Electromagnetic detection of a 410-km-deep melt layer in the southwestern United States

A deep-seated melt or fluid layer on top of the 410-km-deep seismic discontinuity in Earth's upper mantle, as proposed in the transition-zone 'water filter' hypothesis, may have significant bearing on mantle dynamics and chemical differentiation. The geophysical detection of such a layer has, however, proved difficult. Magnetotelluric and geomagnetic depth sounding are geophysical methods sensitive to mantle melt. Here we use these methods to search for a distinct structure near 410-km depth. We calculate one-dimensional forward models of the response of electrical conductivity depth profiles, based on mineral physics studies of the effect of incorporating hydrogen in upper-mantle and transition-zone minerals. These models indicate that a melt layer at 410-km depth is consistent with regional magnetotelluric and geomagnetic depth sounding data from the southwestern United States (Tucson). The 410-km-deep melt layer in this model has a conductance of 3.0 x 10(4) S and an estimated thickness of 5-30 km. This is the only regional data set that we have examined for which such a melt layer structure was found, consistent with regional seismic studies. We infer that the hypothesized transition-zone water filter occurs regionally, but that such a layer is unlikely to be a global feature.     

Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States

Stable continental regions, free from tectonic activity, are generally found only within ancient cratons-the centres of continents which formed in the Archaean era, 4.0-2.5 Gyr ago. But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stable, whereas some older (late Archaean) regions have been tectonically disturbed, suggesting that age alone does not determine lithospheric strength and crustal stability. Here we report rhenium-osmium isotope and mineral compositions of peridotite xenoliths from two regions of the Cordilleran mountain belt. We found that the younger, undeformed Colorado plateau is underlain by lithospheric mantle that is 'depleted' (deficient in minerals extracted by partial melting of the rock), whereas the older (Archaean), yet deformed, southern Basin and Range province is underlain by 'fertile' lithospheric mantle (not depleted by melt extraction). We suggest that the apparent relationship between composition and lithospheric strength, inferred from different degrees of crustal deformation, occurs because depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock). This allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface. The inference that not all Archaean crust developed a strong and thick thermal boundary layer leads to the possibility that such ancient crust may have been overlooked because of its intensive reworking or lost from the geological record owing to preferential recycling.     

Millennial-scale storminess variability in the northeastern United States during the Holocene epoch

For the purpose of detecting the effects of human activities on climate change, it is important to document natural change in past climate. In this context, it has proved particularly difficult to study the variability in the occurrence of extreme climate events, such as storms with exceptional rainfall. Previous investigations have established storm chronologies using sediment cores from single lakes, but such studies can be susceptible to local environmental bias. Here we date terrigenous inwash layers in cores from 13 lakes, which show that the frequency of storm-related floods in the northeastern United States has varied in regular cycles during the past 13,000 years (13 kyr), with a characteristic period of about 3 kyr. Our data show four peaks in storminess during the past 14 kyr, approximately 2.6, 5.8, 9.1 and 11.9 kyr ago. This pattern is consistent with long-term changes in the average sign of the Arctic Oscillation, suggesting that modulation of this dominant atmospheric mode may account for a significant fraction of Holocene climate variability in North America and Europe.     

我的博士后生活

<正>我在国内拿的博士学位。5年如期毕业,也发了几篇第一作者文章。在毕业前一年的年底开始联系博士后。当时(2004年)我认为中国生命科学的科研水平和美国的差距非常大,而且可以通过做博士后来换一下研究方向,扩展     

美国怎样实施教育产业化

教育始终是美国人所关心的国内社会政策的首要问题，其程度超过了对其他诸如经济、医疗保健及社会保障等问题的关心。人们关心教育，关心他们的孩子，在有关教育问题的全国性大讨论中，尽管充斥各种言辞且常常意见不一，但却创造了一种环境，使富有创见的思想在这个环境中滋生并迅速成长。从事教育产业的企业家们了解这些，并抓住这种机会，促进了以产业方式进行的教育改革。     

Names of Legal Profession in the United Kingdom and the United States

In the paper, I try to analyze the different divisions and titles in the United Kingdom and the United States. In the United Kingdom, the legal profession is traditionally divided into two branches, barristers and solicitors (the advocate in Scotland is equivalent to barrister); in the United States, there was never a formal separation of barrister and solicitor as in England, lawyers usually deal with a certain field of legal matters. In addition, I also try to speak something about the development and changes of "proctor", as well as the two kinds of "attorney" and their different development in the Great Britain and the United States.     

美国金融危机评析

2008年9月,美国金融危机井喷式爆发,并迅速波及其他国家。受其影响,全球股市剧烈波动,实体经济也不断遭受冲击。本文将就本次金融危机产生的原因、谁该为这次危机负责、政府是否应该救市以及应该从中吸取哪些教训等一系列问题进行研究、探讨。     

美国技术转移政策的要点及借鉴

 美国已经建立起完善的转移技术体系,包括联邦政府与地方技术转移的治理结构、各联邦组成部门的技术转移战略、行动计划以及概念验证中心、技术转移中心等组织体系的设立。梳理了美国技术转移政策的发展历程、构成体系、典型制度,分析了美国技术转移政策的要点,对中国技术转移具体工作的开展提出了对策建议。     

美国技术转移政策的要点及借鉴

美国已经建立起完善的转移技术体系,包括联邦政府与地方技术转移的治理结构、各联邦组成部门的技术转移战略、行动计划以及概念验证中心、技术转移中心等组织体系的设立。梳理了美国技术转移政策的发展历程、构成体系、典型制度,分析了美国技术转移政策的要点,对中国技术转移具体工作的开展提出了对策建议。     

美国早期的图像广告与月份牌

在美国,印刷月份牌广告开始于19世纪.70年代新发展的彩色平版印刷技术促进了彩色月份牌迅速发展.月份牌上的图像常常与售卖的产品或服务关系不大,甚至毫不相干.印刷从业者们还在专业杂志上登广告声称可以提供包括月份牌在内的宣传新产品的图像.美国海报广告与月份牌给人印象最深的是那些丰富到惊人的人物衣着、华丽的装饰性文字搭配以及具象的形象,尽管两者有广泛的创造性,它们共享着某些共同的图像元素.     

透视美国选举人团制

选举人团制是美国总统大选的中心环节.选举人团制创始于美国建国之初,历经二百多年的演变和发展,选举人团制的内容和运行方式发生了许多深刻变化.选举人团制在设计和运作上存在诸多局限性和弊端,人们提出了许多改革选举人团制的建议.     

美国社区图书馆信息服务简介

介绍了20世纪70年代以来美国社区信息服务的产生、美国社区网络的建立及公共计划的实施以及美国社区信息服务的发展趋势。     

中美战略定位浅析

本文通过对中美两国战略定位研究,剖析两国战略定位缘由和目的,并对两国战略前景进行预测,从而探略两国战略的内涵实质。