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   

Supervised Mineral Classification with Semiautomatic Training and Validation Set Generation in Scanning Electron Microscope Energy Dispersive Spectroscopy Images of Thin Sections     

Harald Flesche  Allan Aasbjerg Nielsen  Rasmus Larsen 《Mathematical Geology》2000,32(3):337-366

This paper addresses the problem of classifying minerals common in siliciclastic and carbonate rocks. Twelve chemical elements are mapped from thin sections by energy dispersive spectroscopy in a scanning electron microscope (SEM). Extensions to traditional multivariate statistical methods are applied to perform the classification. First, training and validation sets are grown from one or a few seed points by a method that ensures spatial and spectral closeness of observations. Spectral closeness is obtained by excluding observations that have high Mahalanobis distances to the training class mean. Spatial closeness is obtained by requesting connectivity. Second, class consistency is controlled by forcing each class into 5–10 subclasses and checking the separability of these subclasses by means of canonical discriminant analysis. Third, class separability is checked by means of the Jeffreys–Matusita distance and the posterior probability of a class mean being classified as another class. Fourth, the actual classification is carried out based on four supervised classifiers all assuming multinormal distributions: simple quadratic, a contextual quadratic, and two hierarchical quadratic classifiers. Overall weighted misclassification rates for all quadratic classifiers are very low for both the training (0.25–0.33%) and validation sets (0.65–1.13%). Finally, the number of rejected observations in routine runs is checked to control the performance of the SEM image acquisition and the classification. Although the contextual classifier performs marginally best on the validation set, the simple quadratic classifier is chosen in routine classifications because of the lower processing time required. The method is presently used as a routine petrographical analysis method at Norsk Hydro Research Centre. The data can be approximated by a Poisson distribution. Accordingly, the square root of the data has constant variance and a linear classifier can be used. Near orthogonal input data, enable the use of a minimum distance classifier. Results from both linear and quadratic minimum distance classifications are described briefly.  相似文献   

Young massive star clusters in M51     

Søren S. Larsen 《Monthly notices of the Royal Astronomical Society》2000,319(3):893-901

A search for young massive star clusters (YMCs) in the nearby face-on spiral galaxy M51 (NGC 5194) has been carried out using UBV CCD images from the prime focus camera on the Lick 3-m Shane telescope. The YMC population is found to be quite rich with a specific U -band luminosity     consistent with the high current star formation rate (SFR) of this galaxy. The brightest clusters have     far brighter than any young clusters currently known in the Milky Way and even surpassing the luminosity of the R136 cluster in the 30 Dor complex in the Large Magellanic Cloud. A few of the YMCs are examined on archive images from the Wide Field Planetary Camera (WF/PC2) on board the Hubble Space Telescope ( HST ), confirming their cluster nature and providing estimates of their effective radii of 2–3 pc. The number of YMCs in M51 is compatible with extrapolation of a power-law luminosity function with exponent ∼−2 from a Milky Way-like population of open clusters. Both the SFR and T _L ( U ) of M51 are similar to those of other cluster-rich spiral galaxies like NGC 1313 and M83.  相似文献   

CHOSEN: A synthesis of hydrometeorological data from intensively monitored catchments and comparative analysis of hydrologic extremes     

Liang Zhang  Edom Moges  James W. Kirchner  Elizabeth Coda  Tianchi Liu  Adam S. Wymore  Zexuan Xu  Laurel G. Larsen 《水文研究》2021,35(11):e14429

Comparative hydrology has been hampered by limited availability of geographically extensive, intercompatible monitoring data on comprehensive water balance stores and fluxes. These limitations have, for example, restricted comprehensive assessment of multiple dimensions of wetting and drying related to climate change and hampered understanding of why widespread changes in precipitation extremes are uncorrelated with changes in streamflow extremes. Here, we address this knowledge gap and underlying data gap by developing a new data synthesis product and using that product to detect trends in the frequencies and magnitudes of a comprehensive set of hydroclimatic and hydrologic extremes. CHOSEN (Comprehensive Hydrologic Observatory Sensor Network) is a database of streamflow, soil moisture, and other hydroclimatic and hydrologic variables from 30 study areas across the United States. An accompanying data pipeline provides a reproducible, semi-automated approach for assimilating data from multiple sources, performing quality assurance and control, gap-filling and writing to a standard format. Based on the analysis of extreme events in the CHOSEN dataset, we detected hotspots, characterized by unusually large proportions of monitored variables exhibiting trends, in the Pacific Northwest, New England, Florida and Alaska. Extreme streamflow wetting and drying trends exhibited regional coherence. Drying trends in the Pacific Northwest and Southeast were often associated with trends in soil moisture and precipitation (Pacific Northwest) and evapotranspiration-related variables (Southeast). In contrast, wetting trends in the upper Midwest and the Rocky Mountains showed few univariate associations with other hydroclimatic extremes, but their latitudes and elevations suggested the importance of changing snowmelt characteristics. On the whole, observed trends are incompatible with a ‘drying-in-dry, wetting-in-wet’ paradigm for climate-induced hydrologic changes over land. Our analysis underscores the need for more extensive, longer-term observational data for soil moisture, snow and evapotranspiration.  相似文献   

Holocene volcanic activity at Grímsvötn,Bárdarbunga and Kverkfjöll subglacial centres beneath Vatnajökull,Iceland     

Bergrún Arna Óladóttir  Gudrún Larsen  Olgeir Sigmarsson 《Bulletin of Volcanology》2011,73(9):1187-1208

Assessment of potential future eruptive behaviour of volcanoes relies strongly on detailed knowledge of their activity in the past, such as eruption frequency, magnitude and repose time. The eruption history of three partly subglacial volcanic systems, Grímsvötn, Bárdarbunga and Kverkfjöll, was studied by analysing tephra from soil profiles around the Vatnajökull ice-cap, which extend back to ~7.6 ka. Well known regional Holocene marker tephra (e.g. H3, H4, H5) were utilized to correlate profiles. Stratigraphic positions and geochemical compositions were used for fine-scale correlation of basaltic tephra. Around Vatnajökull ice-cap 345 tephra layers were identified, of which 70% originated from Grímsvötn, Bárdarbunga or Kverkfjöll. The eruption frequency of each volcanic system was estimated; Grímsvötn has been the most active with an average of ~7 eruptions/100 years (range 4–14) during prehistoric time (before ~870 AD); Bárdarbunga has been the second most active with ~5 eruptions/100 years (range 1–8); and Kverkfjöll has remained essentially calm with 0–3 eruptions/100 years but showing periodic activity with repose times of >1000 years. All three volcanic systems experienced lulls in activity from 5 ka to 2 ka, referred to as the “Mid-Holocene low”. This reduced eruption frequency appears to have resulted from a decrease in magma generation and delivery from the mantle plume rather than from changes in ice-load/glacier thickness. In prehistoric time, there was a time lag of 1000–3000 years between a peak of activity at volcanoes directly above the mantle plume versus at volcanoes located in the non-rifting part of the Eastern Volcanic Zone, closer to the periphery of the island. This time-space relationship suggests that a significant future increase in volcanism can be expected there, following increased levels of volcanism above the plume.  相似文献