Natural Remanent Magnetization Research Articles

Susceptibility and remanent magnetization inversion of magnetic data with a priori information of the Köenigsberger ratio

SUMMARY Magnetic susceptibility and natural remanent magnetization of rocks are useful parameters to study geological structures and geodynamic processes. Traditional widely used algorithms for the inversion of magnetic data can recover the distribution of the apparent susceptibility or total magnetization intensity, but do not provide information on the remanent magnetization. In this paper, we propose a framework to directly invert for the magnetic susceptibility and the natural remanent magnetization vector using surface or airborne magnetic data, assuming that the Köenigsberger ratio of the rock is known or approximately deducible. The susceptibility and remanence are computed using two different approaches: (1) the susceptibility, intensity, and direction of the remanent magnetization are continuously recovered for each discretized cell and (2) the remanence direction is assumed to be uniform in each subzone and is iteratively computed as discrete values. Both processes are implemented using the preconditioned conjugate gradient algorithm. The method is tested on three synthetic models and one field data set from the Zaohuohexi iron-ore deposit, Qinghai Province, northwest China. The results of the continuous inversion show the trend of the remanent magnetization directions, while the discrete inversion yields more specific values. This inversion framework can determine the source bodies’ geometry and position, and also provide superposed and comprehensive information on the natural remanent magnetization, which may be useful to investigate geological bodies bearing stable primary remanent magnetization.

Detailed palaeomagnetic record at Rose Cottage Cave, South Africa: Implications for the Holocene geomagnetic field behaviour and chronostratigraphy

Palaeomagnetic data from a sedimentary section spanning the Holocene and terminal Pleistocene (~13 kya) from Rose Cottage Cave, eastern Free State (South Africa), are reported. The palaeomagnetic analysis took into account rock magnetism and directional analysis. The former reveals that most samples show stable single domain and superparamagnetic particles of Ti-poor magnetite and haematite. Natural remanent magnetisation directions were determined by progressive alternating field demagnetisation methodology. Directional analysis shows normal directions between samples 18 to 39 and 85 to 92; however, during the Early and Late Holocene in samples principally from RC40 to 84 ‘anomalous’ directions occurred. There is a significant westward shift in declination of ~80°, and a conspicuous fluctuating inclination in the lower part of the section during the Early Holocene at ≥9.5 kya and before ~12.0/13.0 kya. This palaeomagnetic record might become a chronostratigraphical marker for latest Pleistocene/Holocene sedimentary deposits in South Africa. Our two new accelerator mass spectrometry radiocarbon dates for the sampled deposit are 9500±50 BP and 1115±30 BP.
 Significance
 
 The study provides new accelerator mass spectrometry dates on the chronological sequence existing in Rose Cottage Cave.
 The findings contribute to the knowledge of the geomagnetic field behaviour since the terminal Pleistocene to the Late Holocene in a period spanning the last 13 000/12 000 years.
 This palaeomagnetic record might become a chronostratigraphical marker for the latest Pleistocene/ Holocene sedimentary deposits in South Africa.

Magnetite magnetofossils record biogeochemical remanent magnetization in hydrogenetic ferromanganese crusts

AbstractRecords of natural remanent magnetization (NRM) and geomagnetic polarity reversals preserved within ferromanganese (Fe-Mn) crusts, together with the application of fine-resolution magnetostratigraphic analysis, have been successfully demonstrated. However, because Fe-bearing precipitates or minerals are thought to be either oxides and/or hydroxides precipitated from ambient oxic seawater or detrital minerals, the magnetic properties of the ferromagnetic minerals and the genetic mechanisms remain controversial; moreover, the origin of the NRM is unclear. Here, we show that nanometer-scale magnetite crystals found in Fe-Mn crusts from the Pacific Ocean and South China Sea are magnetosome fossils based on their narrow size range, chain arrangement, chemical purity, and crystallographic perfection, as indicated by transmission electron microscopy. Furthermore, our new data from rock magnetic and electron paramagnetic resonance analyses, combined with a previously reported micro-magnetostratigraphic sequence, indicate that magnetotactic bacteria and their post-mortem remains contribute to a biogeochemical remanent magnetization of Fe-Mn crusts. In addition, the results provide evidence for a previously unappreciated pathway for the biogeochemical cycling of iron in the deep ocean.

Верхнедевонские отложения (франский ярус) северо-запада Русской платформы: палеомагнитные данные

Paleomagnetic determinations for the Devonian strata of the Main Devonian Field, available in the international paleomagnetic data base, do not make it possible to construct both detailed magnetostratigraphic scales and reliable trajectories of the apparent polar wander path of the paleomagnetic pole. This is primarily due to the insufficient amount of data that determine modern reliability criteria. Obtaining more complete series of reliable paleomagnetic determinations is one of the important tasks of paleomagnetic studies. The paper presents new paleomagnetic determinations from a collection of rocks of the Frasnian stage of the Upper Devonian (Ilmen and Bureg Beds of the Semiluky Formation), sampled on the southern shore of Lake Ilmen, east of the village. Korostyn in 2009. Magnetomineralogical studies were carried out to determine the magnetic minerals of carriers of natural remanent magnetization. Three characteristic components of natural remanent magnetization have been identified. Two components correspond to the Late Paleozoic magnetization reversal (in the Carboniferous and Permian times). The third, bipolar component of the Frasnian age passes the reverse polarity test of the geomagnetic field.. The obtained position of the paleomagnetic pole by the bipolar component of the natural remanent magnetization is consistent with the available data on the section of the Late Upper Devonian of the Main Devonian Field.

Rock-magnetic and archaeomagnetic investigations on archaeological artefacts from Maharashtra, India

Abstract Archaeointensity and rock-magnetic studies were undertaken on 49 baked clay artefacts from four archaeological sites (Ter, Junnar, Nalasopara and Kanheri) in Maharashtra, India. Rock-magnetic properties, including bulk magnetic susceptibility, magnetic remanence and thermomagnetic analysis, indicate the presence of a low-coercivity magnetite in fine (superparamagnetic, single domain) grain-sizes. The ratio of anhysteretic remanent magnetization to saturation isothermal remanent magnetization, the reversible high-temperature susceptibility curves and the 3-axes isothermal remanent magnetization tests also indicate that the artefacts dominantly possess fine-grained magnetic particles, carrying a stable thermoremanent magnetization (TRM). Archaeointensity was estimated using Coe's modified Thellier method corresponding to the linear behaviour of natural remanent magnetization loss and TRM gained plots, which were evaluated with ThellierTool4.0 software. Cooling rate and anisotropy of the TRM corrections were applied and the corrected intensities were used to calculate a mean archaeointensity value for each one of the four sites. The new archaeointensity values were plotted along the existing Indian archaeointensity values derived only from archaeological artefacts, and were compared with the SHA.DIF.14k and ARCH10k.1 global models’ predictions. The present study aims to improve the overall understanding of Indian geomagnetic field variation in the past by providing new high-quality archaeointensity results. However, still more archaeointensity values are required to develop a reliable secular variation curve for India.

Obsidians of Pantelleria (Strait of Sicily): A Petrographic, Geochemical and Magnetic Study of Known and New Geological Sources

Abstract This paper provides new petrochemical and paleomagnetic data from obsidian sub-sources on the island of Pantelleria, exploited since the Neolithic. Data has been obtained from 14 obsidian samples from 4 locations: Fossa della Pernice (2 sites), Salto la Vecchia and Balata dei Turchi. Here, we aim to better characterize these obsidians using a cross-disciplinary and multi-analytical approach, to further understand their archaeological significance. Major element analyses (EMP) have enabled two compositional super-groups to be distinguished: (i) Fossa della Pernice, less peralkaline and (ii) Balata dei Turchi–Salto la Vecchia, distinctly more peralkaline and having almost identical chemical patterns. Trace element analyses (LA-ICP-MS) corroborate major element groupings, with the Balata dei Turchi–Salto la Vecchia super-group being further characterized by a pronounced negative europium anomaly. Glass H2O contents (FT-IR) reveal an overlap among all the sub-sources (H2O = 0.1–0.3 wt. %). Magnetic methods have refined the petrochemical groupings, permitting further distinction between Balata dei Turchi–Salto La Vecchia and the Fossa della Pernice super-groups. The occurrence of sub-microscopic (< 1 μm) ferromagnetic minerals results in different magnetic susceptibility and Natural Remanent Magnetization values and allows the best distinction among the products from the chosen sites. When compared with obsidian tools excavated from Bronze-age settlements on the island of Ustica (230 km NE of Pantelleria), 12% are distinctly peralkaline, indicating their provenance to be from the Balata dei Turchi sub-source.

A test of the pseudo-Thellier technique for determining relative palaeointensity in the Tirna Basin, Osmanabad, Maharashtra, India

Abstract A relative palaeointensity determination was obtained using the pseudo-Thellier technique on sediment from Ther, Tirna Basin, Latu-Osmanabad District, Maharashtra, India. The stability of the natural remanent magnetization was investigated by alternating field (AF) demagnetization. Rock magnetic studies suggest that the main carriers of magnetization are ferrimagnetic minerals, predominantly pseudo-single-domain magnetite. To varying degrees, the smoothed palaeoinclination and palaeodeclination patterns of the Tirna Basin are similar to other Asian palaeosecular variation records CALS3k.4, CALS10k.1 and SED3k.1, with an age offset. Measurements of intensity of the natural remanent magnetization left after AF demagnetization v. intensity of anhysteric remanent magnetization gained at the same peak were carried out on a set of samples. A jackknife re-sampling scheme provides error estimates for the palaeointensity. A good agreement pattern can be observed between the Tirna Basin relative palaeointensity proxy and other global curves with an age shift. Although some temporal offsets of palaeointensity features between different records have been recognized, their similar shape suggests that the palaeointensity can give a globally coherent signal and may be used as a relative dating technique. For the first time, relative palaeointensity data for the past 2 kyr from India is presented here, which complement the existing archaeological records but with an additional input.

Fabrics, facies, and flow through a large-volume ignimbrite: Pampa De Oxaya, Chile

The Miocene Oxaya Formation, exposed along the western Andean slope in northern Chile, represents one of the largest ignimbrite provinces on earth. In this study, magnetic fabric data were acquired from a ~ 1-km-long core drilled vertically through a single cooling unit of the Oxaya Formation the ca.22 Ma Cardones ignimbrite. Samples for magnetic analysis were obtained every 20 m from the fine-grained matrix of the core. Detailed measurements of the variation in bulk magnetic properties, including natural remanent magnetization (NRM), susceptibility, and anisotropy of magnetic susceptibility (AMS), were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow. AMS ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large-volume ignimbrite. After utilizing the magnetic remanence to correct for rotation about the core axis, well-grouped and imbricated petrofabric orientations reveal a well-defined SW (247°) transport direction down a proto-Western Andean slope indicating syn- or post-welding flow and confirming the deformed Lauca caldera as the likely source of the eruption. Systematic variations in fabric shape (T) and intensity (P) with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center. These vertical changes in fabric reveal that this massive, apparently homogeneous, deposit has a systematic layering. This layering reflects depth-dependent temperature variations, temporal changes in the flow boundary zone during deposition and post-depositional processes.

Chronostratigraphic framework of the East China Sea since MIS 6 from geomagnetic paleointensity and environmental magnetic records

Abstract Continental shelf is a globally important reservoir for terrigenous sediments, and provides sediment transportation pathways that extend from the shoreline to abyssal basins. Due to the strong land-sea interaction in response to frequent sea level change during the late Quaternary and methods' constraint, it is a big challenge to establish a reliable chronostratigraphy on the continental margin with terrigenous sediments dominance. Relative geomagnetic paleointensity (RPI) is quite useful for high-resolution chronostratigraphy in Brunhes epoch. In this study, relative paleointensity record for a sediment core (SFK-1, 82.9 m) from the outer East China Sea (ECS) shelf was studied. Rock magnetic results show that coarse vortex state magnetite is the dominant magnetic carrier throughout the core and that these sediments generally meet the accepted criteria for constructing relative paleointensity (RPI) record. RPI was established by normalizing the natural remanent magnetization (NRM) with isothermal remanent magnetization (IRM), both after 30 mT alternating demagnetization. Further, our RPI record was correlated with PISO-1500 and 7 age tie points were obtained. An age model is established for SFK-1 core with a basal age of 165 ka by combining the radiocarbon dates, relative geomagnetic paleointensity, and tandem correlation of environmental magnetic parameters to foraminiferal stable oxygen isotopes, which provides a reliable chronological framework for the later paleoenvironmental studies of ECS. The consistency of RPI record of SFK-1 and other worldwide observations suggest that global-scale pattern of geomagnetic paleointensity behavior is also documented in the sediments of the East China Sea, and the RPI of SFK-1 can be used as a reference curve for the RPI studies of other cores of ECS area, and even the whole west Pacific margin seas.

Nanogoethite as a Potential Indicator of Remagnetization in Red Beds

AbstractRed beds are well‐known for recording stable natural remanent magnetization (NRM). However, discriminating primary NRM from secondary remanence in red beds is difficult. The Paleogene Nangqian red beds in eastern Tibetan Plateau variably record an overprint related to nearby magmatism and thus provide a great opportunity to characterize remagnetization in red beds. Through comprehensive rock magnetic, Mössbauer spectroscopic, and petrographic analyses, we find that remagnetization was controlled by temperature. Remagnetized red beds contain abundant authigenic hematite and goethite, with some larger grains unblocking at the Néel temperatures and the remainder (nanoparticles) unblocking at lower temperatures. In contrast, red beds retaining primary NRM are characterized by dominance of detrital hematite and magnetite, presence of fine‐grained authigenic hematite, and absence of authigenic goethite and magnetite. High temperature behaviors of NRM/susceptibility are indicative of remagnetization, but the presence of goethite appears to be a more sensitive criterion for diagnosing remagnetization in red beds.

Last 3000 years of geomagnetic field intensity from India: New reference palaeointensity data from two east coast archaeological sites and archaeomagnetic dating insights

A comprehensive rock magnetic and absolute archaeointensity (AI) investigation was carried out on baked bricks from the east coast of India with an aim to obtain a set of well-defined palaeointensity (PI) data. The new PI data from India comprise a significant first step to improve the regional intensity reference curve, which can be used as reliable dating tool mainly for archaeomagnetic material. Standard Thellier-Coe experiments were applied on 12 independent brick samples collected from a Buddhist monument at Adurru in the Godavari delta and a Medieval Fort at Kolleti Kota site situated on an island in the Kolleru Lake. The samples thus collected represent two different archaeological periods, one around the 3rd century BCE and the other between the 4th and 7th centuries CE respectively. The data obtained from thermal demagnetization of Saturation Isothermal Remanent Magnetization (SIRM), temperature dependent magnetic susceptibility (χ–T), Isothermal Remanent Magnetization (IRM) acquisition curves, and hysteresis measurements showed that the primary carriers of Natural Remanent Magnetization (NRM) are fine-grained Pseudo-Single Domain (PSD) (Stable Single Domain (SSD) + Superparamagnetic (SP)) titanomagnetite with various titanium contents. This implies that the samples had significant stable Thermoremanent Magnetization (TRM), and, therefore, good candidates for PI analysis. Strict selection and quality-control criteria for detecting magnetomineralogical alteration and intensity data corrected for anisotropy and the cooling rate effects on TRM acquisition, offer high quality PI determinations with respect to the previous available data that missed such corrections.A total 33 specimen level PI determinations that correspond to linear NRM–TRM plots, were made using the Thellier GUI software. The corrected data yielded mean PI values of 44.6 ± 1.4 μT for the Buddhist Monument, and 36.6 ± 2.9 μT for the Fort site. By comparison and evaluation of new PI data with Global Geomagnetic Field Model predictions, the sampled bricks were archaeomagnetically dated using the Matlab tool developed by Pavón-Carrasco et al. (2011). Both ARCH3K.1 and SHA.DIF.14 K global models were used to evaluate the age estimates. The SHA.DIF.14 K model provided the most accepted age estimates that varied between 305 and 182 BCE for the early historic Buddhist Structure at Adurru, and 516–889 CE for the Medieval Fort at Kolleti Kota. These new reference archaeointensity data highlight the potential of archaeomagnetic dating to improve the archaeologically determined time periods of bricks, and also show the need for new high quality reference data to improve the geomagnetic field models from India.

Challenges in isolating primary remanent magnetization from Tethyan carbonate rocks on the Tibetan Plateau: Insight from remagnetized Upper Triassic limestones in the eastern Qiangtang block

Carbonate rocks, widely used for paleomagnetically quantifying the drift history of the Gondwana-derived continental blocks of the Tibetan Plateau and evolution of the Paleo/Meso/Neo-Tethys Oceans, are prone to pervasive remagnetization. Identifying remagnetization is difficult because it is commonly undetectable through the classic paleomagnetic field tests. Here we apply comprehensive paleomagnetic, rock magnetic, and petrographic studies to upper Triassic limestones in the eastern Qiangtang block. Our results reveal that detrital/biogenic magnetite, which may carry the primary natural remanent magnetization (NRM), is rarely preserved in these rocks. In contrast, authigenic magnetite and hematite pseudomorphs after pyrite, and monoclinic pyrrhotite record three episodes of remagnetization. The earliest remagnetization was induced by oxidation of early diagenetic pyrite to magnetite, probably related to the collision between the northeastern Tibetan Plateau and the Qiangtang block after closure of the Paleo-Tethys Ocean in the Late Triassic. The second remagnetization, residing in hematite and minor goethite, which is the further subsurface oxidation product of pyrite/magnetite, is possibly related to the development of the localized Cenozoic basins soon after India-Asia collision in the Paleocene. The youngest remagnetization is a combination of thermoviscous and chemical remanent magnetization carried by authigenic magnetite and pyrrhotite, respectively. Our analyses suggest that a high supply of organic carbon during carbonate deposition, prevailing sulfate reducing conditions during early diagenesis, and widespread orogenic fluid migration related to crustal shortening during later diagenesis, have altered the primary remanence of the shallow-water Tethyan carbonate rocks of the Tibetan Plateau. We emphasize that all paleomagnetic results from these rocks must be carefully examined for remagnetization before being used for paleogeographic reconstructions. Future paleomagnetic investigations of the carbonate rocks in orogenic belts should be accompanied by thorough rock magnetic and petrographic studies to determine the origin of the NRM.

Magnetic properties indicate the sources of hadal sediments in the Yap Trench, northwest Pacific Ocean

Magnetic minerals in marine sediments are often masked by the primary natural remanent magnetization and material source signals. In order to understand sedimentary environment and sources of sediments in the abyss, we studied 126 samples of five bottom surface cores collected by the Jiaolong Submersible at 4000–7000 m in depth during the third stage of the China’s 38th Ocean Voyage. The magnetic properties of the sediments were analyzed using Thermosusceptibility (k-T) curves and Day plot. The results show that the magnetic minerals in the sediments of the Yap Trench are mainly maghemite, and the overall magnetic and soft magnetic properties were strong. The magnetic particles of sediments are dominated by pseudo single domains (PSD) grains. The main source of sediment is locally-derived basalt debris and volcanic debris, and the process of sedimentation is gravity-like flow deposition.

How natural remanent magnetization of basaltic units can dominate the reduced to pole magnetic value: A case study from the Faroe Islands

ABSTRACTEarly attempts to utilize magnetic data to understand the volcanic and subvolcanic succession on the Faroese Continental Shelf have shown that conventional interpretation and modelling of magnetic data from this area leads to ambiguous results. Interpretation of the aeromagnetic data on the Faroese Continental Shelf shows that some previously identified basement highs coincide with reduced‐to‐pole magnetic highs, whereas others coincide with negative or mixed magnetic features. Similarly, igneous centres are characterized by different polarity magnetic anomalies. Palaeomagnetic analysis of the onshore volcanic succession has demonstrated that the thermoremanent magnetization of the basaltic lavas is stronger than the induced magnetism, and both reversely and normally magnetized units are present. We have tested this with 2½D profile modelling using the palaeomagnetic information to correlate high‐amplitude magnetic anomalies with basalt successions containing changes in magnetic polarity. This approach has enabled us to map the termination of the differently magnetized units offshore and thereby extend the mapping of the Faroe Island Basalt Group on the Faroese Platform and into adjacent areas.

Paleomagnetic Secular Variation and Relative Paleointensity During the Holocene in South China—Huguangyan Maar Lake Revisited

AbstractThe scarcity of reliable paleomagnetic secular variation (PSV) records from East Asia especially from low‐latitude regions impedes better understanding of global PSV mechanisms. Here we report on a radiocarbon‐dated Holocene PSV record from a composite ~6.7‐m‐long core collected from the high‐sedimentation‐rate Huguangyan Maar Lake (HML) in subtropical‐tropical South China. Rock magnetic results demonstrate that the natural remanent magnetization resides in single‐domain magnetite. Alternating field demagnetization experiments at 1‐cm spacing on u‐channel samples reveal six distinct inclination highs at ~7,500 BCE, ~5,100 BCE, ~4,600 BCE, ~3,600–3,400 BCE, ~1,600–1,200 BCE, and 600–800 CE; three inclination lows at ~4,800 BCE, ~600–300 BCE, and ~1,000–1,300 CE; and three eastward declination trends at ~3,600–3,200 BCE, ~2,600–2,400 BCE, and 400 BCE to 200 CE. The similarity between the HML PSV record and other independently dated records from East Asia and geomagnetic field models corroborates the robustness of our age model and Holocene PSV record. Strikingly, centennial‐ to millennial‐scale PSV features of the HML are comparable, within age uncertainties, with other Holocene records from Europe, North America, and Canada, suggesting that such directional patterns are likely to be hemispheric in scale. Although relative paleointensity data of HML are affected by environmental factors (e.g., organic matter diagenesis), the record still provides a regionally important new PSV reference curve whose conspicuous features may serve as stratigraphic markers for East Asian paleorecords.

A survey of the natural remanent magnetization and magnetic susceptibility of Apollo whole rocks

Magnetic properties of lunar rocks may help understand the evolution of the past magnetic field of the Moon. However, these properties are often measured on small subsamples, which may not be representative of the bulk lunar rocks. In this study, a rough magnetic characterization of a large fraction of the Apollo collection was performed using non-destructive methods, with a focus on large samples (mean mass 500 g). The natural remanent magnetization of 123 Apollo samples and the magnetic susceptibility of 154 Apollo samples were measured in the Apollo storage vault at the Lunar Sample Laboratory Facility in Houston. Magnetic susceptibility is strongly controlled by lithology and is clearly enhanced by impact processing and associated incorporation of meteoritic metal, as well as formation of nanometric iron particles by space weathering. The natural remanent magnetization is lower by a factor of ~3 than that measured on small samples (~100 mg), suggesting magnetic contamination during small-sample preparation or handling, or directional heterogeneity in remanence within large samples. The ratio of natural remanence magnetization to magnetic susceptibility may be used as selection tool for further paleomagnetic study. It is a crude paleointensity proxy and its overall temporal evolution is coherent with the existence of a high magnetic field from ~4 Ga to 3.5 Ga and a subsequent weaker field epoch as previously defined in the literature. Our data also raise the possibility that the intensity of the paleomagnetic field during the high field epoch may have been more variable and lower on average than previously suggested.

Rock-magnetic and paleomagnetic study on a 27-m-long core from Lake Chapala, western Mexico: Paleoenvironmental implications for the last 10 ka

We report detailed rock-magnetic and paleomagnetic results from a 27-m-long sedimentary core collected at depocenter of Lake Chapala, western Trans-Mexican Volcanic Belt. Rock-magnetic parameters point to PSD grain-size titanomagnetites as the dominant magnetic mineralogy. Observed variations, in both concentration and grain-size dependent magnetic parameters, reflect changes in the sediment supply through the studied depth, but mainly during the last half century. Stable characteristic remanent magnetization, representative of secular variation of the geomagnetic field, was retrieved after alternating field demagnetization of the natural remanent magnetization (NRM). The comparison of the obtained magnetic inclination and declination logs against the corresponding curves predicted by the Global Geomagnetic Field Model SHA.DIF.14k enables to establish an alternative sedimentation-rate independent age-depth model for the whole core, in reasonably good agreement with the one calculated using conventional radiocarbon dating. This approach works not only for Lake Chapala, but also for Lake Chalco. The correlation established between rock-magnetic parameters and paleoenvironments, based on diatom identification of representative samples within specific zones, can be considered as a novel proxy methodology. This approach opens a new perspective for new, more detailed rock-magnetic, paleomagnetic and paleoenvironmental investigations for better paleoclimatic reconstructions.

МАГНИТОСТРАТИГРАФИЧЕСКИЕ ИССЛЕДОВАНИЯ ОТЛОЖЕНИЙ ВЕРХНЕГО САРМАТА-НИЖНЕГО МЭОТИСА РАЗРЕЗА МЫС ПАНАГИЯ (ТАМАНСКИЙ П-ОВ)

In order to obtain the missing data on the magnetostratigraphy of transitional layers between Sarmatian and the Maeotian parts in the Eastern Paratethys, the authors studied the Upper Sarmatian ― the Lower Maeotian deposits in the Panagia Cape section (the Taman Peninsula, Russia) using petromagnetic and magnetostratigraphic methods. The Panagia Cape section is located on the Black Sea coast of Taman Peninsula. The total thickness of the studied sediments is about 29 m. The section is composed mainly of clays characterized by weak magnetization. Natural remanent magnetization is caused mainly by monoclinic pyrrhotite. Magnetic anisotropy was studied in order to substantiate reliability of the results. Comparison with the ATNTS 2012 magnetochronological scale shows that the studied transitional layers between Sarmatian and Maeotian parts probably correspond to the 4n, 3Bn and 3Br Сhrones. The obtained results agree with biostratigraphic and radiometric data.

Correlated Relative and Absolute Geomagnetic Paleointensities From a Pliocene N‐R Polarity Reversal Record in Basaltic Lava Flows on Kauai, Hawaii

AbstractNew paleomagnetic results from basaltic lava flows erupted 4 Myr ago on Kauai (Hawaii) reveal how the geomagnetic intensity changed as the field switched from normal to reverse (N‐R) polarity. Following stepwise alternating field demagnetization of each sample's natural remanent magnetization, an anhysteretic remanent magnetization (ARM) was imparted in a 50 μT bias field and then alternating field demagnetized. Very similar behavior of the 35 to 90 mT coercivity fractions suggest that the natural remanent magnetization (interpreted to be a primary thermoremanent magnetization, TRM) and ARM reside in the same subpopulation of magnetic grains. With the ARM intensity serving as an analog for TRM susceptibility, the ratio TRM/ARM provides a measure of relative paleointensity. Flow‐averaged TRM/ARM ratios correlate with a small set of double‐heating paleointensities determined on companion samples, allowing a scaling of the ratios to absolute ancient field intensities. During the N‐R transition, the field strength dropped by 91% compared to the maximum recorded by the flows. This decrease is comparable to that observed in other paleomagnetic studies and numerical simulations. During an interval of NE down directional stasis recorded by 10 flows in the lower half of the section, the geomagnetic intensity rose to over 20 μT and then decreased to near 5 μT. This distinctive behavior could mark the waxing and waning of a stationary, inward directed flux patch on the core‐mantle boundary centered NE of Kauai.

Near-seafloor magnetic mapping of off-axis lava flows near the Kairei and Yokoniwa hydrothermal vent fields in the Central Indian Ridge

The Kairei (KHF) and Yokoniwa hydrothermal fields (YHF) are hosted in mafic as well as ultramafic rocks distributed at an off-axis volcanic knoll of the Central Indian Ridge. Despite intensive investigations, their geological and geophysical background is still debated. Here, we show the results of near-seafloor magnetic anomaly surveys conducted using a submersible. We investigated the bulk magnetization of the hydrothermally altered zone and the surrounding lava flows and evaluated their intensities compared with previously reported values at axial areas of seafloor spreading environments. The KHF is characterized by low coherence between observed and modeled anomalies and low values of magnetization. This result suggests that magnetic minerals within basaltic lava flows were likely altered by hydrothermal fluid circulation. The variation pattern in the observed magnetic anomalies above the lava flows is in phase with that of the modeled magnetic anomalies for the simple assumption that the magnetization direction is parallel to the geomagnetic field. This result suggests that these lava flows preserve normal magnetic polarity corresponding to the Brunhes Chron. The estimated magnetic-anomaly-derived absolute magnetizations show a reasonable correlation with the natural remanent magnetizations of rock samples collected from the seafloor of the same region; their relationship is consistent with previously reported datasets from the Mariana Trough and Mid-Atlantic Ridge. The estimated magnetization intensity reaches 20 A/m in the study area, which is clearly greater than those of previously reported off-axis areas, suggesting that recent volcanic eruption may have occurred in these off-axis areas. The high magnetization distributions are commonly observed at the bottoms of the western slope from the KHF and YHF. This finding provides new insight into the distribution of highly magnetized lava flows in the off-axis areas and indicates the distribution of recent off-axis volcanic activities, which is potentially linked to the sub-seafloor hydrothermal circulation.