Magnetic Memory of Rocks: the Kazakhstan Orocline and Climatic Record of the Indian Monsoon.

The Kazakhstan orocline, a pair of concentric horseshoe-shaped volcanic arcs of Devonian (external) and late Paleozoic (internal) age, is thought to have formed during the amalgamation of Eurasia. Paleomagnetic investigations of several volcanic complexes ranging in age from Silurian to Permian are described in Chapters 2, 3 and 4 of this thesis. These studies have resulted in the construction of a tectonic model for the formation of the Kazakhstan orocline. Our results suggest that in the Middle Devonian, a nearly straight, northwest-southeast trending volcanic arc delineated the northeastern margin of the Kazakhstania landmass. In the Late Devonian, an initial collision with Tarim pinned Kazakhstania’s southern corner, while dextral shear motion and the considerable clockwise rotation of Siberia dragged its northern end. Relative convergence between Siberia and Tarim caused initial buckling of the Kazakhstania continent trapped between them, subdividing the volcanic arc into three (southwestern, middle, and northeastern) segments. Continued subduction under the established limbs of the orocline, with an estimated outward-directed subduction velocity of ~6mm/yr, gradually led to the closure of the intervening Junggar-Balkhash Ocean and tightening of the orocline. By the Late Permian, the Junggar-Balkhash Ocean no longer existed and the Kazakhstan orocline had obtained its present-day strongly curved shape. The ratio of two pedogenic iron oxides, goethite and hematite, has been demonstrated to be a good proxy for precipitation in soil-forming (terrestrial) environments. A similar interpretation of the mineral ratio has been increasingly applied to studies of marine sediments, in which variation in the goethite to hematite ratio is thought to reflect variation in the precipitation regime at the source area of the sediments. The rock magnetic study of Bengal Fan sediments described in Chapter 5 of this thesis suggests that in some intervals of the studied sedimentary section, variation in the ratio reflects a change in the degree of diagenetic alteration of the initial detrital assemblage rather than a climatic signal. Therefore, when assigning climatic interpretation to changes in the relative abundance of goethite and hematite in marine sediments, the possibility of diagenetic modification should be evaluated.Ph.D.GeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60781/1/alexabra_1.pd

Editorial: Magnetic Records of Extreme Geological Events

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Magnetic Properties and Paleomagnetism of Zebra Rock, Western Australia: Chemical Remanence Acquisition in Hematite Pigment and Ediacaran Geomagnetic Field Behavior

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.We acknowledge the Australian Research Council (ARC) through grant FS100100076 to APR and colleagues that provided a Super Science Fellowship to AA

Bainang Terrane, Yarlung-Tsangpo suture, southern Tibet (Xizang, China): A record of intra-Neotethyan subduction-accretion processes preserved on the roof of the world

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.published_or_final_versio

Precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the Dazhuqu terrane, Yarlung-Tsangpo suture zone, Tibet

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 arc-derived volcaniclastic sediment up-section indicates nearby volcanic are activity. The Dazhuqu terrane developed in an intra-oceanic setting within Tethys where it was isolated from any continental influence.published_or_final_versio

Hematite (a-Fe2O3) quantification in sedimentary magnetism: limitations of existing proxies and ways forward

Determination of hematite contributions to sedimentary magnetizations is an important but difficult task in quantitative environmental studies. The poorly crystalline and fine-grained nature of hematite nanoparticles makes quantification of their concentrations in natural environments challenging using mineralogical and spectroscopic methods, while the weak magnetization of hematite and often significant superparamagnetic nanoparticle concentrations make quantification difficult using magnetic remanence measurements. We demonstrate here that much-used magnetic parameters, such as the S-ratio and "hard" isothermal remanent magnetization (HIRM), tend to significantly underestimate relative and absolute hematite contents, respectively. Unmixing of isothermal remanent magnetization (IRM) acquisition curves is among the more suitable approaches for defining magnetic mineral contributions, although it has under-appreciated uncertainties that limit hematite quantification. Diffuse reflectance spectroscopy and other methods can enable relative hematite and goethite content quantification under some conditions. Combined use of magnetic, mineralogical, and spectroscopic approaches provides valuable cross-checks on estimated hematite contents; such an integrated approach is recommended here. Further work is also needed to rise to the challenge of developing improved methods for hematite quantification

НОВЫЕ ПАЛЕОМАГНИТНЫЕ ДАННЫЕ ПО СИЛУРИЙСКИМ И ДЕВОНСКИМ ВУЛКАНИТАМ ЧИНГИЗСКОЙ ОСТРОВНОЙ ДУГИ КАЗАХСТАНА И ИХ ВКЛАД В ПРЕДСТАВЛЕНИЯ О ТЕКТОНИЧЕСКОЙ ЭВОЛЮЦИИ УРАЛО-МОНГОЛЬСКОГО ПОЯСА

The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is the key to the formation of the Eurasian supercontinent during Paleozoic time, but the views on this complicated process remain very disparate and sometimes controversial. Three volcanic formations of the Middle Silurian, LowertoMiddle 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 singlepolarity characteristic component in midSilurian andesites yields a positive intraformational conglomerate test, whereas dualpolarity prefolding components are isolated from the two Devonian collections. These new data were evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confidence the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward 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 Late Carboniferous time. A comparison of the Chingiz paleolatitudes with those of Siberia shows similarities between the northward motion and rotational history of the Chingiz unit and those of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt.Тектоническая эволюция Урало-Монгольского подвижного пояса (УМП) многие десятилетия является предметом исследования огромного количества авторов. Однако, несмотря на все усилия, тектонические реконструкции разных авторов различаются самым радикальным образом, а во многом являются взаимоисключающими. Один из способов прояснить ситуацию – получить последовательности разновозрастных палеомагнитных определений и на их основе оценить кинематику ключевых структур УМП. При палеомагнитных исследованиях среднепалеозойских вулканитов Чингизской палеоостровной дуги на северо-востоке Казахстана в андезитах середины силура была выделена первичная компонента намагниченности, что подтверждается положительным тестом галек для внутриформационного конгломерата. В двух среднедевонских объектах также была выделена первичная намагниченность, для которой тест складки и тест обращения положительны. Объединив все имеющиеся данные по этому региону, мы получили последовательность палеомагнитных определений в интервале с позднего кембрия до поздней перми, что позволило уверенно определить, в каком полушарии находилась Чингизская палеодуга. Сделан вывод, что эта структура устойчиво смещалась к северу и пересекла экватор в силуре. Имеющиеся данные так же уверенно указывают на вторичную природу изгиба вулканических поясов Казахстана, имеющих подковообразные очертания. Сравнение этих данных с кривой кажущейся миграции полюса Сибирской платформы позволяет говорить о том, что большую часть палеозоя Чингизская палеодуга двигалась согласованно с Сибирской платформой, что накладывает жесткие ограничения на эволюцию УМП

Low-temperature magnetic properties of pelagic carbonates: Oxidation of biogenic magnetite and identification of magnetosome chains

Pelagic marine carbonates provide important records of pastenvironmental change. We carried out detailed low-temperature magneticmeasurements on biogenic magnetite-bearing sediments from the SouthernOcean (Ocean Drilling Program (ODP) Holes 738B, 738C, 689D, and 690C)and on samples containing whole magnetotactic bacteria cells. Wedocument a range of low-temperature magnetic properties, includingreversible humped low-temperature cycling (LTC) curves. Differentdegrees of magnetite oxidation are considered to be responsible for theobserved variable shapes of LTC curves. A dipole spring mechanism inmagnetosome chains is introduced to explain reversible LTC curves. Thisdipole spring mechanism is proposed to result from the uniaxialanisotropy that originates from the chain arrangement of biogenicmagnetite, similar to published results for uniaxial stable singledomain (SD) particles. The dipole spring mechanism reversibly restoresthe remanence during warming in LTC measurements. This supports aprevious idea that remanence of magnetosome chains is completelyreversible during LTC experiments. We suggest that this magneticfingerprint is a diagnostic indicator for intact magnetosome chains,although the presence of isolated uniaxial stable SD particles andmagnetically interacting particles can complicate this test. Magneticmeasurements through the Eocene section of ODP Hole 738B reveal aninterval with distinct magnetic properties that we interpret tooriginate from less oxidized biogenic magnetite and enrichment of abiogenic hard component. Co-occurrence of these two magneticfingerprints during the late Eocene in the Southern Ocean indicates lessoxic conditions, probably due to increased oceanic primary productivityand organic carbon burial

Magnetic fingerprint of southern Portuguese speleothems and implications for paleo- and environmental magnetism

Environmental magnetism of speleothems is still in its early stage of development. Here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems, and what are the factors that control its preservation and reliability. To address these issues, we used a multidisciplinary approach, including rock magnetism, petrography, scanning electron microscopy, stable carbon and oxygen isotope compositions, and major and trace element concentrations. We applied this to a set of samples from different stages of speleothem evolution: present-day dripwater (glass plates), a weathered stalactite, a fresh stalagmite, cave sediments, and terra rossa soils. These samples come from the Penico and Excentricas caves, located in two distinct aquifers of the Algarve region, South Portugal. Our results show that the main magnetic carriers of the speleothems under study are primary (detrital) and consist of maghemite (and magnetite?). Similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater. Hence, they represent a regional environmental signature. Interestingly, a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite, whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities. We propose that these low intensities can be the result from(i) different remanence acquisition mechanisms between stalagmite and stalactite and/or (ii) iron dissolution by fungal activity. We also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock (soil)-atmosphere, while oxygen isotope composition and alkaline-earth element concentrations inform about calcite-water interaction processes. These results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo-environmental signal.info:eu-repo/semantics/acceptedVersio

Iron oxide minerals in dust-source sediments from the Bodélé Depression, Chad: Implications for radiative properties and Fe bioavailability of dust plumes from the Sahara

Atmospheric mineral dust can influence climate and biogeochemical cycles. An important component of mineral dust is ferric oxide minerals (hematite and goethite) which have been shown to influence strongly the optical properties of dust plumes and thus affect the radiative forcing of global dust. Here we report on the iron mineralogy of dust-source samples from the Bodélé Depression (Chad, north-central Africa), which is estimated to be Earth’s most prolific dust producer and may be a key contributor to the global radiative budget of the atmosphere as well as to long-range nutrient transport to the Amazon Basin. By using a combination of magnetic property measurements, Mössbauer spectroscopy, reflectance spectroscopy, chemical analysis, and scanning electron microscopy, we document the abundance and relative amounts of goethite, hematite, and magnetite in dust-source samples from the Bodélé Depression. The partition between hematite and goethite is important to know to improve models for the radiative effects of ferric oxide minerals in mineral dust aerosols. The combination of methods shows (1) the dominance of goethite over hematite in the source sediments, (2) the abundance and occurrences of their nanosize components, and (3) the ubiquity of magnetite, albeit in small amounts. Dominant goethite and subordinate hematite together compose about 2% of yellow-reddish dust-source sediments from the Bodélé Depression and contribute strongly to diminution of reflectance in bulk samples. These observations imply that dust plumes from the Bodélé Depression that are derived from goethite-dominated sediments strongly absorb solar radiation. The presence of ubiquitous magnetite (0.002-0.57 wt. %) is also noteworthy for its potentially higher solubility relative to ferric oxide and for its small sizes, including PM<0.1m. For all examined samples, the average iron apportionment is estimated at about 33% in ferric oxide minerals, 1.4 % in magnetite, and 65% in ferric silicates. Structural iron in clay minerals may account for much of the iron in the ferric silicates. We estimate that the mean ferric oxides flux exported from the Bodélé Depression is 0.9 Tg/yr with greater than 50% exported as ferric oxide nanoparticles (<0.1m). The high surface-to-volume ratios of ferric oxide nanoparticles once entrained into dust plumes may facilitate increased atmospheric chemical and physical processing and affect iron solubility and bioavailability to marine and terrestrial ecosystems