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91.
A technique for identifying non‐visible basaltic tephra‐rich horizons of Younger Dryas (YD)/Greenland Stadial (GS) 1 age in northeast Atlantic sediments using rapid, non‐destructive magnetic measurements is presented. Three high‐resolution marine sediment cores have been studied in an E–W transect across the Hebridean margin: St Kilda Basin (MD95‐2007), Barra Fan (MD95‐2006) and Rockall Trough (MD04‐2822). Magnetic susceptibilities and remanent magnetisations were measured at contiguous 1 cm resolution on bulk sediments. In all three cores, an interval with higher proportions of hard magnetic minerals coincides with a clearly defined peak in basaltic tephra shard (>250 µm) counts, which can be constrained to the early part of the YD/GS1 based on faunal climate proxies. Electron microprobe analyses of the magnetically distinct basaltic tephra interval, in all three cores, displays the same major element geochemistry as published for the Vedde basaltic (I Tab. 1), i.e. sourced from the Icelandic volcano Katla. The identification of transitional alkalic basaltic tephras within marine sediments could potentially be facilitated by magnetic analysis as a useful chronostratigraphic screening tool. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
92.
The upwelling region off northwest Africa is one of the most productive regions in the world ocean. This study details the response of surface‐ and deep‐water environments off Mauritania, northwest Africa, to the rapid climate events of the last deglaciation, especially the Bølling–Allerød (15.5–13.5 ka BP) and Younger Dryas (13.5–11.5 ka BP). A high accumulation rate gravity core GeoB7926‐2, recovered at ~20° N, 18° W, was analysed for the grain size distribution of the terrigenous sediment fraction, the organic carbon content, diatom and benthic foraminifera communities. Humid conditions were observed during the Bølling–Allerød with a high contribution of fluvial sediment input. During the Younger Dryas intensified trade winds caused a larger sediment input of aeolian dust from the Sahara and more intense upwelling with higher primary productivity, as indicated by high diatom concentrations. The abrupt and large increase of organic matter caused low oxygen conditions at the sea floor, reflected by the poor benthic foraminiferal fauna and the dominance of the low‐oxygen‐tolerant foraminiferal species Bulimina exilis. This is surprising since low‐oxygen conditions have not been recorded during modern times at the sea floor in this region, despite present‐day intensive upwelling and high primary productivity. After the Younger Dryas, more humid conditions returned, diatom abundance decreased and B. exilis was replaced by typical deep‐sea species as found in the region today, indicating the return of more oxygenated conditions at the sea floor. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
93.
Clarification of the temporal relationships amongst records of environmental change is dependent on accurate timescales. Event markers such as tephra layers are extremely important for constraining chronologies and providing tie points. In this report we present evidence of a previously unknown early Holocene Icelandic cryptotephra from a lake in northern Scotland—the ‘An Druim Tephra’. The calibrated radiocarbon age of 9560 cal. yr BP for this new cryptotephra makes it an important addition to the suite of cryptotephras now recorded from the last glacial termination and early Holocene in northwest Europe. In addition we report evidence in support of a ‘Younger Borrobol Tephra’ from Lateglacial sediments of Allerød age. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
94.
针对BDS/GPS载波相位定位的病态性问题,该文提出了采用条件数法对比分析BDS/GPS在我国不同地区、不同时段载波相位定位病态性及其在8个地面站的定位精度因子PDOP值变化情况,其中PDOP值能够反映单点定位的病态性。研究结果表明:BDS在我国大部分地区可用性良好,且其平均PDOP值随纬度的增大而线性增加;然而,BDS导航定位的病态性程度比GPS更为严重,BDS载波相位定位病态性程度不仅随观测时长变化,还与所处的位置变化有关,而GPS几乎不受观测地点的影响。研究成果可在BDS/GPS模糊度的快速、准确固定方面提供参考。 相似文献
95.
着重介绍了GPS数据的单历元解算方法,以及监测点变形量的分析和预测,探讨了这种方法在变形监测系统数据处理中的应用情况及数据相关性对变形精度的影响。阐述了对变形量进行分析和预测的各种方法,提出用整体方差法对变形量进行分析,并用卡尔曼滤波方法对变形值进行预测,试算结果表明这些方法是可行的。最后,设计了一套GPS自动变形监测系统,包括软件功能设计、数据库实现等,并对系统安全性和可靠性进行了分析。 相似文献
96.
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98.
The inflow of Atlantic Water to the Nordic seas from mid–late Younger Dryas to earliest Holocene (12 450–10 000 a BP) is reconstructed on the basis of a high‐resolution core (LINK14) from 346 m water depth on the east Faroe shelf. We have analysed the distribution of planktic and benthic foraminifera, stable isotopes and ice‐rafted debris (IRD), and calculated absolute temperatures and salinities by transfer functions. During the investigated time period there was almost continuous inflow of Atlantic Water to the Nordic seas. Deposition of IRD during the mid–late Younger Dryas and Pre‐Boreal coolings indicates the presence of melting icebergs and that summer sea surface temperatures were low. The east–west temperature gradient across the Faroe–Shetland Channel was much steeper than today. The cold conditions around the Faroe Islands are attributed to stronger East Greenland and East Icelandic currents than at present. The near‐continuous inflow of Atlantic Water is consistent with published evidence suggesting that deep convection took place in the Nordic seas, although the convection sites probably had shifted to a more easterly position than at present. Around the time of deposition of the Saksunarvatn Tephra c. 10 350 a BP, sea surface temperatures increased to the present level. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
99.
新仙女木事件在热带湖光岩玛珥湖的记录 总被引:12,自引:6,他引:12
湖光岩玛珥湖沉积物总有机碳,总氮,总氢,生物硅等有机地球化学指标以及AMS^14C年代明确地指示了新仙女木事件在中国南方地区的存在,它不仅表现为降水的显著减少,而且表现为气温的降低,显示了凉干的气候环境,反映了季风系统在很短的时间内进行了大规模重组,夏季风强度显著减弱的事实,这为西北太平洋低续度区存在YD事件提供了新的来自于陆地的证据。 相似文献