The Devensian/Weichselian Late-glacial in northwest Europe (Ireland,Britain, north Belgium,The Netherlands,northwest Germany)

A synthesis of environmental changes during the Devensian (Weichselian) Lateglacial period (14-9 ka BP) is presented for an area extending from Ireland in the west to northwest Germany in the east. Following a brief reference to the problems of chronology, the principal changes in geomorphology and soils, vegetation history and climate experienced in the region during the Late-glacial are described. Reconstructions of thermal variations during the Late-glacial period are attempted independently for Ireland, England and Wales, Scotland (Highlands and Islanads), north Belgium, The Netherlands and northwest Germany. The collective palaeotemperature data, based mainly uppoln pollen data but also in Britain and The Netherlands on coleopteran data using the ‘mutual climatic range’ approach, provide an overview of regional differences along an east-west transect in northwest Europe.     

Tephra‐linked peat humification records from Irish ombrotrophic bogs question nature of solar forcing at 850 cal. yr BC

This paper investigates evidence for palaeoclimatic changes during the period ca. 1500–500 cal. yr BC through peat humification studies on seven Irish ombrotrophic bogs. The sites are well‐correlated by the identification of three mid‐first millennium BC tephras, which enable the humification records at specific points in time to be directly compared. Phases of temporarily increased wetness are suggested at ca. 1300–1250 cal. yr BC , ca. 1150–1050 cal. yr BC , ca. 940 cal. yr BC and ca. 740 cal. yr BC . The last of these is confirmed to be synchronous at five sites, suggesting external forcing on a regional scale. The timing of this wet‐shift is constrained by two closely dated tephras and is demonstrated to be distinct from the widely reported changes to cooler/wetter conditions associated with a solar minimum at 850–760 cal. yr BC , at which time the Irish sites appear instead to experience drier conditions. The results suggest the possibility of either non‐uniform responses to solar forcing in northwest Europe at this time, or the existence of unrelated climate events in the early first millennium BC . The findings caution against the correlation of loosely dated palaeoclimate data if the effects of forcing mechanisms are to be understood. Copyright © 2006 John Wiley & Sons, Ltd.     

Wisconsinan Late-glacial environmental change in Nova Scotia: A regional synthesis

A synthesis of the interpreted vegetational and climatic history of Nova Scotia during the Wisconsinan Late-glacial period (14-9 ka BP) is presented and described in 500-year intervals. The concomitant regional deglaciation history of the region is also described. A synthesis chart is provided which includes a qualitative curve of temperature reconstructions for the Lateglacial period.     

An alternative Pangea reconstruction for Middle America with the Chortis Block in the Gulf of Mexico: tectonic implications

Understanding Pangea breakup requires a robust reconstruction, and this article focuses on the Middle America sector of the supercontinent. Although most Pangean reconstructions locate the Yucatan Block along the southern USA, the Chortis Block is generally placed off southern Mexico (Pacific model), undergoing sinistral relative motion during the Mesozoic and Cenozoic. However, the Pacific model is inconsistent with the absence of a Cenozoic fault linking the Cayman transforms and the Middle America Trench. We present an alternative Pangean reconstruction, where both the Yucatan and Chortis Blocks are placed in the future Gulf of Mexico, moving Mexico westwards along the Mojave–Sonora megashear to accommodate overlap with South America. Subsequent Mesozoic and Cenozoic evolution is inferred to have occurred in two stages: (i) Jurassic clockwise rotation along the Mojave–Sonora and West Florida megashears, followed by (ii) Cenozoic anticlockwise rotation along the Sierra Madre Oriental and East Yucatan megashears. The first stage is linked to the breakup of Pangea where the Gulf of Mexico formed as a pull-apart basin. The second stage is related to the evolution of the Caribbean where the Chortis and Yucatan Blocks rotated into the trailing side of the Caribbean Plate (pirate model). The new reconstruction is consistent with major parameters, such as (i) gravity, magnetic, and palaeomagnetic data; (ii) the westward continuation of the Cayman transform faults through the Chiapas foldbelt and along the N–S front of the Sierra Madre Oriental foldbelt; (iii) the 27–19 Ma removal of the southern Mexican forearc; (iv) offset of the Cretaceous volcanic arc (Guerrero-Suina); (v) the deflection of the Laramide orogen (Sierra Madre Oriental–Zongolica–Colon); and (vi) the continuity of Cretaceous platformal carbonates containing Caribbean fauna across Middle America. In this latter context, the Motagua high-pressure belt is interpreted as a Cretaceous extrusion zone into the upper plate above a subduction zone rather than as an oceanic suture.     

Climate variability and change in the drylands of Western North America

We argue that it is important to expand the consideration of climate in the context of provision of ecosystem services in drylands. In addition to climate change, it is necessary to include climate variability on timescales relevant to human and ecological considerations, namely interannual to decadal and multidecadal. The period of global instrumental record (about a century and a half long at the very most) is neither an adequate nor an unbiased sample of the range and character of natural climate variability that might be expected with the climate system configured as it is now. We base this on evidence from W. N. America, where there has recently been a major multi-year drought, of a scale and intensity that has occurred several times in the last 2000 years, and on attempts to provide explanations of these phenomena based on physical climatology. Ensembles of runs of forced climate system models suggest the next 50 years will bring much more extensive and intense drought in the continental interior of North America. The trajectory followed by the supply of ecosystem services will be contingent not only on the genotypes available and the antecedent soil, economic and social conditions but also on climate variability and change. The critical features of climate on which patterns of plant growth and water supply depend may vary sharply during and between human generations, resulting in very different experiences and hence, expectations.     

螺旋CT MPR重建对退行性腰椎管狭窄的诊断价值及临床意义

目的:探讨螺旋CT常规椎间盘扫描,结合兴趣区多平面图像重建对退行性腰椎管狭窄的诊断价值及临床意义.方法:对有临床症状且符合退行性腰椎管狭窄诊断的190例患者CT资料进行回顾性分析.结果:退行性椎管狭窄CT表现为椎间盘膨出/突出、椎体后缘骨质增生、椎体滑脱,椎小关节肥大、黄韧带肥厚和后纵韧带骨化至中央椎管矢状前后径及横径变小,侧隐窝、椎间孔变窄.190例患者中以Ⅱ、Ⅲ型狭窄为主,椎管狭窄的部位主要分布在L4～L5及L5～SI.MPR图像直观的显示了脱出的椎间盘与神经根的位置关系,以及椎体、小关节增生所致椎管狭窄失状及冠状影像表现.结论:螺旋CT轴位扫描结合兴趣区MPR重建对退行性椎管狭窄的发现、评价及病因诊断方面具有很高的敏感性,对临床治疗计划的制定有重要的参考价值.     

Global scale patterns of continental fragmentation: Wilson's cycles as a constraint for long-term sea-level changes

We propose to characterize land–ocean distributions over Late Proterozoic to Phanerozoic times from measurement of perimeters and areas of continental fragments, based on paleomagnetic reconstructions. These measurements serve to calculate geophysically constrained breakup and scatter indexes of continental land masses from 0 to 1100 Ma. We then provide quantitative investigation and modelling of relationships between scatter of continental landmasses and mean age of the oceanic lithosphere during Mesozoic times, which appears to range from 56 to 62 Ma over the last 170 My. We then inverse the scatter of continental landmasses in terms of global oceanic crust mean age over the last 600 My, i.e. back in times where no measurement of seafloor accretion history is possible because of subduction. We finally show that the inferred evolution of oceanic lithosphere mean age over the Phanerozoic remarkably correlates in time with long-term sea-level changes since the Cambrian.     

On the ‘Divergence Problem’ in Northern Forests: A review of the tree-ring evidence and possible causes

An anomalous reduction in forest growth indices and temperature sensitivity has been detected in tree-ring width and density records from many circumpolar northern latitude sites since around the middle 20th century. This phenomenon, also known as the “divergence problem”, is expressed as an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings. The divergence problem has potentially significant implications for large-scale patterns of forest growth, the development of paleoclimatic reconstructions based on tree-ring records from northern forests, and the global carbon cycle. Herein we review the current literature published on the divergence problem to date, and assess its possible causes and implications. The causes, however, are not well understood and are difficult to test due to the existence of a number of covarying environmental factors that may potentially impact recent tree growth. These possible causes include temperature-induced drought stress, nonlinear thresholds or time-dependent responses to recent warming, delayed snowmelt and related changes in seasonality, and differential growth/climate relationships inferred for maximum, minimum and mean temperatures. Another possible cause of the divergence described briefly herein is ‘global dimming’, a phenomenon that has appeared, in recent decades, to decrease the amount of solar radiation available for photosynthesis and plant growth on a large scale. It is theorized that the dimming phenomenon should have a relatively greater impact on tree growth at higher northern latitudes, consistent with what has been observed from the tree-ring record. Additional potential causes include “end effects” and other methodological issues that can emerge in standardization and chronology development, and biases in instrumental target data and its modeling. Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century, more research is needed to confirm these observations.