475 research outputs found
Formation of iron sulfide at faecal pellets and other microniches within suboxic surface sediment.
Faecal pellet deposition and bioturbation may lead to heterogeneously distributed particles of localized highly reactive organic matter (microniches) being present below the oxygen penetration depth. Where O2, NO3-, and Fe/Mn oxyhydroxides become depleted within these microniches or where they exist in zones of sulfate reduction, significant localized peaks in sulfide concentration can occur. These discrete zones of sulfide evolution can cause formation of iron sulfides that would not be predicted by analysis of the ‘bulk’ sediment. Using a reaction-transport model developed specifically for investigating spherical microniches, and incorporating 3D diffusion, we investigated how the rate constants of organic matter (OM) degradation, particle porosity and niche lifetime, affect dissolved sulfide and iron concentrations, and formation of iron sulfide at such niches. For all of the modelled scenarios the saturation index for iron sulfide is positive, indicating favourable conditions for FeS precipitation in all niches. Those simulations within the microniche lifetime range of 2.5 to 5 days gave comparable concentration ratios of sulfide to iron in solution within the niche to experimentally observed values. Our model results provide insight into the mechanisms of preservation of OM, including soft tissue, in the paleo record, by predicting the conditions that result in preferential deposition of precipitates at the edge of microniches. Decreases in porosity, shorter niche lifetimes and increases in OM degradation rate constants, all tend to increase the likelihood that FeS precipitation will preferentially occur at the edges of a niche, rather than uniformly throughout the niche volume
Spatial modeling of extreme snow depth
The spatial modeling of extreme snow is important for adequate risk
management in Alpine and high altitude countries. A natural approach to such
modeling is through the theory of max-stable processes, an infinite-dimensional
extension of multivariate extreme value theory. In this paper we describe the
application of such processes in modeling the spatial dependence of extreme
snow depth in Switzerland, based on data for the winters 1966--2008 at 101
stations. The models we propose rely on a climate transformation that allows us
to account for the presence of climate regions and for directional effects,
resulting from synoptic weather patterns. Estimation is performed through
pairwise likelihood inference and the models are compared using penalized
likelihood criteria. The max-stable models provide a much better fit to the
joint behavior of the extremes than do independence or full dependence models.Comment: Published in at http://dx.doi.org/10.1214/11-AOAS464 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Model misspecification in peaks over threshold analysis
Classical peaks over threshold analysis is widely used for statistical
modeling of sample extremes, and can be supplemented by a model for the sizes
of clusters of exceedances. Under mild conditions a compound Poisson process
model allows the estimation of the marginal distribution of threshold
exceedances and of the mean cluster size, but requires the choice of a
threshold and of a run parameter, , that determines how exceedances are
declustered. We extend a class of estimators of the reciprocal mean cluster
size, known as the extremal index, establish consistency and asymptotic
normality, and use the compound Poisson process to derive misspecification
tests of model validity and of the choice of run parameter and threshold.
Simulated examples and real data on temperatures and rainfall illustrate the
ideas, both for estimating the extremal index in nonstandard situations and for
assessing the validity of extremal models.Comment: Published in at http://dx.doi.org/10.1214/09-AOAS292 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
2D simultaneous measurement of the oxyanions of P, V, As, Mo, Sb, W and U.
Previous work used the sampling technique diffusive gradients in thin-films analysed by laser ablation mass spectrometry to measure sulphide, P, V and As at a microniche of reactive organic carbon in a freshwater sediment. Here we present new developments of this technique. The number of analytes has been extended and we demonstrate the technique for depth profiling of analytes in both one and two dimensions. The physical dimensions of the cell in the laser ablation unit restrict the maximum length of gel that can be analysed. We address this problem by proposing a method for obtaining better data continuity when analysing multiple segments of gel from the same probe. 13C is used as the internal standard for each gel segment. For the cross standardisation of different gel segments 58Fe signals are obtained from ablation of a small piece of standard ferrihydrite gel analysed during the same run as the sample gel. As the ferrihydrite gel is a subsection of a much larger gel (i.e. the Fe concentration is consistent for all subsection), then any difference in signal can be attributed to changes in detector sensitivity and gels across different runs and performed on different days can be standardised
Bayesian Inference from Composite Likelihoods, with an Application to Spatial Extremes
Composite likelihoods are increasingly used in applications where the full
likelihood is analytically unknown or computationally prohibitive. Although the
maximum composite likelihood estimator has frequentist properties akin to those
of the usual maximum likelihood estimator, Bayesian inference based on
composite likelihoods has yet to be explored. In this paper we investigate the
use of the Metropolis--Hastings algorithm to compute a pseudo-posterior
distribution based on the composite likelihood. Two methodologies for adjusting
the algorithm are presented and their performance on approximating the true
posterior distribution is investigated using simulated data sets and real data
on spatial extremes of rainfall
Likelihood estimators for multivariate extremes
The main approach to inference for multivariate extremes consists in
approximating the joint upper tail of the observations by a parametric family
arising in the limit for extreme events. The latter may be expressed in terms
of componentwise maxima, high threshold exceedances or point processes,
yielding different but related asymptotic characterizations and estimators. The
present paper clarifies the connections between the main likelihood estimators,
and assesses their practical performance. We investigate their ability to
estimate the extremal dependence structure and to predict future extremes,
using exact calculations and simulation, in the case of the logistic model
The association of cobalt with iron and manganese (oxyhydr)oxides in marine sediment.
Formation and dissolution of authigenic Fe and Mn (oxyhydr)oxides influence cycling of trace metals in oxic/suboxic surface sediments.We used the diffusive gradients in thin films technique (DGT) to estimate the association of cobalt with iron and manganese oxides. We compared Co, Fe and Mn maxima measured by DGT in the pore waters of fresh and aged marine sediment cores and estimated the Co/Fe and Co/Mn ratios in the metal oxides. A Mn maximum was not visible in DGT concentration profiles of freshly collected sediment cores, but after ageing the sediment we observed a distinct Mn peak, presumably due to broadening of the depth range over which the various electron acceptors occur. Estimated Co/Mn ratios from both experiments are within the range of literature values for marine sediments, but the value from the aged experiment is at the lower end of the range. This is attributed to stimulation of sulphate reduction and precipitation of cobalt sulfides. The good correlation between Co and Fe maxima in the fresh sediments is attributed to the similarity of their reactions with sulphide rather than Co being released during authigenic Fe oxide reduction
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