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31.
R. A. Reyment 《Cretaceous Research》1980,1(4):299-327
The first trans-Saharan epicontinental transgression began in the Late Cenomanian and peaked in the Early Turonian. The sea entered through rifts (Nigeria) and subsiding areas (north Africa) and, at maximum extent, spilled out onto stable craton. The central Sahara was land in late Early Turonian time; the sea seems to have remained in the central part of the Benue rift, to sally forth again during the minor Coniacian transgressive pulse. The Coniacian transgression went no further than eastern and central Niger Republic. For a short time during the Cenomanian-Turonian link-up between the Tethys and the South Atlantic, some organisms (mainly ammonites and pelecypods) passed across the Sahara. The Coniacian marine deposits of eastern Niger contain several elements typical of the Mungo River Formation of Cameroun, and coastal Nigeria. These faunistic indications imply that the topographical obstacle formed by the Zambuk ridge of northeastern Nigeria was swamped at maximum transgression. A third, very extensive epicontinental transgression began in latest Campanian time and reached its acme in the Early Maastrichtian. In the central Sahara, at least, there was a retreat of the sea in later Maastrichtian times. A final transgression peaked during the Paleocene, after which the sea withdrew permanently from the northwest African hinterland. The same marine ostracod associations occur in the Paleocene of coastal Nigeria, the Sokoto embayment, Mali and Libya, thus providing strong evidence of a marine connexion, albeit brief, between the South Atlantic and the Tethys, despite the lack of outcrop evidence in the crucial Niger valley region. 相似文献
32.
Amar Messaïtfa 《Environmental Geology》2008,55(2):377-383
In the Algeria South, the dental fluorosis is a “silent” epidemic spreading within the populations. The present study was
aimed to determine the fluoride content in water, dates and tea that are widely consumed. The results reveal that 35% of water
wells had excessive fluoride levels (>1.5 mg/L). The eastern areas where dental decay is the most widespread pathology present
the highest contents. Dates, tea and water are considerable sources of fluoride, with a daily contribution of 10, 20 and up
to 70%, respectively. For these three sources, the daily intake of fluoride ingested by an adult exceeds the proposed safe
threshold of 0.05–0.07 mg/kg day. 相似文献
33.
基于91个气象站观测值和大气环流模型CCSM3,对南岭同纬度典型区域的气候特征参数进行分析,并对未来不同区域的温度和降水进行预测,进一步探索净初级生产力对温度和降水的敏感性,并分析导致南岭同纬度带典型区域气候差异的可能原因。结果表明：1）南岭和同纬度其它区域呈现不同的干旱期和湿润期,撒哈拉沙漠、阿拉伯半岛沙漠和塔尔沙漠仅存在干旱期,墨西哥荒漠和南岭均存在湿润期,但两者湿润期出现的季节有差异。南岭在植物生长的春季和夏季雨热同期,而墨西哥荒漠雨水充沛的季节多在秋季。墨西哥除湿润期外,一年有两次干旱期,其中冬旱持续的时间较长。2）撒哈拉沙漠、阿拉伯半岛沙漠和塔尔沙漠的潜在蒸散、风速和日照百分率均高于墨西哥荒漠和南岭,但降水正好相反。墨西哥荒漠干季的水汽压与撒哈拉沙漠和阿拉伯半岛沙漠接近,低于该区域湿季及塔尔沙漠与南岭的值。3）在B1气候情景下,对2000―2099年5个阶段（每20年为一个阶段）与1981―2010年的温度和降水数据进行T-test检验发现,南岭同纬度5个典型区域的未来温度均呈极显著上升趋势（P＜0.001）,降水总体呈增加趋势,然而在不同阶段和区域也存在不同比例的显著减少和显著增加情况。4）通过比较5个区域是否存在温度或降水成为潜在生产力限制因子发现,南岭及同纬度的其它区域均为降水限制因子区域。其中,撒哈拉沙漠和阿拉伯半岛沙漠对降水的敏感性显著高于墨西哥荒漠、塔尔沙漠和南岭。5）南岭与同纬度其它区域气候迥异的原因除气候变化外,还包括海陆位置、人类活动干扰的强弱、地形地貌特征等因素。 相似文献
34.
J.C. BritoS. Fahd P. GeniezF. Martínez-Freiría J.M. PleguezuelosJ.-F. Trape 《Journal of Arid Environments》2011,75(11):1029-1037
North-West Africa is an outstanding region to study biogeographic patterns in biodiversity distribution. This study identifies biogeographic affinities and areas of probable occurrence for seven viperid snakes through the combination of high resolution presence data and environmental factors. Vipers exhibited distinct biogeographical affinities: Bitis arietans was mostly found along savannahs, Echis leucogaster along the Sahel/savannahs, Cerastes cerastes and C. vipera throughout most desertic areas, Daboia mauritanica in coastal and hilly regions and Vipera latastei and V. monticola in almost only mountains. Suitable habitats were predicted for B. arietans in southern Senegal and Mali, and coastal southern Morocco, for E. leucogaster in southern Mauritania, Senegal, Mali, and fragmented habitats in Saharan mountains and south-western Morocco, for C. cerastes from the Atlantic coast to northern Algeria, for C. vipera in the central region, for D. mauritanica in northern-western Morocco and northern Algeria, for V. latastei in the Rif, Middle Atlas and coastal Morocco, and for V. monticola in the High Atlas. In potential sympatric areas, competition is probably limiting distribution, resulting in parapatric ranges. Northern Saharan populations of B. arietans and E. leucogaster are isolated. Saharan mountains and coastal south-western Morocco constitute isolated suitable areas for sub-Saharan relicts. 相似文献
35.
We describe in this paper a traditional charcoal making technology occasionally adopted by Tuareg people in the hyper-arid central Sahara (SW Libya). This methodology (called esed) has been identified thanks to ethnobotanic and ethnographic interviews with people living in the Tadrart Acacus massif (the kel Tadrart Tuareg) and confirmed by macroscopic and micromorphological analyses of residual field evidence. Esed consisted of burning in situ trunks of dead Acacia tortilis (Forssk.) Hayne; subsequently, small portions of collapsed and still-burning wood were buried with sand in an attempt to ensure combustion under anoxic conditions. Our interviews elucidated that the esed technology is well known by people living in the region and has possibly been practiced for a long time. Surprisingly, local informants highlighted that esed was employed to made charcoal only during years marked by abrupt reduction of precipitation and consequently of natural resources. The last employment of esed dates to the dry phase of the mid-1970s, when charcoal was used as an exchangeable good. The discovery of this charcoal making technology shed new light on the capability of Tuareg to manage the few natural resources available in marginal environments and to cope with rapid climate changes. 相似文献
36.
37.
Endorheic basins, filled by a rhythmic stratification of fluvio-lacustrine sediments, are useful archives for studying the interaction of palaeoenvironment and prehistoric development. Archaeological evidence indicates an occupational maximum during the Early to Mid-Holocene humid phase that can be linked to geochronological dates. A wadi has cut these sediments and a wadi profile of about 6 m was sampled for luminescence dating.
Palaeodose estimates are based on single aliquot measurements of the sand-sized quartz fraction using the regenerative dose technique. The comparison of equivalent dose results from different aliquot sizes (<100 and >1000 grains) provides interesting insights into heterogeneous degrees of bleaching. Problems arising from substantial variations in environmental dose rates of such heterogeneous sediments are discussed.
The luminescence ages indicate two major phases of deposition, both of Holocene age. The lower 2 m of sediments were deposited about 9400 years ago. The sedimentation of the upper 2 m started about 7200 and terminated around 6300 years ago. Radiocarbon ages of charcoals contained in the upper part of the section disagree with that OSL dating. This discrepancy is explained by re-deposition of the charcoal fragments. 相似文献
38.
E. Klitzsch 《International Journal of Earth Sciences》1994,83(3):475-483
The major contributions to our knowledge of the geological history of the East Saharan countries are described. Much work has been published on the history and progress of geographical, socioeconomic and archaeological exploration in North Africa. Little attention, however, has been paid to the question of how our knowledge of the geology of this vast area was developed. Thus, even though progress in geology is vital to the development of Saharan countries, little has been recorded of this progress. Without an understanding of regional geology and without a good knowledge of groundwater, mineral and petroleum potentials, these countries would be in very different situations today. The author has, over the last 35 years, been involved in scientific research and exploration of petroleum, mineral and groundwater resources in Libya, Egypt, Sudan and Chad. 相似文献
39.
An approach combining the use of water dissolved chemical species and isotopic fingerprints has been used to understand the behavior of a phreatic aquifer and to determine the origin of its different water components. This aquifer is located in the large sedimentary basin of the Great Oriental Erg (Algeria) and overlies two deeper aquifers: the Complexe Terminal (CT) and the Continental Intercalaire (CI). Besides the deterioration of its groundwater quality, its water table has risen during the last 20 years. A water budget surplus between 950 and 2500 l s?1 was estimated. Down-gradient groundwater evolution (south-north) has shown that the mineralisation increases from 1.23 to 5.20 g l?1 due to evaporite minerals dissolution. Chemical and isotopic data demonstrated that in addition to rainfall there is a contribution from the CT and CI aquifers. The latter are tritium-free and less mineralized than the phreatic aquifer. Their radiocarbon contents are very low (<10 pmC, percent modern Carbon) (Pleistocene recharge) whereas quite the contrary is observed for the superficial aquifer which exhibits fairly high and variable C-14 activities (50–100 pmC), evidence of recent recharge. On the basis of tritium contents, two groundwater groups were identified for the phreatic aquifer. 相似文献
40.
撒哈拉沙漠东北部全新世气候环境与人类活动 总被引：2，自引：0，他引：2
现代极端干旱的撒哈拉沙漠东北部,在12000 aB.P.前出现了代表湿润气候记录的湖相沉积;全新世以来,在9900~2400 aB.P.形成了大面积的连续湖泊,并在9800~6450 aB.P.达到鼎盛时期。在6000 aB.P.~3600 aB.P.前,湖泊演化进入波动期,反映了该时期气候环境周期性的变动并具600年的准周期。撒哈拉沙漠东北全新世湿润时期,是该区古人类发展与进步、繁演与分化的时期;也是自然演变史上一个十分重要的时期,其中强烈的风化作用和碎屑物质积累为今日撒哈拉沙漠的形成奠定了物质基础。 相似文献