Contribution of serpentinized ultramafics to marine magnetic anomalies at slow and intermediate spreading centres: insights from the shape of the anomalies

Detailed characteristics of marine magnetic anomalies 33r and 20r suggest that the magnetization of the deeper magnetic layers, including the lower crust and possibly the uppermost mantle, is horizontally displaced with respect to that of the upper crust. We examine the possibility that serpentinization of ultramafics in the lower crust and possibly the uppermost mantle delays the acquisition of magnetization and introduces a shift between the upper- and lower-crustal magnetization patterns. Thermal evolution models and the resulting magnetization patterns of the oceanic lithosphere are calculated for a wide range of physical parameters such as the Nusselt number and the depth of hydrothermal circulation in the crust, and the temperature range of serpentinization. The models with moderate hydrothermal cooling of the whole crust and serpentinization temperatures ranging between 200 and 300 C successfully explain the anomalous skewness and the 'hook shape' of observed sea-level magnetic anomalies created at slow and intermediate spreading rates.     

Multimodal investigation of thermally induced changes in magnetic fabric and magnetic mineralogy

Low-field magnetic susceptibility and its anisotropy (AMS) were measured for a suite of sandstone and siltstone samples. AMS orientations measured on two systems (Bartington and Digico) differed before thermal treatment of the samples but became the same after thermal demagnetization in air to 600 °C. Six position measurement schemes for the Bartington system do not eliminate the effects of specimen inhomogeneity and other errors, whereas 12- and 24-position measurements give good agreement with the Digico anisotropy meter and with the observed petrofabric. Thermal demagnetization from temperatures between 400 and 650 °C had the effect of enhancing both the magnetic susceptibility and AMS. Although the most profound mineralogical change due to heating was the conversion of kaolinite into metakaolin, IRM, XRD, DTA and Mössbauer spectroscopic analysis demonstrate that the changes in magnetic properties were due to the transformation upon heating of trace amounts of sulphides into magnetite and/or maghemite and haematite. Both magnetic susceptibility and the degree of anisotropy decrease with higher-temperature thermal demagnetization due to the oxidation of the newly formed magnetite and/or maghemite into haematite. The magnetic foliation of the newly formed magnetite/maghemite and haematite is parallel to the bedding, possibly following the orientation of the original sulphides.     

国内外大城市的城市森林时空变化对比研究

城市森林在减弱噪声、净化空气、调节城市气温等方面具有十分重要的作用。中国城市正处于快速发展阶段,对国内外大城市的城市森林现状及其时空变化进行对比分析可为中国城市森林的建设提供理论借鉴。跨区域城市森林对比研究的主要困难在于城市区域定义的差异导致不同城市之间可比性差。论文利用强度梯度法,基于夜间灯光影像及GlobeLand30地表覆盖产品,提取12个国内外大城市2000年和2010年的城市森林数据,探究其时空变化。结果表明,从现有城市森林量看,城市森林覆盖率及人均城市森林面积在发达国家城市中普遍偏高,而在亚洲发展中国家城市中非常低,例如纽约的人均城市森林面积高达316.5 m ²,而上海、新德里和雅加达则低于2 m ²。从时间变化看,2000—2010年城市森林面积增长率在发达国家城市中均小于70%,而在大部分发展中国家城市超过了100%。城市森林增加的最主要来源是城市面积扩张,减少的主要原因是转为硬化地面和城市草地。按照基于缓冲区分析得到的城市森林空间分布格局,这些城市可以分为3类：中心城区高、周边城区低(北京、首尔),中心城区低、周边城区高(广州、东京、圣保罗等),整个城区均低(上海、新德里、雅加达)。最后,基于对比结果为中国城市森林未来的发展提供了建议。