A 20‐ka climate record from Central Himalayan loess deposits

The southwest monsoon that dominated Central Himalaya has preserved loessic silt deposits preserved in patches that are proximal to periglacial areas. The occurrence of such silts suggests contemporary prevalence of cold and dry northwesterly winds. Field stratigraphy, geochemistry, mineral magnetism, infrared stimulated luminescence (IRSL) and radiocarbon dating has enabled reconstruction of an event chronology during the past 20 ka. Three events of loess accretion could be identified. The first two events of loess deposition occurred betweem 20 and 9 ka and were separated by a phase of moderate weathering. Pedogenesis at the end of this event gave rise to a well‐developed soil that was bracketed around 9 to > 4 ka. This was followed by the third phase of loess accretion that occurred around 4 to > 1 ka. Episodes of loess deposition and soil formation are interpreted in terms of changes in the strength of the Indian southwest monsoon. Copyright © 2005 John Wiley & Sons, Ltd.     

湖泊沉积物中胶黄铁矿的鉴出及其磁学意义

对青藏高原东部若尔盖盆地钻孔获取的湖泊沉积物进行了详细的岩石磁学调查及矿物学鉴定, 发现大量胶黄铁矿存在, 并认定其为主要的磁载体. 该矿物颗粒细小, 粒度均匀, 可能为生物化学成因. 与大量文献报道的相反, 在长期暴露于空气后, 样品中的胶黄铁矿仍未被氧化. 这可能与钻孔内该矿物被硅质胶结物包裹有关. 研究结果初步揭示了湖泊沉积物磁学研究的复杂性, 同时也为环境磁学机理的研究提供了新的线索.     

Where did rotational shortening occur in the Himalayas? - Inferences from palaeomagnetic remagnetisations

In metacarbonates of the Lesser (LH) and Tethyan (TH) Himalayas of Kumaon/Garhwal (N-India) characteristic remanent magnetisations carried by pyrrhotite (unblocking temperatures: 250-330°C) and magnetite (demagnetising spectra: 15-50 mT) have been identified. Negative fold tests indicate remanence acquisition after the main folding phase, which is of short-wavelength character and occurs during the early orogenese of the Himalayas. A thermal or thermochemical origin of magnetisation is likely and the age of remanence acquisition is indicated to be about 40 Ma by ⁴⁰K/³⁹Ar cooling and ⁴⁰Ar/³⁹Ar crystallisation ages. In the Kumaon LH a long-wavelength tilting is indicated by a distribution of the remanence directions along a small-circle in N-S direction. Steepening of the remanence directions in the TH related to ramping on the Main Central Thrust (MCT) was not observed, in contrast to other related studies. In the Alaknanda valley of LH a 38±8 Ma age of remanence acquisition is supported by comparison of observed inclinations to the apparent polar wander path of India. Clockwise rotation of 20.3±11.7° (LH/Alaknanda valley) and 11.3±8.5° (TH) with respect to the Indian plate is observed, indicating that there is no significant evidence for rotational shortening along the MCT since about 40 Ma. Our results suggest that most of rotational underthrusting and oroclinal bending has not been accommodated by the MCT, but by the main thrusts south of it. The latest Miocene/Pliocene age of the Main Boundary Thrust indicates that oroclinal bending is a late-orogenic process.     

Multicomponent magnetization in western Dolpo (Tethyan Himalaya, Nepal): tectonic implications

A palaeomagnetic study has been carried out in the Tethyan Himalaya (TH; the northern margin of Greater India). Twenty-six palaeomagnetic sites have been sampled in Triassic low-grade metasediments of western Dolpo. Two remanent components have been identified. A pyrrhotite component, characterized by unblocking temperatures of 270–335 °C, yields an in situ mean direction of D=191.7°, I=−30.9° (k=29.5, α₉₅=5.7°, N=23 sites). The component fails the fold test at the 99% confidence level (k_{in situ}/k_bed=6.9) and is therefore of postfolding origin. For reason of the low metamorphic grade, this pyrrhotite magnetization is believed to be of thermo-chemical origin. Geochronological data and inclination matching indicate an acquisition age around 35 Ma. The second remanence component has higher unblocking temperatures (>400 °C and up to 500–580 °C range) and resides in magnetite. A positive fold test and comparison with expected Triassic palaeomagnetic directions suggest a primary origin.The postfolding character of the pyrrhotite component, and its interpreted age of remanence acquisition, implies that the main Himalayan folding is older than 35 Ma in the western Dolpo area. This study also suggests that the second metamorphic event (Neo-Himalayan) was more significant in the Dolpo area than the first (Eo-Himalayan) one.A clockwise rotation of 10–15° is inferred from the pyrrhotite component, which is compatible with oroclinal bending and/or rotational underthrusting models. This rotation is also supported by the magnetite component, indicating that no rotation of the Tethyan Himalaya relative to India took place before 35 Ma.     

The lacustrine sedimentary records of coal-burning atmospheric pollution

With the acceleration of industrialization, coal- burning atmospheric pollution is getting more and more serious in many regions in China. However, there are only about ten odd years literature records about pollution in some large and mid-sized cities, where the atmosphere has perhaps been polluted for several decades, and therefore it is difficult to know the process and characteristic of pollution, and diffi-cult to evaluate its damage to environment. Lacustrine sediment, a natural record w…     

土壤剖面的磁学特征及其对交通污染的指示意义——以北京首都机场高速公路为例

对北京首都机场高速公路旁采集的土壤柱状T01剖面的磁学参数和金属元素分析,探讨了研究区内现代交通导致的土壤磁学性质的变化及其对环境污染的响应.结果表明,磁参数(χ,ARM和SIRM)与重金属含量呈同步垂向变化趋势.来源于交通运输排放的污染物是土壤剖面上部(0～8 cm)磁性和金属含量增强的主要原因,8cm以下,土壤基本未受到污染,磁性矿物和重金属含量较低,磁性颗粒变化稳定,基本代表了该地区土壤的自然背景.尽管土壤岩石磁学分析表明剖面上下部受污染和未受污染样品的磁载体均是粒度较粗的多畴磁铁矿,但是结合磁参数比值曲线,说明底部样品的磁颗粒的粒度较顶部偏细.磁性矿物的含量变化没有影响磁颗粒的粒度特征.指标聚类等相关分析表明,土壤磁参数(χ,ARM和SIRM)与重金属元素(Pb,Zn和Cu)含量显著相关;结合模糊聚类分析,磁参数可用于追踪、识别交通污染物质在土壤剖面中的富集、迁移状态,揭示不同深度土壤的污染程度.     

The domain structure of synthetic stoichiometric TM10–TM75 and Al-, Mg-, Mn- and V-doped TM62 titanomagnetites

The existence of domain structure has been questioned for titanomagnetites of typical oceanic basalt composition owing to the unusual temperature dependence of their susceptibility, resembling that of spin glasses. In order to make a direct test of domain structure, a series of stoichiometric titanomagnetites between magnetite (TM0) and 75% ulvöspinel content (TM75) as well as a titanomagnetite of typical oceanic basalt composition have been synthesised using the double-sintering technique at 1300°C, in controlled atmospheres. The purity, stoichiometry and homogeneity of these materials were tested by optical, X-ray and microprobe studies as well as by magnetic measurements.Domain structures were observed using the Bitter-pattern technique after ionic polishing to produce stress-free surface of the bulk material. The optimum time required for ionic polishing was found to increase with the ulvöspinel content and to be correlated with the magnetostrictive constant θ. Magnetite showed a domain configuration which is also typical for nickel (mostly lamella-shaped domains, pine-tree-shaped closure domains, high domain wall mobility in small external fields, straight domain walls). The tendency to form lamella-shaped domains is present up to TM75 (which has a Curie temperature of only 40°C), but with an increasing tendency to form curved domain walls and to have fewer and also differently shaped closure domains. This is demonstrated in a series of photographs. The results constitute unequivocal evidence for the existence of a domain structure in the classical sense in a broad range of stoichiometric pure and doped (Al, Mg, Mn, V) titanomagnetites.     

Fidelity and time resolution of the magnetostratigraphic record in Siwalik sediments: high-resolution study of a complete polarity transition and evidence for cryptochrons in a Miocene fluviatile section

The Surai Khola section in southwest Nepal, a 5000 m continuously exposed record of fluvial sedimentation since Middle Miocene, was revisited for high-resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field. Polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions. Polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column. Almost antiparallel palaeoremanence directions, residing in different haematite phases in the same specimens, could generally not be separated properly by thermal demagnetization. Differing demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions, but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization. Accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization (DRM) with considerable inclination errors and scattered directions for individual specimens.     

On an unmetamorphosed iron-formation in the early precambrian of South-West Greenland

During Renzy Mines Ltd. exploration programme in 1972, an iron-formation was found in the Midternæs area north east of Ivigtut, South-West Greenland. It contains up to 32.9% Fe. The dominant mineral is greenalite, with minor amounts of magnetite, siderite and quartz. It was found in the northernmost part of the Ketilidian supracrustals (? 1800 m.y.), which are non-metamorphosed or very weakly metamorphosed. The iron-formation is suggested to be of submarine-exhalative origin, and precipitated in a moderately deep sea at low atmospheric O₂ pressure.