Organotin accumulation in an estuarine food chain: comparing field measurements with model estimations

The bioaccumulation model OMEGA (optimal modelling for ecotoxicological applications) is used to explore accumulation of organotins in the Western Scheldt food chain, consisting of herbi-detritivores, primary and secondary carnivorous fish and a piscivorous bird. Organotins studied are tributyltin (TBT) and triphenyltin (TPT) and the respective di- and mono-organotin metabolites. Empirical elimination rate constants are compared to model predictions for organic substances and metals. It is found that field bioaccumulation ratios are higher than predicted based on elimination kinetics relevant for organic compounds. The results indicate that uptake of organotins mainly occurs via hydrophobic mechanisms, whereas elimination may occur via metal-like kinetics. This results in very low elimination rates, which are comparable to model predictions for metals.     

Magnetic hydrogen atmosphere models and the neutron star RX J1856.5–3754

RX J1856.5−3754 is one of the brightest nearby isolated neutron stars (INSs), and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5−3754, within the uncertainties. In our simplest model, the best-fitting parameters are an interstellar column density   N _H≈ 1 × 10²⁰ cm⁻²  and an emitting area with   R ^∞≈ 17 km  (assuming a distance to RX J1856.5−3754 of 140 pc), temperature   T ^∞≈ 4.3 × 10⁵ K  , gravitational redshift   z _g ∼ 0.22  , atmospheric hydrogen column   y _H≈ 1 g cm⁻²  , and magnetic field   B ≈ (3–4) × 10¹² G  ; the values for the temperature and magnetic field indicate an effective average over the surface. We also calculate a more realistic model, which accounts for magnetic field and temperature variations over the NS surface as well as general relativistic effects, to determine pulsations; we find that there exist viewing geometries that produce pulsations near the currently observed limits. The origin of the thin atmospheres required to fit the data is an important question, and we briefly discuss mechanisms for producing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5−3754.     

Quantifying nitrogen cycling beneath a meander of a low gradient,N‐impacted,agricultural stream using tracers and numerical modelling

In watersheds impacted by nitrate from agricultural fertilizers, nitrification and denitrification may be decoupled as denitrification in the hyporheic zone is not limited to naturally produced nitrate. While most hyporheic research focuses on the 1–2 m of sediment beneath the stream bed, there are a limited number of studies that quantify nitrogen (N) cycling at larger hyporheic scales (10s of metres to kms). We conducted an investigation to quantify N cycling through a single meander of a low gradient, meandering stream, draining an agricultural watershed. Chemistry (major ions and N species) and hydrologic data were collected from the stream and groundwater beneath the meander. Evidence indicates that nearly all the shallow groundwater flowing beneath the meander originates as stream water on the upgradient side of the meander, and returns to the stream on the downgradient side. We quantified the flux of water beneath the meander using a numerical model. The flux of N into and out of the meander was quantified by multiplying the concentration of the important N species (nitrate, ammonium, dissolved organic nitrogen (DON)) by the modelled water fluxes. The flux of N into the meander is dominated by nitrate, and the flux of N out of the meander is dominated by ammonium and DON. While stream nitrate varied seasonally, ammonium and DON beneath the meander were relatively constant throughout the year. When stream nitrate concentrations are high (>2 mg litre^?1), flow beneath the meander is a net sink for N as more N from nitrate in stream water is consumed than is produced as ammonium and DON. When stream nitrate concentrations are low (<2 mg litre^?1), the flux of N entering is less than exiting the meander. On an annual basis, the meander hyporheic flow serves as a net sink for N. Copyright © 2007 John Wiley & Sons, Ltd.     

Spectral evolution of V838 Monocerotis in the optical and near-infrared in early 2002

We report optical and near-infrared spectroscopy, and optical spectropolarimetry, of the peculiar variable V838 Mon during the multiple outburst phase in early 2002. The spectral evolution is exceptional. Our earliest spectra (2002 January) are noteworthy for their strong absorption lines of barium and strontium in the optical, and bands of CO and circumstellar H₂O in the near-infrared. All but the CO weaken or are absent in later spectra. The behaviour of the CO band during this phase is extraordinary: initially in absorption, it was observed two months later in optically thick emission. The excitation of the CO is probably the result of the propagation of a shock wave at the third maximum. The two spectropolarimetric epochs were taken 6 and 27 d after the second outburst on 2002 February 8. The polarization at both times was measured to be   p_V ≈ 2.7  per cent. Nearly all of the measured polarization is believed to be due to interstellar dust, a conclusion that is consistent with previous studies. At both epochs, however, a weak and variable intrinsic component is thought to be present. Between January and March of 2002 the luminosity of V838 Mon increased by a factor of 15 and the apparent diameter increased fourfold.     

Micromorphological aspects of glaciolacustrine sediments in Northern Patagonia,Argentina

In this study the sedimentology, micromorphology and structure of four deformation es in end moraines or near-end moraine settings are presented. Micromorphology has proven be a powerful tool in the interpretation of the sediments. The four sites are related to three of the four major glaciations known from this part of Patagonia: Nahuel Huapi, Anfiteatro and Pichileufú. All four sites are characterised by glaciolacustrine vironments in which sedimentation occurred at least partly on and/or against dead ice. With the exception of one section in the San Martin de los Andes area, where deformation occurred active ice-push, all major disturbances (faults and folds) are the result of dead ice collapse. Comparison with micromorphological observations on basal tills shows that the sediments all lack clear subglacial imprint.