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401.
Abundant evidence exists for glaciation being an important geomorphic process in the mid-latitude regions of both hemispheres of Mars, as well as in specific environments at near-equatorial latitudes, such as along the western flanks of the major Tharsis volcanoes. Detailed analyses of glacial landforms (lobate-debris aprons, lineated valley fill, concentric crater fill, viscous flow features) have suggested that this glaciation was predominantly cold-based. This is consistent with the view that the Amazonian has been continuously cold and dry, similar to conditions today. We present new data based on a survey of images from the Context Camera (CTX) on the Mars Reconnaissance Orbiter that some of these glaciers experienced limited surface melting, leading to the formation of small glaciofluvial valleys. Some of these valleys show evidence for proglacial erosion (eroding the region immediately in front of or adjacent to a glacier), while others are supraglacial (eroding a glacier’s surface). These valleys formed during the Amazonian, consistent with the inferred timing of glacial features based on both crater counts and stratigraphic constraints. The small scale of the features interpreted to be of glaciofluvial origin hindered earlier recognition, although their scale is similar to glaciofluvial counterparts on Earth. These valleys appear qualitatively different from valley networks formed in the Noachian, which can be much longer and often formed integrated networks and large lakes. The valleys we describe here are also morphologically distinct from gullies, which are very recent fluvial landforms formed during the last several million years and on much steeper slopes (∼20-30° for gullies versus ?10° for the valleys we describe). These small valleys represent a distinct class of fluvial features on the surface of Mars (glaciofluvial); their presence shows that the hydrology of Amazonian Mars is more diverse than previously thought. 相似文献
402.
Anna Wachnicka Evelyn Gaiser Laurel Collins Thomas Frankovich Joseph Boyer 《Estuaries and Coasts》2010,33(5):1080-1098
The composition and distribution of diatom algae inhabiting estuaries and coasts of the subtropical Americas are poorly documented,
especially relative to the central role diatoms play in coastal food webs and to their potential utility as sentinels of environmental
change in these threatened ecosystems. Here, we document the distribution of diatoms among the diverse habitat types and long
environmental gradients represented by the shallow topographic relief of the South Florida, USA, coastline. A total of 592
species were encountered from 38 freshwater, mangrove, and marine locations in the Everglades wetland and Florida Bay during
two seasonal collections, with the highest diversity occurring at sites of high salinity and low water column organic carbon
concentration (WTOC). Freshwater, mangrove, and estuarine assemblages were compositionally distinct, but seasonal differences
were only detected in mangrove and estuarine sites where solute concentration differed greatly between wet and dry seasons.
Epiphytic, planktonic, and sediment assemblages were compositionally similar, implying a high degree of mixing along the shallow,
tidal, and storm-prone coast. The relationships between diatom taxa and salinity, water total phosphorus (WTP), water total
nitrogen (WTN), and WTOC concentrations were determined and incorporated into weighted averaging partial least squares regression
models. Salinity was the most influential variable, resulting in a highly predictive model (r
apparent2 = 0.97, r
jackknife2 = 0.95) that can be used in the future to infer changes in coastal freshwater delivery or sea-level rise in South Florida
and compositionally similar environments. Models predicting WTN (r
apparent2 = 0.75, r
jackknife2 = 0.46), WTP (r
apparent2 = 0.75, r
jackknife2 = 0.49), and WTOC (r
apparent2 = 0.79, r
jackknife2 = 0.57) were also strong, suggesting that diatoms can provide reliable inferences of changes in solute delivery to the coastal
ecosystem. 相似文献
403.
Phytoplankton Biomass and Production in Subtropical Hong Kong Waters: Influence of the Pearl River Outflow 总被引:1,自引:0,他引:1
Alvin Y. T. Ho Jie Xu Kedong Yin Yuelu Jiang Xiangcheng Yuan Lei He Donald M. Anderson Joseph H. W. Lee Paul J. Harrison 《Estuaries and Coasts》2010,33(1):170-181
The size-fractionated phytoplankton biomass and primary production were investigated in four contrasting areas of Hong Kong
waters in 2006. Phytoplankton biomass and production varied seasonally in response to the influence of the Pearl River discharge.
In the dry season, the phytoplankton biomass and production were low (<42 mg chl m−2 and <1.8 g C m−2 day−1) in all four areas, due to low temperatures and dilution and reduced light availability due to strong vertical mixing. In
contrast, in the wet season, in the river-impacted western areas, the phytoplankton biomass and production increased greater
than five-fold compared to the dry season, especially in summer. In summer, algal biomass was 15-fold higher than in winter,
and the mean integrated primary productivity (IPP) was 9 g C m−2 day−1 in southern waters due to strong stratification, high temperatures, light availability, and nutrient input from the Pearl
River estuary. However, in the highly flushed western waters, chl a and IPP were lower (<30 mg m−2 and 4 g C m−2 day−1, respectively) due to dilution. The maximal algal biomass and primary production occurred in southern waters with strong
stratification and less flushing. Spring blooms (>10 μg chl a L−1) rarely occurred despite the high chl-specific photosynthetic rate (mostly >10 μg C μg chl a
−1 day−1) as the accumulation of algal biomass was restricted by active physical processes (e.g., strong vertical mixing and freshwater
dilution). Phytoplankton biomass and production were mostly dominated by the >5-μm size fraction all year except in eastern
waters during spring and mostly composed of fast-growing chain-forming diatoms. In the stratified southern waters in summer,
the largest algal blooms occurred in part due to high nutrient inputs from the Pearl River estuary. 相似文献
404.
Detlef P. van Vuuren Jae Edmonds Steven J. Smith Kate V. Calvin Joseph Karas Mikiko Kainuma Nebojsa Nakicenovic Keywan Riahi Bas J. van Ruijven Rob Swart Allison Thomson 《Climatic change》2010,103(3-4):635-642
Long-term scenarios developed by integrated assessment models are used in climate research to provide an indication of plausible long-term emissions of greenhouse gases and other radiatively active substances based on developments in the global energy system, land-use and the emissions associated with these systems. The phenomena that determine these long-term developments (several decades or even centuries) are very different than those that operate on a shorter time-scales (a few years). Nevertheless, in the literature, we still often find direct comparisons between short-term observations and long-term developments that do not take into account the differing dynamics over these time scales. In this letter, we discuss some of the differences between the factors that operate in the short term and those that operate in the long term. We use long-term historical emissions trends to show that short-term observations are very poor indicators of long-term future emissions developments. Based on this, we conclude that the performance of long-term scenarios should be evaluated against the appropriate, corresponding long-term variables and trends. The research community may facilitate this by developing appropriate data sets and protocols that can be used to test the performance of long-term scenarios and the models that produce them. 相似文献
405.
An observational and modeling study of a sea fog event over the Yellow Sea on 1 August 2003 总被引:1,自引:0,他引:1
Gang Fu Pengyuan Li Joseph G. Crompton Jingtian Guo Shanhong Gao Suping Zhang 《Meteorology and Atmospheric Physics》2010,107(3-4):149-159
A dense sea fog episode that occurred near the coastal city of Qingdao in the Shandong Peninsula of China on 1 August 2003 is investigated by using all of the available observational data and high-resolution modeling results from the Regional Atmospheric Modeling System (RAMS). This fog event reduced the horizontal visibility to be less than 60 m in some locations and caused several traffic accidents locally. In this paper, all of the available observational data, including visible satellite imagery of Geostationary Operational Environmental Satellite (GOES)-9 and MODerate-resolution Imaging Spectroradiometer (MODIS), objectively reanalyzed Final Analysis (FNL) data issued by the National Centers for Environmental Prediction (NCEP), sounding data at the Qingdao and Dalian stations, and the latest 4.4 version of the RAMS model, were employed to study this sea fog case. We begin with the analyses of the environmental conditions of the sea fog event, including the large-scale conditions, the difference between T 2m (air temperature at 2 m altitude) and sea surface temperature (SST), and the atmospheric sounding profiles of the two stations. The characteristics of this sea fog event was documented by using visible satellite imagery of GOES-9 and MODIS. In order to better understand the fog formation mechanism, a high-resolution RAMS model of dimensions 4 km × 4 km was designed, which was initialized and validated by FNL data. A 54-h modeling period that started from 18 UTC 31 July 2003 reproduced the main characteristics of this sea fog event. The simulated lower visibility area agreed well with the sea fog area identified from the satellite imagery. It is shown that advection cooling effect plays a significant role in the fog formation. 相似文献
406.
Yuki Kimura Joseph A. Nuth Subrata Chakraborty Mark H. Thiemens 《Meteoritics & planetary science》2007,42(7-8):1429-1439
Abstract— We report the first production of non‐mass‐dependently fractionated silicate smokes from the gas phase at room temperature from a stream of silane and/or pentacarbonyl iron in a molecular hydrogen (or helium) flow mixed with molecular oxygen (or nitrous oxide). The smokes were formed at the Goddard Space Flight Center (GSFC) at total pressures of just under 100 Torr in an electrical discharge powered by a Tesla coil, were collected from the surfaces of the copper electrodes after each experiment and sent to the University of California at San Diego (UCSD) for oxygen isotopic analysis. Transmission electron microscopy studies of the smokes show that they grew in the gas phase rather than on the surfaces of the electrodes. We hypothesize at least two types of fractionation processes occurred during formation of the solids: a mass‐dependent process that made isotopically lighter oxides compared to our initial oxygen gas composition followed by a mass‐independent process that produced oxides enriched in 17O and 18O. The maximum Δ17O observed is + 4.7‰ for an iron oxide produced in flowing hydrogen, using O2 as the oxidant. More typical displacements are 1–2‰ above the equilibrium fractionation line. The chemical reaction mechanisms that yield these smokes are still under investigation. 相似文献
407.
Joseph Silk 《Astronomy& Geophysics》2007,48(2):2.30-2.38
Joseph Silk of Oxford University presents his 2006 George Darwin Lecture on the cosmological case for dark matter and dark energy. 相似文献
408.
Saleem Zaroubi Rajat M. Thomas Naoshi Sugiyama Joseph Silk 《Monthly notices of the Royal Astronomical Society》2007,375(4):1269-1279
A simple analytical model is used to calculate the X-ray heating of the intergalactic medium (IGM) for a range of black hole masses. This process is efficient enough to decouple the spin temperature of the IGM from the cosmic microwave background (CMB) temperature and produce a differential brightness temperature of the order of ∼ 5–20 mK out to distances as large as a few comoving Mpc, depending on the redshift, black hole mass and lifetime. We explore the influence of two types of black holes, those with and without ionizing ultraviolet radiation. The results of the simple analytical model are compared to those of a full spherically symmetric radiative transfer code. Two simple scenarios are proposed for the formation and evolution of black hole mass density in the Universe. The first considers an intermediate mass black hole that form as an end-product of pop III stars, whereas the second considers supermassive black holes that form directly through the collapse of massive haloes with low spin parameter. These scenarios are shown not to violate any of the observational constraints, yet produce enough X-ray photons to decouple the spin temperature from that of the CMB. This is an important issue for future high-redshift 21-cm observations. 相似文献
409.
Charles Joseph Byrne 《Earth, Moon, and Planets》2007,101(3-4):153-188
The differences between the surface structure of the near side and the far side of the Moon have been topics of interest ever
since photographs of the far side have been available. One recurrent hypothesis is that a large impact on the near side has
deposited ejecta on the far side, resulting in thicker crust there. Specific proposals were made by P.H. Cadogan for the Gargantuan
Basin and by E.A. Whitaker for the Procellarum Basin. Despite considerable effort, no consensus has been reached on the existence
of these basins. The problem of searching for such a basin is one of finding its signature in a somewhat chaotic field of
basin and crater impacts. The search requires a model of the topographic shape of an impact basin and its ejecta field. Such
a model is described, based on elevation data of lunar basins collected by the Lidar instrument of the Clementine mission
and crustal thickness data derived from tracking Clementine and other spacecraft. The parameters of the model are scaled according
to the principles of dimensional analysis and isostatic compensation in the early Moon. The orbital dynamics of the ejecta
and the curvature of the Moon are also taken into account. Using such a scaled model, a search for the best fit for a large
basin led to identification of a basin whose cavity covers more than half the Moon, including the area of all of the impact
basins visible on the near side. The center of this basin is at 22 degrees east longitude and 8.5 degrees north latitude and
its average radius is approximately 3,160 km. It is a megabasin, a basin that contains other basins (the far side South Pole-Aitken
Basin also qualifies for that designation). It has been called the Near Side Megabasin. Much of the material ejected from
the basin escaped the Moon, but the remainder formed an ejecta blanket that covered all of the far side beyond the basin rim
to a depth of from 6 to 30 km. Isostatic compensation reduced the depth relative to the mean surface to a range of 1–5 km,
but the crustal thickness data reveals the full extent of the original ejecta. The elevation profile of the ejecta deposited
on the far side, together with modification for subsequent impacts by known basins (especially the far side South Pole-Aitken
Basin) matches the available topographic data to a high degree. The standard deviation of the residual elevations (after subtracting
the model from the measured elevations) is about one-half of the standard deviation of the measured elevations. A section
on implications discusses the relations of this giant basin to known variations in the composition, mineralogy, and elevations
of different lunar terranes. 相似文献
410.
Marios Kampakoglou Joseph Silk 《Monthly notices of the Royal Astronomical Society》2007,380(2):646-656
Galaxy discs are characterized by star formation histories that vary systematically along the Hubble sequence. We study global star formation, incorporating supernova feedback, gas accretion and enriched outflows in discs modelled by a multiphase interstellar medium in a fixed gravitational potential. The star formation histories, gas distributions and chemical evolution can be explained in a simple sequence of models which are primarily regulated by the cold gas accretion history. 相似文献