39,578 research outputs found
Impact of nitrogenous fertilizers on carbonate dissolution in small agricultural catchments: Implications for weathering CO2 uptake at regional and global scales
The goal of this study was to highlight the occurrence of an additional proton-promoted weathering pathway of carbonate rocks in agricultural areas where N-fertilizers are extensively spread, and to estimate its consequences on riverine alkalinity and uptake of CO2 by weathering. We surveyed 25 small streams in the calcareous molassic Gascogne area located in the Garonne river basin (south-western France) that drain cultivated or forested catchments for their major element compositions during different hydrologic periods. Among these catchments, the Hay and the Montousse´, two experimental catchments,
were monitored on a weekly basis. Studies in the literature from other small carbonate catchments in Europe were dissected in the same way. In areas of intensive agriculture, the molar ratio (Ca + Mg)/HCO3 in surface waters is significantly higher (0.7 on average) than in areas of low anthropogenic pressure (0.5). This corresponds to a decrease in riverine alkalinity, which can reach 80% during storm events. This relative loss of alkalinity correlates well with the NO3 content in surface waters. In cultivated areas, the contribution of atmospheric/soil CO2 to the total riverine alkalinity (CO2 ATM-SOIL/HCO3) is less than 50% (expected value for carbonate basins), and it decreases when the nitrate concentration increases. This loss of alkalinity can be attributed to the substitution of carbonic acid (natural weathering pathway) by protons produced by nitrification of Nfertilizers (anthropogenic weathering pathway) occurring in soils during carbonate dissolution. As a consequence of these processes,
the alkalinity over the last 30 years shows a decreasing trend in the Save river (one of the main Garonne river tributaries, draining an agricultural catchment), while the nitrate and calcium plus magnesium contents are increasing.
We estimated that the contribution of atmospheric/soil CO2 to riverine alkalinity decreased by about 7–17% on average for all the studied catchments. Using these values, the deficit of CO2 uptake can be estimated as up to 0.22–0.53 and 12–29 Tg1 yr1 CO2 on a country scale (France) and a global scale, respectively. These losses represent up to 5.7–13.4% and only
1.6–3.8% of the total CO2 flux naturally consumed by carbonate dissolution, for France and on a global scale, respectively. Nevertheless, this loss of alkalinity relative to the Ca + Mg content relates to carbonate weathering by protons from N-fertilizers nitrification, which is a net source of CO2 for the atmosphere. This anthropogenic CO2 source is not negligible since it
could reach 6–15% of CO2 uptake by natural silicate weathering and could consequently partly counterbalance this natural CO2 sink
The forgotten monoid
We study properties of the forgotten monoid which appeared in work of Lascoux
and Schutzenberger and recently resurfaced in the construction of dual
equivalence graphs by Assaf. In particular, we provide an explicit
characterization of the forgotten classes in terms of inversion numbers and
show that there are n^2-3n+4 forgotten classes in the symmetric group S_n. Each
forgotten class contains a canonical element that can be characterized by
pattern avoidance. We also show that the sum of Gessel's quasi-symmetric
functions over a forgotten class is a 0-1 sum of ribbon-Schur functions.Comment: 13 pages; in version 3 the proof of Proposition 3 is correcte
Nilpotent bicone and characteristic submodule of a reductive Lie algebra
The nilpotent bicone of a finite dimensional complex reductive Lie algebra g
is the subset of elements in g x g whose subspace generated by the components
is contained in the nilpotent cone of g. The main result of this note is that
the nilpotent bicone is a complete intersection. This affirmatively answers a
conjecture of Kraft-Wallach concerning the nullcone. In addition, we introduce
and study the characteristic submodule of g. The properties of the nilpotent
bicone and the characteristic submodule are known to be very important for the
understanding of the commuting variety and its ideal of definition. In order to
study the nilpotent bicone, we introduce another subvariety, the principal
bicone. The nilpotent bicone, as well as the principal bicone, are linked to
jet schemes. We study their dimensions using arguments from motivic
integration. Namely, we follow methods developed in
http://arxiv.org/abs/math/0008002v5 .Comment: 48 pages. Remark 8 has been modified; one sentence was not correct.
We thank Kari Vilonen for pointing out this erro
Membrane properties revealed by spatiotemporal response to a local inhomogeneity
We study theoretically the spatiotemporal response of a lipid membrane
submitted to a local chemical change of its environment, taking into account
the time-dependent profile of the reagent concentration due to diffusion in the
solution above the membrane. We show that the effect of the evolution of the
reagent concentration profile becomes negligible after some time. It then
becomes possible to extract interesting properties of the membrane response to
the chemical modification. We find that a local density asymmetry between the
two monolayers relaxes by spreading diffusively in the whole membrane. This
behavior is driven by intermonolayer friction. Moreover, we show how the ratio
of the spontaneous curvature change to the equilibrium density change induced
by the chemical modification can be extracted from the dynamics of the local
membrane deformation. Such information cannot be obtained by analyzing the
equilibrium vesicle shapes that exist in different membrane environments in
light of the area-difference elasticity model.Comment: 11 pages, 4 figure
Kernel Inverse Regression for spatial random fields
In this paper, we propose a dimension reduction model for spatially dependent
variables. Namely, we investigate an extension of the \emph{inverse regression}
method under strong mixing condition. This method is based on estimation of the
matrix of covariance of the expectation of the explanatory given the dependent
variable, called the \emph{inverse regression}. Then, we study, under strong
mixing condition, the weak and strong consistency of this estimate, using a
kernel estimate of the \emph{inverse regression}. We provide the asymptotic
behaviour of this estimate. A spatial predictor based on this dimension
reduction approach is also proposed. This latter appears as an alternative to
the spatial non-parametric predictor
Orthogonal involutions on central simple algebras and function fields of Severi-Brauer varieties
An orthogonal involution on a central simple algebra , after
scalar extension to the function field of the Severi--Brauer
variety of , is adjoint to a quadratic form over
, which is uniquely defined up to a scalar factor. Some
properties of the involution, such as hyperbolicity, and isotropy up to an
odd-degree extension of the base field, are encoded in this quadratic form,
meaning that they hold for the involution if and only if they hold for
. As opposed to this, we prove that there exists non-totally
decomposable orthogonal involutions that become totally decomposable over
, so that the associated form is a Pfister form. We
also provide examples of nonisomorphic involutions on an index algebra that
yield similar quadratic forms, thus proving that the form does not
determine the isomorphism class of , even when the underlying algebra
has index . As a consequence, we show that the invariant for
orthogonal involutions is not classifying in degree , and does not detect
totally decomposable involutions in degree , as opposed to what happens for
quadratic forms
The Arason invariant of orthogonal involutions of degree 12 and 8, and quaternionic subgroups of the Brauer group
Using the Rost invariant for torsors under Spin groups one may define an
analogue of the Arason invariant for certain hermitian forms and orthogonal
involutions. We calculate this invariant explicitly in various cases, and use
it to associate to every orthogonal involution with trivial discriminant and
trivial Clifford invariant over a central simple algebra of even co-index a
cohomology class of degree 3 with coefficients. This invariant
is the double of any representative of the Arason invariant; it vanishes
when the algebra has degree at most 10, and also when there is a quadratic
extension of the center that simultaneously splits the algebra and makes the
involution hyperbolic. The paper provides a detailed study of both invariants,
with particular attention to the degree 12 case, and to the relation with the
existence of a quadratic splitting field.Comment: A mistake pointed out by A. Sivatski in Section 5.3 has been
corrected in the new version of this preprin
Saturation spaces for regularization methods in inverse problems
The aim of this article is to characterize the saturation spaces that appear in inverse problems. Such spaces are defined for a regularization method and the rate of convergence of the estimation part of the inverse problem depends on their definition. Here we prove that it is possible to define these spaces as regularity spaces, independent of the choice of the approximation method. Moreover, this intrinsec definition enables us to provide minimax rate of convergence under such assumptionsLinear inverse problems, regularization methods, structural econometrics
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