Sesquiterpenoids in sediments of a hypersaline lagoon: A possible algal origin     

V. O. Elias  A. M. A. De Barros  A. B. de Barros  B. R. T. Simoneit  J. N. Cardoso 《Organic Geochemistry》1997,26(11-12)

Core samples were collected in Lagoa Vermelha, a hypersaline lagoon located about 100 km east of Rio de Janeiro (Brazil). The sediment composition is predominantly carbonate in amounts up to 93%. Analysis of δ¹³C of the total organic matter in the sediments showed that marine organic matter predominates throughout the core (δ¹³C ranges from −15.84 to −22.64‰ vs. PDB). Organic carbon contents (TOC) ranged from 0.81 to 13.28%. A series of cadinane-type sesquiterpenoids can be recognized in the gas chromatography-mass spectrometry data. Essentially the same components are present in all the samples, with variations only in their relative abundances. The most abundant compounds are α- and β-cubebene, α- and β-cedrene, cadinenes (different isomers), α-curcumene and calamenene, with minor amounts of calarene, humulene, calacorene and cadalene. Since this lagoon is surrounded by dunes with only minor vegetation typical of this environment (grasses, small non-resinous shrubs and no forest) with no potential source for sesquiterpenoids, a terrestrial origin for these compounds is excluded and an algal origin is more consistent with the locale and the recognition of sesquiterpenoids (including cadinol) in microbial mats from the lagoon. Only the natural product precursor sesquiterpenoids are present in the microbial mats with no detectable diagenetic derivatives (e.g. calamenene and cadalene). This indicates that the compounds in the mats are from recent input and those found in the sediments are most likely derived from former algal biomass in this lagoon, a fact confirmed by the recognition of a series of diagenetic aromatic components in the sediments. Surface sediments contain n-alkanes with no even-to-odd predominance indicating that microbial activity is higher in shallower sediments. Moreover, mass fragmentograms (m/z 191) of biomarkers revealed the presence of 17α(H),21β(H)-hopanes, the mature isomers, together with their ββ precursors and low amounts of the intermediates with the βα configuration (moretanes). This indicates a contribution of mature organic matter to these immature sediments.  相似文献   

Comments on the origin of dust in East Antarctica for present and ice age conditions     

A. Gaudichel  M. De Angelis  S. Joussaume  J. R. Petit  Y. S. Korotkevitch  V. N. Petrov 《Journal of Atmospheric Chemistry》1992,14(1-4):129-142

We have studied the distribution of 327 clay mineral particles retrieved from four Antaretic ice smaples corresponding to present and Last Glacial Maximum (LGM) climate conditions. Illite, chlorite, smectite and kaolinite were identified in all samples. Focusing on kaolinite, because of its use as a possible tracer of low latitude soils, we find a significantly smaller amount for LGM samples while the dust concentration in snow during the LGM was about 30 times higher than for present climate conditions. This can be interpreted as change in the contribution of the Australian source with climate.A second approach was based on the modeling of the desert dust cycle using an Atmospheric General Circulation Model (AGCM) under both present-day and ice age conditions. Unlike mineralogical results, the model suggests the prevalence of the Australian dust source in the deposits over East Antarctica under both present-day and LGM climate conditions. However the model fails to reproduce the strong increase in dust deposits during the LGM. This discrepancy could be partly due to the lack of a higher latitude dust source in the model.The stronger dust input recorded in ice cores for the LGM could be related to an additional active high latitude source (possibly close to South America) overlapping the atmospheric background coming from low latitude areas.  相似文献   

Hillslope erosion by rainstorms–a magnitude-frequency analysis     

Jan De Ploey  Michael J. Kirkby  Frank Ahnert 《地球表面变化过程与地形》1991,16(5):399-409

Daily rainfall data for four stations in Europe and east Africa are used to obtain, by means of magnitude-frequency analysis, a measure of the Cumulative Erosion Potential (CEP) that takes into account rainfall characteristics, soil water storage, and granulometric properties of soils. The CEP has the advantage of being calculated from generally available published rainfall data, so that wide regional coverage is possible. Together with additional data on the surface configuration and on seasonal variations of rainfall and plant cover, the CEP can provide a basis for estimating hillslope erosion by overland flow.  相似文献   

Regional modelling of groundwater flow and salt and environmental tracer transport in deep aquifers in the Paris Basin     

Huai Fu Wei  Emmanuel Ledoux  Ghislain De Marsily 《Journal of Hydrology》1990,120(1-4):341-358

A hydrodynamic model which takes into account the aquitard storage effect was developed for the deep multilayered system including the Dogger aquifer and its surroundings in the Paris Basin. It provides a good explanation for a series of observations in the Dogger concerning, for instance, the hydraulic head, the salinity and the transmissivity. The calibration of the model also makes it possible to estimate some unmeasured parameters such as the aquifer and aquitard storage coefficients. Finally, the results of simulations of the transport of ⁴He and ¹⁴C strengthen the representativeness of the model. The Darcy average horizontal velocity in the Dogger, obtained with the model, is of the order of 0.33m year⁻¹.  相似文献   

Structural Geology, principles, concepts, and problems: Hatcher, R. D. 1989. Merrill Publishing Co., Columbus, Ohio, U.S.A. 531 pp. Price $49.95 (hardback)     

Declan G. De Paor 《Journal of Structural Geology》1990,12(8)

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Nonlinear kinetic Alfvén wave with Poisson equation correction in the low aurora     

De Yu Wang  Guang Li Huang  C. -G. Fälthammar  P. A. Lindqvist  L. Blomberg  Li Ting Song 《Astrophysics and Space Science》1996,240(2):175-186

Nonlinear kinetic Alfvén waves where m _e /m _i , have been solved both with and without the Poisson equation correction. It is found that the ratio of the perpendicular electric field and magnetic field, and the ratio of parallel and perpendicular electric field increase with deepening of the depressive density soliton. The former ratio may be larger than the Alfvén velocity in the case of a large amplitude solitary kinetic Alfvén wave. The Poisson equation correction is important for the nonlinear kinetic Alfvén wave propagating along the magnetic field, which solves a puzzle of Sagdeev potential to approach infinity in the limit ofK _x 0. This correction causes the solitary KAW possessing an electrostatic character along the direction of wave moving frame. These results have been compared with the observations from the Freja satellite in the low aurora.  相似文献   

Observing molecular clouds in halo dark clusters through absorption lines     

F. De Paolis  G. Ingrosso  Ph. Jetzer  M. Roncadelli 《Astrophysics and Space Science》1996,235(2):329-335

In addition to Massive Astrophysical Compact Halo Objects (MACHOs), as detected by microlensing, cold molecular clouds (mainly ofH ₂) may well contribute substantially to the galactic halo dark matter. Here, we argue that the existence of such halo clouds can be inferred from the observation of absorption lines (due to heavy molecules located in the clouds themselves) towards stars of the Large Magellanic Cloud, which lie very close, within 1, to a previously microlensed one.The possibility of clusters of MACHOs has been investigated by several authors (see e.g. Carr and Lacey, 1987; Ashman, 1990; Eichler and Silk, 1992 and Wasserman and Salpeter, 1994).  相似文献   

The Intermittently Embedded Herbig Ae/Be Stars ISO and Ground-Based IR Observations     

C.A. Grady  M.R. Pérez  K.S. Bjorkman  M.L. Sitko  P.S. Thé  D. De Winter  V.P. Grinin  R.W. Russell  D.K. Lynch  M.S. Hanner 《Astrophysics and Space Science》1997,255(1-2):35-41

We discuss silicate emission profiles observed with the ISO SWS and ground-based IR observations in a population of intermittently embedded Herbig Ae/Be stars which are viewed edge-on to their polarimetrically identified dust and gas disks. The ISO SWS observations confirm the lack of a simple correlation between system age and the profile shape. Comparison with laboratory silicates suggests that much of the observed variation is due to different annealing histories of the grains. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

ISO-SWS Spectra of OH/IR Stars     

J. Cami  T. De Jong  K. Justtannont  I. Yamamura  L.B.F.M. Waters 《Astrophysics and Space Science》1997,255(1-2):339-340

We present the spectra of 5 OH/IR stars observed with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). The spectra are dominated by dust features, both in the amorphous and crystalline state and illustrate the influence of the mass loss rate on the observed spectral features. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

STRUCTURE AND ROTATION OF THE SOLAR INTERIOR: INITIAL RESULTS FROM THE MDI MEDIUM-L PROGRAM     

Kosovichev  A. G.  Schou  J.  Scherrer  P. H.  Bogart  R. S.  Bush  R. I.  Hoeksema  J. T.  Aloise  J.  Bacon  L.  Burnette  A.  De Forest  C.  Giles  P. M.  Leibrand  K.  Nigam  R.  Rubin  M.  Scott  K.  Williams  S. D.  Basu  Sarbani  Christensen-dalsgaard  J.  DÄppen  W.  Duvall  T. L.  Howe  R.  Thompson  M. J.  Gough  D. O.  Sekii  T.  Toomre  J.  Tarbell  T. D.  Title  A. M.  Mathur  D.  Morrison  M.  Saba  J. L. R.  Wolfson  C. J.  Zayer  I.  Milford  P. N. 《Solar physics》1997,170(1):43-61

The medium-l program of the Michelson Doppler Imager instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l, from 0 to 300. The data for the program are partly processed on board because only about 3% of MDI observations can be transmitted continuously to the ground. The on-board data processing, the main component of which is Gaussian-weighted binning, has been optimized to reduce the negative influence of spatial aliasing of the high-degree oscillation modes. The data processing is completed in a data analysis pipeline at the SOI Stanford Support Center to determine the mean multiplet frequencies and splitting coefficients. The initial results show that the noise in the medium-l oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. This is important for inferring the Sun's internal structure and rotation. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean frequencies gives evidence for a sharp variation at the edge of the energy-generating core. The results also confirm the previous finding by the GONG (Gough et al., 1996) that, in a thin layer just beneath the convection zone, helium appears to be less abundant than predicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer. This layer is likely to be the place where the solar dynamo operates. In order to understand how the Sun works, it is extremely important to observe the evolution of this transition layer throughout the 11-year activity cycle.  相似文献