Anaerobic bacterial degradation of pristenes and phytenes in marine sediments does not lead to pristane and phytane during early diagenesis     

《Organic Geochemistry》2013

Anaerobic degradation of mixtures of isomeric pristenes and phytenes by sedimentary bacterial communities was investigated. These isoprenoid alkenes were quickly metabolized without formation of the corresponding alkanes. Identification of several bacterial metabolites allowed us to confirm the key role played by hydration in the metabolism of alkenes. Despite the increasing production of 5α-stanols during incubation, attesting to the involvement of biohydrogenation, this process did not act significantly on the double bond of pristenes and phytenes. Their resistance is attributed to the lack of binding polar group(s) to anchor the substrate to the enzyme and thus allow the double bond to reach the reductive catalytic site. It therefore seems that microbial hydrogenation of pristenes and phytenes does not account for the presence of pristane and phytane in recent sediments during early diagenesis.  相似文献   

On stress and strain in a continuous-discontinuous shear zone undergoing simple shear and volume loss     

《Journal of Structural Geology》2013

I summarise observations within a continuous-discontinuous shear zone to discuss the local stress and strain conditions experienced within a mixed rheology shear zone undergoing volume loss and deformation approximating simple shear. The Chrystalls Beach Complex, New Zealand, comprises phacoids formed from dismembered beds by layer-parallel extension, enclosed within a relatively incompetent matrix. Local extension is generally subparallel to the regional direction of shortening, and overall it appears that layer-parallel extension is a geometrical necessity in low angle shear zones where significant flattening occurs in response to simple shear accompanied by volume loss.Preferential stress loading of phacoids is predicted by fibre-loading theory, and the failure of phacoids by brittle fracture is thereby governed by fibre stresses transferred from the matrix. The principal stress orientations in a phacoid are likely rotated relative to the matrix, and either parallel or perpendicular to the phacoid-matrix interface. As preferential loading of phacoids decreases the stress level in the matrix, an increased volume fraction of phacoids increases the strength of the shear zone as a whole. However, only small matrix volume fractions are required for the composite to act nearly as weak as the matrix.  相似文献   

Assessing 4D-VAR for dynamical mapping of coastal high-frequency radar in San Diego     

《Dynamics of Atmospheres and Oceans》2010,49(1-3):175-197

The problem of dynamically mapping high-frequency (HF) radar radial velocity observations is investigated using a three-dimensional hydrodynamic model of the San Diego coastal region and an adjoint-based assimilation method. The HF radar provides near-real-time radial velocities from three sites covering the region offshore of San Diego Bay. The hydrodynamical model is the Massachusetts Institute of Technology general circulation model (MITgcm) with 1 km horizontal resolution and 40 vertical layers. The domain is centered on Point Loma, extending 117 km offshore and 120 km alongshore. The reference run (before adjustment) is initialized from a single profile of T and S and is forced with wind data from a single shore station and with zero heat and fresh water fluxes. The adjoint of the model is used to adjust initial temperature, salinity, and velocity, hourly temperature, salinity and horizontal velocities at the open boundaries, and hourly surface fluxes of momentum, heat and freshwater so that the model reproduces hourly HF radar radial velocity observations. Results from a small number of experiments suggest that the adjoint method can be successfully used over 10-day windows at coastal model resolution. It produces a dynamically consistent model run that fits HF radar data with errors near the specified uncertainties. In a test of the forecasting capability of the San Diego model after adjustment, the forecast skill was shown to exceed persistence for up to 20 h.  相似文献   

Early stage of critical clusters growth in phenomenological and molecular dynamics simulation models     

《Atmospheric Research》2007,83(3-4):465-480

The growth of critical clusters is discussed in the paper according to the classical and molecular dynamics (MD) approaches. A new formula for molecule numbers in critical clusters has been derived within the framework of the classical approach. A set of equations controlling the early stage of growth in a neighborhood of a critical size is presented. As far as molecular dynamics simulation is concerned, a computational technique based on the DL_POLY code is described in brief. Computation results are presented concerning cluster formation of H₂O vapor, distribution of clusters versus time, cluster growth and radial density distribution of isolated clusters. A comparison with the classical results is made for a case of dense vapor, where the mechanism of strong condensation is predominant. The Hertz–Knudsen formula seems to be verified by the molecular dynamics results.  相似文献   

Upwelling-enhanced seasonal stratification in a semiarid bay     

Peng Cheng  Arnoldo Valle-Levinson  Clinton D. Winant  Aurelien L.S. Ponte  Guillermo Gutierrez de Velasco  Kraig B. Winters 《Continental Shelf Research》2010,30(10-11):1241-1249

The role of wind-driven upwelling in stratifying a semiarid bay in the Gulf of California is demonstrated with observations in Bahía Concepción, Baja California Sur, Mexico. The stratification in Bahía Concepción is related to the seasonal heat transfer from the atmosphere as well as to cold water intrusions forced by wind-driven upwelling. During winter, the water column is relatively well-mixed by atmospheric cooling and by northwesterly, downwelling-favorable, winds that typically exceed 10 m/s. During summer, the water column is gradually heated and becomes stratified because of the heat flux from the atmosphere. The wind field shifts from downwelling-favorable to upwelling-favorable at the beginning of summer, i.e., the winds become predominantly southeasterly. The reversal of wind direction triggers a major cold water intrusion at the beginning of the summer season that drops the temperature of the entire water column by 3–5 °C. The persistent upwelling-favorable winds during the summer provide a continuous cold water supply that helps maintain the stratification of the bay.  相似文献   

Tuning a physically-based model of the air–sea gas transfer velocity     

C.D. Jeffery  I.S. Robinson  D.K. Woolf 《Ocean Modelling》2010,31(1-2):28-35

Air–sea gas transfer velocities are estimated for one year using a 1-D upper-ocean model (GOTM) and a modified version of the NOAA–COARE transfer velocity parameterization. Tuning parameters are evaluated with the aim of bringing the physically based NOAA–COARE parameterization in line with current estimates, based on simple wind-speed dependent models derived from bomb-radiocarbon inventories and deliberate tracer release experiments. We suggest that A = 1.3 and B = 1.0, for the sub-layer scaling parameter and the bubble mediated exchange, respectively, are consistent with the global average CO₂ transfer velocity k. Using these parameters and a simple 2nd order polynomial approximation, with respect to wind speed, we estimate a global annual average k for CO₂ of 16.4 ± 5.6 cm h^?1 when using global mean winds of 6.89 m s^?1 from the NCEP/NCAR Reanalysis 1 1954–2000. The tuned model can be used to predict the transfer velocity of any gas, with appropriate treatment of the dependence on molecular properties including the strong solubility dependence of bubble-mediated transfer. For example, an initial estimate of the global average transfer velocity of DMS (a relatively soluble gas) is only 11.9 cm h^?1 whilst for less soluble methane the estimate is 18.0 cm h^?1.  相似文献   

Book reviews     

John S. Perry  A. Henderson-Sellers  P. M. Morrisette 《Climatic change》1989,15(3):487-494

The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

Fram Strait ice flux calculations and associated Arctic ice conditions     

Ronald E. Englebretson  Dr. John E. Walsh 《GeoJournal》1989,18(1):61-67

The areal outflow of ice through Fram Strait during the period 1953–1984 is estimated on a monthly basis from the geostrophic wind and the ice concentration. Summer ice coverage in various sectors of the Arctic is then compared with the computed outflow through Fram Strait in various antecedent periods. Lag correlations indicate that interannual variations of summer ice severity in the Pacific side of the Arctic Basin are consistent with fluctuations of Fram Strait outflow during the previous 3–9 months. The findlings suggest that above-normal outflow of multiyear ice during the winter/spring months may precondition the large-scale pack ice to respond more directly to offshore flow events during the ensuing months.The areas of highest correlation with Fram Strait outflow undergo a pronounced shift in the early 1970's. Coincident changes in the large-scale circulation pattern imply that the source region of the Transpolar Drift Stream shifted westward from the Alaskan to the Siberian waters during this period.  相似文献   

Atmosphere-ocean interaction in mid-latitude storms     

Prof. Dr. R. G. Fleagle  Dr. N. A. Bond  Dr. W. A. Nuss 《Meteorology and Atmospheric Physics》1988,38(1-2):50-63

Summary Surface fluxes of heat, latent heat, and momentum, and entrainment fluxes and vertical motion at the top of the boundary layer have been calculated for limited regions of several mid-latitude ocean storms. Results have been combined to describe distributions of boundary layer processes which are characteristic of such storms. Surface heat fluxes have important effects in the region west of cold or occluded fronts and are relatively unimportant within a band of about 200 km width east of fronts. Entrainment in pre-frontal regions is driven largely by vertical shear at the top of the boundary layer, while in post-frontal regions it is driven largely by surface heat flux. Boundary layers are well defined in regions more than roughly 200 km east or west of fronts; but closer to fronts boundary layers are not well defined due to the combined effects of entrainment, condensation, and vertical motion associated with the distribution of surface stress.With 12 Figures  相似文献   

Statistical distribution of natural fractures and the possible physical generating mechanism     

F. K. Boadu  L. T. Long 《Pure and Applied Geophysics》1994,142(2):273-293

We have fitted field measurements of fracture spacings (from the vicinity of Lake Strom Thurmond, Georgia, U.S.A.) to the Weibull, Schuhmann and fractal distributions. The fracture spacings follow a fractal and Weibull distribution which implies that they were formed as a result of a repetitive fragmentation process. The limited variation of the fracture density with orientation in the study area suggests that the stress distribution generating these fractures may be uniform.  相似文献