tsangpo suture zone of Tibet. The lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central Tethyan domain from Late Triassic to mid-Cretaceous time. These rocks are preserved within a south-verging imbricate thrust stack of thin (<<1 km thick) northward younging tectonic slices. Five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts. The northern tract, which accumulated on the north side of Neotethys, was probably separated from its southern counterpart by a mid-ocean ridge. Detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract. Oceanic plate stratigraphy of the northern tract records its northward travel and mid-Cretaceous (late Aptian) approach towards a south-facing intra-oceanic subduction zone. Rocks in the southern tract developed closer to the Indian subcontinent and experienced thermotectonic subsidence and Mid-Jurassic basic alkaline intraplate magmatism. They were probably accreted late in the Cretaceous. Variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion. The overall tectonostratigraphy of the terrane reflects its development in a remote intra-oceanic setting.
  • Ali, JR, DJ Ward, C King, A Abrajevitch GEOPHYSICAL JOURNAL INTERNATIONAL 154 (2) 463 -470 2003年08月 
    A palaeomagnetic investigation of lower Eocene (ca. 52 Ma) London Clay Formation cemented mudstones from Sheppey (SE England) has yielded a mean direction of Dec. = 1.1degrees, Inc. = 43.2degrees, where N = 9, alpha95 = 6.8degrees and K = 58.5. This apparently high-quality direction (Q-factor = 5) has an associated palaeopole of 178.6degreesE, 63.7degreesN, where A (95) = 6.8degrees. The data represent the first pole from post Jurassic stable Eurasia rocks outside of the European North Atlantic Igneous Province (NAIP), of which most results have been obtained from NW Britain and the Faroe Islands. The data can in part be used to constrain the position of Palaeogene Eurasia, in particular the zero-offset declination implying negligible rotation of western Eurasia since the early Cenozoic. This is in contrast with data derived from the European NAIP, which imply small to moderate clockwise rotations for this part of the plate. The inclination angle may provide less useful information as it appears to be anomalously shallow when compared with that associated with the NAIP derived poles. In an attempt to understand the shallowing, we re-examined data from Palaeocene-Eocene sediments recovered in several boreholes (bathyal sediments in DSDP Hole 550, four cores through fluvio-delatic to middle shelf sequences in the London area, and one borehole sequence from East Anglia). In all cases, the sediments show systematic inclination shallowing similar in magnitude to that reported from Sheppey. Tectonic and geomagnetic explanations can be discounted; sediment compaction appears to be the likely cause. In light of the current controversy surrounding the 'stable Asia shallow inclination problem', the result reinforces the suggestion that tectonic modelling needs to be done carefully when the supporting data are based exclusively on palaeomagnetic studies of sedimentary rocks.
  • SV Ziabrev, JC Aitchison, AV Abrajevitch, Badengzhu, AM Davis, H Luo JOURNAL OF THE GEOLOGICAL SOCIETY 160 591 -599 2003年07月 
    Well-preserved, abundant radiolarians provide high-precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the Yarlung-Tsangpo suture zone in southern Tibet. Dazhuqu terrane ophiolites were generated in an intra-oceanic supra-subduction zone setting within a relatively short (<10 Ma) interval from late Barremian to late Aptian. Accumulation of sediments upon the newly generated ophiolite initially occurred in a series of discrete rift-controlled sub-basins associated with various spreading centres. An increasing flux of are-derived volcaniclastic sediment up-section indicates nearby volcanic arc activity. The Dazhuqu terrane developed in an intra-oceanic setting within Tethys where it was isolated from any continental influence.
  • JC Aitchison, A Abrajevitch, Ali, JR, Badengzhu, AM Davis, H Luo, JB Liu, IRC McDermid, S Ziabrev EPISODES 25 (2) 90 -94 2002年06月 
    Detailed investigations along the Yarlung Tsangpo suture zone, Tibet result in the following conclusions. (1) Arc, forearc, and subduction complex elements of at least one intra-oceanic island arc which once lay within Tethys and accreted to India prior to its collision with Asia are preserved within the suture. (2) Collision-related tectonic melange is widespread. (3) Temporally distinct conglomeratic molasse units were developed; they are related to different collision events. (4) Improved radiolarian fossil data place constraints on timing of the development of intra-oceanic terranes and melange formation. (5) A simple unconformable contact occurs between Upper Oligocene-Lower Miocene conglomerates and the southern Lhasa terrane; the so-called south-directed "Gangdese thrust" does not exist. These new results suggest the former existence of an intra-oceanic subduction system within Tethys and indicate that considerable revision of existing models for the evolution of the Yarlung Tsangpo suture zone is required.

愛媛大学教員活動実績

教育活動(B)

担当授業科目(B01)

  • 2019, 前期, 学部, 地球科学野外実習Ⅰ
  • 2019, 前期, 学部, 地球科学野外研究
  • 2019, 前期, 修士, 地質学特論
  • 2019, 前期, 修士, 地球科学プレゼンテーション特別実習Ⅱ
  • 2019, 前期, 学部, 地球科学野外実習I
  • 2019, 前期, 学部, 地球科学野外研究


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