Rainfall Conditions in Equatorial East Africa during the Nineteenth Century as Inferred from the Record of Lake Victoria     

Sharon E. Nicholson  Xungang Yin 《Climatic change》2001,48(2-3):387-398

The East African lakes have exhibited dramaticfluctuations on both historical and paleo-climatictime scales. Levels of these lakes, and otherhistorical indicators in Africa, suggested thatenvironmental conditions in the nineteenth centurywere much more extreme than anything evident in themodern record. In this study, a water balance modelis used to estimate the rainfall associated with theseconditions, based on the Lake Victoria record. Theresults suggest that the conditions were not unlikeanomalous periods found during the twentieth century,but they may have persisted for somewhat longerperiods of time. The paper also demonstrates ageneric tool that can be applied to interpretinghistorical and paleo-lake levels in quantitative termsof rainfall.  相似文献   

Origin of Dissolved Groundwater Sulphate in Coastal Plain Sediments of the Rio de la Plata, Eastern Argentina     

Williams S. Logan  Ronald V. Nicholson 《Aquatic Geochemistry》1997,3(4):305-328

Groundwater of the coastal plain of the Rio de La Plata, Argentina,contains up to 17 g L^-1 SO₄ and 37 gL^-1 TDS. Some of this SO₄ is from paleo-seawater intrusion; however, SO₄ : Cl ratios can be>2 : 1, and most of the SO₄ must, therefore, have another source. Three possible sources were investigated: gypsum, organic matter,and iron sulphides. Dissolved SO₄ showed δ³⁴S valuesfrom -7 to 0‰, typical values for S from iron sulphides or organicmatter, but distinct from that of seawater (+22‰). To test whetherthe SO₄ was derived from oxidation of reduced S, four 4-mcores were taken from marine sediments of the coastal plain. Two were takenfrom higher, drier areas where the highest dissolved SO₄values were encountered, and two were taken from lower, wetter areas thathad much lower SO₄ concentrations. Pore waterSO₄, Cl and alkalinity were determined; solids were analyzedfor SO₄, sulphide-S and organic-S. Sulphide-S was the dominant form of reduced S, averaging about0.5% S in the lower interval (2.5–4 m) of the cores. Sulphidewas absent in the upper 2.5 m in both topographically higher and lowerareas. Sulphate was present in the entire unit in the higher, drier areas,but almost absent in lower areas. Organic-S was insignificant. Our model for the origin of dissolved SO₄ is: fine-grainedpyrite was oxidized during hotter or drier periods. Some resulting dissolvedSO₄ was precipitated as gypsum. Iron from the pyriteprecipitated as FeOOH. Lower, wetter areas formed over time where recharginggroundwater dissolved most of the gypsum. In higher areas with low hydraulicgradients and high net evapotranspiration, SO₄ remained asgypsum and in the dissolved phase. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Relaxation Oscillations in Tidally Evolving Satellites     

Dane Quinn  Brett Gladman  Phil Nicholson  Richard Rand 《Celestial Mechanics and Dynamical Astronomy》1997,67(2):111-130

We study the rotational evolution under tidal torques of axisymmetric natural satellites in inclined, precessing orbits. In the spin- and orbit-averaged equations of motion, we find that a global limit cycle exists for parameter values near the stability limit of Cassini state . The limit cycle involves an alternation between states of near-synchronous spin at low obliquity, and strongly subsynchronous spin at an obliquity near 90°. This dynamical feature is characterized as a relaxation oscillation, arising as the system slowly traverses two saddle-node bifurcations in a reduced system. This slow timescale is controlled by ε, the nondimensional tidal dissipation rate. Unfortunately, a straightforward expansion of the governing equations for small ε is shown to be insufficient for understanding the underlying structure of the system. Rather, the dynamical equations of motion possess a singular term, multiplied by ε, which vanishes in the unperturbed system. We thus provide a demonstration that a dissipatively perturbed conservative system can behave qualitatively differently from the unperturbed system. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Temporal and spatial variability of SSTs in the tropical Atlantic and Indian Oceans     

S. E. Nicholson  B. S. Nyenzi 《Meteorology and Atmospheric Physics》1990,42(1):1-17

Summary The objective of this study is to describe spatial and temporal patterns of sea-surface temperature (SST) variability in the Atlantic and Indian Oceans. The analysis domain extends from 40°S to 25°N and 50°W to 80°E, hence the tropical and most of the South Atlantic and central and western Indian Oceans. The investigation, covering the years 1948 to 1979, utilizes the COADS marine data set. Empirical orthogonal functions and spectral analysis are used to analyze SST fields.A major finding of this investigation is that SSTs vary coherently throughout most of the analysis domain. The greatest coherence is evident from 10°N to 30°S in the Atlantic and from 20°N to 35°S in the western Indian Ocean. Spectral analysis of regional time series shows that throughout this region the time scale of 5–6 years is the dominant one in the fluctuations; this is also the case for the Southern Oscillation and for equatorial rainfall. SST variations are roughly in-phase within each ocean and the two oceans are roughly in-phase with each other, i.e., the lags which exist are much smaller than the dominant time scale of the fluctuations. The SST anomalies appear to propagate eastward from NE Brazil; the eastern Atlantic lags the western by two to six months and the Indian Ocean lags the western Atlantic by four to eight months.With 15 Figures  相似文献   

A mark-recapture estimator of stock sizes from unrefined data applied to a stock of Dover sole (Solea solea L.)     

Horwood  J. W.; Nicholson  M. D. 《ICES Journal of Marine Science》1991,48(1):33-39

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Independent radio-occultation studies of venus' atmosphere     

Philip D. Nicholson  Duane O. Muhleman 《Icarus》1978,33(1):89-101

Two independent analyses of the dual-frequency radio-occultation experiment performed by Mariner 10 at Venus are presented. Using closed-loop frequency data obtained at NASA's Goldstone facility, we have computed S- and X-band pressure-temperature profiles for Venus' neutral atmosphere, and an S-band profile of the nightside ionosphere. Neutral atmosphere dispersion between the two frequencies is negligible (less than 0.1% in refractivity), as expected for a CO₂ atmosphere. The results confirm those obtained by Howard et al. (1974) from the same S-band data with an accuracy of ±5°K at a given pressure level, though there is a discrepancy of 1 km in the radial scale between the two analyses. These two Mariner 10 profiles are compared with the Mariner 5 occultation profile and in situ measurements by Veneras 8, 9, and 10. The occultation was also monitored at the Owens Valley Radio Observatory, though only at X-band. Despite the much lower quality of these data, a reasonable neutral atmosphere refractivity profile above 65 km was obtained from the occultation entry. Uncertainties in the calculated temperatures, however, are too large to permit useful comparison with previous results. The existence of real anomalies in both the amplitude and frequency of the signal during exit from occultation is confirmed.  相似文献   

Rock control on microweathering of bedrock surfaces in a periglacial environment     

Dawn T. Nicholson   《Geomorphology》2008,101(4):655-665

Microweathering of ice-smoothed bedrock surfaces was investigated in the Røldal area of Hardanger Plateau (60°), southern Norway. Postglacial rates of weathering were determined from surface lowering using quartz veins as reference surfaces. Weathering processes are inferred from assessment of weathering rind formation, surface hardness, and the preservation of small-scale glacial erosional features.Surface lowering rates for a range of metamorphic rocks vary from 0.05 to 2.20 mm ka^− 1 and are broadly comparable with those obtained from crystalline rocks in other periglacial environments. The mean rate of surface lowering at 0.55 mm ka^− 1 is low and demonstrates the relatively small impact of microweathering on postglacial landscape evolution. Variations in bedrock microweathering can be explained by lithological variation. Amphibolite and mica-rich bedrock surfaces experience greater denudation and weakening, least weathering rind formation, and abundant preservation of glacial striae, despite greater surface lowering. Conversely, quartz-rich bedrock surfaces are most resistant to denudation and weakening, but have greater weathering rind formation and fewer preserved striae. Postglacial microweathering is achieved primarily through granular disintegration involving detachment and removal of mineral grains and weakening from increased porosity. Granular decomposition is manifest in the formation of weathering rinds. Analysis of interactions between weathering indices indicates that rind accumulation is limited by microerosion.A conceptual model is proposed that illustrates the temporal interrelationships between in situ and erosional facets of microweathering in two contrasting mineral assemblages. The model proposes that cyclic processes of in situ disintegration, decomposition, and erosion are at work. The relative balance between these processes varies with lithology so that in more resistant quartz-rich rocks the net effect is minimal surface lowering and accumulation of weathering rind. In weaker, amphibolitic and micaceous rocks, the net effect is greater surface lowering and minimal accumulation of weathering rind. The results of the research demonstrate the important influence of rock properties, notably mineral composition, in postglacial microweathering of crystalline bedrock in a periglacial environment.  相似文献   

Using smoothers for comprehensive assessments of contaminant time series in marine biota   总被引：1，自引：1，他引：0  

Fryer  R. J.; Nicholson  M. D. 《ICES Journal of Marine Science》1999,56(5):779-790

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Geomorphological significance of Ontario Lacus on Titan: Integrated interpretation of Cassini VIMS,ISS and RADAR data and comparison with the Etosha Pan (Namibia)     

T. Cornet  O. Bourgeois  S. Le Mouélic  S. Rodriguez  T. Lopez Gonzalez  C. Sotin  G. Tobie  C. Fleurant  J.W. Barnes  R.H. Brown  K.H. Baines  B.J. Buratti  R.N. Clark  P.D. Nicholson 《Icarus》2012,218(2):788-806

Ontario Lacus is the largest lake of the whole southern hemisphere of Titan, Saturn’s major moon. It has been imaged twice by each of the Cassini imaging systems (Imaging Science Subsystem (ISS) in 2004 and 2005, Visual and Infrared Mapping Spectrometer (VIMS) in 2007 and 2009 and RADAR in 2009 and 2010). We compile a geomorphological map and derive a “hydrogeological” interpretation of Ontario Lacus, based on a joint analysis of ISS, VIMS and RADAR SAR datasets, along with the T49 altimetric profile acquired in December 2008. The morphologies observed on Ontario Lacus are compared to landforms of a semi-arid terrestrial analog, which resembles Titan’s lakes: the Etosha Pan, located in the Owambo Basin (Namibia). The Etosha Pan is a flat-floored depression formed by dissolution, under semi-arid conditions, of a surface evaporitic layer (calcretes) controlled by groundwater vertical motions. We infer that Ontario Lacus is an extremely flat and shallow depression lying in an alluvial plain surrounded by small mountain ranges under climatic conditions similar to those of terrestrial semi-arid regions. Channels are seen in the southern part of Ontario Lacus in VIMS and RADAR data, acquired at a 2-years time interval. Their constancy in location with time implies that the southern portion of the depression is probably not fully covered by a liquid layer at the time of the observations, and that they most probably run on the floor of the depression. A shallow layer of surface liquids, corresponding to the darkest portions of the RADAR images, would thus cover about 53% of the surface area of the depression, of which almost 70% is located in its northern part. These liquid-covered parts of the depression, where liquid ethane was previously identified, are interpreted as topographic lows where the “alkanofer” raises above the depression floor. The rest of the depression, and mostly its southern part, is interpreted as a flat and smooth exposed floor, likely composed of a thick and liquid-saturated coating of photon-absorbing materials in the infrared. This hypothesis could explain its dark appearance both in the infrared and radar data and the persistence of channels seen on the depression floor over the time. Shorelines are observed on the border of Ontario Lacus suggesting past high-stand levels of the alkanofer table. The analogy with the Etosha Pan suggests that Ontario Lacus’ depression developed at the expense of a soluble layer covering the region. Dissolution of this layer would be controlled by vertical motions of the alkanofer table over the time. During flooding events, liquid hydrocarbons covering the depression floor would dissolve the surface layer, increasing progressively the diameter of the depression on geological timescales. During drought episodes, liquid hydrocarbons of the underground alkanofer would evaporate, leading to crystallization of “evaporites” in the pores and at the surface of the substratum, and to the formation of the regional soluble layer. The presence of specific landforms (lunette dunes or evaporites) is compatible with such evaporitic regional settings. Alternatively, but not exclusively, the surface soluble layer might have formed by accumulation on the ground of soluble compounds formed in the atmosphere.  相似文献   

Dissipation of Titan's north polar cloud at northern spring equinox     

Stéphane Le Mouélic  Pascal Rannou  Sébastien Rodriguez  Christophe Sotin  Caitlin A. Griffith  Lucille Le Corre  Jason W. Barnes  Robert H. Brown  Kevin H. Baines  Bonnie J. Buratti  Roger N. Clark  Philip D. Nicholson  Gabriel Tobie 《Planetary and Space Science》2012,60(1):86-92

Saturn's Moon Titan has a thick atmosphere with a meteorological cycle. We report on the evolution of the giant cloud system covering its north pole using observations acquired by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft. A radiative transfer model in spherical geometry shows that the clouds are found at an altitude between 30 and 65 km. We also show that the polar cloud system vanished progressively as Titan approached equinox in August 2009, revealing at optical wavelengths the underlying sea known as Kraken Mare. This decrease of activity suggests that the north-polar downwelling has begun to shut off. Such a scenario is compared with the Titan global circulation model of Rannou et al. (2006), which predicts a decrease of cloud coverage in northern latitudes at the same period of time.  相似文献