Beneath the surface: Characteristics of oceanic ecosystems under weak mixing conditions – A theoretical investigation     

Aike Beckmann  Inga Hense   《Progress in Oceanography》2007,75(4):771-796

This study considers an important biome in aquatic environments, the subsurface ecosystem that evolves under low mixing conditions, from a theoretical point of view. Employing a conceptual model that involves phytoplankton, a limiting nutrient and sinking detritus, we use a set of key characteristics (thickness, depth, biomass amplitude/productivity) to qualitatively and quantitatively describe subsurface biomass maximum layers (SBMLs) of phytoplankton. These SBMLs are defined by the existence of two community compensation depths in the water column, which confine the layer of net community production; their depth coincides with the upper nutricline. Analysing the results of a large ensemble of simulations with a one-dimensional numerical model, we explore the parameter dependencies to obtain fundamental steady-state relationships that connect primary production, mortality and grazing, remineralization, vertical diffusion and detrital sinking. As a main result, we find that we can distinguish between factors that determine the vertically integrated primary production and others that affect only depth and shape (thickness and biomass amplitude) of this subsurface production layer. A simple relationship is derived analytically, which can be used to estimate the steady-state primary productivity in the subsurface oligotrophic ocean. The fundamental nature of the results provides further insight into the dynamics of these “hidden” ecosystems and their role in marine nutrient cycling.  相似文献   

Lateral tidal mixing in the Antarctic marginal seas     

Aike Beckmann  Adriene F. Pereira 《Ocean Dynamics》2003,53(1):21-26

Tidal mixing plays an important role in the modification of dense water masses around the Antarctic continent. In addition to the vertical (diapycnal) mixing in the near-bottom layers, lateral mixing can also be of relevance in some areas. A numerical tide simulation shows that lateral tidal mixing is not uniformly distributed along the shelf break. In particular, strong mixing occurs all along the Ross Sea and Southern Weddell Sea shelf breaks, while other regions (e.g., the western Weddell Sea) are relatively quiet. The latter regions correspond surprisingly well to areas where indications for cross-shelf exchange of dense water masses have been found. The results suggest that lateral tidal mixing may account for the relatively small contribution of Ross Sea dense water masses to Antarctic Bottom Water.  相似文献   

Monitoring Subsurface Microbial Ecology in a Sulfate-Amended, Gasoline-Contaminated Aquifer   总被引：3，自引：0，他引：3  

Kerry Sublette  Aaron Peacock  David White  Greg Davis  Dora Ogles  David Cook  Ravi Kolhatkar  Dennis Beckmann  Xiaomin Yang 《Ground Water Monitoring & Remediation》2006,26(2):70-78

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基于数据融合的SRTM数据空洞填补方法   总被引：7，自引：0，他引：7  

阚瑷珂  朱利东  张瑞军  郑文锋  银正彤  王绪本  王成善 《地理空间信息》2007,5(3):62-64

总结国内外对SRTM数据空洞的填补方法,指出基于内插的填补方法无法满足大区域SRTM数据处理的需求。提出在内插的基础上进行数据融合的空洞填补方法,并以实例验证了该方法是一种获取完整地形数据的有效途径。  相似文献   

Ocean/ice shelf interaction in the southern Weddell Sea: results of a regional numerical helium/neon simulation     

Christian B. Rodehacke  Hartmut H. Hellmer  Oliver Huhn  Aike Beckmann 《Ocean Dynamics》2007,57(1):1-11

Ocean/ice interaction at the base of deep-drafted Antarctic ice shelves modifies the physical properties of inflowing shelf waters to become Ice Shelf Water (ISW). In contrast to the conditions at the atmosphere/ocean interface, the increased hydrostatic pressure at the glacial base causes gases embedded in the ice to dissolve completely after being released by melting. Helium and neon, with an extremely low solubility, are saturated in glacial meltwater by more than 1000%. At the continental slope in front of the large Antarctic caverns, ISW mixes with ambient waters to form different precursors of Antarctic Bottom Water. A regional ocean circulation model, which uses an explicit formulation of the ocean/ice shelf interaction to describe for the first time the input of noble gases to the Southern Ocean, is presented. The results reveal a long-term variability of the basal mass loss solely controlled by the interaction between waters of the continental shelf and the ice shelf cavern. Modeled helium and neon supersaturations from the Filchner–Ronne Ice Shelf front show a “low-pass” filtering of the inflowing signal due to cavern processes. On circumpolar scales, the simulated helium and neon distributions allow us to quantify the ISW contribution to bottom water, which spreads with the coastal current connecting the major formation sites in Ross and Weddell Seas.

Christian B. RodehackeEmail:

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The nature of the hard state of Cygnus X-3     

L. Hjalmarsdotter  A. A. Zdziarski  S. Larsson  V. Beckmann  M. McCollough  D. C. Hannikainen  O. Vilhu 《Monthly notices of the Royal Astronomical Society》2008,384(1):278-290

The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ∼20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf–Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE /ASM and CGRO /BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, provided the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher   L / L _E  than in the usual advection models, or the mass of the compact object is  ≳20 M_⊙  , possibly making it the most-massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlation.  相似文献   

Deep Geophysical Structure Research on Middle Section of the Longmen Mountain Tectonic Belt     

YU Nian  WANG Xuben  CAI Xuelin  KAN Aike  LI Jian 《《地质学报》英文版》2013,87(Z1):90-94

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Wake model of hydrodynamic forces on pipelines     

K. F. Lambrakos  J. C. Chao  H. Beckmann  H. R. Brannon 《Ocean Engineering》1987,14(2)

The hydrodynamic force model for pipelines presented includes flow history effects (wake effects) and time dependence in the force coefficients. These two features in the model were necessary to obtain satisfactory agreement between model predictions and full scale field measurements of pipeline forces. Conventional force models which represent adaptations of Morison's equation with ambient velocity and constant coefficients give predictions which for the lift component of the force in particular are in very poor agreement with the measurements. The parameters in the new model have been estimated on the basis of the full scale measurements and reflect a wide range of flow conditions. The model can be used in pipeline on-bottom stability design calculations for regular or irregular waves.  相似文献   

四川茶马古道交通变迁对沿线商贸型古村镇演化的影响     

项清  阚瑷珂  黄弘  于欢  何杰 《世界地理研究》2021,30(6):1320-1329

茶马古道作为西南地区重要的贸易线路和移民通道,长期进行的贸易活动和人口迁移促进了沿线地区商贸型古村镇的形成和演化。以茶马古道四川段为例,基于最邻近指数、Ripley's K函数、核密度分析、重心分析等空间分析和计量地理方法,探讨古道交通变迁对商贸型古村镇分布演化的影响及其作用机制。研究结果表明：①四川茶马古道的交通变迁过程可分为两个阶段,明清以前主要分布在岷江上游地区,沟通川西盆地和黄河上游的河谷草原,明清以后主要分布在川西藏区,连接着川西和藏东南。②四川茶马古道交通变迁影响了沿线商贸型古村镇的分布格局、演化重心和演化方向。从先秦两汉至隋唐五代时期,商贸型古村镇呈现出从雅安一带往都江堰,沿岷江北上向茂县、马尔康方向演化,直至宋元明清时期,转向川西地区集聚。演化重心呈现出从雅安往北、再回到雅安一带并往西迁移,Ripley's K函数定量描述了商贸型古村镇这一外向扩散和内向集聚的过程。③古道的空间路径、地域环境、贸易政策、古村镇居民等因素分别对商贸型古村镇的演化产生主导、驱动、调节和反馈的作用机制。  相似文献   

Model evaluation experiments in the North Atlantic Basin: simulations in nonlinear terrain-following coordinates   总被引：7，自引：0，他引：7  

Dale B. Haidvogel  Hernan G. Arango  Kate Hedstrom  Aike Beckmann  Paola Malanotte-Rizzoli  Alexander F. Shchepetkin 《Dynamics of Atmospheres and Oceans》2000,32(3-4)

A primitive equation ocean circulation model in nonlinear terrain-following coordinates is applied to a decadal-length simulation of the circulation in the North Atlantic Ocean. In addition to the stretched sigma coordinate, novel features of the model include the utilization of a weakly dissipative, third-order scheme for tracer advection, and a conservative and constancy-preserving time-stepping algorithm. The objectives of the study are to assess the quality of the new terrain-following model in the limit of realistic basin-scale simulations, and to compare the results obtained with it against those of other North Atlantic models used in recent multi-model comparison studies.The new model is able to reproduce many features of both the wind-driven and thermohaline circulation, and to do so within error bounds comparable with prior model simulations (e.g., CME and DYNAMO). Quantitative comparison with comparable results obtained with the Miami Isopycnic Coordinate Model (MICOM) show our terrain-following solutions are of similar overall quality when viewed against known measures of merit including meridional overturning and heat flux, Florida Straits and Gulf Stream transport, seasonal cycling of temperature and salinity, and upper ocean currents and tracer fields in the eastern North Atlantic Basin. Sensitivity studies confirm that the nonlinear vertical coordinate contributes significantly to model fidelity, and that the global inventories and spatial structure of the tracer fields are affected in important ways by the choice of lateral advection scheme.  相似文献