The marine prochlorophyte Prochlorococcus contributes significantly to phytoplankton biomass and primary production in the Sargasso Sea     

Ralf Goericke  Nicholas A. Welschmeyer   《Deep Sea Research Part I: Oceanographic Research Papers》1993,40(11-12)

The newly-discovered prochlorophyte Prochlorococcus marinus is often numerically dominant in the euphotic zone of the tropical and subtropical ocean; however, its contribution to phytoplankton biomass and primary production is largely unknown. Using its unique pigment divinyl-chlorophyll a (Chl a₂) as a chemosystematic marker, we show that Prochlorococcus is present at a station in the Sargasso Sea throughout most of the year. Whereas it is only found at depth during the early summer, it can be found throughout the euphotic zone during the rest of the year. Averaged over the year Prochlorococcus pigment-biomass constitutes about 30% of the total. Its growth rate, estimated from the incorporation of ¹⁴C into Chl a₂ ranged from values of 0.3 day⁻¹ in the surface layer to values less than 0.1 day⁻¹ at the bottom of the euphotic zone. Averaged over the seasons, approximately 25% of the total productivity was due to Prochlorococcus. Prochlorococcus clearly is an important component of the ecosystem in the Sargasso Sea, and perhaps the world ocean.  相似文献   

Analysis of dune erosion processes in large-scale flume experiments   总被引：1，自引：0，他引：1  

J.S.M. van Thiel de Vries  M.R.A. van Gent  D.J.R. Walstra  A.J.H.M. Reniers   《Coastal Engineering》2008,55(12):1028-1040

Large-scale physical model tests were conducted with different wave periods to examine the physical processes driving dune erosion. The model tests have been carried out in a flume (2DV) with a sandy dune exposed to extreme surge and wave conditions [Van Gent, M.R.A., Van Thiel de Vries, J.S.M., Coeveld, E.M., De Vroeg, J.H. and Van de Graaff, J., 2008. Large-scale dune erosion tests to study the effect of wave periods. Coastal Engineering. doi:10.1016/j.coastaleng.2008.04.003.]. Detailed measurements in time and space of water pressure, flow velocities and sediment concentrations were performed in the near shore area. The data revealed that both short- and long waves are important to inner surf hydrodynamics. Depth averaged flows are directed offshore and increase towards the shore line. The corresponding mean sediment concentrations rise sharply towards the dune face (up to 50 g/l near the bed). The strong increase in the mean sediment concentration towards the dune face correlates well with the maximum wave surface slope which in turn is coupled to both the pressure gradient and the near-bed wave-breaking induced turbulence. Analysis shows that the pressure gradient is only partially coupled to the flow acceleration suggesting that the latter cannot always be used as a proxy for the first. Weak correlation is obtained with the near-bed flows related to the bed shear stress. Tests with a larger wave period resulted in a larger dune erosion volume. During these tests more wave energy (combined incident and infragravity waves) reached the dune face, but more importantly, this wave energy is dissipated by fewer waves resulting in more intense wave breakers and steeper wave fronts. It is therefore expected that the wave-breaking induced near-bed turbulence increases resulting in significantly higher (O(100%)) mean sediment concentrations. In addition the mean flow velocities are comparable, yielding a substantially larger offshore directed sediment transport capacity. This increase in offshore directed transport is only partially compensated by a concurrent increase in the wave related onshore transport capacity associated with intrawave processes, resulting in a net increase in the dune erosion rate.  相似文献   

Early Archean spherule layers from the Barberton Greenstone Belt,South Africa: Mineralogy and geochemistry of the spherule beds in the CT3 drill core          下载免费PDF全文

Seda Ozdemir  Toni Schulz  Christian Koeberl  Wolf Uwe Reimold  Tanja Mohr‐Westheide  Desiree Hoehnel  Ralf Thomas Schmitt 《Meteoritics & planetary science》2017,52(12):2586-2631

Little is known about the Hadean and the Archean impact record on Earth. In the CT3 drill core from the Fig Tree Group of the northern Barberton Greenstone Belt, 17 spherule layer intersections occur, which, provide an outstanding new opportunity to gain insights into meteorite bombardment of the early Earth. CT3 spherules, as primary features, mostly exhibit textural patterns similar to those of the other Barberton spherule layers, but locally mineralogical and chemical compositional differences are observed, likely as a result of various degrees of alteration. The observed mineralogy of the spherule layers is of secondary origin and comprises K‐feldspar, phyllosilicates, carbonates, sulfides, and oxides, with the exception of secondary Ni‐Cr spinel that is of primary origin. Our petrographic investigations suggest alteration by K‐metasomatism, sericitization, silicification, and carbonatization. Siderophile element contents of bulk samples show significant enrichments in Ni (up to 2 wt%) and Ir (up to ~3 ppm), similar to previously studied Archean spherule layers. These values are indicative of the presence of a meteoritic component. On the other hand, lithophile and chalcophile element abundances indicate hydrothermal overprint on the CT3 samples; this may also have influenced the redistribution of the meteoritic component(s). Last, we group the CT3 spherule layers, which occur in three intervals (A, B, and C), according to their petrographic and geochemical features, which indicate evidence for at least three distinct impact events before tectonic overprint that affected the original deposits.  相似文献   

Wind-wave variability in a shallow tidal sea—Spectral modelling combined with neural network methods     

Agnieszka Herman  Ralf Kaiser  Hanz D. Niemeyer 《Coastal Engineering》2009

In this paper the wind-wave variability in the tidal basins of the German Wadden Sea is modelled with combined numerical and neural-network (NN) methods. First, the wave propagation and transformation in the study area are modelled with the state-of-the-art third-generation spectral wave model SWAN. The ability of SWAN to accurately reproduce the phenomena of interest in nonstationary conditions governed by highly variable winds, water levels and currents is shown by comparisons of the modelled and measured mean wave parameters at four stations. The principal component analysis of the SWAN results is then used to reveal the dominating spatial patterns in the data and to reduce their dimensionality, thus enabling an efficient and relatively straightforward NN modelling of mean wave parameters in the whole study area. It is shown that the data produced with the approach developed in this work have statistical properties (discrete probability distributions of the mean wave parameters etc.) very close to the properties of the data obtained with SWAN, thus proving that this approach can be used as a reliable tool for wind wave simulation in coastal areas, complementary to (often computationally demanding) spectral wave models.  相似文献   

HydrIS:一个用于地下水管理的开放的GIS决策支持系统——以Morocco为例(英文)   总被引：1，自引：0，他引：1  

JARAR OULIDI Hassane  LWNER Ralf  BENAABIDATE Lahcen  WCHTER Joachim 《地球空间信息科学学报》2009,12(3):212-216

A regional groundwater management system has been elaborated, integrating Relational Database Management System (RDBMS) and various web services. It consists of web geospatial application so-called HydrIS ( Hydrogeological Information System) based on Open Source components and technologies, leading to a feasible and low-cost solution. Therefore, HydrIS permits delivery of data from a number of heterogeneous sources to standards supported by the Open Geospatial Consortium (OGC). The protocols used for exchanging data are also derived from OGC standards, i.e., WMS (Web Mapping Service), WFS (Web Feature Service), and WCS (Web Coverage Service). Finally, a geoportal was developed, which consists of client-applications that communicate with different Web Services (WMS, WCS, and WFS) through http-requests. A prototype for web-based GIS application was designed using the deegree Framework to provide systematic interfaces and functions. This system was developed to demonstrate the value of making hydrogeological data more widely accessible through client/server architecture. This experience and knowledge already gained in this project will be a source for technology transfer and policy decisions. Otherwise, this will enable user groups to improve the management of their groundwater resources and contribute to enhanced decision support capabilities.   相似文献