A MESO-β SCALE SIMULATION OF THE EFFECTS OF BOREAL FOREST ECOSYSTEM ON THE LOWER ATMOSPHERE

Based on the Intensive Field Campaign(IFC-1)data of Boreal Ecosystem-Atmosphere Study(BOREAS).a three-dimensional meso-β scale model is used to simulate the effect of boreal forests onthe lower atmosphere.A fine horizontal resolution of 2 km×2 km is used in order to distinguish thevegetative heterogeneity in the boreal region.A total of 20×25 grid points cover the entire sub-modeling area in BOREAS' South Study Area(SSA).The ecosystem types and their coverage ineach grid square are extracted from the North American Land Cover Characteristics Data Base(NALCCD)generated by the U.S.Geographical Survey(USGS)and the University of Nebraska-Lincoln(UNL).The topography of the study area is taken from the Digital Elevation Map(DEM)of USGS.The model outputs include the components of the energy balance budget within the canopyand at the ground.the turbulence parameters in the atmospheric boundary layer and the wind.temperature and humidity profiles extending up to a height of 1500 m.In addition to the fine timeand spatial step,the unique feature of the present model is the incorporation of both dynamic andbiological effects of the Boreal forest into the model parameterization scheme.The model resultscompare favorably with BOREAS' IFC-1 data in 1994 when the forest was in the luxuriant growingperiod.     

High resolution paleoenvironmental and chronological investigations of Norse landnám at Tasiusaq, Eastern Settlement, Greenland

High-resolution paleoenvironmental data from a peat profile with a small pollen source area are used to reconstruct the impacts of landnám on vegetation and soils at a Norse farm complex (∅2 at Tasiusaq) comprising two farms in the Eastern Settlement of Greenland. Analyses include the AMS ¹⁴C dating of plant macrofossil samples and the use of Bayesian radiocarbon calibration to construct improved age-depth models for Norse cultural horizons. The onset of a regional landnám may be indicated by the clearance of Betula pubescens woodland immediately prior to local settlement. The latter is dated to AD 950-1020 (2σ) and is characterised by possible burning of Betula glandulosa scrub to provide grassland pasture for domestic stock. Clearance and grazing resulted in accelerated levels of soil erosion at a westerly farm. This was followed by an easterly migration of settlement and agriculture. Site constraints prevent an assessment of the demise of the easterly farm, but pressures of overgrazing and land degradation may have been the major factors responsible for the abandonment of the earlier farm.     

Asymmetrically zoned reaction rims: assessment of grain boundary diffusivities and growth rates related to natural diffusion-controlled mineral reactions

This study explores garnet coronas around hedenbergite, which were formed by the reaction plagioclase + hedenbergite→garnet + quartz, to derive information about diffusion paths that allowed for material redistribution during reaction progress. Whereas quartz forms disconnected single grains along the garnet/hedenbergite boundaries, garnet forms ～20‐μm‐wide continuous polycrystalline rims along former plagioclase/hedenbergite phase boundaries. Individual garnet crystals are separated by low‐angle grain boundaries, which commonly form a direct link between the reaction interfaces of the plagioclase|garnet|hedenbergite succession. Compositional variations in garnet involve: (i) an overall asymmetric compositional zoning in Ca, Fe²⁺, Fe³⁺ and Al across the garnet layer; and (ii) micron‐scale compositional variations in the near‐grain boundary regions and along plagioclase/garnet phase boundaries. These compositional variations formed during garnet rim growth. Thereby, transfer of the chemical components occurred by a combination of fast‐path diffusion along grain boundaries within the garnet rim, slow diffusion through the interior of the garnet grains, and by fast diffusion along the garnet/plagioclase and the garnet/hedenbergite phase boundaries. Numerical simulation indicates that diffusion of Ca, Al and Fe²⁺ occurred about three to four, four and six to seven orders of magnitude faster along the grain boundaries than through the interior of the garnet grains. Fast‐path diffusion along grain boundaries contributed substantially to the bulk material transfer across the growing garnet rim. Despite the contribution of fast‐path diffusion, bulk diffusion through the garnet rim was too slow to allow for chemical equilibration of the phases involved in garnet rim formation even on a micrometre scale. Based on published garnet volume diffusion data the growth interval of a 20‐μm‐wide garnet rim is estimated at ～10³–10⁴ years at the inferred reaction conditions of 760 ± 50 °C at 7.6 kbar. Using the same parameterization of the growth law, 100‐μm‐ and 1‐mm‐thick garnet rims would grow within 10⁵–10⁶ and 10⁶–10⁷ years respectively.     

Modified mild slope equation and open boundary conditions

A numerical model to compute wave field is developed. It is based on the Berkhoff diffraction-refraction equation, in which an energy dissipation term is added, to take into account the breaking and the bottom friction phenomena. The energy dissipation function, by breaking and by bottom friction, is introduced in the Berkhoff equation to obtain a new equation of propagation.The resolution is done with the hybrid finite element method, where lagrangians elements are used.     

Study on the characteristics of the marine boundary layer in the Equatorial Pacific

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渐新世/中新世分界的地层学事件

介绍渐新世/中新世分界面(即古近纪/新近纪分界面)附近的浮游有孔虫和钙质超微化石事件及其新年龄。2004年国际地层年代表确定该分界面位于第58长偏心率周期弱振幅处,古地磁事件C6Cn.2n底面,经天文调谐后的年龄为23.03 Ma。发生在该界面附近的生物地层学事件主要是浮游有孔虫Paragloborotalia kugleri的始现面(22.96 Ma)和钙质超微化石Sphenolithus delphix的末现面(23.11 Ma)。在南海北部东沙群岛附近的大洋钻探ODP1148站,渐新世/中新世界面以崩塌沉积物为特征,有明显沉积间断,标志南海当时有较大范围的构造运动。     

Mesozoic transtensional basin history of the Eastern Cordillera, Colombian Andes: Inferences from tectonic models

Backstripping analysis and forward modeling of 162 stratigraphic columns and wells of the Eastern Cordillera (EC), Llanos, and Magdalena Valley shows the Mesozoic Colombian Basin is marked by five lithosphere stretching pulses. Three stretching events are suggested during the Triassic–Jurassic, but additional biostratigraphical data are needed to identify them precisely. The spatial distribution of lithosphere stretching values suggests that small, narrow (<150 km), asymmetric graben basins were located on opposite sides of the paleo-Magdalena–La Salina fault system, which probably was active as a master transtensional or strike-slip fault system. Paleomagnetic data suggesting a significant (at least 10°) northward translation of terranes west of the Bucaramanga fault during the Early Jurassic, and the similarity between the early Mesozoic stratigraphy and tectonic setting of the Payandé terrane with the Late Permian transtensional rift of the Eastern Cordillera of Peru and Bolivia indicate that the areas were adjacent in early Mesozoic times. New geochronological, petrological, stratigraphic, and structural research is necessary to test this hypothesis, including additional paleomagnetic investigations to determine the paleolatitudinal position of the Central Cordillera and adjacent tectonic terranes during the Triassic–Jurassic. Two stretching events are suggested for the Cretaceous: Berriasian–Hauterivian (144–127 Ma) and Aptian–Albian (121–102 Ma). During the Early Cretaceous, marine facies accumulated on an extensional basin system. Shallow-marine sedimentation ended at the end of the Cretaceous due to the accretion of oceanic terranes of the Western Cordillera. In Berriasian–Hauterivian subsidence curves, isopach maps and paleomagnetic data imply a (>180 km) wide, asymmetrical, transtensional half-rift basin existed, divided by the Santander Floresta horst or high. The location of small mafic intrusions coincides with areas of thin crust (crustal stretching factors >1.4) and maximum stretching of the subcrustal lithosphere. During the Aptian–early Albian, the basin extended toward the south in the Upper Magdalena Valley. Differences between crustal and subcrustal stretching values suggest some lowermost crustal decoupling between the crust and subcrustal lithosphere or that increased thermal thinning affected the mantle lithosphere. Late Cretaceous subsidence was mainly driven by lithospheric cooling, water loading, and horizontal compressional stresses generated by collision of oceanic terranes in western Colombia. Triassic transtensional basins were narrow and increased in width during the Triassic and Jurassic. Cretaceous transtensional basins were wider than Triassic–Jurassic basins. During the Mesozoic, the strike-slip component gradually decreased at the expense of the increase of the extensional component, as suggested by paleomagnetic data and lithosphere stretching values. During the Berriasian–Hauterivian, the eastern side of the extensional basin may have developed by reactivation of an older Paleozoic rift system associated with the Guaicáramo fault system. The western side probably developed through reactivation of an earlier normal fault system developed during Triassic–Jurassic transtension. Alternatively, the eastern and western margins of the graben may have developed along older strike-slip faults, which were the boundaries of the accretion of terranes west of the Guaicáramo fault during the Late Triassic and Jurassic. The increasing width of the graben system likely was the result of progressive tensional reactivation of preexisting upper crustal weakness zones. Lateral changes in Mesozoic sediment thickness suggest the reverse or thrust faults that now define the eastern and western borders of the EC were originally normal faults with a strike-slip component that inverted during the Cenozoic Andean orogeny. Thus, the Guaicáramo, La Salina, Bitúima, Magdalena, and Boyacá originally were transtensional faults. Their oblique orientation relative to the Mesozoic magmatic arc of the Central Cordillera may be the result of oblique slip extension during the Cretaceous or inherited from the pre-Mesozoic structural grains. However, not all Mesozoic transtensional faults were inverted.     

Crustal structure of the Eastern Alps along the TRANSALP profile from wide-angle seismic tomography

The objective of the TRANSALP project is an investigation of the Eastern Alps with regard to their deep structure and dynamic evolution. The core of the project is a 340-km-long seismic profile at 12°E between Munich and Venice. This paper deals with the P-wave velocity distribution as derived from active source travel time tomography. Our database consists of Vibroseis and explosion seismic travel times recorded at up to 100 seismological stations distributed in a 30-km-wide corridor along the profile. In order to derive a velocity and reflector model, we simultaneously inverted refractions and reflections using a derivative of a damped least squares approach for local earthquake tomography. 8000 travel time picks from dense Vibroseis recordings provide the basis for high resolution in the upper crust. Explosion seismic wide-angle reflection travel times constrain both deeper crustal velocities and structure of the crust–mantle boundary with low resolution. In the resulting model, the Adriatic crust shows significantly higher P-wave velocities than the European crust. The European Moho is dipping south at an angle of 7°. The Adriatic Moho dips north with a gentle inclination at shallower depths. This geometry suggests S-directed subduction. Azimuthal variations of the first-break velocities as well as observations of shear wave splitting reveal strong anisotropy in the Tauern Window. We explain this finding by foliations and laminations generated by lateral extrusion. Based on the P-wave model we also localized almost 100 local earthquakes recorded during the 2-month acquisition campaign in 1999. Seismicity patterns in the North seem related to the Inn valley shear zone, and to thrusting of Austroalpine units over European basement. The alignment of deep seismicity in the Trento-Vicenza region with the top of the Adriatic lower crust corroborates the suggestion of a deep thrust fault in the Southern Alps.