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231.
Paola Ridge, along the NW Calabrian margin (southern Tyrrhenian Sea), is one of the few reported deep sea sites of precipitation of authigenic carbonates in the Tyrrhenian Sea. Here, the changing composition of the seeping fluids and the dynamic nature of the seepage induced the precipitation of pyrite, siderite and other carbonate phases. The occurrence of this array of authigenic precipitates is thought to be related to fluctuation of the sulfate-methane transition zone (SMTZ).Concretions of authigenic minerals formed in the near sub-bottom sediments of the Paola Ridge were investigated for their geochemical and isotopic composition. These concretions were collected in an area characterized by the presence of two alleged mud volcanoes and three mud diapirs. The mud diapirs are dotted by pockmarks and dissected by normal faults, and are known for having been a site of fluid seepage for at least the past 40 kyrs. Present-day venting activity occurs alongside the two alleged mud volcanoes and is dominated by CO2-rich discharging fluids. This discover led us to question the hypothesis of the mud volcanoes and investigate the origin of the fluids in each different domed structure of the study area.In this study, we used stable isotopes (carbon and oxygen) of carbonates coupled with rare earth element (REE) composition of different carbonate and non-carbonate phases for tracing fluid composition and early diagenesis of authigenic precipitates. The analyses on authigenic precipitates were coupled with chemical investigation of venting gas and sea-water.Authigenic calcite/aragonite concretions, from surficial sediments on diapiric structures, have depleted 13C isotopic composition and slightly positive δ18O values. By contrast, siderite concretions, generally found within the first 6 m of sediments on the alleged mud volcanoes, yielded positive δ13C and δ18O values. The siderite REE pattern shows consistent LREE (light REE) fractionation, MREE (medium REE) enrichment and positive Gd and La anomalies. As shown by the REE distribution, the 13C-depleted composition and their association with chemosymbiotic fauna, calcite/aragonite precipitated at time of moderate to high methane flux close to the seafloor, under the influence of bottom seawater. Authigenic siderite, on the other hand, formed in the subseafloor, during periods of lower gas discharges under prolonged anoxic conditions within sediments in equilibrium with 13C-rich dissolved inorganic carbon (DIC) and 18O-rich water, likely related to methanogenesis and intermittent venting of deep-sourced CO2. 相似文献
232.
The temporal and spatial variability of the Yellow Sea Cold Water Mass in the southeastern Yellow Sea, 2009-2011 总被引:1,自引:1,他引:0
The Yellow Sea Cold Water Mass(YSCWM) is one of the important water mass in the Yellow Sea(YS).It is distributed in the lower layer in the Yellow Sea central trough with the temperature less than 10 C and the salinity lower than 33.0.To understand the variability of the YSCWM,the hydrographic data obtained in April and August during 2009–2011 are analyzed in the southeastern Yellow Sea.In August 2011,relatively warm and saline water compared with that in 2009 and 2010 was detected in the lower layer in the Yellow Sea central area.Although the typhoon passed before the cruise,the salinity in the Yellow Sea central trough is much higher than the previous season.It means that the saline event cannot be explained by the typhoon but only by the intrusion of saline water during the previous winter.In April 2011,actually,warm and saline water(T >10 C,S >34) was observed in the deepest water depth of the southeastern area of the Yellow Sea.The wind data show that the northerly wind in 2011 winter is stronger than in 2009 and 2010 winter season.The strong northerly wind can trigger the intrusion of warm and saline Yellow Sea Warm Current.Therefore,it is proposed that the strong northerly wind in winter season leads to the intrusion of the Yellow Sea Warm Current into the Yellow Sea central trough and influenced a variability of the YSCWM in summer. 相似文献
233.
《The Journal of geography》2012,111(1):323-329
Abstract The geopolitics of the Cold War is illustrated by the diplomatic ties between countries, particularly the two superpowers and their respective allies. Specifically, the location of embassies abroad and those hosted by countries are examined to determine what they reflect about larger geopolitical issues. This article grew out of class projects used in introductory human geography courses to teach place name, library, writing, analytical, computer, and cartographic skills. The global patterns of embassy locations for all countries are first examined and then the particular patterns of the United States, the Soviet Union, and NATO and Warsaw Pact countries. A comparison between the location of U.S. and Soviet embassies shows that the Cold War was geographically visible only for “minor” states. The Warsaw Pact countries were ideologically more unified in the location of their embassies than the NATO countries. With the collapse of Cold War geopolitics, embassy locations are already changing, signaling major transformations to a New World Order. Geographers will need to monitor these newly emerging patterns and teach their students how to examine them. 相似文献
234.
The tropopause height and tropopause temperature are sensitive to temperature changes in troposphere and stratosphere. These
are the measures of global climatic variability. Atmospheric profiles of temperature, refractivity and water vapour are always
needed for communication, navigation and atmospheric modeling studies. The tropopause characteristics over the Indian region
have been studied using radio occultation measurements (CHAMP) on the basis of cold point criterion. Tropopause height shows
large variation in the latitude range ∼30°–40°N during winter. Tropopause temperature less than −82°C, assumed to facilitate
troposphere to stratosphere air transport, is observed at a number of tropical Indian locations and no seasonal pattern is
observed in its occurrence. The bias in temperature and refractivity deduced from radiosonde and radio occultation measurements
is also presented. 相似文献
235.
非均匀下垫面边界层的观测和数值模拟研究(I):冷岛效应和逆湿现象的完整物理图像 总被引:11,自引:4,他引:7
利用HEIFE、DHEX和GAME/Tibet 3个观测试验和部分常规气象站观测资料,分析了下垫面非均匀条件下产生的冷岛效应和逆湿现象的完整物理图像以及它们对地表能量和五水转化的影响.分析结果表明,下垫面非均匀特征对五水转化有着明显的影响;冷岛效应和逆湿现象是下垫面非均匀性的必然结果,它们在自然界普遍存在;现有陆面过程模式很难刻画冷岛效应和逆湿现象,通常现有陆面过程模式很难将冷岛效应中的热量平流转化成蒸发潜热,同时陆面过程模式也混淆了逆湿现象和发生在下垫面上的大气水的冷却凝结.这些评述将有助于今后非均匀陆面过程的进一步研究. 相似文献
236.
长江下游的酷暑与冷夏是旱涝以外的另一种重要灾害性天气,本文以南京为代表,确定了长江下游酷暑年和冷夏年的三条标准。盛夏(7—8月)温度主要决定于7月平均温度T,并与副热带高压和中高纬度的冷空气活动以及印度季风低压—ITCZ有关。在酷暑年,印度低压更为频繁而强烈,位置略偏南;副热带高压较强,且直接控制江淮流域。100hPa青藏高压明显偏强,位置偏北偏东;青藏高原上常有500hPa暖中心东移。冷夏年三种成因:一是冷空气较强,梅雨明显而持久;二是ITCZ和热带系统强而活跃,盛夏频繁影响长江流域;三是以上两种天气系统的影响,形成混合型冷夏。 相似文献
237.
238.
高原及冷空气对1998和1991年夏季西太副高及雨带的影响 总被引:15,自引:17,他引:15
为了寻找西太副高南北急剧摆动的中、短期预报着眼点 ,本文利用 1998和 1991年 6~ 8月的逐日天气图资料、第二次青藏高原气象科学试验的实测资料 ,以及NCEP/NCAR的 2 .5°× 2 .5°经 /纬度格点的日平均u、v及高度再分析资料等 ,分析了高原涡东移、南亚高压南北摆动 ,以及北方冷空气入侵等对西太副高脊线及其相伴雨带摆动的影响。主要结论如下 :1)凡高原东部气柱平均厚度ΔH3 0 050 0 ≥392gpdm时 ,未来高原涡将东移出高原 ,激发西南涡产生东移 ,使西太副高南落 ,长江中、下游流域产生暴雨 ;2 )南亚高压东段脊线和西太副高西段脊线的南北摆动趋势相同 ,但前者更稳定 ,且常提前 3天 ,可据此预报西太副高的摆动及其相伴雨带的位置 ;3)若北方有冷空气持续南下 ,华北与华南、江南分别出现 2m·s-1以上的南风负、正距平 ,将易在该南风正、负距平过渡区且偏于负距平区一侧产生暴雨 ,并使西太副高稳定少动。 相似文献
239.
新仙女木事件在河西走廊地区开始于 12 5 8ka·B·P ,结束于 9 4ka·B·P ,极盛期为11 0ka·B·P左右。新仙女木事件使河西走廊地区晚更新世末转温暖湿润的气候在 12 0~ 10 0ka·B·P之间发生间断。降温期间 ,突发性洪水、干旱、风暴、沙暴、尘暴极端事件和突发性火灾频繁发生。降水减少 ,湖泊干涸 ,生态环境恶劣 ,沙漠化日益严重。古地理环境的演变 ,影响和限制了动物群的繁衍和人类文化的发展。 相似文献
240.
Understanding the hydrology of cold seep environments is crucial to perform accurate estimates of fluid and chemical fluxes at sedimentary wedges. Shallow convection processes may affect fluid flux estimates and could favor the destabilization of gas hydrate accumulations, increasing the sediment-ocean methane flux. Evidence for the occurrence of convection at cold seeps, however, is still limited. We use the concentration of 14C (D14C) in carbonate crusts formed at cold seeps of the eastern Mediterranean Sea as a tracer for convective recirculation of seawater-derived fluids. A numerical model is applied to investigate the controls on 14C incorporation in cold seep carbonates. Our simulations show that increased amounts of CH4 in the expelled fluids result in elevated crust D14C, while high Ca2+ and HCO3− concentrations produce the opposite effect. Convection is the only transport process that can significantly increase crust D14C. Advection, bioirrigation, eddy diffusion and bioturbation instead, have little effect on, or produce a decrease of, crust D14C. In addition, the presence of old or modern carbon (MC) in host sediments prior to cementation and the 14C-decay associated to the time needed to form the crust contribute in defining the D14C of carbonate crusts. We then use the model to reproduce the 14C content of the eastern Mediterranean Sea crusts to constrain the chemical and hydrological conditions that led to their formation. Some crusts contain relatively low amounts of 14C (−945.0<D14C ‰<−930.2) which, assuming no ageing after crust formation, can be reproduced without considering convection. Other crusts from two sites (the Amsterdam and Napoli mud volcanoes), instead, have a very high 14C-content (−899.0<D14C ‰<−838.4) which can only be reproduced by the model if convection mixes deep fluids with seawater. Order-of-magnitude calculations using the Rayleigh criterion for convection suggest that the slow seepage (about 10 cm year−1) of low salinity (20‰) fluids at the Amsterdam sites could trigger haline convection there. On the Napoli mud volcano, where high-density brines are expelled, density-driven convection cannot take place and other processes, possibly involving the rapid movement of free gas in the sediment, could be important. 相似文献