3,151 research outputs found
Gender, genre and authority in seventeenth-century religious writing: Anna Maria van Schurman and Antoinette Bourignon as contrasting examples
Controllers for high-performance nuclear fusion plasmas
A succesful nuclear fusion reactor will confine plasma at hig temperatures and densities, with low thermal losses. The workhorse of the nuclear fusion community is the tokamak, a toroidal device in which plasmas are confined by poloidal and toroidal magnetic fields. Ideally, the confirming magnetic fields form a set of nested tori. A repetive magnetohydrodynamic (MHD) event in the plasma core (the sawteeth instability) perturbs the confirming magnetic field by producing seed islands. In low-pressure plasmas the seed islands will self-heal. In high-pressure plasmas the seed islands can grow and saturate. These neoclassical tearing models (NTMs) reduce the plasma performance or lead to plasma disruption. This sets the resistive pressure limit in tokamaks. High-performance operation in tokamaks therefore implies the control or amelioration of the NTMs. Controllers for the sawteeth and the NTMs will be discussed, with special emphasis on the development of dedicated sensors and models for MHD control
Living as a Spiritual Virgin and Claiming Prophetic Authority:The Parallel Lives of Maria Petyt and Antoinette Bourignon
The construction of knowledge-based economies versus knowledge societies: The cases of Germany and Singapore
In the past decades, terms such as knowledge-based economy (KBE)\u27, and \u27information/knowledge society\u27 have been adopted by governments worldwide in order to underline their interest in developing their economies and societies further and assure future growth. Many governments used these catchwords as labels for government programs and action plans aiming at economic and social prosperity. This aim of national governments to construct knowledge-based economies, information/knowledge societies, the actions taken and especially the ability or disability to do so, is the topic of this paper. As two cases of comparison act Singapore and Germany. (DIPF/Orig.
A rapid transition from ice covered CO2–rich waters to a biologically mediated CO2 sink in the eastern Weddell Gyre
Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25° E to 30° E. In December 2002 and January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO2) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. Ultimately the fCO2 difference across the sea surface drives air-sea fluxes of CO2. Deep CTD sections and surface transects of fCO2 were made along the Prime Meridian, a northwest-southeast section, and along 17° E to 23° E during cruise ANT XX/2 on FS Polarstern. Upward movement and entrainment of WDW into the winter mixed layer had significantly increased DIC and fCO2 below the sea ice along 0° W and 17° E to 23° E, notably in the southern Weddell Gyre. Nonetheless, the ice cover largely prevented outgassing of CO2 to the atmosphere. During and upon melting of the ice, biological activity rapidly reduced surface water fCO2 by up to 100 µatm, thus creating a sink for atmospheric CO2. Despite the tendency of the surfacing WDW to cause CO2 supersaturation, the Weddell Gyre may well be a CO2 sink on an annual basis due to this effective mechanism involving ice cover and ensuing biological fCO2 reduction. Dissolution of calcium carbonate (CaCO3) in melting sea ice may play a minor role in this rapid reduction of surface water fCO2
The marine geochemistry of the rare earth elements
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1983Novel methods were developed for the determination of 12 of the 14
Rare Earth Elements (REE) in seawater. Initial extractions of the REE by
chelating ion exchange chromatography is followed by cation exchange for
removal of co-extracted U and remaining traces of major ions. Finally
traces of U are removed by anion exchange before irradiation for 8 hours
at a flux of 5 x 1013 neutrons.cm-2.sec-l. After post-irradiation
separation of 24 Na, the gamma spectra are recorded over four different
time intervals with a Ge(Li) detector. An internal standard (144Ce) is
carried all along the procedure for improved precision by avoidance of
counting geometry errors. Vertical profiles are reported for three stations in respectively the
Northwest Atlantic Ocean, the Eastern Equatorial Pacific Ocean and the
Cariaco Trench, an anoxic basin. This data set represents the first
detailed profiles of Pr, Tb, Ho, Tm and Lu in seawater, together with
profiles of La, Ce, Nd, Sm, Eu, Gd and Yb. The first observations of
positive Ce anomalies in seawater are ascribed to regeneration of Ce
under reducing conditions. The first reported positive Gd anomalies are
ascribed to the unique chemical properties of the Gd(III)-cation, which
has an exactly half-filled 4f electron shell. Concentrations of the REE range from 0.3 pmol.kg-l (Lu) to 86
pmol.kg-l (Ce) and are among the lowest reported so far for trace
elements in seawater. The REE as a group typically exhibit a quasi-linear
increase with depth. In the deep water there appears to be some degree of
correlation with silicate. Concentration levels in the deep Pacific Ocean
are 2-4 times those in deep Atlantic waters. Ce has an opposite
behaviour, with very strong depletions in deep Pacific waters. In the
Cariaco Trench all REE, but especially Ce, are strongly affected by the
chemical changes across the oxic/anoxic interface. The REE distributions normalized versus shales (crustal abundance)
exhibit four major features:
i) a gradual enrichment of the heavy REE, most strongly developed
in the deep Pacific Ocean. This is compatible with the
stabilization of heavy REE by stronger inorganic complexation in
seawater as predicted by the TURNER- WHITFIELD-DICKSON
speciation model.
ii) the first description of positive Gd anomalies, in agreement
with the anomalously strong complexation of the Gd(III)-cation
predicted by the same speciation model.
iii) most commonly negative, but sometimes positive, Ce anomalies.
iv) a linear Eu/Sm relation for all samples. Distributions of the dissolved REE in ocean waters seem to be
dominated by their internal cycling within the ocean basins. With a few
notable exceptions, the ultimate external sources (riverine, aeolian,
hydrothermal) and sinks (authigenic minerals) appear to have little
impact on the spatial distribution of the REE in oceanic water masses.
Analogies with distributions of other properties within the oceans
suggest that the REE as a group are controlled by two simultaneous
processes:
A) cycling like or identical to opal and calcium-carbonate, with
circumstantial evidence in support of the latter as a possible carrier.
B) adsorptive scavenging, possibly by manganese-oxide phases on settling
particles. The latter mechanism is strongly supported by the parallels between
REE(III) speciation in seawater and the 'typical 1 seawater REE pattern.
This general correspondence is highlighted by the very distinct
excursions of Gd in both Gd(III) speciation and the observed seawater REE
patterns.
Combination of both apparent mechanisms, for instance scavenging of
REE by adsorptive coatings (Mn oxides) on settling skeletal material, is
very well conceivable. Upon dissolution of the shells at or near the
seafloor the adsorbed REE fraction would be released into the bottom
waters. The observations of
positive Ce anomalies in Northwest Atlantic surface waters,
enhanced Ce anomalies and Mn levels in the OZ-minimum zone of the
Eastern Equatorial Pacific Ocean, and
enhanced Ce concentrations in anoxic waters
all support the contention that a vigorous cycling driven by oxidation
and reduction reactions dominates both Ce and Mn in the ocean basins.
Under conditions of thermodynamic equilibrium, Ce tends to become
depleted in well-oxygenated open ocean waters, and normal or enriched in
waters below a pOZ threshold of about 0.001-0.010 atm partial pressure.
The latter threshold level generally lies below the sediment/water
interface.
However, the kinetics of oxidation (and reduction) of Ce appears to
be slow relative to various transport processes. This leads to
disequilibria, i.e. a major uncoupling of the pOZ threshold level and
the Ce anomaly distribution. The REE are definitely non-conservative in seawater and in general
the REE pattern or 143Nd/144Nd isotopic ratio cannot be treated as
ideal water mass tracers. The continuous redistribution of Ce within the
modern ocean, combined with the likelihood of active diagenesis,
precludes the use of Ce anomalies as indicators of oxic versus anoxic
conditions in ancient oceans. On the other hand, the Eu/Sm ratio,
possibly combined with 143Nd/144Nd , would have potential as a tracer
for understanding modern and ancient processes of hydrothermal
circulation.This research was supported by Department of Energy contract
DE-AS02-76EV03566 and Office of Naval Research Contract NOOOl 4-82-C-00l 9
NR 083-004
Dissolved carbon dioxide in Dutch coastal waters
The role of shelf seas in global carbon cycling is poorly understood. The dissolved inorganic carbon system and air-sea exchange of carbon dioxide (CO2) are described for the Dutch coastal zone in September 1993. The inorganic carbon chemistry was affected by tidal mixing, wind speed, wind direction, freshwater input, stratification and coastal upwelling. Surface water had a variable fugacity of carbon dioxide (fCO2) between 300 and 800 μatm with short-term changes partly related to the tidal cycle. High contents of dissolved inorganic carbon (DIC) and CO2 in relatively saline water probably originated from mineralisation of accumulated organic matter in water and sediments farther out at sea and transport of water enriched in DIC into the coastal zone by upwelling. Air-sea exchange of CO2 ranged from —20 to 60 mmol m−2 day−1. These fluxes are critically discussed in the light of potential stratification. It is not possible to assess from this study whether the Dutch coastal zone is a net sink or source for atmospheric CO2
Neodymium isotopic composition and concentration in the western North Atlantic Ocean: results from the GEOTRACES GA02 section
The neodymium (Nd) isotopic composition of seawater is commonly used as a proxy to study past changes in the thermohaline circulation. The modern database for such reconstructions is however poor and the understanding of the underlying processes is incomplete. Here we present new observational data for Nd isotopes and concentrations from twelve seawater depth profiles, which follow the flow path of North Atlantic Deep Water (NADW) from its formation region in the North Atlantic to the northern equatorial Atlantic. Samples were collected during two cruises constituting the northern part of the Dutch GEOTRACES transect GA02 in 2010. The results show that the different water masses in the subpolar North Atlantic Ocean, which ultimately constitute NADW, have the following Nd isotope characteristics: Upper Labrador Sea Water (ULSW), εNd = -14.2 ± 0.3; Labrador Sea Water (LSW), εNd = -13.7 ± 0.9; Northeast Atlantic Deep Water (NEADW), εNd = -12.5 ± 0.6; Northwest Atlantic Bottom Water (NWABW), εNd = -11.8 ± 1.4. In the subtropics, where these source water masses have mixed to form NADW, which is exported to the global ocean, upper-NADW is characterised by εNd values of -13.2 ± 1.0 (2sd) and lower-NADW exhibits values of εNd = -12.4 ± 0.4 (2sd). While both signatures overlap within error, the signature for lower-NADW is significantly more radiogenic than the traditionally used value for NADW (εNd = -13.5) due to the dominance of source waters from the Nordic Seas (NWABW and NEADW). Comparison between the concentration profiles and the corresponding Nd isotope profiles with other water mass properties such as salinity, silicate concentrations, neutral densities and chlorofluorocarbon (CFC) concentration provides novel insights into the geochemical cycle of Nd and reveals that different processes are necessary to account for the observed Nd characteristics in the subpolar and subtropical gyres and throughout the vertical water column. While our data set provides additional insights into the contribution of boundary exchange in areas of sediment resuspension, the results for open ocean seawater demonstrate, at an unprecedented level, the suitability of Nd isotopes to trace modern water masses in the strongly advecting western Atlantic Ocean
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