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排序方式: 共有121条查询结果,搜索用时 250 毫秒
51.
Nina D. ROSENBERG Lauren BROWNING William L. BOURCIER 《Meteoritics & planetary science》2001,36(2):239-244
Abstract— Results from an inorganic geochemical modeling study support a scenario in which low‐temperature aqueous alteration of an anhydrous CM asteroidal parent body and melt water from H2O and CO2 ices produces the altered assemblage observed in CM carbonaceous chondrites (chrysotile, greenalite, tochilinite, cronstedtite and minor calcite and magnetite). We consider a range of possible precursor mineral assemblages, varying with respect to the Fe‐oxidation state of the initial anhydrous phases. The aqueous solutions produced by this alteration are generally strongly basic and reducing and a large quantity of H2, and possible CH4, gas can be released during aqueous alteration. 相似文献
52.
As climate change has driven dramatic changes in Northern sea ice regimes, marine mammals have gained iconic status around the world reflecting the perils of global warming. There is a tension between policies that have international support like a ban on seal hunting or whaling, and the adoption of adaptive, flexible rules that are likely to work in Northern places. Whereas most wildlife policy focuses on biological information to inform policy strategy, this analysis focuses on the “human dimensions” of Northern marine mammal management. This research examines ways in which human relationships and modes of governance affect conservation success. Standard analyses of risk to animal populations focused on direct sources of take are inadequate to address multi-causal, complex problems such as climate-induced habitat loss or increased industrialization of the Arctic Ocean. Early conservation policy strategies focusing on the moratorium of take have eliminated or reduced such practices as commercialized hunting and high levels of fisheries bycatch, but may be less relevant in an era in which habitats and climate changes are key drivers of population dynamics. This paper argues that effective adaptive policy requires new ways of learning about and governing human interactions with marine mammals. Through an exploration of marine mammal management in three Northern regions (Alaska, Nunavut, and the Finnish Baltic Sea coast), the paper analyzes the extent to which these marine mammal management regimes are practicing adaptive governance, that is, building cross-scale (local to international) understanding while allowing actors at the local scale the flexibility to direct the creation of rules that are ecologically robust and likely to succeed. Lessons are taken from these examples and used to propose selected policy and research recommendations for the marine mammal policy community. 相似文献
53.
Arctic coastal populations share a close relationship with their environment consisting of linkages among communities, landscapes and seascapes, and the social institutions developed to sustain the system. This cultural–biogeophysical dynamic is termed throughout the section as a social–ecological system (SES). Marine mammals constitute a large portion of the subsistence diet for these communities, and as such represent key ecological services provided by the system. At the same time, marine mammals have gained iconic status as symbols for climate change in the North. A tension results between the demands of balancing on one hand good policy optics consistent with national and international norms and, on the other hand, flexible and adaptive institutions able to take on the task of managing resources in a dynamic, changing North. This tension and associated policy solutions such as co-management are explored in a series of papers focusing on marine mammal management dilemmas and policy practices around the circumpolar North. This introduction communicates the problem context and describes the five papers making up this special section. A guiding premise to this work is that new international pressures to implement moratoria on marine mammal hunting in the North ignore critical human dimensions of marine mammal management. Such policy proposals are unlikely to succeed in areas that require collaboration across scales. Simultaneously, new local-scale participation in multi-level management regimes holds promise for creating more resilient marine mammal SESs. 相似文献
54.
Lauren H. Wyatt Alan L. Baker David L. Berlinsky 《Aquatic Sciences - Research Across Boundaries》2010,72(3):361-369
The decline in anadromous rainbow smelt (Osmerus mordax) populations may be due to anthropogenic causes including spawning habitat degradation. The purpose of this study was to
assess the survival of rainbow smelt embryos incubated under sediment layers of different depths (0.00, 0.25, 1.00, and 6.00 g/45.6 cm2) and in contact with periphyton communities of different biomass. Embryo survival was also assessed when cultured on periphyton
in combination with sterilized sediment or eutrophying compounds (nitrates and phosphates). Oxygen consumption was monitored
from embryos cultured alone, on periphyton layers, and under a sediment layer. Survival was significantly reduced under the
highest sediment treatment and attributed to low oxygen availability to the embryos. Embryonic survival was also significantly
reduced when incubated on the highest periphyton biomass. Embryos under the sediment layer consumed oxygen at a significantly
greater rate than the controls, and the dissolved oxygen concentration below the sediment–water interface decreased to near
anoxic. These results suggest that embryonic survival could be impacted in rivers with heavy sedimentation or a high standing
biomass of periphyton. 相似文献
55.
Kathryn M. Stack John P. Grotzinger Michael P. Lamb Sanjeev Gupta David M. Rubin Linda C. Kah Lauren A. Edgar Deirdra M. Fey Joel A. Hurowitz Marie McBride Frances Rivera‐Hernndez Dawn Y. Sumner Jason K. Van Beek Rebecca M. E. Williams Robin Aileen Yingst 《Sedimentology》2019,66(5):1768-1802
Recent robotic missions to Mars have offered new insights into the extent, diversity and habitability of the Martian sedimentary rock record. Since the Curiosity rover landed in Gale crater in August 2012, the Mars Science Laboratory Science Team has explored the origins and habitability of ancient fluvial, deltaic, lacustrine and aeolian deposits preserved within the crater. This study describes the sedimentology of a ca 13 m thick succession named the Pahrump Hills member of the Murray formation, the first thick fine‐grained deposit discovered in situ on Mars. This work evaluates the depositional processes responsible for its formation and reconstructs its palaeoenvironmental setting. The Pahrump Hills succession can be sub‐divided into four distinct sedimentary facies: (i) thinly laminated mudstone; (ii) low‐angle cross‐stratified mudstone; (iii) cross‐stratified sandstone; and (iv) thickly laminated mudstone–sandstone. The very fine grain size of the mudstone facies and abundant millimetre‐scale and sub‐millimetre‐scale laminations exhibiting quasi‐uniform thickness throughout the Pahrump Hills succession are most consistent with lacustrine deposition. Low‐angle geometric discordances in the mudstone facies are interpreted as ‘scour and drape’ structures and suggest the action of currents, such as those associated with hyperpycnal river‐generated plumes plunging into a lake. Observation of an overall upward coarsening in grain size and thickening of laminae throughout the Pahrump Hills succession is consistent with deposition from basinward progradation of a fluvial‐deltaic system derived from the northern crater rim into the Gale crater lake. Palaeohydraulic modelling constrains the salinity of the ancient lake in Gale crater: assuming river sediment concentrations typical of floods on Earth, plunging river plumes and sedimentary structures like those observed at Pahrump Hills would have required lake densities near freshwater to form. The depositional model for the Pahrump Hills member presented here implies the presence of an ancient sustained, habitable freshwater lake in Gale crater for at least ca 103 to 107 Earth years. 相似文献
56.
Revised mapping along the Dent Fault (northwest England) has improved the resolution of folds and faults formed during Variscan (late Carboniferous) sinistral transpression. A NNE-trending east-down monocline, comprising the Fell End Syncline and Taythes Anticline, was forced in Carboniferous cover above a reactivated precursor to the Dent Fault within the Lower Palaeozoic basement. The Taythes Anticline is periclinal due to interference with earlier Acadian folds. The steep limb of the monocline was eventually cut by the west-dipping Dent Fault. The hangingwall of the Dent Fault was dissected by sub-vertical or east dipping faults, together forming a positive flower structure in cross-section and a contractional duplex in plan view. The footwall to the Dent Fault preserves evidence of mostly dip-slip displacements, whereas strike-slip was preferentially partitioned into the hangingwall faults. This pattern of displacement partitioning may be typical of transpressive structures in general. The faults of the Taythes duplex formed in a restraining overlap zone between the Dent Fault and the Rawthey Fault to the west. The orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly. Kinematic constraints provided by the Dent and neighbouring Variscan faults yield a NNW–SSE regional shortening direction in this part of the Variscan foreland. 相似文献
57.
We used regression analyses of water samples from 18 lakes, nine rivers, and one spring in Ethiopia to (a) test the hypothesis that water bodies of relatively higher salinity (K25>1000 μS cm−1) have a different conductivity to salinity relationship than waters of lower salinity (K25 < 1000 μS cm−1), and (b) develop models to predict total cations and salinity from conductivity that can be used for Ethiopian waters and other African aquatic systems of similar chemical composition. We found no statistical difference in the bilogarithmic relationships (total cations vs. conductivity; salinity vs. conductivity) for waters of higher salinity (K25 > 1000 μS cm−1) and waters of lower salinity (K25 < 1000 μS cm−1). However, comparison among our models and models from the literature suggests that developing separate equations for low and high salinity water bodies has some merit. We believe that the equations developed in this study can be used for Ethiopian waters and other African waters within the range of conductivity in this study. 相似文献
58.
Geochemistry of Thermal and Mineral Waters in the Cascade Mountains of Western North America 总被引:1,自引:0,他引:1
Lauren S. Forcella 《Ground water》1982,20(1):39-47
59.
Flat-pebble conglomerate: its multiple origins and relationship to metre-scale depositional cycles 总被引:8,自引:0,他引:8
Paul M. Myrow Lauren Tice Bonny Archuleta† Bryn Clark‡ John F. Taylor§ Robert L. Ripperdan¶ 《Sedimentology》2004,51(5):973-996
Carbonate flat‐pebble conglomerate is an important component of Precambrian to lower Palaeozoic strata, but its origins remain enigmatic. The Upper Cambrian to Lower Ordovician strata of the Snowy Range Formation in northern Wyoming and southern Montana contain abundant flat‐pebble conglomerate beds in shallow‐water cyclic and non‐cyclic strata. Several origins of flat‐pebble conglomerate are inferred for these strata. In one case, all stages of development of flat‐pebble conglomerate are captured within storm‐dominated shoreface deposits of hummocky cross‐stratified (HCS) fine carbonate grainstone. A variety of synsedimentary deformation structures records the transition from mildly deformed in situ stratification to buckled beds of partially disarticulated bedding to fully developed flat‐pebble conglomerate. These features resulted from failure of a shoreface and subsequent brittle and ductile deformation of compacted to early cemented deposits. Failure was induced by either storm or seismic waves, and many beds failed along discrete slide scar surfaces. Centimetre‐scale laminae within thick amalgamated HCS beds were planes of weakness that led to the development of platy clasts within partly disarticulated and rotated bedding of the buckled beds. In some cases, buckled masses accelerated downslope until they exceeded their internal friction, completely disarticulated into clasts and transformed into a mass flow of individual cm‐ to dm‐scale clasts. This transition was accompanied by the addition of sand‐sized echinoderm‐rich debris from local sources, which slightly lowered friction by reducing clast–clast interactions. The resulting dominantly horizontal clast orientations suggest transport by dense, viscous flow dominated by laminar shear. These flows generally came to rest on the lower shoreface, although in some cases they continued a limited distance beyond fairweather wave base and were interbedded with shale and grainstone beds. The clasts in these beds show no evidence of extensive reworking (i.e. not well rounded) or condensation (i.e. no rinds or coatings). A second type of flat‐pebble conglomerate bed occurs at the top of upward‐coarsening, metre‐scale cycles. The flat‐pebble conglomerate beds cap these shoaling cycles and represent either lowstand deposits or, in some cases, may represent transgressive lags. The clasts are well rounded, display borings and have iron‐rich coatings. The matrix to these beds locally includes glauconite. These beds were considerably reworked and represent condensed deposits. Thrombolites occur above the flat‐pebble beds and record microbial growth before initial transgression at the cycle boundaries. A third type of flat‐pebble conglomerate bed occurs within unusual metre‐scale, shale‐dominated, asymmetric, subaqueous cycles in Shoshone Canyon, Wyoming. Flat‐pebble beds in these cycles consist solely of clasts of carbonate nodules identical to those that are in situ within underlying shale beds. These deeper water cycles can be interpreted as either upward‐shoaling or ‐deepening cycles. The flat‐pebble conglomerate beds record winnowing and reworking of shale and carbonate nodules to lags, during either lowstand or the first stages of transgression. 相似文献
60.