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341.
Paul Tréguer 《Comptes Rendus Geoscience》2002,334(1):3-11
Silicon is a key element whose major role in the control of the cycle of carbon in the ocean has been recently revealed. This is first illustrated through the resolution of the ‘opal paradox’ in the modern Southern Ocean. Second, the ‘silica hypothesis’ explains why, during the Last Glacial Maximum, the atmospheric concentration of CO2 was about 40 % below that of the interglacial period. Increased deliveries of wind-borne silica to the surface ocean as well as of riverine inputs favoured the dominance of diatoms over coccolithophorids, resulting in a net emission decrease of CO2 by coccolithophorids from the ocean surface to the atmosphere. To cite this article: P. Tréguer, C. R. Geoscience 334 (2002) 3–11 相似文献
342.
Glacial mélange in the open-cast mine at Amsdorf, central Germany, consists of several square meters of large, sorted sediment blocks embedded in till. The blocks are composed of largely intact to slightly deformed glaciofluvial and glaciolacustrine sand, silt and clay, initially deposited in a proglacial lake (2–3 km up-ice) and subsequently overridden by a glacier. The blocks typically have cuboid to subrounded outlines, are randomly distributed in the till, and the contacts with the surrounding till are distinctly sharp. Underneath the mélange are varved clays which exhibit strong deformations occasionally intervening with entirely undisturbed areas. It is suggested that the blocks were entrained into debris-rich basal-ice by bulk freeze-on when the glacier sole was lowered onto the bottom of an overridden lake. After entrainment the blocks were transported englacially and re-deposited (with far-traveled till matrix) as a melt-out till from stagnant ice. The glacier moved mainly by sliding enhanced by low-permeability varved clays in the substratum. The glacier is believed to have been of a polythermal type. These results show that bulk freeze-on can lead to entrainment of soft sediment blocks at least 20 m2 in size, and that these blocks can be englacially transported with little or no deformation for several kilometers and more. The occurrence of deformed and undeformed clays under the till mélange indicates a possible mosaic of coupled and decoupled ice, the latter caused by a thin, transient subglacial water film separating the bed from the glacier. 相似文献
343.
来自东亚夏季风北界内蒙古一个盐湖的岩心沉积物内发现粗粒沉积物 ,既可沉积在湖泊收缩期 ,也可以沉积在沙丘再次活动时期。这些沉积物有机磷含量低而碎屑组分高 ,说明它沉积在一种干旱的环境中 ;反之 ,有机质含量高而碎屑组分低的细粒沉积物则最终沉积在高湖面期 ,这是因为砂坪呈围迁状态及风成尘埃较好地保存于水体中。所揭示的三个湿期分别是 13.4~ 8kaBP、6 .4~ 5 .8kaBP和4.2~ 3.1kaBP ,第一期最湿 ,其次是第三期、第二期。上述干和湿期自始至终和早先发现的同一个干旱和半干旱转换带一致 ,但是 ,这和中国大陆东部、台湾岛、冲绳海槽和中国南海所揭示的湿润的全新世大暖期不一样 ,而且 ,全新世最冷期 (4~ 2kaBP)则对应内蒙古的一个湿期 (即 4.2~ 3.1kaBP) ,这些差别可能是由于高蒸发量远远地超过了较高的季风降水量 ,这是确定夏季风北界在该地区有效湿度的一个关键标志。 相似文献
344.
贡嘎山地区全新世的冰川变化、气候变化与河谷阶地发育 总被引:11,自引:0,他引:11
本区冰川变化与河谷阶地发育关系密切,全新世初气候转暖,冰川逐步后退,形成第四级河流阶地(T4下部),全新世中期的高温期(7500-4000aBP)冰川强烈消融,形成第四级阶地(T4中上部)<7420±90aBP;全新世晚期气候转冷,新冰期可分出三次冰进:前观景台冰进>3080±80aBP,观景台冰进1550±80-2350±65aBP,海螺沟冰进940±85-780±90aBP,相应出现T3>3000aBP,T21200±70-2170±60aBP,T1930±70aBP,现在小冰期最早出现在400年前,有3次波动,相当于公元1603-1650;1751-1800;1851-1900;三次气候寒冷期。 相似文献
345.
天山中段及祁连山东段小冰期以来的冰川及环境 总被引:22,自引:0,他引:22
本文通过考察、航片判读、及AAR法测算了冰川变化,用地衣、文史资料、及树轮年表进行了小冰期的测年,使用实测资料分析、冰岩芯及树轮资料等估算了小冰期的气温与降水、并进而计算了地表径流的变化。在全部讨论中,还引用了一些试验性的方法和公式。 相似文献
346.
西藏喜马拉雅山区危险冰湖及其溃决特征 总被引:28,自引:1,他引:28
喜马拉雅山区的冰湖溃决主要属终碛湖溃决。野外考察和编目研究的结果表明,终碛湖占各类高山冰湖总数的1/2和总水体的3/4。鉴别出34个危险终碛湖,其水体容量在10×10~6—30×10~6m~3之间,平均水深31m,实测最大水深在70m左右。危险终碛湖绝大多数形成于小冰期最后一次冰退阶段,由于其形态特征和现代冰川的直接影响而具有潜在的溃决危险;讨论了终碛湖溃决的特征,主要因素及其周期性。 相似文献
347.
Derek Ford 《地球表面变化过程与地形》1987,12(5):507-521
Canadian examples suggest that karst landforms may be divided into eight types in terms of their temporal relationships to the record of repeated Quaternary glaciations. Two types are postglacial, two are subglacial, one type occurs where glacial features are adapted to karstic drainage, and three types display sequences of karstic and glacial action. Glacier effects upon karst landforms and their underlying aquifers display the gamut of possibilities. They may destroy, inhibit, preserve, or stimulate karst development. Where continuous permafrost is maintained when covered by glacier ice, postglacial karst is limited to the active layer epikarst. Where permafrost is thawed beneath ice or during deglaciation there are a variety of postglacial karst developments, depending in part upon climate and in part upon local lithologic and relief conditions. 相似文献
348.
Limited modification of mid-latitude landscapes by ice sheets: The case of northeast Scotland 总被引:1,自引:0,他引:1
The paper uses a case study in Scotland to examine the amount and processes of landscape modification by Quaternary ice sheets. There is an inverse correlation between the distribution of landforms of glacial erosion and pre-glacial landscape remnants in northeast Scotland. The implication is that in places ice sheets can preserve a pre-glacial landscape unscathed, while elsewhere they remove the pre-glacial weathered rock. The location of glacial protection or erosion is strongly influenced by the topography and its influence on former ice sheet flow and basal thermal regime. The classic glacially eroded landscape of areal scouring can be produced by the removal of only 10–50 m of weathered rock. Furthermore rock basins, often regarded as the hallmark of glacial erosion, may be directly inherited from the pre-glacial pattern of deep weathering. 相似文献
349.
S. Lorenz B. Grieger Ph. Helbig K. Herterich 《International Journal of Earth Sciences》1996,85(3):513-524
We present atmospheric simulations of three different time slices of the late Quaternary using the ECHAM 3 general circulation
model in T42 resolution. In this work we describe the results of model runs for the time slices 6000 years BP (last climate
optimum), 21 000 BP (last glacial maximum) and 115 000 years BP (glacial inception). Although the solar insolation is known
for all time slices, a complete data set of the other boundary conditions which are necessary for running the atmospheric
model exists only for the last glacial maximum in the form of the CLIMAP reconstruction. For the other two time slices, which
are interglacial states like the modern climate, sea surface temperatures, land albedo and ice sheet topography are kept at
modern values and only the solar insolation is changed appropriately. The response of the model to solar insolation changes
is quite reasonable. The modelled anomalies are small and roughly opposite in sign for 6000 BP and 115 000 BP, respectively.
In the case of last glacial maximum, the glacial ice sheet topography and ice albedo produce a much larger climate anomaly
in the model. However, to enable a real test of model performance under glacial boundary conditions, the CLIMAP sea surface
temperatures, which are now known to be partly inaccurate, should be replaced by an updated “state-of-the-art” reconstruction. 相似文献
350.
采场底板突水相似材料模拟研究 总被引:15,自引:0,他引:15
利用相似材料模拟试验,模拟了采场底板突水机理。实验表明,在无断层构造条件下,底板在承压水作用下呈O-X型破坏,并且在O与X的交点处最容易产生突水通道;在有断层构造条件下,断层突水的实质是在承压水作用下断层两盘的关键层向上产生相对移差。当断层越易产生最大位移差时,越容易突水;在突水的同时还伴随着断层带充填的冲刷。 相似文献