262 research outputs found
Major impact of dust deposition on the productivity of the Arabian Sea
In the Arabian Sea (AS), spatiotemporal nutrient limitation patterns of primary production and the possible role of nutrient inputs from the atmosphere are still not well understood. Using a biogeochemical model forced by modeled aerosol deposition, we show that without high atmospheric iron inputs through dust deposition during the summer monsoon, primary production over the AS would be reduced by half. Atmospheric iron deposition also supports most of the nitrogen fixation over the AS. However, our ocean biogeochemistry modeling results suggest that dinitrogen fixation constitutes a negligible fraction of the primary production. Finally, we show that atmospheric inputs of nitrogen, mostly from anthropogenic activities in India, have a negligible impact on primary production
Modelo de cabalgamiento profundo para el Alto Atlas (Marruecos). Implicaciones sísmicas en la zona de colisión entre Eurasia y Africa
Previous crustal models of the High Atlas suppose the existence of a mid-crustal detachment where all the surface thrusts merged and below which the lower crust was continuous. However, both seismic refraction data and gravity modeling detected a jump in crustal thickness between the High Atlas and the northern plains. Here we show that this rapid and vertical jump in the depth of Moho discontinuity suggests that a thrust fault may penetrate the lower crust and offset the Moho (deep-rooted “thick skinned” model). The distribution of Neogene and Quaternary volcanisms along and at the northern part of the High Atlas lineament can be related to the beginning of a partial continental subduction of the West African plate to the north underneath Moroccan microplate. Allowing from the complex problem of the plate boundary in the western zone of the Mediterranean, we propose to interpret the South-Atlasic fault zone as the actual northwestern boundary of the stable part of the African plate rather than the Azores-Gibraltar fault currently used.Los modelos geodinámicos existentes sobre la estructura profunda del alto Atlas suponen la existencia de un despegue medio-cortical donde convergen los cabalgamientos superficiales y bajo el cual la corteza inferior es continua. Los datos de sísmica de refracción y gravimetría, sin embargo, indican la existencia de una discontinuidad en el grosor de la corteza (profundidad del Moho) bajo el Alto Atlas. En este artículo ponemos de manifiesto que este salto rápido en la profundidad del Moho puede ser causado por un cabalgamiento que penetra la corteza inferior, desplazando la base de la misma (“deeprooted thick skinned model”). La distribución del volcanismo Neógeno y Cuaternario a lo largo de y al norte de la alineación del Alto Atlas pueden estar relacionados con el comienzo de una subducción continental parcial de la placa Africana occidental hacia el norte, bajo la microplaca marroquí. La expresión en superficie de este cabalgamiento, la zona de falla sud-atlásica, refleja la influencia de una sutura continental heredada de orogenias anteriores (panafricana, hercínica y rifting Jurásico). Por tanto, proponemos que este frente heredado representa el límite meridional de la zona de colisión mediterránea y el margen noroccidental de la porción estable de la placa africana
Gamma ray production in inelastic scattering of neutrons produced by cosmic muons in Fe
We report on the study of the intensities of several gamma lines emitted
after the inelastic scattering of neutrons in Fe. Neutrons were produced
by cosmic muons passing the 20t massive iron cube placed at the Earth's surface
and used as a passive shield for the HPGe detector. Relative intensities of
detected gamma lines are compared with the results collected in the same iron
shield by the use of Cf neutrons. Assessment against the published data
from neutron scattering experiments at energies up to 14 MeV is also provided
Biogeographic classification of the Caspian Sea
Like other inland seas, the Caspian Sea (CS) has been influenced by climate
change and anthropogenic disturbance during recent decades, yet the
scientific understanding of this water body remains poor. In this study, an
eco-geographical classification of the CS based on physical information
derived from space and in situ data is developed and tested against a set of
biological observations. We used a two-step classification procedure,
consisting of (i) a data reduction with self-organizing maps (SOMs) and (ii) a
synthesis of the most relevant features into a reduced number of marine
ecoregions using the hierarchical agglomerative clustering (HAC) method. From
an initial set of 12 potential physical variables, 6 independent variables
were selected for the classification algorithm, i.e., sea surface temperature
(SST), bathymetry, sea ice, seasonal variation of sea surface salinity
(DSSS), total suspended matter (TSM) and its seasonal variation (DTSM). The
classification results reveal a robust separation between the northern and
the middle/southern basins as well as a separation of the shallow nearshore
waters from those offshore. The observed patterns in ecoregions can be
attributed to differences in climate and geochemical factors such as distance
from river, water depth and currents. A comparison of the annual and monthly
mean Chl <i>a</i> concentrations between the different ecoregions shows
significant differences (one-way ANOVA, <i>P</i> < 0.05). In particular, we
found differences in phytoplankton phenology, with differences in the date of
bloom initiation, its duration and amplitude between ecoregions. A first
qualitative evaluation of differences in community composition based on
recorded presence–absence patterns of 25 different species of plankton, fish
and benthic invertebrate also confirms the relevance of the ecoregions as
proxies for habitats with common biological characteristics
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