Wetter or colder during the Last Glacial Maximum? Revisiting the pluvial lake question in southwestern North America     

Kirsten M. Menking  Roger Y. Anderson  Nabil G. Shafike  Kamran H. Syed  Bruce D. Allen 《Quaternary Research》2004,62(3):174-288

Well-preserved shorelines in Estancia basin and a relatively simple hydrologic setting have prompted several inquiries into the basin's hydrologic balance for the purpose of estimating regional precipitation during the late Pleistocene. Estimates have ranged from 86% to 150% of modern, the disparity largely the result of assumptions about past temperatures. In this study, we use an array of models for surface-water runoff, groundwater flow, and lake energy balance to examine previously proposed scenarios for late Pleistocene climate. Constraints imposed by geologic evidence of past lake levels indicate that precipitation for the Last Glacial Maximum (LGM) may have doubled relative to modern values during brief episodes of colder and wetter climate and that annual runoff was as much as 15% of annual precipitation during these episodes.  相似文献   

The Use Of Environmental Isotopes And Chloride As Natural Tracers To Investigate The Effects Of Depressurisation Of A Coastal Aquifer For Lignite Mining,India     

B.S. Sukhija  D.V. Reddy  P. Nagabhushanam  Syed Hussain  V.Y. Giri 《Hydrogeology Journal》1996,4(2):70-88

In order to mine lignite from the Neyveli opencast mines, about 200 km south of Madras, India, the Neyveli aquifer has been undergoing depressurisation continuously for almost three decades. To gain insight into the aquifer hydrodynamics, environmental-isotope and chloride measurements on selected groundwater samples were initiated two decades ago, and detailed studies were undertaken during 1985–91. This article demonstrates the effect of large-scale groundwater withdrawals on the hydrologic regime and the effects on environmental chloride and isotopic distributions. A feature common to both the radiocarbon and chloride data of 1991 is their marked variation in a small area between the recharge zone and the mine area where pumping is concentrated. Furthermore, an excellent correlation exists between the temporal increase in the apparent radiocarbon ages of groundwater sampled in 1991 and 1985 and the corresponding lowering of potentiometric levels. Isochlors based on 1991 and 1985 data also indicate a changed hydrologic regime. In certain areas, heavy withdrawals have resulted in increased groundwater salinity.  相似文献   

Recharge processes: piston flow vs preferential flow in semi-arid aquifers of India   总被引：3，自引：4，他引：3  

B. S. Sukhija  D. V. Reddy  P. Nagabhushanam  Syed Hussain 《Hydrogeology Journal》2003,11(3):387-395

Study of groundwater recharge processes is vital for quantification of total natural recharge to the aquifers. One of the recharge processes demonstrated earlier by tracer experiments in the unsaturated zone is that of piston flow movement of soil moisture. Based on this recharge process, environmental tritium, chloride and injected tritium studies have been carried out extensively in various geological environs of India. The purpose of this paper is to evaluate the validity of the piston flow concept in different geological environs viz. consolidated fractured and weathered granites, semi-consolidated sandstones and unconsolidated alluvial tracts, and quantify the contribution from this process as well as that from the preferential flow mechanism using different tracers. Analysis of tracer data demonstrates that the preferential flow recharge process contributes very significantly (an average of 75% of total recharge) in the case of fractured granites and is important (an average of 33% of total recharge) for semi-consolidated sandstones, whereas the preferential flow recharge component is minimal in unconsolidated alluvial tracts (piston flow model is applicable). These findings necessitate re-evaluation of the total natural recharge potential of the above mentioned geological environs in view of the significant preferential flow recharge that is evidenced and estimated. Electronic Publication  相似文献   

A Special Issue dedicated to Robert N. Clayton     

Munir Humayun  James R. O'Neil 《Geochimica et cosmochimica acta》2003,67(17):3097-3099

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Landscape dynamics in Hokersar Wetland, Jammu & Kashmir—An application of geospatial approach     

P K Joshi  Humayun Rashid  P S Roy 《Journal of the Indian Society of Remote Sensing》2002,30(1-2):1-5

Geospatial presentation of habitat has become a key issue for planning conservation and management of any ecosystem. Hokersar wetland, one of the best resorts of migratory waterfowl in Kashmir, is under anthropogenic pressure and siltation due to floods. This has resulted in the degradation and change in the habitat quality of varied aquatic flora and fauna. Moreover, the seasonal changes affect the water level and land cover characteristics of the landscape. In the present study temporal mapping of the wetland has been carried out using the data sets for the autumn and spring seasons to assess the land cove/land use dynamics. The temporal change analysis, in the urban sprawl and the wetland, has been carried out to assess the rate of changes in the wetland and its environs. The wetland initially comprised of patch of marshy waterfowl habitat with some open water bodies. It has been fragmented into a large number of land uses because of anthropogenic activities. The increase in the settlement has been observed proportionate to the rate of fragmentation in the wetland. This study has created an information base, which will help to design conservation schemes for long term maintenance of the wetland.  相似文献   

Lithologic mapping of a forested montane terrain from Landsat 5 TM image     

Taufique H. Mahmood  Khaled Hasan  Syed Humayun Akhter 《国际地球制图》2019,34(7):750-768

Thick forest cover and poor infrastructures are the major hindrances for detailed lithologic mapping in an inaccessible montane landscape. To overcome these limitations, we utilize a Landsat 5 TM image to map lithology using vegetation and drainage pattern as an indicator of underlying rock types in a heavily forested region of the Chittagong Hill Tracts area located in southeastern Bangladesh. We use supervised and unsupervised classifiers for a vegetation-based approach while on-screen digitization is used for drainage patterns-based mapping. Field observations were used for mapping lithology and evaluating accuracy. Overall, our results agree well with the current geologic map and improve it by providing a more spatially detailed distribution of the sandstone and shale. The performances of all approaches are good at the inner and outer flanks of anticlines located in the study area while the drainage pattern mapping performs best at the mid-flank area.  相似文献   

Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars          下载免费PDF全文

Roger H. Hewins  Brigitte Zanda  Munir Humayun  Alexander Nemchin  Jean‐Pierre Lorand  Sylvain Pont  Damien Deldicque  Jeremy J. Bellucci  Pierre Beck  Hugues Leroux  Maya Marinova  Laurent Remusat  Christa Göpel  Eric Lewin  Marion Grange  Allen Kennedy  Martin J. Whitehouse 《Meteoritics & planetary science》2017,52(1):89-124

Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. 2013 ) and a ¹⁴⁷Sm‐¹⁴³Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. 2016 ) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ¹⁷O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014 ). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015 ), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015 ).  相似文献   

Propagation of Coronal Mass Ejections Observed During the Rising Phase of Solar Cycle 24     

M. Syed Ibrahim  P. K. Manoharan  A. Shanmugaraju 《Solar physics》2017,292(9):133

In this study, we investigate the interplanetary consequences and travel time details of 58 coronal mass ejections (CMEs) in the Sun–Earth distance. The CMEs considered are halo and partial halo events of width \({>}\,120\)°. These CMEs occurred during 2009?–?2013, in the ascending phase of the Solar Cycle 24. Moreover, they are Earth-directed events that originated close to the centre of the solar disk (within about \(\pm30\)° from the Sun’s centre) and propagated approximately along the Sun–Earth line. For each CME, the onset time and the initial speed have been estimated from the white-light images observed by the LASCO coronagraphs onboard the SOHO space mission. These CMEs cover an initial speed range of \({\sim}\,260\,\mbox{--}\,2700~\mbox{km}\,\mbox{s}^{-1}\). For these CMEs, the associated interplanetary shocks (IP shocks) and interplanetary CMEs (ICMEs) at the near-Earth environment have been identified from in-situ solar wind measurements available at the OMNI data base. Most of these events have been associated with moderate to intense IP shocks. However, these events have caused only weak to moderate geomagnetic storms in the Earth’s magnetosphere. The relationship of the travel time with the initial speed of the CME has been compared with the observations made in the previous Cycle 23, during 1996?–?2004. In the present study, for a given initial speed of the CME, the travel time and the speed at 1 AU suggest that the CME was most likely not much affected by the drag caused by the slow-speed dominated heliosphere. Additionally, the weak geomagnetic storms and moderate IP shocks associated with the current set of Earth-directed CMEs indicate magnetically weak CME events of Cycle 24. The magnetic energy that is available to propagate CME and cause geomagnetic storm could be significantly low.  相似文献   

The design of visco‐acoustic weighted L1‐error frequency–space wavefield extrapolators          下载免费PDF全文

Syed Abdul Salam  Wail A. Mousa 《Geophysical Prospecting》2018,66(2):300-310

Seismic imaging is an important step for imaging the subsurface structures of the Earth. One of the attractive domains for seismic imaging is explicit frequency–space (f – x) prestack depth migration. So far, this domain focused on migrating seismic data in acoustic media, but very little work assumed visco‐acoustic media. In reality, seismic exploration data amplitudes suffer from attenuation. To tackle the problem of attenuation, new operators are required, which compensates for it. We propose the weighted L ₁ ‐error minimisation technique to design visco‐acoustic f – x wavefield extrapolators. The L ₁ ‐error wavenumber responses provide superior extrapolator designs as compared with the previously designed equiripple L ₄ ‐norm and L_∞‐norm extrapolation wavenumber responses. To verify the new compensating designs, prestack depth migration is performed on the challenging Marmousi model dataset. A reference migrated section is obtained using non‐compensating f – x extrapolators on an acoustic dataset. Then, both compensating and non‐compensating extrapolators are applied to a visco‐acoustic dataset, and both migrated sections are then compared. The final images show that the proposed weighted L ₁ ‐error method enhances the resolution and results in practically stable images.  相似文献   

Geo-Spatial Dynamics of Snow Cover and Hydro-Meteorological Parameters of Hunza Basin,HKH Region,Pakistan     

Ijaz Ahmad  Ahmad  Zulfiqar  Lisa  Mona  Mahmood  Syed Amer  Ali  Asad  Rehman  Obaid Ur 《Water Resources》2019,46(6):894-909

Water Resources - Snow cover dynamics play an important role in the hydrological characteristics of Upper Indus Basin (UIB) of Pakistan in terms of seasonal accumulation and depletions. The current...  相似文献