Sensitivity of glaciation to initial snow cover,CO2, snow albedo,and oceanic roughness in the NCAR CCM     

Robert J Oglesby 《Climate Dynamics》1990,4(4):219-235

We present results from numerical experiments made with a GCM, the NCAR CCM1, that were designed to estimate the annual balance between snow-fall accumulation and ablation for geographically important land regions for a variety of conditions. We also attempt to assess the reliability of these results by investigating model sensitivity to changes in prescribed physical parameters. Experiments were run with an initial imposition of 1 m of (midwinter) snowcover over all northern hemisphere land points. Over Alaska, western Canada, Siberia, and the Tibetan Plateau the model tended to retain this snow cover through the summer and in some cases increase its depth as well. We define these regions as glaciation sensitive and note some correspondence between them and source regions for the Pleistocene ice sheets. An experiment with greatly reduced CO₂ (100 ppm) showed a tendency towards spontaneous glaciation, i.e., the model remained snow-covered throughout the summer over the same geographic regions noted above. With 200 ppm CO₂ (roughly equal to values at the last glacial maximum), snow cover over these regions did not quite survive the summer on a consistent basis. Combining 200 ppm CO₂ and 1 m of initial northern hemisphere snow cover yielded glaciation-sensitive conditions, agreeing remarkably well with locations undergoing glaciation during the Pleistocene. To assess the reliability of these results, we have determined minimal model uncertainty by varying two of the empirical coefficients in the model within physically plausible ranges. In one case surface roughness of all ocean gridpoints was reduced by an order of magnitude, leading to local 10% reductions in precipitation (snowfall), a change hard to distinguish from inherent model variability. In the other case, the fraction of a land grid square assumed to be occupied by snow cover for albedo purposes was varied from one-half to unity. Large changes occurred in the degree of summer melting, and in some cases the sign of the net balance changed as fractional snow cover was changed. We conclude that the model may be able to reveal regions sensitive to glaciation, but that it cannot yield a reliable quantitative computation of the magnitude of the net snow accumulation that can be implicitly or explicitly integrated through time.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dilmenil  相似文献   

Magmatism in the eastern Aleutian Arc: temporal characteristic of igneous activity on Akutan Island   总被引：1，自引：0，他引：1  

Jay D. Romick  Michael R. Perfit  Samuel E. Swanson  Robert D. Shuster 《Contributions to Mineralogy and Petrology》1990,104(6):700-721

Lavas from Akutan Island, located in the eastern Aleutian arc at the transition between continental and oceanic crust, show a gradual change in their petrologic and chemical characteristics over the last 4 million years. The oldest lavas exposed on the island, the Hot Springs Bay Volcanics (HSBV), range from magnesian basalt to dacite (45%–62% SiO₂). The most mafic basalts contain salitic clinopyroxene, Cr- and Al-rich spinel, and pargasitic amphibole suggesting that they were derived from relatively hydrous magmas at greater pressures than lavas from the younger Akutan Volcanics (AKV) and the modern volcano (MOD). AKV lavas also range between basalt and dacite (46%–63% SiO₂), but contain no hydrous phenocrysts and seem to have fractionated within a shallow level magma chamber. Lavas from the modern volcano are andesitic (52%–57% SiO₂) and have a mineral assemblage similar to that of AKV lavas of similar composition. With the exception of clinopyroxene and spinel in the most mafic lavas, the compositions of plagioclase (An_92?45), olivine (Fo_88?51), orthopyroxene (En_69?56), and titanomagnetite (15%–21% TiO₂) phenocrysts found in these lavas are within the range observed in lavas from other Aleutian volcanoes. Variations in the major element chemistry of the older lavas can be reproduced by fractional crystallization of the observed mineral assemblages, however closed system crystal fractionation models are inadequate to explain the trace element variations. During the last 4 million years, La/Yb ratios have decreased (6.5–3.3 for HSBV lavas and 2.9–1.9 for MOD lavas) whereas Ba/La ratios appear to have increased slightly (37–43 for HSBV and AKV, and 41–45 of MOD). The lower La/Yb ratios of MOD lavas correspond with lower total abundances of the REE and slightly higher Sr and Pb isotopic ratios. The increased⁸⁷Sr/⁸⁶Sr ratios and Pb isotopic ratios in the MOD lavas, the less enriched LREE, and the higher Ba/La ratios may result from partial melting of an arc source which has experienced previous melting events but has continued to be contaminated by a component from the subducting slab. It may also indicate a change in the degree of partial melting of the underlying mantle, which corresponds to a different percentage of a slab derived component being incorporated into the overlying mantle.  相似文献   

Monitoring underground nuclear explosions | O. Dahlman and H. Israelson, 1977. Elsevier,Amsterdam, 440 pp., Dfl. 120.00, U.S. $ 49.00     

Robert Uhrhammer 《Earth》1978,14(2):184-185

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The depth vs. depth plot: An aid to selecting piezometer installation depths in boreholes     

Robert C. Dick 《Engineering Geology》1976,10(1):37-42

Dick, R.C., 1976. The depth vs. depth plot: an aid to selecting piezometer installation depths in boreholes. Eng. geol., 10: 37–42.The location of the piezometric surface within a soil or rock mass is often the critical factor in stability analysis and in evaluations of reservoir watertightness. Sealed piezometers are now almost universally used to measure water pressure within the mass. The location of these piezometers must be decided immediately upon completion of each borehole. A common aid to making this decision is a plot of borehole depth and borehole water level as functions of time. This paper introduces the reader to another way to evaluate the same data, with the borehole water depth plotted as a function of borehole depth. Anomalies can be detected more easily, and piezometers installed within these zones. Examples are given.  相似文献   

THE MORMON VILLAGE: ANALYSIS OF A SETTLEMENT TYPE     

Richard H. Jackson  Robert L. Layton 《The Professional geographer》1976,28(2):136-141

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A maximal background spectrum of gravitational radiation     

Leonard A. Rosi  Robert L. Zimmerman 《Astrophysics and Space Science》1976,45(2):447-466

A maximal spectrum of gravitational radiation from sources outside our galaxy is calculated. The sources are galaxies, quasars and events that occur in the early history of the universe. The major contribution is from galaxies whose effect extends over the frequency region 10^–810⁺⁴Hz, peaking at 10^–110 Hz, with a spectral flux of 10 erg cm^–2, s^–1. The main processes of gravitational radiation in the galaxies are stellar collapse into a black hole and dying binary systems. In the region 10^–410⁴ Hz the background spectrum is well above the detection levels of currently proposed detectors. FromMinimal considerations of this spectrum it is determined that the density of gravitational radiation is 10^–39g cm^–3. This background spectrum is sensitive to galactic evolution and especially sensitive to the upper mass limits and mass distribution of stars in galactic models. Therefore, the spectrum could provide information about galactic evolution complementary to that obtained by electromagnetic investigations.  相似文献   

Evaluation Of The Epsilon Generalization When Applied To Rural-Urban Boundaries     

Robert M. Ward  Carole K. Wortley 《The Professional geographer》1975,27(2):205-210

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Transport and emplacement of crater and basin deposits     

Verne R. Oberbeck  Robert H. Morrison  Friedrich Hörz 《Earth, Moon, and Planets》1975,13(1-3):9-26

Material is ejected from impact craters in ballastic trajectories; it impacts first near the crater rim and then at progressively greater ranges. Ejecta from craters smaller than approximately 1 km is laid predominantly on top of the surrounding surface. With increasing crater size, however, more and more surrounding surface will be penetrated by secondary cratering action and these preexisting materials will be mixed with primary crater ejecta. Ejecta from large craters and especially basin forming events not only excavate preexisting, local materials, but also are capable of moving large amounts of material away from the crater. Thus mixing and lateral transport give rise to continuous deposits that contain materials from within and outside the primary crater. As a consequence ejecta of basins and large highland craters have eroded and mixed highland materials throughout geologic time and deposited them in depressions inside and between older crater structures.Because lunar mare surfaces contain few large craters, the mare regolith is built up by successive layers of predominantly primary ejecta. In contrast, the lunar highlands are dominated by the effects of large scale craters formed early in lunar history. These effects lead to thick fragmental deposits which are a mixture of primary crater material and local components. These deposits may also properly be named regolith though the term has been traditionally applied only to the relatively thin fine grained surficial deposit on mare and highland terranes generated during the past few billion year. We believe that the surficial highland regolith - generated over long periods of time - rests on massive fragmental units that have been produced during the early lunar history.  相似文献   

Diurnal variation in rates of calcification and carbonate sediment dissolution in Florida Bay   总被引：2，自引：0，他引：2  

Kimberly K. Yates  Robert B. Halley 《Estuaries and Coasts》2006,29(1):24-39

Water quality and criculation in Florida Bay (a shallow, subtropical estuary in south Florida) are highly dependent upon the development and evolution of carbonate mud banks distributed throughout the Bay. Predicting the effect of natural and anthropogenic perturbations on carbonate sedimentation requires an understanding of annual, seasonal, and daily variations in the biogenic and inorganic processes affecting carbonate sediment precipitation and dissolution. In this study, net calcification rates were measured over diurnal cycles on 27 d during summer and winter from 1999 to 2003 on mud banks and four representative substrate types located within basins between mud banks. Substrate types that were measured in basins include seagrass beds of sparse and intermediate densityThalassia sp., mud bottom, and hard bottom communities. Changes in total alkalinity were used as a proxy for calcification and dissolution. On 22 d (81%), diurnal variation in rates of net calcification was observed. The highest rates of net carbonate sediment production (or lowest rates of net dissolution) generally occurred during daylight hours and ranged from 2.900 to −0.410 g CaCO₃ m⁻²d⁻¹. The lowest rates of carbonate sediment production (or net sediment dissolution) occurred at night and ranged from 0.210 to −1.900 g CaCO₃ m⁻² night⁻¹. During typical diurnal cycles, dissolution during the night consumed an average of 29% of sediment produced during the day on banks and 68% of sediment produced during the day in basins. Net sediment dissolution also occurred during daylight, but only when there was total cloud cover, high turbidity, or hypersalinity. Diurnal variation in calcification and dissolution in surface waters and surface sediments of Florida Bay is linked to cycling of carbon dioxide through photosynthesis and respiration. Estimation of long-term sediment accumulation rates from diurnal rates of carbonate sediment production measured in this study indicates an overall average accumulation rate for Florida Bay of 8.7 cm 1000 yr⁻¹ and suggests that sediment dissolution plays a more important role than sediment transport in loss of sediment from Florida Bay.  相似文献   

Early diagenesis of trace metals (Cd, Cu, Co, Ni, U, Mo, and V) in the freshwater reaches of a macrotidal estuary     

Stéphane Audry  Sébastien Robert 《Geochimica et cosmochimica acta》2006,70(9):2264-2282

Vertical profiles from the water column, including the maximum turbidity zone (MTZ) to the consolidated sediment were sampled in September 2000 in the freshwater reaches of the Gironde Estuary during a complete neap tide-spring tide cycle. The vertical distributions of dissolved major redox parameters and metals (Mn, Fe, Cd, Cu, V, Co, Ni, Mo, and U) were determined. Reactive particulate metal fractions were also determined from selective leaching. The studied system is characterized by density layers functioning at different time-scales, consisting of two mobile layers, i.e., the liquid (LM) and the soft mud (SM), overlying consolidated sediments (CS). This results in a three-zone diagenetic regime where (1) O₂ dynamics are fast enough to show depletion in the rapidly mixed LM sequence (tidal time-scale), (2) denitrification occurs on the weekly time-scale mixing SM sequence, and (3) the Mn, Fe, and sulfate cycling occurs in the CS layer (annual time-scale). The studied trace metals show differential behavior during early diagenesis: (1) Cd, Cu, and V are released into pore water preferentially from organic matter in the SM, (2) Co, Ni, and U are released in the CS from Mn and Fe oxides during reductive dissolution, and (3) Mo from both processes. Transient conditions (i.e., oscillations of redox fronts and reoxidation processes), due to the dynamics of the mobile layers, strongly influence the trace metal distributions as inducing resolubilization (Cd, Cu, and Mo). In the CS, authigenic metal phases accumulate, either by direct precipitation with sulfides (Cu, Cd) or co-precipitation with Fe-sulfides (Mo). Microbially mediated reduction of Fe oxides is proposed to control U removal from pore water by reduction of U(VI) to U(IV) at depth. However, a significant fraction of the trace metals is trapped in the sediment in exchangeable forms, and therefore is susceptible to be mobilized due to resuspension of estuarine sediment during strong river flood periods and/or dredging activities.  相似文献