Bedform topography and basal conditions beneath a fast-flowing West Antarctic ice stream

A grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath ～140 km² of Rutford Ice Stream, West Antarctica. The subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water-saturated sediments. The two troughs are filled with deforming sediment, whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream. The deforming bed is very flat along flow, but undulates across flow. Sliding areas show rougher bed topography. Cross-stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction. These bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation. In places, they form finger-like mounds of material, which extend into the sliding region further downstream. Mean bedform height is 22 m, mean width is 267 m, and many of them extend for at least 1–2 km along flow. We interpret most of these bedforms as drumlins and one as a mega-scale glacial lineation. The juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas, variable both spatially and temporally. Any theory of subglacial sediment rheology must also be able to account for our conclusion that, at any given time, pervasive deformation extends at least a few metres into the bed and can persist over a considerable area (many km²). Bedform geometry and basal conditions concur with interpretations of former ice streams, with evidence for increasing elongation ratio with distance downstream. However, those studies also identified bedrock cropping out at the ice-bed interface, for which there is no evidence on Rutford Ice Stream.     

Estimates of effective stress beneath a modern West Antarctic ice stream from till preconsolidation and void ratio

Preconsolidation stress recorded in subglacial sediments provides important information about subglacial effective stresses. It is commonly used to reconstruct past effective stresses from sediments left after ice retreat. In this article, we use properties of sub‐ice‐stream till samples to estimate effective stresses beneath a modern West Antarctic ice stream. Two previous estimates of sub‐ice‐stream effective stress were derived for the Upstream B (UpB) area of Ice Stream B from shear wave velocities (50 ± 40 kPa, Blankenship et al 1987) and borehole water level measurements (63 ± 24 kPa, Engelhardt & Kamb 1997). However, geotechnical tests performed on samples of the UpB till have shown that if subjected to effective stress of 50–63 kPa this till would have significantly lower porosity (?0.32–0.35) and higher strength (?‐22–28 kPa) than it apparently has in situ (?0.4 and ?2kPa). We derive new estimates of sub‐ice‐stream effective stress using: (1) Casagrande's construction applied to the results of six confined uniaxial tests, and (2) a combination of void‐ratio data for 51 till samples and 3 experimentally constrained equations describing compressibility of the UpB till under normal consolidation, overconsolidation and in the critical state. Casagrande's method yields an upper bound on effective stress of 25 kPa for four till samples and values of 13, and 4.4kPa for two other samples. The void‐ratio approach gives 11.7 ± 2.6 (normal consolidation), 18.3 ± 4.4 (overconsolidation) and 2.0 ± 0.8 kPa (critical state). These new, lower estimates of effective stress are consistent with the low till strength that has been independently measured and inferred from recent theoretical ice‐stream models. Our interpretation of data on till void ratio in terms of sub‐ice‐stream effective stress means that we can qualitatively evaluate the nature of the vertical distribution of this stress in the UpB till layer. We infer that in the sampled top 3 m of till the effective‐stress distribution is non‐hydrostatic, probably close to lithostatic. The results may be useful in future modeling of ice‐stream behavior and may aid efforts to delineate paleo‐ice streams based on their geologic record.     

坡面细沟侵蚀产沙时空分布规律试验研究

利用放水冲刷试验结合稀土元素(REE)示踪技术对坡面侵蚀产沙特征及其时空变化进行了研究。结果表明:在径流冲刷下,坡面侵蚀产沙可分为三个不同的阶段并对应不同的产沙特征。其空间变化表现为,在6°、9°情况下,坡面不同坡段的相对侵蚀量沿下坡方向呈现出先减小后增大的趋势;在12°小流量情况下,坡面各坡段的相对侵蚀量与坡度较小的情况相似,当流量较大时,坡面各不同坡段的相对侵蚀量随坡长的增大而减小。但坡面最下端坡段的相对侵蚀量总是随着冲刷历时的延长而减小,其余各坡段的相对侵蚀量则呈现出缓慢的波动式上升。     

Ice stream sticky spots: A review of their identification and influence beneath contemporary and palaeo-ice streams

Rapidly-flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance. Recent observations have revealed that ice stream flow exhibits considerable variability, with relatively rapid changes taking place in speed and direction. This spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction, known as ‘sticky spots’. In this paper, we provide a detailed review of sticky spot observations from both contemporary and palaeo-ice stream beds in order to better understand their nature and influence. Observations and theoretical considerations reveal four primary causes of ‘stickiness’: (i), bedrock bumps; (ii), till-free areas; (iii), areas of ‘strong’ (well drained) till; and (iv), freeze-on of subglacial meltwater. These may act together in one location, or in isolation; and a progressive increase in their distribution could lead to ice stream shut-down. Bedrock bumps are influential under active ice streams, where they provide form drag and can create thinner ice which increases the likelihood of basal freeze-on. Increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut-down because, over long time-scales, ice stream erosion might be expected to reduce their amplitude. The influence of till-free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas. They are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams. If an ice stream operates solely by till deformation, it is conceivable that a progressive increase in the exposure of till-free areas could lead to shut-down through a process of sediment exhaustion. Areas of strong, well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater. The collapse of an inefficient ‘cannalised’ system to a more efficient ‘channelised’ system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut-down in both contemporary and palaeo-settings. Basal freeze-on has also been observed and inferred from beneath modern and palaeo-ice streams, and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream. A paucity of data from ice stream sticky spots limits a better understanding of their nature, distribution and evolution beneath ice streams. Future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well-preserved palaeo-ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification. Such data will considerably enhance the basal boundary condition in ice stream models which will, ultimately, refine our predictions of the response of contemporary ice sheets to future changes in climate.     

The Dubawnt Lake palaeo-ice stream: evidence for dynamic ice sheet behaviour on the Canadian Shield and insights regarding the controls on ice-stream location and vigour

We report evidence for a major ice stream that operated over the northwestern Canadian Shield in the Keewatin Sector of the Laurentide Ice Sheet during the last deglaciation 9000–8200 (uncalibrated) yr BP. It is reconstructed at 450 km in length, 140 km in width, and had an estimated catchment area of 190000 km². Mapping from satellite imagery reveals a suite of bedforms ('flow-set') characterized by a highly convergent onset zone, abrupt lateral margins, and where flow was presumed to have been fastest, a remarkably coherent pattern of mega-scale glacial lineations with lengths approaching 13 km and elongation ratios in excess of 40:1. Spatial variations in bedform elongation within the flow-set match the expected velocity field of a terrestrial ice stream. The flow pattern does not appear to be steered by topography and its location on the hard bedrock of the Canadian Shield is surprising. A soft sedimentary basin may have influenced ice-stream activity by lubricating the bed over the downstream crystalline bedrock, but it is unlikely that it operated over a pervasively deforming till layer. The location of the ice stream challenges the view that they only arise in deep bedrock troughs or over thick deposits of 'soft' fine-grained sediments. We speculate that fast ice flow may have been triggered when a steep ice sheet surface gradient with high driving stresses contacted a proglacial lake. An increase in velocity through calving could have propagated fast ice flow upstream (in the vicinity of the Keewatin Ice Divide) through a series of thermomechanical feedback mechanisms. It exerted a considerable impact on the Laurentide Ice Sheet, forcing the demise of one of the last major ice centres.     

Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

The presence of a complex bedform arrangement on the sea floor of the continental shelf in the western Amundsen Sea Embayment, West Antarctica, indicates a multi-temporal record of flow related to the activity of one or more ice streams in the past. Mapping and division of the bedforms into distinct landform assemblages reveals their time-transgressive history, which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo-ice velocity, as suggested previously. A main control on the bedform imprint is the geology of the shelf, which is divided broadly between rough bedrock on the inner shelf, and smooth, dipping sedimentary strata on the middle to outer shelf. Inner shelf bedform variability is well preserved, revealing information about local, complex basal ice conditions, meltwater flow, and ice dynamics over time. These details, which are not apparent at the scale of regional morphological studies, indicate that past ice streams flowed across the entire shelf at times, and often had onset zones that lay within the interior of the Antarctic Ice Sheet today. In contrast, highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow, and may be a truer representation of fast palaeo-ice flow in these locations. A revised model for ice streams on the shelf captures complicated multi-temporal bedform patterns associated with an Antarctic palaeo-ice stream for the first time, and confirms a strong substrate control on a major ice stream system that drained the West Antarctic Ice Sheet during the Late Quaternary.     

Controls on spatial and temporal distribution of Precambrian eolianites

Inversely graded stratification, generated by the migration of wind ripples, and adhesion structures permit unequivocal identification of Precambrian eolianites. These criteria, in combination with scale of cross-beds, angle of inclination of foresets, geometry of depositional units, and associated non-eolian facies, are used to discriminate between Precambrian dune/draa, dune-plinth, sand-sheet, and interdune deposits that formed in inland and coastal settings. Based on an analysis of published literature, fundamental conclusions can be drawn on the spatial and temporal distribution of Precambrian eolianites. The oldest reported eolianites are from the ca. 2.1 Ga Deweras Group in Zimbabwe and Hurwitz Group in Canada and numerous examples of eolianites are reported from the 1.8 Ga and younger rock record. Lack of Archean and early Paleoproterozoic eolianites and their widespread development after 1.8 Ga are examined with respect to: absence of vegetation, crustal growth and tectonic setting, relative sea-level fluctuations, unfavorable atmospheric and/or climatic change, and non-recognition. The lack of pre-2.2 Ga eolianites may be related to reworking by braided rivers combing across non-vegetated floodplains, reworking of coastal eolianites during transgression or their non-recognition in the Early Precambrian record. The temporal concentration of eolianites at 1.8 Ga may best be related to the early stages of breakup and the assembly phases of supercontinents.     

The spatial and temporal variability of sand erosion across a stabilizing coastal dune field

This study aimed at quantifying the temporal and spatial variability in sand erosion and deposition over a coastal dune field in Israel. These were measured monthly over 2 years using 315 erosion pins over four transects that were placed perpendicular to the coastline. Vegetation cover was estimated based on aerial photographs and Landsat satellite images, whereas the relative height was based on a digital elevation model. These variables were calculated for the area upwind (south west) of the erosion pins, at various lengths, ranging from 15 to 400 m. Nine geomorphologic units were defined, five related to active units, and four to stabilized units. In active units at least 65% of the temporal variance in the annual absolute changes in sand level was explained by the index of Resultant Drift Potential, with most of the sand movement occurring during winter storms. Local rainfall had no apparent impact on sand mobility, due to the low coincidence of sand carrying winds and rainfall in Israel during the passage of frontal cyclones. As for the spatial variables, only a weak correlation was found between sand mobility with the distance from the coastline (R² = 18%). Rather, sand erosion and deposition were influenced by vegetation cover and the relative height of an area of 100–200 m upwind. The values of Soil Adjusted Vegetation Index were significantly negatively correlated with annual absolute changes (R² = 40%), whereas the relative height was significantly positively correlated (R² = 36%). Applying a multiple regression model, 68% of the spatial variability in sand mobility was explained. The resulting map of sand activity clearly shows that at this stage of the stabilization process, most of the dunes are now disconnected, and movement of sand grains from the beach or between the dunes, is very limited. These methods can be applied into spatial and temporal models of sand mobility, thus assessing the impact of different management practices on coastal dunes.     

The study of rockfall trajectory and kinetic energy distribution based on numerical simulations

Natural Hazards - Rockfall trajectory and kinetic energy distribution are the main components of rockfall research. In many cases, limited by site conditions, field experiments cannot be conducted...     

喜马拉雅降水势能与剥蚀强度时空对应的定量研究

在活动造山带的研究中,剥蚀作用被认为与大气降水有着密切的关系,并且越来越 多的研究证据发现,气候因子在山脉的演化中承载了重要的影响力,降水被认为是活动造山 带剥蚀作用的动因。研究者用降水量和剥蚀强度的对应来阐释气候对山脉演化的制约。然而 也有学者提出反面例证来否定降水量与山脉剥蚀强度之间的耦合关系,进而否定气候因子在 活动造山带构造演化中的影响。由此而提出的科学问题是：降水究竟与剥蚀强度有没有关 联？降水怎样影响剥蚀强度,遵循怎样的机理？在详细分析活动造山带剥蚀作用过程和机理 的基础上,本文提出了降水势能的概念,在校验和整合TRMM 降水量数据与Etopo1 数字地貌 数据体之后,计算了喜马拉雅造山带的降水势能空间分布。同时,收集整理了喜马拉雅造山 带内已公开发表的磷灰石裂变径迹（AFT）年龄数据,以此为数据库获取了喜马拉雅剥蚀强度 时空分布格局。通过这两个数据体的定量分析,研究发现,喜马拉雅造山带内,剥蚀强度与 降水势能之间有很好的空间耦合性。这说明, 在喜马拉雅构造带, 现今的AFT 年龄空间分 布,是受气候作用主导的。进一步的研究发现,喜马拉雅带大约2.8 Ma 以来的AFT 年龄与现 今的降水势能秩相关性显著,且有局部最优的线性相关性,说明喜马拉雅带的剥蚀空间分布 受第四纪以来很年轻的地质历史时期的地表降水过程所控制。最新的气候样式决定了现今的 降水过程,控制了造山带剥蚀强度的空间分布。     

A virtual voyage through 3D structures generated by chaotic mixing of magmas and numerical simulations: a new approach for understanding spatial and temporal complexity of magma dynamics

In this contribution, we present a virtual voyage through 3D structures generated by chaotic mixing of magmas and numerical simulations with the aim to highlight the power of 3D representations in the understanding of this geological phenomenon. In particular, samples of mixed juveniles from Salina island (Southern Italy) are reconstructed in 3D by serial lapping and digital montage and numerical simulations are performed by using a 3D chaotic dynamical system. Natural and simulated magma mixing structures are visualized by using several multimedia tools including animations and “virtual reality” models. It is shown that magma interaction processes can generate large spatial and temporal compositional heterogeneities in magmatic systems. The same topological structures are observed in both 3D reconstructed rock samples and chaotic numerical simulations, indicating that the mixing of magmas is governed by chaotic dynamics. The use of 3D multimedia models gives the opportunity to penetrate into magma mixing structures and to understand their significance in the context of magma dynamics. Such an approach is very powerful since multimedia tools can strongly capture the attention of the reader bringing him/her into an interactive and memorable geological experience. Electronic supplementary material  enclosed:     

阿勒泰市地质灾害时空分布规律研究

新疆阿勒泰骆驼峰崩塌位于阿勒泰市重点景区中,对游客、坡前公路及过往人员车辆构成威胁。新疆阿勒泰复向斜褶皱发育复杂,岩体在内外地质作用下劈理和节理等地质构造较为发育,导致该地区崩塌地质灾害危害程度较高。为有效防控阿勒泰市骆驼峰崩塌灾害,综合利用野外地质调查、InSAR监测、节理统计及理论分析等方法,探明了骆驼峰崩塌危岩体的几何形态和变形破坏特征,揭示了其成因和破坏机理。结果表明,研究区以滑移式崩塌为主,共分为崩塌区、陡崖斜坡和堆积区3个区段;得到最优势节理结构面曲线产状为37°∠70°,总结出该崩塌共存在3种岩体破坏模式;崩塌失稳模式为卸荷-拉裂-滑移,破坏机理可总结为地质构造-地形高差-节理结构面剪切-岩性差异性风化-水、冻融作用等因素诱发-岩体拉裂卸荷等多方面共同作用导致崩塌的变形破坏。研究成果可为阿勒泰骆驼峰景区地质灾害防灾减灾、景区开发规划提供参考,也为类似地区的凝灰岩崩塌灾害的发育特征及破坏机理研究提供借鉴。     

山东铁矿时空分布与成矿规律研究

山东省铁矿资源较丰富,根据山东省矿产资源储量表,截止2015年底,累计查明资源储量6.267Gt,其中鲁西地区保有资源储量占全省总量的94%左右,且以大、中型矿床居多;鲁东地区矿床规模一般较小,多分散分布,保有资源储量占全省总量的6%左右。共发现铁矿矿产地278处,其中大型矿产地18处,中型矿产地72处,小型矿产地188处,形成了莱芜、金岭、苍峄、韩旺等重要铁矿供应基地。近年来,山东省铁矿勘查工作发展迅猛,发现多个大型、超大型铁矿床,尤其在隐伏区和主要成矿区带的的深部及外围勘查找矿成果显著。主要成果有济宁颜店超大型铁矿（1.579Gt）、临沂兰陵超大型铁矿（0.622Gt）、苍峄铁矿东端超大型铁矿（王埝沟铁矿、凤凰山铁矿、沟西铁矿和宋楼铁矿,总储量达0.6Gt）、东阿单庄中型铁矿（0.069Gt）等。最近山东省煤田地质局和山东化工地质勘查院在禹城李屯和齐河县仁里集镇大张村地区进行铁矿勘查工作,发现隐覆在第四系之下的富铁矿矿床,类型为接触交代型铁矿。矿体分布在闪长岩与奥陶纪灰岩接触带上,矿体厚近百米,TFe品位56%～65%,资源量预计超过亿吨,是目前我省品位最高的铁矿床,具有良好的勘查前景。     

Reconstructing the basal thermal regime of an ice stream in a landscape of selective linear erosion: Glen Avon, Cairngorm Mountains, Scotland

The Cairngorm Mountain area of Scotland is a classic example of a landscape of selective linear glacial erosion, with sharp contrasts in the intensity of glacial erosion between the deeply incised troughs and valleys and the undulating high plateau. This article examines the Quaternary development of Glen Avon, a 200 m deep glacial trough set within the high plateau of the mountains. Evidence concerning the aggregate basal thermal regimes of the topographically controlled ice streams that formerly developed in this area is reconstructed from the geomorphological record, including bedforms indicative of wet-based, sliding ice and of dry-based ice frozen to its bed. This mapping indicates that basal sliding was not confined exclusively to the troughs but extended towards valley heads and on to parts of the plateau adjacent to troughs. The extent of basal sliding appears to have been greatest beneath pre-Late Devensian ice sheets. Basal ice temperatures are modelled under steady-state conditions for the last ice sheet at c. 18 ka BP. Basal thermal regimes are predicted using a reconstruction of the preglacial relief and for the current topography of the area. Convergent flow of ice through the preglacial valley system appears to have been sufficient to induce basal melting and therefore to initiate valley deepening. This effect is enhanced when the model is run across the present topography. Comparison of results of the geomorphological mapping and the modelling reveals significant differences between the actual and predicted extent of basal sliding outside the main ice stream. The overall conclusion is that many ice streams in mountainous terrain are inherited from the locations of preglacial valleys, which serve to accelerate ice flow and promote frictional heating beneath ice sheets.     

2001-2012年新疆融雪型洪水时空分布特征

利用2001-2012年新疆区域内发生的融雪型洪水资料,分析研究了近12 a新疆融雪型洪水时空分布特征.结果表明:新疆融雪型洪水与前一年10月至当年3月新疆全站总累计降水量大小相关,降水量大的年份,对应的融雪型洪水发生次数也多;冬末至夏季融雪型洪水在北疆地区基本上是从西向东、从南向北的先后顺序出现,而在南疆地区的融雪洪水基本上是从西向东、从北向南先后顺序出现.新疆融雪型洪水主要集中出现在春夏季,其中,北疆地区在3月,南疆地区在7月发生较多;伊犁河谷、昌吉、阿勒泰、和田等地区及青河、乌鲁木齐、阿克陶、民丰等市县是新疆融雪型洪水的高发区.     

Glacial geomorphology of Teesdale,northern Pennines,England: Implications for upland styles of ice stream operation and deglaciation in the British-Irish Ice Sheet

The glacial geomorphology of Teesdale and the North Pennines uplands is analysed in order to decipher: a) the operation of easterly flowing palaeo-ice streams in the British-Irish Ice Sheet; and b) the style of regional deglaciation. Six landform categories are: i) bedrock controlled features, including glacitectonic bedrock megablocks or ‘rubble moraine’; ii) discrete mounds and hills, often of unknown composition, interpreted as weakly streamlined moraines and potential ‘rubble moraine’; iii) non-streamlined drift mounds and ridges, representing lateral, frontal and inter-ice stream/interlobate moraines; iv) streamlined landforms, including drumlins of various elongation ratios and bedrock controlled lineations; v) glacifluvial outwash and depositional ridges; and vi) relict channels and valleys, related to glacial meltwater incision or meltwater re-occupation of preglacial fluvial features. Multiple tills in valley-floor drumlin exposures indicate that the subglacial bedform record is a blend of flow directions typical of areas of discontinuous till cover and extensive bedrock erosional landforms. Arcuate assemblages of partially streamlined drift mounds are likely to be glacially overridden latero-frontal moraines related to phases of “average glacial conditions” (palimpsests). Deglacial oscillations of a glacier lobe in mid-Teesdale are marked by five inset assemblages of moraines and associated drift and meltwater channels, named the Glacial Lake Eggleshope, Mill Hill, Gueswick, Hayberries and Lonton stages. The Lonton stage moraines are thought to be coeval with bedrock-cored moraines in the central Stainmore Gap and likely record the temporary development of cold-based or polythermal ice conditions around the margins of a plateau-based icefield during the Scottish Readvance.     

2000～2016年陕西省地质灾害时空分布规律及变化趋势

本文利用2000~2016年陕西省已经造成人员伤亡和财产损失的地质灾害灾情资料,分析研究了近十七年陕西省地质灾害时空分布规律及变化趋势。结果表明:(1)时间上,年内地质灾害主要发生在5~10月,而7~9月是重灾期,非汛期地质灾害数量占比呈明显增高态势,由2013年以前的不足10.00%上升至2015年的39.47%、2016年的26.67%;年际上每3~5年会发生一次特大型地质灾害;“十二五”期间,地质灾害的发生数量、死亡失踪人数、直接经济损失总体上呈明显下降趋势。(2)空间上,陕南地区的地质灾害数量最多,但近年来陕北地区的地质灾害数量明显增多,呈现出由陕南向陕北逐渐扩展的趋势,2013年和2016年地质灾害数量占比由2003年的最低值0.25%分别上升至87.54%和46.67%,远远高于同年陕南地区。(3)引发因素上,由单一的自然因素向多因素交织转变,因人为活动引发的地质灾害数量呈现上升趋势。