110 research outputs found
Random interactions and spin-glass thermodynamic transition in the hole-doped Haldane system YCaBaNiO
Magnetization, DC and AC bulk susceptibility of the =1 Haldane chain
system doped with electronic holes, YCaBaNiO
(0x0.20), have been measured and analyzed. The most striking
results are (i) a sub-Curie power law behavior of the linear susceptibility,
, for temperature lower than the Haldane gap
of the undoped compound (x=0) (ii) the existence of a spin-glass thermodynamic
transition at = 2-3 K. These findings are consistent with (i) random
couplings within the chains between the spin degrees of freedom induced by hole
doping, (ii) the existence of ferromagnetic bonds that induce magnetic
frustration when interchain interactions come into play at low temperature.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Sea ice meltwater and circumpolar deep water drive contrasting productivity in three Antarctic polynyas
In the Southern Ocean, polynyas exhibit enhanced rates of primary productivity and represent large seasonal sinks for atmospheric CO2. Three contrasting east Antarctic polynyas were visited in late December to early January 2017: the Dalton, Mertz, and Ninnis polynyas. In the Mertz and Ninnis polynyas, phytoplankton biomass (average of 322 and 354 mg chlorophyll a (Chl a)/m2, respectively) and net community production (5.3 and 4.6 mol C/m2, respectively) were approximately 3 times those measured in the Dalton polynya (average of 122 mg Chl a/m2 and 1.8 mol C/m2). Phytoplankton communities also differed between the polynyas. Diatoms were thriving in the Mertz and Ninnis polynyas but not in the Dalton polynya, where Phaeocystis antarctica dominated. These strong regional differences were explored using physiological, biological, and physical parameters. The most likely drivers of the observed higher productivity in the Mertz and Ninnis were the relatively shallow inflow of iron‐rich modified Circumpolar Deep Water onto the shelf as well as a very large sea ice meltwater contribution. The productivity contrast between the three polynyas could not be explained by (1) the input of glacial meltwater, (2) the presence of Ice Shelf Water, or (3) stratification of the mixed layer. Our results show that physical drivers regulate the productivity of polynyas, suggesting that the response of biological productivity and carbon export to future change will vary among polynyas
Dissolved iron in the Arctic shelf seas and surface waters of the Central Arctic Ocean: Impact of Arctic river water and ice-melt
Key Points
- DFe in the Arctic shelves and surface is linked to freshwater and alkalinity
- Fluvial input main contributor to high DFe, low alkalinity in Central Arctic
- Remineralisation and biological depletion determine DFe in the Arctic Shelf Seas
Abstract
Concentrations of dissolved (10 nM) in the bottom waters of the Laptev Sea shelf may be attributed to either sediment resuspension, sinking of brine or regeneration of DFe in the lower layers. A significant correlation (R2 = 0.60) between salinity and DFe is observed. Using δ18O, salinity ,nutrients and total alkalinity data, the main source for the high (>2 nM) DFe concentrations in the Amundsen and Makarov Basins is identified as (Eurasian) river water, transported with the Transpolar Drift (TPD). On the North American side of the TPD, the DFe concentrations are low ( 4) above the shelf and low ( < 4) off the shelf)
Temporal evolution of decaying summer first-year sea ice in the Western Weddell Sea, Antarctica
The evolution of the main physico-chemical properties of the unflooded 90-cm-thick first-year sea-ice cover at the Ice Station POLarstern (ISPOL) "clean site" is described. ISPOL was an international experiment of the German research icebreaker R.V. Polarstern. The vessel was anchored to an ice floe for an observation period of 5 weeks, during the early summer melt onset in the Western Weddell Sea. The "clean site" was specially designed and accessed so as to prevent any trace metal contamination of the sampling area. Observations were made at 5-day intervals during December 2004 in the central part of the main floe. Results show the succession of two contrasting phases in the behavior of the brine network (brine channels, pockets, and tubes). Initially, brine salinity was higher than that of sea-water, leading to brine migration and a decrease in the mean bulk salinity of the ice cover. This process is highly favored by the already high bulk porosity (14%), which ensures full connectivity of the brine network. Gravity drainage rather than convection seems to be the dominant brine transfer process. Half-way through the observation period, the brine salinity became lower than that of the sea-water throughout the ice column. The brine network therefore switched to a "stratified" regime in which exchange with sea-water was limited to molecular diffusion, strongly stabilizing the bulk mean sea-ice salinity. During the transition between the two regimes, and in areas closer to ridges, slush water (resulting from a mixture of snow meltwater and sea water accumulated at the snow-ice interface) penetrated through the growing "honeycomb-like structure" and replaced the downward draining brines. This resulted in a slight local replenishment of nutrients (as indicated by dissolved silicic acid). However, as a whole, the described decaying regime in this globally unflooded location with limited snow cover should be unfavorable to the development of healthy and active surface and internal microbial communities. The switch from gravity to diffusion controlled transport mechanisms within the ice column also should affect the efficiency of gas exchange across the sea-ice cover. The observed late build-up of a continuous, impermeable, superimposed ice layer should further significantly hamper gas exchange. Statistical estimates of the evolution of the ice thickness during the observation period and salinity trends of the under-ice water salinity down to 30m corroborate model predictions of a moderate bottom melting (5-10cm) from ocean heat fluxes. (C) 2008 Elsevier Ltd. All rights reserved.SIBCLIM - BELCANT
Temporal evolution of decaying summer first-year sea ice in the Western Weddell Sea, Antarctica
The evolution of the main physico-chemical properties of the unflooded 90-cm-thick first-year sea-ice cover at the Ice Station POLarstern (ISPOL) "clean site" is described. ISPOL was an international experiment of the German research icebreaker R.V. Polarstern. The vessel was anchored to an ice floe for an observation period of 5 weeks, during the early summer melt onset in the Western Weddell Sea. The "clean site" was specially designed and accessed so as to prevent any trace metal contamination of the sampling area. Observations were made at 5-day intervals during December 2004 in the central part of the main floe. Results show the succession of two contrasting phases in the behavior of the brine network (brine channels, pockets, and tubes). Initially, brine salinity was higher than that of sea-water, leading to brine migration and a decrease in the mean bulk salinity of the ice cover. This process is highly favored by the already high bulk porosity (14%), which ensures full connectivity of the brine network. Gravity drainage rather than convection seems to be the dominant brine transfer process. Half-way through the observation period, the brine salinity became lower than that of the sea-water throughout the ice column. The brine network therefore switched to a "stratified" regime in which exchange with sea-water was limited to molecular diffusion, strongly stabilizing the bulk mean sea-ice salinity. During the transition between the two regimes, and in areas closer to ridges, slush water (resulting from a mixture of snow meltwater and sea water accumulated at the snow-ice interface) penetrated through the growing "honeycomb-like structure" and replaced the downward draining brines. This resulted in a slight local replenishment of nutrients (as indicated by dissolved silicic acid). However, as a whole, the described decaying regime in this globally unflooded location with limited snow cover should be unfavorable to the development of healthy and active surface and internal microbial communities. The switch from gravity to diffusion controlled transport mechanisms within the ice column also should affect the efficiency of gas exchange across the sea-ice cover. The observed late build-up of a continuous, impermeable, superimposed ice layer should further significantly hamper gas exchange. Statistical estimates of the evolution of the ice thickness during the observation period and salinity trends of the under-ice water salinity down to 30m corroborate model predictions of a moderate bottom melting (5-10cm) from ocean heat fluxes. (C) 2008 Elsevier Ltd. All rights reserved.SIBCLIM - BELCANT
Chlorophyll a in Antarctic sea ice from historical ice core data
Sea ice core chlorophyll a data are used to describe the
seasonal, regional and vertical distribution of algal biomass in
Southern Ocean pack ice. The Antarctic Sea Ice Processes and
Climate – Biology (ASPeCt – Bio) circumpolar dataset consists
of 1300 ice cores collected during 32 cruises over a period
of 25 years. The analyses show that integrated sea ice chlorophyll
a peaks in early spring and late austral summer, which
is consistent with theories on light and nutrient limitation. The
results indicate that on a circum-Antarctic scale, surface,
internal and bottom sea ice layers contribute equally to integrated
biomass, but vertical distribution shows distinct differences
among six regions around the continent. The vertical
distribution of sea ice algal biomass depends on sea ice
thickness, with surface communities most commonly associatedwith
thin ice (<0.4m), and ice ofmoderate thickness (0.4–
1.0 m) having the highest probability of forming bottom
communities
Original Crosslinking of Poly(vinylidene fluoride) via Trialkoxysilane-Containing Cure-Site Monomers
The synthesis of two original fluorinated monomers bearing an x-trialkoxysilane function {4,5,5-trifluoropent-4-ene-1-trimethoxysilane [CF2¼CF(CH2)3Si (OCH3)3] and 4,5,5-trifluoropent-4-ene-1-triethoxysilane [CF2¼CF(CH2)3Si(OCH2 CH3)3]}, their radical copolymerization with vinylidene fluoride (VDF), and the study of the crosslinking of the resulting copolymers are presented. The silicon-containing fluoromonomers were prepared from a three-step reaction in a good overall yield. The first one dealt with the addition of ClCF2CFClI to allyl acetate, whereas the next phase consisted of the dehalogenation/deiodoacetalization of ClCF2CFClCH2CHICH2 OAc into 1,1,2-trifluoro-1,4-pentadiene by the use of zinc dust under sonication. Finally, the syntheses of both original monomers were carried out by a hydrosilylation reaction with trialkoxysilane in the presence of Karstedt or Speier platinum catalysts with the obtained trifluorodiene. The copolymerizations of these silicon-containing fluoromonomers with VDF were initiated by organic peroxide and led to original poly(vinylidene fluoride) bearing pendant trialkoxysilane functions. Their microstructures were characterized with 1H and 19F NMR, which showed that VDF was the more incorporated comonomer in the copolymers. These latter ones were crosslinked in the presence of moisture at 200 8C, and this led to insoluble materials stable in solvents, oils, water, and acid
Central nervous system infections in a tropical area: Influence of emerging and rare infections
International audienceBackground and purpose: The frequency of infectious encephalitis and the distribution of causative pathogens in the tropical areas are poorly known and may be influenced by emerging and rare infections. The aim was to characterize a large series of acute infectious encephalitis and myelitis in immunocompetent patients from the Caribbean island of Guadeloupe identifying clinical, biological and radiological features according to pathogens.Methods: Using a hospital database, we retrospectively collected detailed information on a comprehensive series of immunocompetent patients with acute infectious myelitis and encephalitis over the 2012-2018 period.Results: From 259 suspected cases with acute central nervous system (CNS) infection, we included 171 cases for analysis, comprising 141 encephalitis, 22 myelitis, and eight encephalomyelitis. The annual incidence peaked at 15.0/100 000 during the Zika 2016 outbreak. Children accounted for 22.2% of cases. Eight adults died during hospital stay, all encephalitis. Seventeen infectious agents, two of which had never been described in Guadeloupe so far, were identified in 101 cases (59.1%), including 35 confirmed cases (34.7%), 48 probable cases (47.5%), 15 possible cases (14.9%) and three clinical cases (3.0%). The most frequent etiologic agents were zika virus in 23 cases (13.5%), herpes simplex in 12 (7.0%), varicella-zoster virus in 11 (6.4%), dengue virus in 11 (6.4%) and leptospirosis in 11 (6.4%).Conclusions: Zika outbreak had a major influence on the annual incidence of acute CNS infection. Acute neuroleptospirosis is over-represented in our series. Further efforts are mandatory to develop new diagnostic tools for pathogen profiling
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