Palaeosecular variation in Central Mexico over the last 30 000 years: the record from lavas

13 lava flows of known age (ages from ¹⁴C dating), which have been erupted in the last 30 000 years, have been studied to determine the palaeosecular variation of the geomagnetic field in Central Mexico. Samples were taken from two different monogenetic volcanic fields: the Michoacan-Guanajuato volcanic field (six sites) and the Chichinautzin Formation (seven sites), both part of the Transmexican Volcanic Belt. The lavas were studied in detail using rock magnetic methods (magnetic susceptibility at room temperature, low-temperature susceptibility behaviour, hysteresis loops, Curie temperatures), combined with reflected light microscopy, in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals. The magnetic carriers are titanomagnetites, which show differing degrees of high-temperature deuteric oxidation, and seem to be predominantly pseudo-single domain (PSD), though in many cases are probably a mixture of domain states. Mean palaeomagnetic directions and palaeointensity values using Shaw and Thellier techniques were obtained using several specimens from each flow. Our data seem to indicate a sharp easterly swing in declination about 5000 years ago, which is also observed in lake sediments from Central Mexico. The calculated values of the virtual dipole moment (VDM) range from 3.1 to 14.9 × 10²² A m². Our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr, with a peak at about 9000 years BP. These are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field.     

Constraints on a plume in the mid-mantle beneath the Iceland region from seismic array data

Teleseismic P waves passing through low-wave-speed bodies in the mantle are refracted, causing anomalies in their propagation directions that can be measured by seismometer arrays. Waves from earthquakes in the eastern Pacific and western North America arriving at the NORSAR array in Norway and at seismic stations in Scotland pass beneath the Iceland region at depths of ∼ 1000–2000 km. Waves arriving at NORSAR have anomalous arrival azimuths consistent with a low-wave-speed body at a depth of ∼ 1500 km beneath the Iceland–Faeroe ridge with a maximum diameter of ∼250 km and a maximum wave-speed contrast of ∼ 1.5 per cent. This agrees well with whole-mantle tomography results, which image a low-wave-speed body at this location with a diameter of ∼ 500 km and a wave-speed anomaly of ∼ 0.5 per cent, bearing in mind that whole-mantle tomography, because of its limited resolution, broadens and weakens small anomalies. The observations cannot resolve the location of the body, and the anomaly could be caused in whole or in part by larger bodies farther away, for example by a body imaged beneath Greenland by whole-mantle tomography.     

Obstructed minibasins on a salt-detached slope: An example from above the Sigsbee canopy,northern Gulf of Mexico

Salt-detached gravity gliding/spreading systems having a rugose base-of-salt display complex strain patterns. However, little was previously known about how welding of supra-salt minibasins to the sub-salt may influence both the downslope translation of minibasins on salt-detached slopes and the regional pattern of supra-salt strain. Using a regional 3D seismic reflection data set, we examine a large salt-stock canopy system with a rugose base on the northern Gulf of Mexico slope, on which minibasins both subside and translate downslope. Some minibasins are welded at their bases and others are not. We suggest that basal welds obstruct downslope translation of minibasins and control regional patterns of supra-canopy strain. The distribution of strain above the canopy is complex and variable. Each minibasin that becomes obstructed modifies the local strain field, typically developing a zone of shortening immediately updip and an extensional breakaway zone immediately downdip of the obstructed minibasin. This finding is corroborated by observations from a physical sandbox model of minibasin obstruction. We also find in our natural example that minibasins can be obstructed to different degrees, ranging from severe (e.g., caught in a feeder) to mild (e.g., welded to a flat or gently dipping base-of-salt). By mapping both the presence of obstructed minibasins and the relative degree of minibasin obstruction, we provide an explanation for the origin of complex 3-D strain fields on a salt-detached slope and, potentially, a mechanism that explains differential downslope translation of minibasins. In minibasin-rich salt-detached slope settings, our results may aid: i) structural restorations and regional strain analyses; ii) prediction of subsalt relief in areas of poor seismic imaging; and iii) prediction of stress fields and borehole stability. Our example is detached on allochthonous salt and where the base-of-salt is rugose, with the findings applicable to other such systems worldwide (e.g., Gulf of Mexico; Scotian Margin, offshore eastern Canada). However, our findings are also applicable to systems where the salt is autochthonous but has significant local basal relief (e.g., Santos Basin, Brazil; Kwanza Basin, Angola).     

Disaggregating the historical demographic sources of regional foreign-born and native-born population growth in the United States: a new method with applications

This paper analyzes the demographic processes that contribute to population growth and redistribution in a multiregional system using a new method. The method incorporates a historical perspective that can be used to trace dynamic population processes as they evolve over time. It uses an open multiregional projection model framework in identifying the contributions to regional growth made by each of the principal demographic components of change: fertility, mortality, immigration, emigration, in-migration, and out migration. At the same time, the method recognizes the importance of disaggregating the native-born and foreign-born populations. Available public data and indirect estimation techniques are used to develop the data inputs for the projection model, with which the regional population changes for each 5-year period between 1950 and 1990 were reconstructed. Regional growth rates for the foreign-born and native-born populations are partitioned into the separate demographic components of change, and the projection model identifies the separate contributions to regional growth made by each population. This allows a direct comparison of the impact of immigration with those of corresponding native-born contributions effected through internal migration and natural increases. Finally, the application of the method allows the identification of the contribution that 'recent' (post-1965) immigrant cohorts have made to the composition of the youngest age groups in each region, and also to simulate the impacts of zero immigration scenarios on regional growth.     

Polyvocalism and the public: 'doing' a critical historical geography of architecture

Loretta Lees' recent call for a 'critical geography of architecture' argued that a more nuanced and truly interpretative methodology for analysing the relationship between space, place and architecture would involve active engagement with such architectural spaces. This paper argues that, in the same way, by broadening our approach to historical architectural geographies more meaningful interpretations will result. Utilizing a 'polyvocal' methodological approach, by which not only architects and planners but also residents of these spaces are considered as important actors, will facilitate this 'critical' engagement with architectural spaces. Further, by taking this polyvocal approach, interesting conclusions can be reached about the production, consumption and re-production of space, both in the past and also in the present.     

Hydrobiology of a salt pan from the Peninsula of Baja California, Mexico

The study site, a salt pan in the northwest of Baja California, Mexico,has a surface area of 33.2 ha and water capacity of 216,072m³. It is intermittently isolated from the adjacent ocean by asand bar, so that flooding via the coastal plain is a low frequency eventassociated with extreme storm conditions. Temporal changes in water volumewere represented by a particular expression of the mass conservationequation having as variables the volumetric capacity of the system, waterinputs by pluvial precipitation, runoff and infiltration, and water loss byevaporation; the numerical model explained over 80 per cent of observedfluctuations. The hydrobiological study covered a three-year period and twohydrological cycles; the first one was of mixed marine and pluvial origin,and the second, continental. Throughout the first cycle, phosphate andnitrate concentrations were up to one order of magnitude higher as weretotal particulate protein, chlorophyll a and phytoplankton abundance. Ingeneral, nannoplankton abundance was up to three orders of magnitude greaterthan the microphlanktonic fraction. In the former, 29 different cell formswere observed; two halophytic bacteria constituted 60–80 per cent ofthat fraction. In the microplankton, there were 27 different forms; mostwere benthic diatoms. The zooplanktonic assemblages of the first cycle hadmarine and continental elements; at 141 per mille, Artemia sp. firstappeared; at 240 per mille, it was the only faunistic element in the system.During the second cycle, Artemia was always present, co-existing with othercontinental species up to a salinity of 99 per mille. The morphology andmorphometry of Artemia were similar to those of Artemia franciscana.     

Useful Products of the Century Plant a Lesson on Mexico

Creating opportunities for students to actively apply hazards theory to real-life situations is often a challenge in hazards geography courses. This article presents a project, the Jocko Lakes Fire Project, that implemented learning strategies to encourage students to be active in wildfire hazards research. Wildfire hazards stand out as an increasing threat to communities in forested areas given current and projected rates of urbanization, the growing concentration of wealth in hazard-prone areas, the increasing costs of forest wildfire reduction, and climate change. Components of the project involved students in problem definition and the articulation of a research plan; identifying and collecting relevant data; and analyzing and documenting the wildfire hazard event. The student-based evaluation of the project and its outcomes highlights the ways in which this approach can increase understanding of local hazard scenarios, familiarity with relevant theory, geographical knowledge, and skills in research.     

基于分层分区法的中国历史耕地数据的网格化重建

网格化历史耕地数据集能为历史时期耕地变化研究提供更精确的支持,并且为全球环境气候变化研究模型模拟提供驱动数据。本文综合考虑了中国历代土地利用开发的特点及自然人文因子对耕地的影响,设计了一套对中国耕地先分区再分层分配的网格化方法。基于国内3个主流区域耕地数据研究成果,采用上述方法建立了1820年（清仁宗嘉庆二十五年）和1936年（民国二十五年）中国10 km×10 km分辨率的耕地数据集,并绘制了分布图。本文还利用国内具有代表性的区域数据集对重建结果进行对比验证。结果表明,该方法可以保证耕地数量的权威性,并且建立具有区域性的高精度历史耕地数据集。     

历史地理视角下丝绸之路空间格局及影响研究

已有的“一带一路”相关研究已逐步引起了学界对历史文化资源于今日发展影响的关注,然而作为“一带一路”倡议的历史依托和出发点,丝绸之路相关量化研究仍处于起步阶段。从文明的角度来说,如何认识丝绸之路是当代中国理解全球的关键。运用OLS、Logistic回归等手段,定量探讨了丝绸之路路线的形成动因以及其对历史及当代的影响。结果表明：① 丝绸之路道路密度分布总体处于波动状态,在以罗马、长安、大都为代表的交通网络起始点以及撒马尔罕、塔什干为代表的内线重要中转枢纽处密度数值大幅提高。② 作为商业贸易通道,丝绸之路路线选择影响最大的因素是地理因子,主要倾向于低坡度、高起伏度的地区,这一点在中亚复杂山地地形中表现得尤为明显。③ 丝绸之路的开辟与通畅,对当时社会经济、城市发展起到了显著的作用。丝绸之路道路密度同距最近海岸距离、纬度、历史文明指数等指标一道构成了古代城市分布的主要影响因素。④ 丝绸之路对区域经济发展的正面影响具有一定的持续性,其对今日经济的发展仍起着难以否认的支撑作用。丝绸之路道路密度每增加1%,所在区域夜间灯光指数增加0.373。本文通过横跨历史学、地理学、经济学的探索能够很好地提供差异化理论视角和技术手段助益,为相关研究的开展起到一定的参考与启发作用,同时也为“一带一路”相关理论的完善提供理论支撑与历史证明。     

Spectral aspects of the Gaussian beam method: reflection from a homogeneous half-space

Summary. Use of paraxially approximated Gaussian beams continues to be actively pursued for construction of synthetic seismograms in complicated environments. How to select the beams in the stack remains a source of difficulty which has primarily been addressed by semi-heuristic considerations. In this paper, the classical example of line-source field reflection from a homogeneous half-space that can sustain a head wave is examined from a plane-wave spectral point of view. The individual beam fields are modelled exactly by the complex source point technique, which emphasizes the complex spectral content of these wave objects. The quality of the paraxial approximation of a typical reflected (Gaussian) beam characterized by different parameters is examined from this perspective, and is compared with uniform and non-uniform asymptotics generated from the exact beam field spectral integral. With this information as background, the reflected field for a real line-source is synthesized by beam superposition. Except for the immediate vicinity of the critical reflection angle, the well-known failure of narrow paraxial beams, no matter how densely stacked, to reproduce the head wave effects is shown to be due to the inadequate spectral content of these beams and not to the failure of beam stacking per se. When the rigorous solutions are used for the narrow-waist beams, even relatively few suffice to yield agreement with the exact solution. This circumstance emphasizes the importance of fully understanding the spectral implications of various beam stacking schemes.