542 research outputs found
Sources and mechanisms for the enrichment of highly reactive iron in euxinic Black Sea sediments
The deep basinal euxinic sediments of the Black Sea are enriched in iron that is highly susceptible to sulfidization compared to oxic/suboxic continental
margin sediments in the Black Sea and oxic/dysoxic continental margin and deep-sea sediments worldwide. A mass balance treatment of iron speciation data from three
deep basin sediment cores shows that this enrichment is due to a combination of (1)highly reactive iron-bearing phases (sulfides and oxides) whose ultimate source was by
diagenetic mobilization from the shelf (Wijsman and others, 2001) and (2) enhanced iron reactivity in the lithogenous component of deep basinal sediments. The cause of the enhanced reactivity of lithogenous iron is problematical. Possible mechanisms include microbial oxidation of non-reactive iron silicates and the preferential deposition of a fine-grained, reactive iron-enriched lithogenous component in the deep basin
by the fractionation of the lithogenous flux during transport across the shelf. The application of paleoredox indicators based on iron reactivity (Degree of Pyritization, Indicator of Anoxicity) should take into account that the availability of highly reactive
iron, and hence the concentration of reactive iron phases in the sediment, is controlled by a variety of chemical, physical and biological factors, some of which are not related directly to redox conditions in the water column
First record of Pseudoryzomys simplex (Cricetidae, Sigmodontinae) in a flooded area of the Pantanal, Brazil
Simple method for excitation of a Bose-Einstein condensate
An appropriate, time-dependent modification of the trapping potential may be
sufficient to create effectively collective excitations in a cold atom
Bose-Einstein condensate. The proposed method is complementary to earlier
suggestions and should allow the creation of both dark solitons and vortices.Comment: 8 pages, 7 figures, version accepted for publication in Phys. Rev.
Could a Shigella vaccine impact long-term health outcomes?: Summary report of an expert meeting to inform a Shigella vaccine public health value proposition, March 24 and 29, 2021.
Shigellosis is a leading cause of diarrhea and dysentery in young children from low to middle-income countries and adults experiencing traveler's diarrhea worldwide. In addition to acute illness, infection by Shigella bacteria is associated with stunted growth among children, which has been linked to detrimental long-term health, developmental, and economic outcomes. On March 24 and 29, 2021, PATH convened an expert panel to discuss the potential impact of Shigella vaccines on these long-term outcomes. Based on current empirical evidence, this discussion focused on whether Shigella vaccines could potentially alleviate the long-term burden associated with Shigella infections. Also, the experts provided recommendations about how to best model the burden, health and vaccine impact, and economic consequences of Shigella infections. This international multidisciplinary panel included 13 scientists, physicians, and economists from multiple relevant specialties. According to the panel, while the relationship between Shigella infections and childhood growth deficits is complex, this relationship likely exists. Vaccine probe studies are the crucial next step to determine whether vaccination could ameliorate Shigella infection-related long-term impacts. Infants should be vaccinated during their first year of life to maximize their protection from severe acute health outcomes and ideally reduce stunting risk and subsequent negative long-term developmental and health impacts. With vaccine schedule crowding, targeted or combination vaccination approaches would likely increase vaccine uptake in high-burden areas. Shigella impact and economic assessment models should include a wider range of linear growth outcomes. Also, these models should produce a spectrum of results-ones addressing immediate benefits for usual health care decision-makers and others that include broader health impacts, providing a more comprehensive picture of vaccination benefits. While many of the underlying mechanisms of this relationship need better characterization, the remaining gaps can be best addressed by collecting data post-vaccine introduction or through large trials
Hall Effect and Resistivity in High-Tc Superconductors: The Conserving Approximation
The Hall coefficient (R_H) of high-Tc cuprates in the normal state shows the
striking non-Fermi liquid behavior: R_H follows a Curie-Weiss type temperature
dependence, and |R_H|>>1/|ne| at low temperatures in the under-doped compounds.
Moreover, R_H is positive for hole-doped compounds and is negative for
electron-doped ones, although each of them has a similar hole-like Fermi
surface. In this paper, we give the explanation of this long-standing problem
from the standpoint of the nearly antiferromagnetic (AF) Fermi liquid. We
consider seriously the vertex corrections for the current which are
indispensable to satisfy the conservation laws, which are violated within the
conventional Boltzmann transport approximation. The obtained total current J_k
takes an enhanced value and is no more perpendicular to the Fermi surface due
to the strong AF fluctuations. By virtue of this mechanism, the anomalous
behavior of R_H in high-Tc cuprates is neutrally explained. We find that both
the temperature and the (electron, or hole) doping dependences of R_H in
high-T_c cuprates are reproduced well by numerical calculations based on the
fluctuation-exchange (FLEX) approximation, applied to the single-band Hubbard
model. We also discuss the temperature dependence of R_H in other nearly AF
metals, e.g., V_2O_3, kappa-BEDT-TTF organic superconductors, and heavy fermion
systems close to the AF phase boundary.Comment: 19 pages, to appear in Phys. Rev. B, No.59, Vol.22, 199
Hypogene Calcitization: Evaporite Diagenesis in the Western Delaware Basin
Evaporite calcitization within the Castile Formation of the Delaware Basin is more widespread and diverse than originally recognized. Coupled field and GIS studies have identified more than 1000 individual occurrences of calcitization within the Castile Formation outcrop area, which includes both calcitized masses (limestone buttes) and laterally extensive calcitized horizons (limestone sheets). Both limestone buttes and sheets commonly contain a central brecciated zone that we attribute to hypogene dissolution. Lithologic fabric of calcitized zones ranges from little alteration of original varved laminae to fabrics showing extensive laminae distortion as well as extensive vuggy and open cavernous porosity. Calcitization is most abundant in the western portion of the Castile outcrop region where surface denudation has been greatest. Calcitization often forms linear trends, indicating fluid migration along fractures, but also occurs as dense clusters indicating focused, ascending, hydrocarbon-rich fluids. Native sulfur, secondary tabular gypsum (i.e. selenite) and hypogene caves are commonly associated with clusters of calcitization. This assemblage suggests that calcium sulfate diagenesis within the Castile Formation is dominated by hypogene speleogemesis
The Dependence of the Superconducting Transition Temperature of Organic Molecular Crystals on Intrinsically Non-Magnetic Disorder: a Signature of either Unconventional Superconductivity or Novel Local Magnetic Moment Formation
We give a theoretical analysis of published experimental studies of the
effects of impurities and disorder on the superconducting transition
temperature, T_c, of the organic molecular crystals kappa-ET_2X and beta-ET_2X
(where ET is bis(ethylenedithio)tetrathiafulvalene and X is an anion eg I_3).
The Abrikosov-Gorkov (AG) formula describes the suppression of T_c both by
magnetic impurities in singlet superconductors, including s-wave
superconductors and by non-magnetic impurities in a non-s-wave superconductor.
We show that various sources of disorder lead to the suppression of T_c as
described by the AG formula. This is confirmed by the excellent fit to the
data, the fact that these materials are in the clean limit and the excellent
agreement between the value of the interlayer hopping integral, t_perp,
calculated from this fit and the value of t_perp found from angular-dependant
magnetoresistance and quantum oscillation experiments. If the disorder is, as
seems most likely, non-magnetic then the pairing state cannot be s-wave. We
show that the cooling rate dependence of the magnetisation is inconsistent with
paramagnetic impurities. Triplet pairing is ruled out by several experiments.
If the disorder is non-magnetic then this implies that l>=2, in which case
Occam's razor suggests that d-wave pairing is realised. Given the proximity of
these materials to an antiferromagnetic Mott transition, it is possible that
the disorder leads to the formation of local magnetic moments via some novel
mechanism. Thus we conclude that either kappa-ET_2X and beta-ET_2X are d-wave
superconductors or else they display a novel mechanism for the formation of
localised moments. We suggest systematic experiments to differentiate between
these scenarios.Comment: 18 pages, 5 figure
Oracle-based optimization applied to climate model calibration
In this paper, we show how oracle-based optimization can be effectively used for the calibration of an intermediate complexity climate model. In a fully developed example, we estimate the 12 principal parameters of the C-GOLDSTEIN climate model by using an oracle- based optimization tool, Proximal-ACCPM. The oracle is a procedure that finds, for each query point, a value for the goodness-of-fit function and an evaluation of its gradient. The difficulty in the model calibration problem stems from the need to undertake costly calculations for each simulation and also from the fact that the error function used to assess the goodness-of-fit is not convex. The method converges to a Fbest fit_ estimate over 10 times faster than a comparable test using the ensemble Kalman filter. The approach is simple to implement and potentially useful in calibrating computationally demanding models based on temporal integration (simulation), for which functional derivative information is not readily available
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