Pattern,style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf     

Tom Bradwell  Derek Fabel  Chris D. Clark  Richard C. Chiverrell  David Small  Rachel K. Smedley  Margot H. Saher  Steven G. Moreton  Dayton Dove  S. Louise Callard  Geoff A. T. Duller  Alicia Medialdea  Mark D. Bateman  Matthew J. Burke  Neil McDonald  Sean Gilgannon  Sally Morgan  David H. Roberts  Colm ó Cofaigh 《第四纪科学杂志》2021,36(5):871-933

Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast-flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice-stream-dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km² of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic-nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector-wide ice-sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice-sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice-sheet sector undergoing large size variations in response to sub-millennial-scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine-terminating ice-sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice-sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice-sheet build-up in NW Scotland, with the Minch ice-stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding-zone wedges and moraines on the mid- and inner continental shelf attest to significant stabilizations of the ice-sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present-day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea-level c. 26 kabp , followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice-shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp , as the grounding-line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding-line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp , ice-sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine-instability-triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid-Greenland Stadial 2, with an ice-accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice-sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp , before significant ice-mass losses thereafter. At the glacial termination, c. 14.5 kabp , glaciers fed outwash sediment to now-abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice-sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice-sheet glaciation dominated by ice-streaming were interspersed with periods of much more restricted ice-cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice-sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice-sheet health but also to the acceleration of ice-sheet demise via marine ice-sheet instabilities.  相似文献   

渤海湾盆地大型基岩潜山储层特征及其控制因素——以渤中19- 6凝析气田为例     

叶涛  牛成民  王清斌  代黎明  李飞 《地质学报》2021,95(6):1889-1902

渤中19-6气田是渤海湾盆地近期发现的规模最大的深层变质基岩凝析气田,探明储量超过千亿立方米,气柱高度超1000 m,现今埋深大于4000 m,储层规模大但非均质性强.基于三维地震、钻井、测井以及薄片资料,明确了基岩潜山储层的发育特征,并探讨了优质储层形成的主控因素.结果表明:基岩潜山以太古宇变质花岗岩为主要的储集岩性,储集空间主要为构造裂缝,同时发育沿裂缝溶蚀形成的孔隙及构造破碎粒间孔,平均孔隙度为5％,平均渗透率为6.48×10-3μm2.脆性岩石、构造挤压以及风化作用共同控制了规模性储层的形成:① 研究区90％的岩石为低抗压强度的变质花岗岩,为裂缝的形成提供了物质基础;② 印支期强烈的挤压为规模性裂缝的形成提供了外部动力,是大规模储层发育最为关键的因素;不同构造位置裂缝发育程度不同,造成了储层平面分布的非均质性;③ 长期的风化作用使得潜山上部300 m以内的储集物性得到进一步改善,控制了潜山垂向上储层的非均质性.  相似文献   

江汉平原东北缘末次冰消期沉积物粒度特征及环境意义     

张玉芬  李长安  赵举兴  毛欣  许应石  魏传义  李亚伟  张岱 《沉积学报》2021,39(4):983-994

麻城剖面位于江汉平原东北缘山地丘陵与平原过渡带半封闭的宽浅碟形洼地,是江汉平原北缘末次冰消期代表性沉积剖面。沉积环境主要由山麓坡洪积相和沼泽相组成,形成年代为距今18.00~8.60 ka。麻城剖面粒度组成主要以粉砂为主,达60.73%,其中,粗粉砂达40.83%。剖面粒度变化可划分为7个变化阶段。结合剖面的岩性与沉积环境特征等综合分析,发现末次冰消期气候环境经历了四个气候期:第一气候期17.90~17.02 ka B.P.,为冰盛期之后气候开始回暖升温期。此时降水相对较多,地表径流强度中等偏强。第二个时期17.02~12.02 ka B.P.,为快速升温期。总体暖湿多雨,是末次冰消期最显著暖湿期,在江汉平原北缘坡麓地带的洼地广泛发育了沼泽。粒度变化进一步表明气候存在波动,早、晚期降雨强度大,时有强降水发生,地表径流稳定性差;中期（14.47~12.64 ka B.P.）降水明显减少,但雨量相对均匀,地表径流变幅小,此与长江中下游地区石笋记录的冰消期气候特征是一致的。第三气候期12.02~10.80 ka B.P.,该期是新仙女木事件之后的快速升温期,平均粒径和分选系数为剖面最大。表明当时降雨量大,气候稳定性较差,强降雨时有发生。第四气候期10.80~8.60 ka B.P.,气候总体为偏温湿环境。地表径流较第三气候期明显降低,早期降水偏多,后期温湿程度降低,地表径流变小。剖面中有两个明显的气候事件记录,一是新仙女木事件,表现为第④层与第⑤层之间的短暂沉积间断和沼泽环境的结束;二是9.8~9.2 ka B.P.干旱期,为厚约15 cm的黄土状土的沉积。研究表明,长江和汉江洪泛影响不到的江汉平原边缘洼地沉积可能是记录本地区气候特征的良好载体。  相似文献   

四川盆地喜马拉雅期张扭性断裂构造特征及形成机制          下载免费PDF全文

苏楠  杨威  苑保国  戴鑫  王小丹  武赛军  马石玉  谢武仁  张光武  裴仰文 《地球科学》2021,46(7):2362-2378

前人对四川盆地盆缘冲断带及川中高-磨地区断裂系统进行了系统性研究,但对于盆地其他区块断裂发育特征、形成演化及应力背景的认识尚不充分.发现了梓潼-成都-威远-华蓥山-广安地区震旦系-下三叠统地层中发育的一套区域性张扭性断裂,主要自震旦系及以下地层向上延伸切穿二叠系地层,多为高陡、小断距正断层,部分形成负花状构造.根据该断裂系统的剖面产状、纵向穿层特征及盆地范围内体现出的分异性,推断该断裂系统形成于喜马拉雅期.物理模拟实验研究发现该断裂体系应发育于扭张性应力环境中,该断裂体系的发育指示了四川盆地内部新生代存在南西-北东向张应力,可能与四川盆地新生代发生的逆时针旋转有关.   相似文献   

无人机遥感技术在甘肃北山地区地质填图中的应用     

戴均豪  薛林福  李忠潭  桑学佳  马建雄 《吉林大学学报(地球科学版)》2021,51(6):1908-1920

甘肃北山地区岩浆岩及变质岩出露广泛,植被稀少,地势平缓,是无人机遥感开展地质填图试验的理想目标区。为解决传统地质填图方法受地形、环境限制和投入高及工作周期长等问题,选取北山长流水地区20 km²化探重点工作区为目标区,利用大疆精灵4专业版无人机采集图像,并采用Photoscan软件合成高分辨率正射影像以及三维模型,建立解译标志,对目标区域进行地质解译,获得比前人1：1万地质图更加精细的地质图。该方法较传统填图方法能够解译出地质体形态、岩脉产状、微小断裂等更详细的地质内容,并能提供划分岩脉期次的证据。  相似文献   

单多普勒雷达反演涡旋风场方法分析     

常亚楠  戴建华  黄兴友  郑石 《气象科技》2019,47(5):719-730

由于单雷达在探测台风临近、登陆时仍具有双部或多部雷达难以具备的优势,目前利用单多普勒雷达反演涡旋风场的方法并不多见,本文对能够反演涡旋风场的3种方法NIVAP(自然坐标系的积分VAP方法)、EVAPTC(反演热带气旋的EVAP方法)、GBVTD(地基速度追踪法)进行了比较,分别从每种方法的假设条件和数学方法以及结果精度来比较3种方法对涡旋风场的适用性。利用中尺度气旋Rankine模式模拟了纯涡旋性气旋、只有环境风场时、环境风场和辐合(辐散)同时存在时的径向速度场及风场,比较了各种情况下的反演风场和模拟风场的相似系数。在对模拟风场比较之后,选取了2014年"威马逊"台风登陆前1h、登陆时及登陆后1h的3个实测雷达资料,对比分析了3种方法反演实测资料的风场特征,3种方法中EVAPTC方法最好,GBVTD方法在应用中有一定的限制,NIVAP方法较差。  相似文献   

重庆市乡村义务教育资源可得性差异及影响因素     

王世琼  刘睿  戴技才  陈希娅  胡秋宁 《热带地理》2022,42(8):1349-1362

以国家统筹城乡教育发展试验区的重庆市乡村地区为例,采用Densi-Graph城乡识别法进行行政区划内部格网尺度的精准识别,在此基础上探究义务教育资源可得性水平的空间差异及影响因素。结果表明：1)基于Densi-Graph城乡识别法的重庆市城乡差异显著,乡村区域面积达79 752 km²,占区域总面积的96.82%。城市部分面积为2 616 km²,占区域总面积的3.18%。2)重庆市义务教育资源可得性水平具有显著的空间集聚与关联特征,并呈现由中心城区向外递减的格局。3)政府重视程度、社会经济发展水平、交通设施水平、自然地理环境均与义务教育资源可得性水平具有显著相关性。4)交通站点密度与教育投入是影响义务教育资源可得性水平的主导因子,二者与各项因子的交互叠加作用增强效果显著;影响机制上,地形起伏度与坡度是乡村义务教育资源可得性水平的限制条件,社会经济是基础,交通是关键,教育投入是核心。  相似文献   

空间正义视角下乡村权的概念框架与比较研究     

戴燕归  周恺 《热带地理》2022,42(8):1314-1323

“乡村权”概念是近年空间正义研究在乡村地域进行理论延伸与实践检验的新领域,其通过揭示以土地为空间媒介的乡村权利剥夺现象,批判性分析乡村空间资源分配和空间生产过程中出现的非正义问题。延续激进地理学视角,文章聚焦“乡村土地”与“村民身份”两大问题,基于突尼斯、印度、黎巴嫩等南方国家,以及英国、法国、意大利等北方国家的案例,揭示村民在生产、生活及意识层面所处的非正义境况（即“被迫非农化”）,并探寻村民为寻求空间正义所采取的个人或集体抗争方式（即“自发再农化”）。通过案例比较发现：1）南方国家与北方国家的乡村案例表现出的被迫非农化现象具有共性,其本质都是资本剥削导致的乡村土地和村民身份被剥夺;2）南方和北方国家的自发再农化手段存在差异,南方国家的村民主要采取法律途径或极端方式,北方国家则依托自上而下的国家政策保障与自下而上的地方能力建设。进而,归纳出乡村权三大内涵,即“占有权（拥有农地并进行农业生产）”“定居权（以村民的生活方式生活在乡村）”“参与权（自主决定是否非农化）”。最后,建议在中国乡村振兴中,通过科学知识共享、社区学习与合作等方式提升村民自身能力,实现村民在乡村空间占有、定居、参与的...  相似文献   

<Superscript>234</Superscript>Th/<Superscript>238</Superscript>U disequilibrium and particulate organic carbon export in the northern South China Sea     

Weifang Chen  Pinghe Cai  Minhan Dai  Junfeng Wei 《Journal of Oceanography》2008,64(3):417-428

We utilized ²³⁴Th, a naturally occurring radionuclide, to quantify the particulate organic carbon (POC) export rates in the northern South China Sea (SCS) based on data collected in July 2000 (summer), May 2001 (spring) and November 2002 (autumn). Th-234 deficit was enhanced with depth in the euphotic zone, reaching a subsurface maximum at the Chl-a maximum in most cases, as commonly observed in many oceanic regimes. Th-234 was in general in equilibrium with ²³⁸U at a depth of ∼100 m, the bottom of the euphotic zone. In this study the ²³⁴Th deficit appeared to be less significant in November than in July and May. A surface excess of ²³⁴Th relative to ²³⁸U was found in the summer over the shelf of the northern SCS, most likely due to the accumulation of suspended particles entrapped by a salinity front. Comparison of the ²³⁴Th fluxes from the upper 10 m water column between 2-D and traditional 1-D models revealed agreement within the errors of estimation, suggesting the applicability of the 1-D model to this particular shelf region. 1-D model-based ²³⁴Th fluxes were converted to POC export rates using the ratios of bottle POC to ²³⁴Th. The values ranged from 5.3 to 26.6 mmol C m⁻²d⁻¹ and were slightly higher than those in the southern SCS and other oligotrophic areas. POC export overall showed larger values in spring and summer than in autumn, the seasonality of which was, however, not significant. The highest POC export rate (26.6 mmol C m⁻²d⁻¹) appeared at the shelf break in spring (May), when Chl-a increased and the community structure changed from pico-phytoplankton (<2 μm) dominated to nano-phytoplankton (2–20 μm) and micro-phytoplankton (20–200 μm) dominated.  相似文献   

A modified analytical method for the shipboard determination of nanomolar concentrations of orthophosphate in seawater     

Jian Ma  Dongxing Yuan  Ying Liang  Minhan Dai 《Journal of Oceanography》2008,64(3):443-449

A method for the determination of nanomolar concentrations of orthophosphate in oligotrophic seawater developed by Liang et al. (2007) has been modified to make it fully feasible for shipboard application and for faster sample throughput with minimized sample volume. The technique is based on the flow injection method with solid phase extraction on a Sep-Pak C18 cartridge and colorimetric detector. The Schlieren effect was minimized by rinsing the cartridge sequentially with 5 mL water and 2 mL 95% ethanol solution. With three micro pumps in parallel, savings of up to 80% in amount of reagents and 25% volume of seawater samples could be achieved in comparison to the previous method. Variation of stopped flow time and sample loading time gave 3 different standard curves, which corresponded to 3 linear ranges within 3.4 and 515 nM. The modified method permits the analysis of samples over a wide range of concentrations, and has been successfully applied to shipboard determination of trace orthophosphate in more than 200 seawater samples during a one-month cruise in the South China Sea. For seawater at concentrations of 20.6, 82.5, 206.2 nM orthophosphate, the relative standard deviations (RSD) (n = 6), determined daily for 6 days on board ship were 4.45%, 4.73% and 6.75%, respectively. Five seawater samples collected in the Station SEATS (South East Asia Time Series Station at 18°N, 116°E) were analyzed using the present method both on board and in a land-based laboratory, as well as with the magnesium hydroxide-induced coprecipitation (MAGIC) method, and showed no significant difference according to the statistical t-test.  相似文献