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71.
Sensitivity of sea ice to wind-stress and radiative forcing since 1500: a model study of the Little Ice Age and beyond 总被引:3,自引:2,他引:1
Three different reconstructed wind-stress fields which take into account variations of the North Atlantic Oscillation, one
general circulation model wind-stress field, and three radiative forcings (volcanic activity, insolation changes and greenhouse
gas changes) are used with the UVic Earth System Climate Model to simulate the surface air temperature, the sea-ice cover,
and the Atlantic meridional overturning circulation (AMOC) since 1500, a period which includes the Little Ice Age (LIA). The
simulated Northern Hemisphere surface air temperature, used for model validation, agrees well with several temperature reconstructions.
The simulated sea-ice cover in each hemisphere responds quite differently to the forcings. In the Northern Hemisphere, the
simulated sea-ice area and volume during the LIA are larger than the present-day area and volume. The wind-driven changes
in sea-ice area are about twice as large as those due to thermodynamic (i.e., radiative) forcing. For the sea-ice volume,
changes due to wind forcing and thermodynamics are of similar magnitude. Before 1850, the simulations suggest that volcanic
activity was mainly responsible for the thermodynamically produced area and volume changes, while after 1900 the slow greenhouse
gas increase was the main driver of the sea-ice changes. Changes in insolation have a small effect on the sea ice throughout
the integration period. The export of the thicker sea ice during the LIA has no significant effect on the maximum strength
of the AMOC. A more important process in altering the maximum strength of the AMOC and the sea-ice thickness is the wind-driven
northward ocean heat transport. In the Southern Hemisphere, there are no visible long-term trends in the simulated sea-ice
area or volume since 1500. The wind-driven changes are roughly four times larger than those due to radiative forcing. Prior
to 1800, all the radiative forcings could have contributed to the thermodynamically driven changes in area and volume. In
the 1800s the volcanic forcing was dominant, and during the first part of the 1900s both the insolation changes and the greenhouse
gas forcing are responsible for thermodynamically produced changes. Finally, in the latter part of the 1900s the greenhouse
gas forcing is the dominant factor in determining the sea-ice changes in the Southern Hemisphere.
相似文献
Jan SedláčekEmail: |
72.
At least three sets of moraines mark distinct glacial stands since the last glacial maximum (LGM) in the Three Sisters region of the Oregon Cascade Range. The oldest stand predates 8.1 ka (defined here as post-LGM), followed by a second between ∼ 2 and 8 ka (Neoglacial) and a third from the Little Ice Age (LIA) advance of the last 300 years. The post-LGM equilibrium line altitudes were 260 ± 100 m lower than that of modern glaciers, requiring 23 ± 9% increased winter snowfall and 1.4 ± 0.5°C cooler summer temperatures than at present. The LIA advance had equilibrium line altitudes 110 ± 40 m lower than at present, implying 10 ± 4% greater winter snowfall and 0.6 ± 0.2°C cooler summer temperatures. 相似文献
73.
John Dodson Xiaoqiang Li Ming Ji Keliang Zhao Xinying Zhou Vladimir Levchenko 《Quaternary Research》2009,(3):309-314
Understanding of the origin and development of bronze technology in eastern Asia remains unresolved. Here we report on the distribution of copper and associated cations in sediments from Huoshiliang in northwestern Gansu, China, strontium and lead isotope analyses of ore and slag samples, and some artifact fragments at archaeological sites at Ganggangwa and Huoshiliang in the Black River valley.We conclude that bronze production began perhaps as early as 2135 BC and that the Baishantang modern mine site at Dingxin was a possible source of copper ore. There was at least one other, but currently unidentified, source of ore. The Bronze Age people were also farmers and planted cereals such as wheat, and they may have abandoned the region when wood was exhausted and desertification took over. 相似文献
74.
古/中元古代界线:1.8Ga 总被引:1,自引:0,他引:1
年代地层表是我们描述地球历史演化的时间框架,也隐含着我们对地球演化过程的基本认识,承载着一系列核心科学问题。现有的国际前寒武纪地质年表存在不少问题,还没有被普遍接受的新方案。在现行国际地层年表(IUGS 1989—2004)中,20~18亿年被称为造山纪,18~16亿年被称为固结纪,16~14亿年被称为盖层纪,也即:造山运动结束到盖层发育之间的过渡时期为固结纪,而将盖层的广泛发育作为中元古代的开始。在我国,由于"吕梁运动"是华北克拉通结晶基底最终形成的标志性构造-热事件,此后发育以长城系-蓟县系-青白口系为代表的地台型沉积盖层,因此我国地质界一直以长城系的底界代表中元古代的开始,并根据"吕梁运动"结束时间,将古/中元古代的时间界线置于18亿年。2012年国际地层委员会提出了一份全新的全球前寒武纪地质年代表划分建议方案,其中2060~1780Ma阶段被称为古元古代哥伦比亚纪,主要以Columbia超大陆的聚合为特征;而之后17. 8~8. 5亿年的近10亿年间,则被定义为罗迪尼亚纪,代表了从Columbia超大陆裂解到Rodinia超大陆聚合的漫长阶段。即这一新建议方案的古/中元古代界线为17. 8亿年,与我国学者长期以来所坚持的古/中元古代分界是基本一致的。在华北克拉通,"吕梁运动"的结束时间在~18亿年,此后整体处于多期裂解的陆内伸展环境,长城系-蓟县系基本上属于连续的陆表海沉积。近年来的研究表明,燕山地区长城系的底界年龄约为17亿年,长城系与蓟县系的分界则为16亿年。如果一味按照现行国际地层划分方案,其古/中元古代的界线(16亿年)将对应于长城系-蓟县系的分界,华北克拉通这套盖层型沉积将被人为分割为两部分,这显然是很不合理的。值得注意的是,在华北克拉通中部吕梁地区发育的小两岭组火山岩和在其南部地区发育的、也是世界范围内同时期最大规模的火山活动——熊耳群火山-沉积岩系,是华北克拉通结晶基底之上最早发育的盖层沉积,其起始形成时间约为18亿年。这与新建议的古/中元古代分界非常接近。从全球地质演化来看,从18到16亿年,造山作用结束,Columbia超大陆开始裂解,岩浆作用方式和岩浆岩组合类型及其地球化学特征发生明显改变,如斜长岩、环斑花岗岩在世界主要克拉通均有发育。与此同时,稳定沉积盖层开始广泛发育,条带状铁建造(BIF)消失,代之以鲕状或粒状矿物集合体组成的浅海富铁沉积,另外海相硫化物沉积、有核原生生物等也首次出现。所有这些都标志着,在此前后,地球岩石圈、水圈和大气圈均发生了重大转折,生物圈也进入新的演化阶段,标志着地球进入"中年期"演化阶段。虽然地球演化发生重要转折的根本原因和细节还有待进一步深入探讨,但这一转变的起点或最重要的时间点,无疑就在18亿年前后,因此,本文认为,应将其作为全球古/中元古代的时间界线。 相似文献
75.
Extensional and compressional structures are globally abundant on Mars. Distribution of these structures and their ages constrain the crustal stress state and tectonic evolution of the planet. Here in this paper, we report on our investigation over the distribution of the tectonic structures and timings of the associated stress fields from the Noachis-Sabaea region. Thereafter, we hypothesize possible origins in relation to the internal/external processes through detailed morphostructural mapping. In doing so, we have extracted the absolute model ages of these linear tectonic structures using crater size-frequency distribution measurements, buffered crater counting in particular. The estimated ages indicate that the tectonic structures are younger than the mega impacts events(especially Hellas) and instead they reveal two dominant phases of interior dynamics prevailing on the southern highlands, firstly the extensional phase terminating around3.8 Ga forming grabens and then compressional phase around 3.5-3.6 Ga producing wrinkle ridges and lobate scarps. These derived absolute model ages of the grabens exhibit the age ca. 100 Ma younger than the previously documented end of the global extensional phase. The following compressional activity corresponds to the peak of global contraction period in Early Hesperian. Therefore, we conclude that the planet wide heat loss mechanism, involving crustal stretching coupled with gravitationally driven relaxation(i.e.,lithospheric mobility) resulted in the extensional structures around Late Noachian(around 3.8 Ga). Lately cooling related global contraction generated compressional stress ensuing shortening of the upper crust of the southern highlands at the Early Hesperian period(around 3.5-3.6 Ga). 相似文献
76.
中世纪暖期和小冰期是近2 000年气候变化中重要的气候事件,厘清其在低纬度地区干湿变化特征及其对古代农业发展及人口增长的影响,具有重要的意义。通过对广西柳州岩溶洞穴中一60 cm岩芯进行孢粉、炭屑分析发现,研究区的气候、稻作农业从唐末以来先后经历了6个阶段演化:1)875―940 a A.D.,C/P值及花粉记录指示:研究区气候整体偏干,水稻种植减少;2)940―1 050 a A.D.,C/P值及花粉记录指示:夏季风增强,水稻种植稍增长,人口增加;3)1 050―1 140 a A.D.,C/P值及花粉记录指示:夏季风减弱,稻作农业及人口较前期有所增长;4)1 140―1370 a A.D.,C/P值及莎草科花粉含量指示:研究区当时气候处于“中世纪暖期”温暖湿润期,稻作农业面积扩大,人口增加;5)1 370―1 900 a A.D.,C/P值及莎草科花粉含量指示:“小冰期”气候变干,水稻种植及人口逐渐减少;6)1 900―2 009 a A.D.气候变暖湿,水稻种植扩大,人口迅速增长。进一步对研究区气候事件与稻作农业及人口关系研究发现:历史时期气候事件与水稻种植、人口具有耦合关系,即中世纪暖期,气候暖湿,水稻种植扩大,人口增殖;小冰期,气候冷干,水稻种植萎缩,人口锐减。在非原地沉积时,莎草科和C/P 对水分反映敏感,可以用来指示气候干湿变化。 相似文献
77.
吉林晚石炭世要期石头口门裂陷槽位于长春东50km。该裂陷槽以海底火山沉积--硅质岩沉积为主。早期以含锰结核的碎屑岩和泥岩、硅质岩为主,并有少量的凝灰碉和玄武岩;中期以海底火山喷发的高钠质细碧岩、角地为主,并伴有规模不大的蛇纹岩;晚期以硅质岩、长石杂砂岩、泥岩、生物碎屑灰岩为主,在生物碎屑灰岩中首次发现Fusulina lanceolata F.sp.、Pseudostaffella khotune 相似文献
78.
Richard G. Klein Kathryn Cruz-Uribe David Halkett Tim Hart John E. Parkington 《Quaternary Research》1999,52(3):393
Boegoeberg 1 (BOG1) is located on the Atlantic coast of South Africa, 850 km north of Cape Town. The site is a shallow rock shelter in the side of a sand-choked gully that was emptied by diamond miners. Abundant coprolites, chewed bones, and partially digested bones implicate hyenas as the bone accumulators. The location of the site, quantity of bones, and composition of the fauna imply it was a brown hyena nursery den. The abundance of Cape fur seal bones shows that the hyenas had ready access to the coast. Radiocarbon dates place the site before 37,000 14C yr ago, while the large average size of the black-backed jackals and the presence of extralimital ungulates imply cool, moist conditions, probably during the early part of the last glaciation (isotope stage 4 or stage 3 before 37,000 14C yr ago) or perhaps during one of the cooler phases (isotope substages 5d or 5b) within the last interglaciation. Comparisons of the BOG1 seal bones to those from regional Middle Stone Age (MSA) and Later Stone Age (LSA) archeological sites suggest (1) that hyena and human seal accumulations can be distinguished by a tendency for vertebrae to be much more common in a hyena accumulation and (2) that hyena and LSA accumulations can be distinguished by a tendency for hyena-accumulated seals to represent a much wider range of individual seal ages. Differences in the way hyenas and people dismember, transport, and consume seal carcasses probably explain the contrast in skeletal part representation, while differences in season of occupation explain the contrast in seal age representation. Like modern brown hyenas, the BOG1 hyenas probably occupied the coast year-round, while the LSA people focused their coastal visits on the August–October interval when nine-to-eleven-month-old seals were abundant. The MSA sample from Klasies River Mouth Cave 1 resembles BOG1 in seal age composition, suggesting that unlike LSA people, MSA people obtained seals more or less throughout the year. 相似文献
79.
Samuli Helama Jianmin Jiang Johanna Korhonen Jari Holopainen Mauri Timonen 《地理学报(英文版)》2013,23(6):1069-1079
Cryophenological records (i.e. observational series of freeze and breakup dates of ice) are of great importance when assessing the environmental variations in cold regions. Here we employed the extraordinarily long observational records of river ice breakup dates and air temperatures in northern Fennoscandia to examine their interrelations since 1802. Historical observations, along with modern data, comprise the informational setting for this analysis carried out using t-test. Temperature history of April-May season was used as cli- matic counterpart for the breakup timings. Both records (temperature and breakup) showed seven sub-periods during which their local means were distinctly different relative to preced- ing and subsequent sub-periods. The starting and ending years of these sub-periods oc- curred in temporal agreement. The main findings of this study are summarized as follows: (1) the synchrony between the temperature and river ice breakup records ruled out the possibility that the changes would have occurred due to quality of the historical series (i.e. inhomoge- neity problems often linked to historical time-series); (2) the studied records agreed to show lower spring temperatures and later river ice breakups during the 19th century, in comparison to the 20th century conditions, evidencing the prevalence of cooler spring temperatures in the study region, in agreement with the concept of the Little Ice Age (1570-1900) climate in North-West Europe; (3) the most recent sub-period demonstrate the highest spring tem- peratures with concomitantly earliest river ice breakups, showing the relative warmth of the current springtime climate in the study region in the context of the past two centuries; (4) the effects of anthropogenic changes in the river environment (e.g. construction and demolition of dams) during the 20th century should be considered for non-climatic variations in the breakup records; (5) this study emphasizes the importance of multi-centurial (i.e. historical) cryo- phenological information for highly interesting viewpoints of climate and environmental his- tory. 相似文献
80.
小冰期福建海岸沙丘的沉积环境 总被引:2,自引:2,他引:0
选取福建东南沿海典型海岸沙丘——东海沙丘,运用GPR(Ground Penetrating Radar)技术探测海岸沙丘内部构造,辅助天然剖面和人工探槽观测,进行系统的年代采样与OSL(Optically Stimulated Luminescence)测年,并选择物理意义明确的粒度参数作为主要环境代用指标,获得了亚热带季风区砂质海岸带典型海岸沙丘的沉积年代和沉积结构。结果表明:(1)东海沙丘是由若干灌丛沙丘组成的横向沙丘,0~6 m主要是~0.45 ka BP以来形成的海岸风成沙丘,可以划分为~0.45 ka BP和0.22~0.12 ka BP两个阶段。(2)东海沙丘沉积过程是多因素共同作用的结果,冷干的气候环境是沙丘发育的适宜条件,入境台风频率增加可能加快了风沙的沉积过程。对比历史文献资料发现,气候异常期往往与社会动荡期、重要历史事件相对应。 相似文献