Reconstruction of the variability of the southwest monsoon during the past 3 ka,from the continental margin of the southeastern Arabian Sea     

Onkar S. Chauhan  Elke Vogelsang  Nathani Basavaiah  U. Syed Abdul Kader 《第四纪科学杂志》2010,25(5):798-807

From temporal variation in δ¹⁸O in Globigerinoides ruber and G. sacculifer and geochemical indices of weathering/erosion (chemical index of alteration, Al and Ti), we infer rapid southwest monsoon (SWM) deterioration with dwindling fluvial and detrital fluxes at ca. 450–650, 1000 and 1800–2200 cal. a BP during the late Holocene. We have evaluated the role of solar influx (reconstructed) and high‐latitude climate variability (archived in GRIP and GISP‐2 cores) on SWM precipitation. Broadly, our δ¹⁸O climate reconstruction is concordant with GRIP and GISP‐2, and supports a teleconnection through atmospheric connection between the SWM and the North Atlantic climate – albeit temporal extents of the Little Ice Age and Medieval Warm Period from high latitude are not entirely coeval. Moreover, there is a humid climate and enhanced precipitation during the terminal stages of the Little Ice Age. The medieval warming (ca. AD 800–1300) is not synchronous either, and is punctuated by an arid event centred at 1000 a BP. Although the delineation of the specific influence of solar influx on SWM precipitation is elusive, we surmise that SWM precipitation is a complex phenomenon and local orography along southwestern India may have a role on the entrapment of moisture from the southwest trade winds, when these hit land. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Evolution of the Archean-Palaeoproterozoic Bundelkhand Massif, central India—evidence from granitoid geochemistry     

M.E.A. Mondal  Syed M. Zainuddin 《地学学报》1996,8(6):532-539

The Bundelkhand massif of Archean-Palaeoproterozoic age is primarily a granite-gneiss complex. Three distinct granitoid suites have been identified within the massif hornblende granitoids, biotite granitoids and leucogranitoids, in order of decreasing age. These granitoids were emplaced in previously deformed basement consisting of gneisses, banded iron formations and other metasediments, mafic to felsic volcanics.
The granitoids exhibit a large compositional range from quartz diorite to syenogranite and show a calc-alkaline trend. They are metaluminous to peraluminous and have I-type characteristics. The SiO₂ content ranges from 49 to 77 wt%. Low K₂O/Na₂O characterizes the granitoids. The oldest hornblende granitoids have low Rb and Yb contents compared to the younger biotite granitoids and leucogranitoids. Rb/Sr values for most of the granitoids are low (< 1). K/Rb ratios range from 95 to 373 which is, in general, comparable with other calc-alkaline suites. Y/Nb ratios of the granitoids are > 1.2 which is a characteristic feature of magmas derived from sources chemically similar to island arc or continental margin basalts.
The features mentioned above coupled with concentrations of Rb, Y, Nb, Yb, Ta and Th indicate a volcanic-arc tectonic setting for the granitoids. It is proposed that the massif represents subduction-related magmatism of an ocean in the southern part of the massif (an Andean plate margin).  相似文献   

Studies of clump structure of photodissociation regions at millimeter and sub-millimeter wavelengths     

Abdul Qaiyum  Syed Salman Ali 《Journal of Astrophysics and Astronomy》2003,24(3-4):69-83

To interpret the millimeter and sub-millimeter line emissions of atomic and molecular species from galactic and extragalactic photodissociation regions, warm gas components and molecular clouds, generally, escape probability formalism of Tielens & Hollenbach (herein referred as TH) are employed which is based on the assumption of plane parallel geometry of infinite slab allowing photons to escape only from the front. Contrary to the assumption observationally it is found that these lines are optically thin except OI(63μ m) and low rotational transitions of CO and some other molecules. This observational evidence led us to assume that emitting regions are finite parallel plane slab in which photons are allowed to escape from both the surfaces (back and front). Therefore, in the present study escape of radiations from both sides of the homogeneous and also clumpy PDR/molecular clouds are taken into consideration for calculating the line intensities at millimeter and sub-millimeter wavelengths (hereinafter referred as QA). Results are compared with that of the TH model. It is found that thermal and chemical structures of the regions are almost similar in both the formalisms. But line intensities are modified by differing factors. Particularly at low density and low kinetic temperature and also for optically thin lines line intensities calculated from TH and QA model differ substantially. But at density higher than the critical density and also for optically thick lines TH and QA models converge to almost same values. An attempt has been made to study the physical conditions of the M17 region employing the present formalism.  相似文献   

Interaction Between Two CMEs During 14 – 15 February 2011 and Their Unusual Radio Signature     

A. Shanmugaraju  S. Prasanna Subramanian  Bojan Vrsnak  M. Syed Ibrahim 《Solar physics》2014,289(12):4621-4632

We report a detailed analysis of an interaction between two coronal mass ejections (CMEs) that were observed on 14?–?15 February 2011 and the corresponding radio enhancement, which was similar to the “CME cannibalism” reported by Gopalswamy et al. (Astrophys. J. 548, L91, 2001). A primary CME, with a mean field-of-view velocity of 669 km?s^?1 in the Solar and Heliospheric Observatory (SOHO)/Large Angle Spectrometric Coronagraph (LASCO), was more than as twice as fast as the slow CME preceding it (326 km?s^?1), which indicates that the two CMEs interacted. A radio-enhancement signature (in the frequency range 1 MHz?–?400 kHz) due to the CME interaction was analyzed and interpreted using the CME data from LASCO and from the Solar Terrestrial Relations Observatory (STEREO) HI-1, radio data from Wind/Radio and Plasma Wave Experiment (WAVES), and employing known electron-density models and kinematic modeling. The following results are obtained: i) The CME interaction occurred around 05:00?–?10:00 UT in a height range 20?–?25 R_⊙. An unusual radio signature is observed during the time of interaction in the Wind/WAVES dynamic radio spectrum. ii) The enhancement duration shows that the interaction segment might be wider than 5 R_⊙. iii) The shock height estimated using density models for the radio enhancement region is 10?–?30 R_⊙. iv) Using kinematic modeling and assuming a completely inelastic collision, the decrease of kinetic energy based on speeds from LASCO data is determined to be 0.77×10²³ J, and 3.67×10²³ J if speeds from STEREO data are considered. vi) The acceleration, momentum, and force are found to be a=?168 m?s^?2, I=6.1×10¹⁸ kg?m?s^?1, and F=1.7×10¹⁵ N, respectively, using STEREO data.  相似文献   

Si‐bearing metal and niningerite in Almahata Sitta fine‐grained ureilites and insights into the diversity of metal on the ureilite parent body     

Marian Horstmann  Munir Humayun  Mario Fischer‐Gödde  Addi Bischoff  Mona Weyrauch 《Meteoritics & planetary science》2014,49(10):1948-1977

A detailed mineralogical and chemical study of Almahata Sitta fine‐grained ureilites (MS‐20, MS‐165, MS‐168) was performed to shed light on the origin of these lithologies and their sulfide and metal. The Almahata Sitta fine‐grained ureilites (silicates <30 μm grain size) show textural and chemical evidence for severe impact smelting as described for other fine‐grained ureilites. Highly reduced areas in Almahata Sitta fine‐grained ureilites show large (up to ～1 mm) Si‐bearing metal grains (up to ～4.5 wt% Si) and niningerite [Mg_>0.5,(Mn,Fe)_<0.5S] with some similarities to the mineralogy of enstatite (E) chondrites. Overall, metal grains show a large compositional variability in Ni and Si concentrations. Niningerite grains probably formed as a by‐product of smelting via sulfidation. The large Si‐Ni variation in fine‐grained ureilite metal could be the result of variable degrees of reduction during impact smelting, inherited from coarse‐grained ureilite precursors, or a combination of both. Large Si‐bearing metal grains probably formed via coalescence of existing and newly formed metal during impact smelting. Bulk and in situ siderophile trace element abundances indicate three distinct populations of (1) metal crystallized from partial melts in MS‐20, (2) metal resembling bulk chondritic compositions in MS‐165, and (3) residual metal in MS‐168. Almahata Sitta fine‐grained ureilites developed their distinctive mineralogy due to severe reduction during smelting. Despite the presence of E chondrite and ureilite stones in the Almahata Sitta fall, a mixing relation of E chondrites or their constituents and ureilite material in Almahata Sitta can be ruled out based on isotopic, textural, and mineral‐chemical reasons.  相似文献   

Behaviour,of stratospheric NO2 during stratospheric warming of January‐February 1979     

M.Q. Syed  A.W. Harrison 《大气与海洋》2013,51(3):216-235

Abstract

Ground‐based measurements of stratospheric NO₂, using absorption spectroscopy of twilight sky in the spectral region 437–451 nm, have been made at Calgary (51.08°N, 114.13°W), Alberta, Canada, during the period December 1978‐March 10, 1979. The NO₂ showed a rather steady behaviour in the month of December when there was no stratospheric warming in progress; the monthly average of the afternoon vertical column abundance for December was 1.7 × 10¹⁵ cm². In the rest of the observation period the NO₂ showed a complex behaviour, but this behaviour appears to be explicable in terms of the stratospheric circulation changes taking place in that period due to the occurrence of a stratospheric warming event.  相似文献   

Analysis of Rigid Short Caisson with Granular Core Incorporating Nonlinear Interface and Base Responses     

Syed M. Ali Jawaid  Madhira R. Madhav 《Geotechnical and Geological Engineering》2009,27(3):391-406

It is difficult to construct a conventional shallow foundation in alluvial lowlands because of soft soils and high ground water table. A rigid short caisson foundation with granular core is being proposed for alluvial lowlands. The proposed foundation is analyzed using non-linear hyperbolic stress–displacement responses of homogeneous alluvial deposits. Extensive parametric studies are carried out to study the effects of length ratio (L/d₀), diameter ratio (d/d₀) of granular core with respect to casing, relative stiffness of shaft (α_τ), relative casing base stiffness (α_b), and friction angle of granular material (ϕ_gp) on the load sharing and the settlement of the proposed foundation.  相似文献   

Primary sedimentary structures and MISS in Gulcheru quartzite along SW part of Cuddapah basin     

Goswami Sukanta  Mukherjee Atanu  Bhattacharjee Purnajit  Zakaulla Syed 《Journal of the Geological Society of India》2017,89(5):511-520

Sedimentary structures are very useful in paleocurrent analysis and interpretation of siliciclastic shallow marine environment. These interesting sedimentary structures such as parallel bedding, cross bedding, ripples, and mud cracks as well as synaeresis cracks, are best studied in the field. They are formed by a variety of sedimentary processes, including fluid flow, sediment-gravity flow, soft-sediment deformation and biogenic activity. Gulcheru Formation has evidence of interaction of microbial communities with clastic sedimentation during Palaeoproterozoic time. Because of high porosity-permeability of siliciclastics of Gulcheru Formation, palaeontologists do not expect many fossils preserved in such rocks and thus they have been overlooked. Microbially induced sedimentary structures (MISS) are highlighted in the Gulcheru Formation. They are indicative of shallow marine environment. Gulcheru Formation shows alluvial fan to shallow marine shelf environments within a limited thickness of about ~ 110m, which indicates an alluvial plain coast where alluvial deposits are modified by wave forces.  相似文献   

Identification of the influencing factors on groundwater drought and depletion in north-western Bangladesh     

Syed?Md.?Touhidul?MustafaEmail authorView author&#;s OrcID profile  Khodayar?Abdollahi  Boud?Verbeiren  Marijke?Huysmans 《Hydrogeology Journal》2017,25(5):1357-1375

Groundwater drought is a specific type of hydrological drought that concerns groundwater bodies. It may have a significant adverse effect on the socio-economic, agricultural, and environmental conditions. Investigating the effect of different climatic and anthropogenic factors on groundwater drought provides essential information for sustainable planning and management of (ground) water resources. The aim of this study is to identify the influencing factors on groundwater drought in north-western Bangladesh, to understand the forcing mechanisms. A multi-step methodology is proposed to achieve this objective. The standardised precipitation index (SPI) and reconnaissance drought index (RDI) have been used to quantify the aggregated deficit between precipitation and the evaporative demand of the atmosphere, i.e. meteorological drought. The influence of land-cover patterns on the groundwater drought has been identified by calculating spatially distributed groundwater recharge as a function of land cover. Groundwater drought is defined by a threshold method. The results show that the evapotranspiration and rainfall deficits are determining meteorological drought, which shows a direct relation with groundwater recharge deficits. Land-cover change has a small effect on groundwater recharge but does not seem to be the main cause of groundwater-level decline (depletion) in the study area. The groundwater depth and groundwater-level deficit (drought) is continuously increasing with little correlation to meteorological drought or recharge anomalies. Overexploitation of groundwater for irrigation seems to be the main cause of groundwater-level decline in the study area. Efficient irrigation management is essential to reduce the growing pressure on groundwater resources and ensure sustainable water management.  相似文献   

Abundance and Ecology of Endemic Asterorotalia trispinosa from the Western Bay of Bengal: Implications for its Application as a Paleomonsoon Proxy     

Rajeev?SARASWAT  Manumalenki?MANASA  Thejasino?SUOKHRIE  Mohammad?Syed?SAALIM  Rajiv?NIGAM 《《地质学报》英文版》2017,91(6):2268-2282

A total of 97 surface sediment samples were collected from the continental shelf and slope of the western Bay of Bengal to assess the potential application of Asterorotalia trispinosa as a paleomonsoon proxy. The relative abundance of living as well as total (living and dead) A. trispinosa ranges from being absent to a maximum of ~31%. The highest abundance of A. trispinosa is close to the outfall region of the Ganga-Brahmaputra Rivers and decreases away from the river mouths. Contrary to that, A. trispinosa is rare in front of both the Cauvery and Pennar river outfall regions, in the central part of the western Bay of Bengal. The living A. trispinosa abundance is the lowest in between two major river systems, viz. Ganga-Brahmaputra-Mahanadi and Krishna-Godavari. The relative abundance of both the total and living A. trispinosa is strongly positively correlated with ambient seawater temperature, and negatively correlated with %Corg and salinity. Based on the spatial distribution, we conclude that A. trispinosa is stenohaline in nature, rather than euryhaline, and further that the increased relative abundance of A. trispinosa indicates warmer and only marginally hyposaline environment. Even though the ecological preference of A. trispinosa suggests it as a potential paleomonsoon proxy, the restricted distribution implies limited application.  相似文献