Detailed PSC formation in a two-dimensional chemical transport model of the stratosphere     

D. Fonteyn  N. Larsen 《Annales Geophysicae》1996,14(3):315-328

A new two-dimensional zonal model of the stratosphere, based on a formulation in an isentropic framework, with complete chemistry has been coupled with a high resolution detailed microphysical model for polar stratospheric clouds (PSCs). The 2D model chemistry includes all presently known heterogeneous processes on sulfate aerosols and PSCs. The coupling of these two models, with inherently different time scales, is discussed. It is demonstrated that in order to obtain a realistic interrelationship between NO_y and N₂O an accurate simulation of the sedimentation by PSC particles is necessary. A good agreement of model PSC presence and observations is found for the Antarctic polar winter without the need to impose additional artificial temperature variations in the model. The calculated occurrence of polar stratospheric clouds and resulting heterogeneous chemistry during the Antarctic winter are discussed. Sensitivity of the polar stratospheric chemical composition and cloud formation for different perturbations is investigated by studying the effects of transport across the polar vortex boundary and heterogeneous processing by an enhanced sulfate aerosol load. The importance of including sedimentation for all cases is also discussed.  相似文献   

Complex exposure histories of chert clasts in the late Pleistocene shorelines of Lake Lisan,southern Israel     

Ari Matmon  Onn Crouvi  Yehouda Enzel  Paul Bierman  Jennifer Larsen  Naomi Porat  Rivka Amit  Marc Caffee 《地球表面变化过程与地形》2003,28(5):493-506

Activities of ²⁶Al and ¹⁰Be in five chert clasts sampled from two beach ridges of late Pleistocene Lake Lisan, precursor of the Dead Sea in southern Israel, indicate low rates of chert bedrock erosion and complex exposure, burial, and by inference, transport histories. The chert clasts were derived from the Senonian Mishash Formation, a chert‐bearing chalk, which is widely exposed in the Nahal Zin drainage basin, the drainage system that supplied most of the material to the beach ridges. Simple exposure ages, assuming only exposure at the beach ridge sampling sites, range from 35 to 354 ky; using the ratio ²⁶Al/¹⁰Be, total clast histories range from 0·46 to 4·3 My, unrelated to the clasts' current position and exposure period on the late Pleistocene beach ridges, 160–177 m below sea level. Optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20·0 ± 1·4 ka and 36·1 ± 3·3 ka. These ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of Lake Lisan, which suggest that the lake reached its highest stand around 27 000 cal. years BP . If the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production, then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50–1300 ky and 390–3130 ky respectively. Alternatively, the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock. Back calculating such rates of rock erosion suggests values between 0·4 and 12 m My^?1. The relatively long burial periods indicate extended sediment storage as colluvium on slopes and/or as alluvial deposits in river terraces. Some clasts may have been stored for long periods in abandoned Pliocene and early Pleistocene routes of Nahal Zin to the Mediterranean before being transported again back into the Nahal Zin drainage system and washed on to the shores of Lake Lisan during the late Pleistocene. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Barrier jets around the Southern Alps of New Zealand and their potential to enhance alpine rainfall     

Michael J. Revell  Jeff H. Copeland  Howard R. Larsen  David S. Wratt 《Atmospheric Research》2002,61(4)

We use new data from the SALPEX'96 campaign to examine the evolution of orographically modified flow over and around New Zealand's Southern Alps. We consider prefrontal situations for which the larger-scale synoptic pattern generates an incoming northwesterly flow perpendicular to the mountain axis, calculate the nondimensional parameters which control the wind flow and determine how long it takes orographic modifications to become established. We place flows over the New Zealand Alps in a Rossby number/Blocking number parameter space and compare them with flows over other mountain ranges on the globe. Using model and aircraft data, we show the region of modified flow extends over 100 km upstream of the Southern Alps.We show 2D model simulations represent many aspects of the flow modification satisfactorily, although they tend to overdevelop the final state of the Barrier Jet flow. In 3D, using real data to force the boundaries, the migratory nature of the weather systems in the area would generally limit the time available for a modified steady state to develop. In addition, in situations where there is significant lateral variation of initial fields, the 2D results may only be applicable for a few hours.From the point of view of potential to influence alpine rainfall processes, for the case studied, the gradual uplift associated with the modified flow between 1 and 2 km above sea level extended far enough upstream in saturated air to enable cloud drops to grow to drizzle drops before reaching the much more substantial updrafts at the foot of the Alps. This allows vigorous prefrontal precipitation to occur over the foothills and mountains due largely to warm rain processes.  相似文献   

Developing a Learning Progression for Place   总被引：1，自引：1，他引：0  

Thomas B. Larsen  John A. Harrington Jr. 《The Journal of geography》2018,117(3):100-118

The concept and characteristics of place within geography education are surveyed in the context of learning progressions. Beginning with place's history and relevance in geographic thought, this review moves on to discuss connections between space and place. From a geographic education perspective, place-based instruction and place characteristics are threaded with ideas of landscape and human encounter with or perception of place. Grade level standards can be problematic when dealing with a complex subject, such as place. Borrowing from educators in the sciences, this review reflects on the challenge of building one or more learning progressions for K–12 education on place.  相似文献   

Seismotectonic implications of sand blows in the southern Mississippi Embayment     

R.T. Cox  A.A. Hill  D. Larsen  T. Holzer  S.L Forman  T. Noce  C. Gardner  J. Morat 《Engineering Geology》2007,89(3-4):278-299

We explore seismically-induced sand blows from the southern Mississippi Embayment and their implications in resolving the question of near or distal epicentral source region. This was accomplished using aerial photography, field excavations, and cone penetration tests. Our analysis shows that three sand blow fields exhibit a distinct chronology of strong ground motion for the southern embayment: (1) The Ashley County, Arkansas sand blow field, near the Arkansas/Louisiana state border, experienced four Holocene sand venting episodes; (2) to the north, the Desha County field experienced at least three episodes of liquefaction; and (3) the Lincoln–Jefferson Counties field experienced at least one episode. Cone penetration tests (CPT) conducted in and between the sand blow fields suggest that the fields may not be distal liquefaction associated with New Madrid seismic zone earthquakes but rather are likely associated with strong earthquakes on local faults. This conclusion is consistent with the differences in timing of the southern embayment sand venting episodes and those in the New Madrid seismic zone. These results suggest that active tectonism and strong seismicity in intraplate North America may not be localized at isolated weak spots, but rather widespread on fault systems that are favorably oriented for slip in the contemporary stress field.  相似文献   

Arsenic in groundwater of the Red River floodplain, Vietnam: Controlling geochemical processes and reactive transport modeling   总被引：4，自引：0，他引：4  

Dieke Postma  Flemming Larsen  Mai Thanh Duc  Pham Quy Nhan 《Geochimica et cosmochimica acta》2007,71(21):5054-5071

The mobilization of arsenic (As) to the groundwater was studied in a shallow Holocene aquifer on the Red River flood plain near Hanoi, Vietnam. The groundwater chemistry was investigated in a transect of 100 piezometers. Results show an anoxic aquifer featuring organic carbon decomposition with redox zonation dominated by the reduction of Fe-oxides and methanogenesis. Enhanced P_CO2 pressure causes carbonate dissolution to take place but mainly in the soil and unsaturated zone. The concentration of As increases over depth to a concentration of up to 550 μg/L. Most As is present as As(III) but some As(V) is always found. Arsenic correlates well with NH₄, relating its release to organic matter decomposition and the source of As appears to be the Fe-oxides being reduced. Part of the produced Fe(II) is apparently reprecipitated as siderite containing less As. Results from sediment extraction indicate most As to be related to the Fe-oxide fractions. The measured amount of sorbed As is low. In agreement, speciation calculations for a Fe-oxide surface suggest As(III) to constitute only 3% of the surface sites while the remainder is occupied by carbonate and silica species. The evolution in water chemistry over depth is homogeneous and a reactive transport model was constructed to quantify the geochemical processes along the vertical groundwater flow component. A redox zonation model was constructed using the partial equilibrium approach with organic carbon degradation in the sediment as the only rate controlling parameter. Apart from the upper meter a constant degradation rate of 0.15 C mmol/L/yr could explain the redox zonation throughout the aquifer. Modeling also indicates that the Fe-oxide being reduced is of a stable type like goethite or hematite. Arsenic is contained in the Fe-oxides and is first released during their dissolution. Our model further suggests that part of the released As is adsorbed on the surface of the remaining Fe-oxides and in this way may be retarded.  相似文献   

The chemical evolution of gas-rich dwarf galaxies     

T. I. Larsen  J. Sommer-Larsen  B. E. J. Pagel 《Monthly notices of the Royal Astronomical Society》2001,323(3):555-576

In an effort to understand the evolution of N, O and He abundances in gas-rich dwarf galaxies, we investigate the dispersion and mixing of supernova ejecta in relation to H  ii region evolution and develop a numerical model of chemical evolution based on a double-bursting mode of star formation (with an interval of the order of 3×10⁷ yr between bursts of a pair) which has been designed to account for the existence of significant scatter in the N/O–O/H relation.
The dependence of the abundances on gas fraction is explored on the basis of this and similar models, in combination with various hypotheses concerning inflow and selective and non-selective outflow. The gas fractions are uncertain within wide limits for blue compact galaxies, but are more well defined for some dwarf irregulars. Selective winds do not give a good fit to N/O, while closed models and models with non-selective winds with or without inflow are all found to be viable.  相似文献   

The agricultural revolution as environmental catastrophe: Implications for health and lifestyle in the Holocene     

Clark Spencer Larsen   《Quaternary International》2006,150(1):12

One of the most fundamental developments in the history of our species—and one having among the most profound impacts on landscapes and the people occupying them—was the domestication of plants and animals. In addition to altering landscapes around the globe from the terminal Pleistocene and early Holocene, the shift from foraging to farming resulted in negative and multiple consequences for human health. Study of human skeletal remains from archaeological contexts shows that the introduction of grains and other cultigens and the increase in their dietary focus resulted in a decline in health and alterations in activity and lifestyle. Although agriculture provided the economic basis for the rise of states and development of civilizations, the change in diet and acquisition of food resulted in a decline in quality of life for most human populations in the last 10,000 years.  相似文献   

Palaeomagnetism and magnetomineralogy of a Holocene lake sediment from Vågsøy,western Norway     

Reidar Løvlie  Eiliv Larsen 《Physics of the Earth and Planetary Interiors》1981,27(2):143-150

Stable remanent directions encountered in three oriented lake-sediment cores, covering 6 m of gyttja deposits, reveal declination swings with amplitudes of the order of ± 50°. Anomalously low inclinations, scattered around the horizontal in the upper 2 m of core, is attributed to compressional disturbances of the alignment of magnetic grains during the coring procedure. Four ¹⁴C dates obtained (1250, 1940, 4600 and 6330 y B.P.) have enabled a correlation with features ‘c’–‘g’ of the British Geomagnetic Master Curve). For the older part of the curve, features ‘h’ and ‘i’, the correlation is questionable. Magneto-mineralogical parameters and qualitative chemical analysis of individual grains by secondary X-ray emission analysis reveal the presence of pure magnetite grains. Scanning electron micrographs show a dominant proportion of concoidal slightly elongated grains (10–80 μm). A few almost-perfect octahedral magnetite crystals can be related to outcrops of augen gneisses in the vicinity confirming a detrital origin of these grains.  相似文献   

The 1996 and 1998 subglacial eruptions beneath the Vatnajökull ice sheet in Iceland: contrasting geochemical and geophysical inferences on magma migration     

O. Sigmarsson  H. R. Karlsson  G. Larsen 《Bulletin of Volcanology》2000,61(7):468-476

87 Sr/⁸⁶Sr (0.70322) and δ ¹⁸O ( ∼2.9‰), whereas significantly lower and higher values, respectively, are found in samples from the Bárdarbunga volcanic system (0.70307 and 3.8‰). These results strongly indicate that the Gjálp magma originated from the Grímsv?tn magma system. The 1996 magma is of an intermediate composition, representing a basaltic icelandite formed by 50% fractional crystallization of a tholeiite magma similar in composition to that expelled by the 1998 Grímsv?tn eruption. The differentiation that produced the Gjálp magma may have taken place in a subsidiary magma chamber that last erupted in 1938 and would be located directly beneath the 1996 eruption site. This chamber was ruptured when a tectonic fracture propagated southward from Bárdarbunga central volcano, as indicated by the seismicity that preceded the eruption. Our geochemical results are therefore not in agreement with lateral magma migration feeding the 1996 Gjálp eruption. Moreover, the results clearly demonstrate that isotope ratios are excellent tracers for deciphering pathways of magma migration and permit a clear delineation of the volcanic systems beneath Vatnaj?kull ice sheet. Received: 1 April 1998 / Accepted: 17 August 1999  相似文献