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Magnetic signals in basalts from the Sylhet traps (ST), Shillong Plateau, India is investigated using rock magnetism as a proxy to explain the potentiality of the ST basalts as an indicator of magnetic mineralogy. A paleomagnetic study was carried out in order to unravel the Paleopole. In detail, rock magnetic and Anisotropy of Magnetic susceptibility (AMS) results have been reported for the first time on the ST basalts. A total of 38 oriented block samples were collected from 8 sites along Cherrapunji-Shella bazaar road section, Meghalaya, India. Samples were subjected to rock magnetic measurements such as magnetic susceptibility (K), Isothermal Remanent Magnetization (IRM), hysteresis loops, backfield (coercivity remanence) and k-T (Susceptibility vs. Temperature) analysis to identify the magnetic properties of basalts. Advanced Variable Field Translation Balance (AVFTB) was used for some rock magnetic studies. A few samples were also subjected to Anisotropy of Magnetic susceptibility (AMS) study to understand the petrofabric orientations and shape anisotropy in the ST basalts. The Remanence ratio (Mrs/Ms) and the Coercivity ratio (Bcr/Bc) ranging from 0.105 to 0.333 and from 1.25 to 1.87 respectively indicate sizes of magnetic grains are mainly in the range of Pseudo Single Domain (PSD) limits with minority of them in the ranges of Single Domain (SD) limits. The results of k-T study in combination with other rock magnetic results shows that magnetite (Fe 3 O 4) [Curie Temperature = 565-590 °C] as the dominant magnetic carrier with a subordinate content of titanomagnetite Fe2+(Fe3+Ti) 2 O 4 [Curie Temperature = 380–450 °C]. From the AMS study, parameters like lineation, foliation are calculated, and results define all the three shape anisotropies present in the ST basalts. The ST basalts display paleomagnetic directions with mean declination = 255°, mean inclination = -57° (α 95 = 3.44°; k = 16.74). The pole position of the ST at 117 Ma is -26.79 °N, 330.75 °E (dp = 3.6; dm = 5) with a paleolatitude 37.6 °S. This infers that the ST is a later volcanic event than Rajmahal traps (RT) which is considered contemporary to the ST. [ABSTRACT FROM AUTHOR]

The mid- to late-Holocene monsoon decline led to aridification of the Indian Peninsula impacting the early agricultural practices in the region. Our analysis of organic carbon, mineral magnetic properties and AMS 14C dating of a 54.2-m-long sediment core (CY) from the Godavari Delta, India, showed changes in the organic carbon source and sediment provenance, which are linked to the changes in vegetation and soil/rock erosion caused by widespread aridification and associated human adaptation in central India. Our results show a decline in the concentration of ferrimagnetic minerals, indicating reduced input from the basalts of the Deccan Plateau after ~6.0 cal. ka BP in response to the weaker Indian monsoon. δ13C values show a distinct increase from ~4.9 cal. ka BP, indicating an increase in C4 plant sources under the continued weak monsoon phase, whereas a higher ferrimagnetic mineral concentration in the sediment suggested an increased Deccan basalt source. Abrupt increase in δ13C values and decrease in TOC content accompanied with a significant increase in ferrimagnetic mineral concentration from ~3.2 to 3.1 cal. ka BP reflected a shift of organic carbon and sediment source and a severe decline in vegetation coverage. Such phenomena indicate intensified deforestation and soil/rock erosion in the Deccan Plateau producing higher ferrimagnetic mineral inputs, which is in agreement with significant expansion of agricultural activities in the Deccan Chalcolithic cultural period. In addition, C3 plants recovered and magnetic concentration declined during the wet events (4.6 and 4.0 cal. ka BP) of Neolithic time, while both C3 plants and magnetic parameters increased during the wet events (3.1-2.8 and 2.1 cal. ka BP) of the Chalcolithic cultural period. This implies increased agricultural activity and the onset of human modification of the ecosystem. [ABSTRACT FROM AUTHOR]

This study provides evidence of the monsoon record over the past 25 ka from the Kilang Sarai palaeolake which is located in NW Himalaya in the rain shadow zone of the Indian Summer Monsoon (ISM). The multi-proxy study by using geomorphology, carbon isotopes, mineral magnetism, clay mineralogy and elemental chemistry of a 8 m thick laminated lacustrine sediments indicates that the area experienced fluctuating precipitation conditions during the last 25 ka. The time period between 12 and 5 ka BP can be regarded as a very wet interval of the Last Glacial to mid Holocene due to a combined effect of ISM and Westerlies, followed by aridity after 5 ka BP. Tectonic triggers may have caused the development of terraces and closure of lake. At the millennial time scale, a correlation of precipitation and vegetational changes between our data and other records from similar geographical settings suggests that Kilang Sarai basin responded to periods of strengthening in precipitation during the Last Glacial to early Holocene. [ABSTRACT FROM AUTHOR]

Detailed rock magnetic measurements such as susceptibility (χ), susceptibility of anhysteric remanent magnetization (ΧARM), saturation isothermal remanent magnetization (SIRM), soft-isothermal remanent magnetization (soft-IRM) and hard-IRM (HIRM) were carried out on the topmost sediment samples of 73 gravity cores collected in the Krishna-Godavari (KG) offshore basin to understand the distribution of magnetic parameters and the associated diagenetic signatures related to upward methane flux. The variation in magnetic parameters in the KG offshore basin fluctuates around the average value (Χ = 105 × 10−8 m³ kg−1, ΧARM = 1.22 × 10−5 m³ kg−1, SIRM = 2040 × 10−5 Am² kg−1, soft-IRM = 1807 and HIRM = 82) except at the location GC-07 (15.767°N, 81.814°E) where the magnetic parameters Χ, ΧARM and SIRM drop to ~1/6th, ~1/14th and ~1/8th of the average near surface value indicating substantial loss in the concentration of primary magnetic iron oxide minerals. Anomalous low value of ΧARM/Χ and ΧARM/SIRM at GC-07 suggests that the magnetic remanence property is governed by coarser pseudo-single domain (PSD) or multi-domain (MD) grains. To understand the nature of the anomalous magnetic parameters at GC-07, complete downcore variation of magnetic parameters was also studied. The magnetic parameters were found to be even more depleted at all depths with susceptibility dropping (at 5.2 mbsf) to ~1/200th of its average near surface value. Interestingly, this core shows a rapid increase in the concentration of dissolved methane from ~0 nM at surface to ∼153 nM at around 5.2 mbsf where the sulfate concentration is also reduced to zero. The anaerobic oxidation of methane (AOM) and the sulfate methane transition zone (SMTZ) occur between ~4.2 and 5.2 mbsf based on the present day sulfate concentration profile. The close association between the dissolution of magnetic iron oxide minerals and AOM at GC-07 suggests that the reduced magnetization is primarily due to the release of HS− by AOM. The magnetic iron oxide minerals are dissolved not only at SMTZ but also above the current depth of the SMTZ. It is likely that the methane flux was higher in the past leading to upward shift of the SMTZ, and the HS− released by AOM facilitated the dissolution of magnetic iron oxide minerals at shallower depths. Alternatively, the HS− diffusing upward from the current depth of SMTZ might have lead to dissolution at shallower depth. Therefore, additional geological/geochemical investigations are warranted at GC-07 to establish the dissolution mechanism of magnetic iron oxide minerals. The variability in different magnetic parameters within the slope basin and channels shows systematic decrease in the concentration of magnetic minerals and increase in the abundance of stable single domain (SSD) magnetic grains with water depth. The observed strong correlation between the silt fraction and the magnetic susceptibility for the slope basin and channels suggests that the variation in magnetic parameters is predominantly controlled by the sediment sorting/transport process as finer particles are deposited in deeper water. [ABSTRACT FROM AUTHOR]

Abstract: We have used rock magnetic techniques in this study to assess atmospheric pollution at five stations in and around Mangalore city on the southwestern coast of India. Samples of dust were collected from two suburban areas (Thokkottu and Pumpwell located respectively ~10km and 3km from the city center), the city center itself (Milagres) and industrial/port areas (Panambur and Mangalore Refinery and Petrochemicals Limited (MRPL)). Low-frequency magnetic susceptibility (χlf), frequency-dependent susceptibility (χfd), susceptibility of anhysteretic remanent magnetization (χARM) and isothermal remanent magnetization (IRM 20 to 1000mT) were determined on 23 dust samples and inter-parametric ratios calculated. Results show that samples from suburban areas (particularly Thokkottu) are characterized by low χlf (<314.1×10−8 m3 kg−1) and up to 6% χfd, suggesting low levels of pollution and the presence of pedogenic magnetite possibly derived from soils by wind erosion. However, the average χlf of Milagres, Panambur and MRPL dust samples is high by factors of 9.2, 3.3 and 2.6 compared to that of the Thokkottu sample. The Milagres sample contains magnetically “soft” minerals like magnetite, possibly indicating its derivation from motor vehicle exhaust. In contrast, the Panambur dust sample is characterized by magnetically “hard” minerals such as hematite and goethite as it has an 8-fold higher HIRM value compared to the Thokkottu sample. This magnetic signature is perhaps the result of dust particles derived from the grinding of hematite-rich iron ore by the Kudremukh Iron Ore Company Limited (KIOCL) at Panambur and its storage and export through the nearby New Mangalore Port. However, the dust sample from MRPL has magnetically “soft” minerals like magnetite. This magnetic mineral may have originated from petroleum refining processes at MRPL. Particulate pollution from industrial activities and motor vehicle exhaust is a threat to human health and is known to cause cardiovascular and respiratory ailments. Therefore, the pollution levels brought out by this study warrant a comprehensive epidemiological study in the area of study. [Copyright &y& Elsevier]

Palaeomagnetic studies were carried out on the basal Khairmalia volcanic rocks of the Proterozoic Vindhyan Supergroup. Two hundred and ten specimens prepared from 30 oriented block samples from 10 sites near Khairmalia, south of Chittorgarh, Rajasthan were studied. They are characterized by mean characteristic remanent magnetization D = 8° and I = 9° (α95 = 16.3; k = 23.11). Virtual geomagnetic pole is estimated at 68.6°N; 231.3°E (dp = 8.3 and dm = 16.4). Plot on the Proterozoic apparent polar wander path¹ indicates that the volcanics may have erupted ca. 1800 Ma. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, field, microstructural and anisotropy of magnetic susceptibility (AMS) data are presented from the ∼2.48Ga Malanjkhand Granite and adjacent gneiss with an aim to evaluate the time relationship between fabric development in the granite and regional tectonics. The Central Indian Suture (CIS) that demarcates the southern margin of the NE–SW to ENE–WSW trending Central Indian Tectonic Zone (CITZ), and is a consequence of the accretion between the northern and southern Indian shields, lies to the north of the granite. The northern margin of the granite that has proximity to the CIS, is mylonitized, and shows a dextral (top-to-the-NE) sense of shear. Field and magnetic foliation in the gneiss are parallel to each other as well as to the field foliation in the western and northwestern margin of the granite. Also, the field fabrics in the granite and gneiss curve clockwise into the CIS. The magnetic foliation trajectories curve from a NS orientation, in areas remote from the CIS, to a NE–SW orientation in parts that are proximal to the CIS in the north. The granite shows synmagmatic deformation fabrics such as preferentially oriented zoned feldspar phenocrysts as well as evidence of high-T solid-state deformation fabric such as chessboard pattern in quartz. Magnetic anisotropy increases towards the CIS, which is inferred to support synmagmatic deformation. It is concluded that the tectonic evolution of the CITZ and emplacement/fabric development in the Malanjkhand Granite were synchronous phenomena. WNW–ESE directed convergence led to transpression, which is inferred to be responsible for the emplacement of the granite. It is concluded that in the Malanjkhand Granite, the development of syndeformational emplacement fabrics to syntectonic cooling was a continuum, and the suturing of the northern and southern Indian shields along the CIS took place during the Palaeoproterozoic period. Thus, the study provides an insight into the amalgamation and stabilization of the Indian shield during the late Archaean/Palaeoproterozoic time. [Copyright &y& Elsevier]

Abstract: Rock magnetic concentration, grain size and mineralogy parameters together with organic carbon, calcium carbonate, redox-sensitive elements, δ 18O of Globigerinoides ruber and radiocarbon dating were carried out on a 445 cm long sediment core collected at 1380 m depth off Mangalore, southwestern margin of India. The top 290 cm sediments of the core correspond to the last 18 kaBP. The δ 18O and magnetic records exhibit major events at ∼16 kaBP, 14.5 kaBP, 11.5 kaBP and 9.8/8.6 kaBP related to start and intensity of the summer monsoon and climate change, and are synchronous with that of the western Arabian Sea and North Atlantic. The sediments with high magnetic susceptibility correlate with high sedimentation rates. The sediments are dominated by fine-grained magnetite, but intervals of 1.2–3.8 kaBP and 10–13.5 kaBP were subjected to diagenetic changes, resulting in the dissolution of fine-grained magnetites and enrichment of redox-sensitive trace elements (Cu, Ni, Zn, V, Mo and U). The sediments between 290 cm and 445 cm correspond to 18–27 kaBP and are characterized by distinct decrease in magnetic concentration, grain size and mineralogy parameters, high organic carbon, low concentrations of redox-sensitive trace elements and abundant pyritized tubules. The reductive diagenetic conditions indicated by rock magnetic properties are in contrast with the weak sub-oxic conditions revealed by low concentrations of trace elements in the sediments. The seasonal organic matter flux produced during the winter monsoon and moderate sedimentation rates favoured reductive diagenesis in the sediments at and below the last glacial maximum (LGM). Intermittent bioturbation, however, allowed oxidants to penetrate into the sediments, remobilized redox-sensitive trace elements into the water column and modified the primary geochemical signal of the sedimentary environment. [Copyright &y& Elsevier]

Abstract: Lonar crater, India, is the best preserved terrestrial impact crater formed in basalt and is a unique terrestrial analogue for small, simple craters on terrestrial planets and the Moon. We investigated the paleomagnetic and rock-magnetic properties of the 1.88 km diameter crater in order to understand the effect of impacts on magnetization in target rocks. The magnetization in the Lonar basalts consists of an original 65 Ma Deccan magnetization and a recent overprint. We constrained the timing of magnetization acquisition at Lonar using a combination of conglomerate tests on ejecta deposit clasts and fold tests on the overturned and jumbled rim fold. In some areas, the recumbent rim fold is preserved and can be approximated as a horizontal cylindrical fold. In other areas, substantial vertical axis rotation may have occurred where tear zones developed during folding. We observed only subtle effects from the impact on the rock-magnetic properties of Lonar materials, which include a slightly elevated coercivity in shocked ejecta blocks. We show that paleomagnetism can provide a constraint on shock heating in the absence of petrographic evidence of shock (in this case, <187±15 °C). At Lonar, viscous (and/or chemical) remanent magnetization acquired in the <50 kyr subsequent to crater formation has obscured any evidence of shock remanent magnetization. We also find no evidence of shock demagnetization or the presence of intense impact-induced or impact-amplified transient magnetic fields that have been proposed around larger impact structures. [Copyright &y& Elsevier]