Injection of Oort Cloud comets: the fundamental role of stellar perturbations

We present Monte Carlo simulations of the dynamical evolution of the Oort cloud over the age of the Solar System, using an initial sample of one million test comets without any cloning. Our model includes perturbations due to the Galactic tide (radial and vertical) and passing stars. We present the first detailed analysis of the injection mechanism into observable orbits by comparing the complete model with separate models for tidal and stellar perturbations alone. We find that a fundamental role for injecting comets from the region outside the loss cone (perihelion distance q > 15 AU) into observable orbits (q < 5 AU) is played by stellar perturbations. These act in synergy with the tide such that the total injection rate is significantly larger than the sum of the two separate rates. This synergy is as important during comet showers as during quiescent periods and concerns comets with both small and large semi-major axes. We propose different dynamical mechanisms to explain the synergies in the inner and outer parts of the Oort Cloud. We find that the filling of the observable part of the loss cone under normal conditions in the present-day Solar System rises from <1% for a < 20 000 AU to about 100% for a ? 100 000 AU.     

Mapping biomass with remote sensing: a comparison of methods for the case study of Uganda

Background  

Assessing biomass is gaining increasing interest mainly for bioenergy, climate change research and mitigation activities, such as reducing emissions from deforestation and forest degradation and the role of conservation, sustainable management of forests and enhancement of forest carbon stocks in developing countries (REDD+). In response to these needs, a number of biomass/carbon maps have been recently produced using different approaches but the lack of comparable reference data limits their proper validation. The objectives of this study are to compare the available maps for Uganda and to understand the sources of variability in the estimation. Uganda was chosen as a case-study because it presents a reliable national biomass reference dataset.     

The Simulation of the Opposing Fluxes of Latent Heat and CO<Subscript>2</Subscript> over Various Land-Use Types: Coupling a Gas Exchange Model to a Mesoscale Atmospheric Model

A mesoscale meteorological model (FOOT3DK) is coupled with a gas exchange model to simulate surface fluxes of CO₂ and H₂O under field conditions. The gas exchange model consists of a C3 single leaf photosynthesis sub-model and an extended big leaf (sun/shade) sub-model that divides the canopy into sunlit and shaded fractions. Simulated CO₂ fluxes of the stand-alone version of the gas exchange model correspond well to eddy-covariance measurements at a test site in a rural area in the west of Germany. The coupled FOOT3DK/gas exchange model is validated for the diurnal cycle at singular grid points, and delivers realistic fluxes with respect to their order of magnitude and to the general daily course. Compared to the Jarvis-based big leaf scheme, simulations of latent heat fluxes with a photosynthesis-based scheme for stomatal conductance are more realistic. As expected, flux averages are strongly influenced by the underlying land cover. While the simulated net ecosystem exchange is highly correlated with leaf area index, this correlation is much weaker for the latent heat flux. Photosynthetic CO₂ uptake is associated with transpirational water loss via the stomata, and the resulting opposing surface fluxes of CO₂ and H₂O are reproduced with the model approach. Over vegetated surfaces it is shown that the coupling of a photosynthesis-based gas exchange model with the land-surface scheme of a mesoscale model results in more realistic simulated latent heat fluxes.     

Hydrological impacts of climate change predicted for an inland lake catchment in Ontario by using monthly water balance analyses

Potential hydrological impacts of climate change on long‐term water balances were analysed for Harp Lake and its catchment. Harp Lake is located in the boreal ecozone of Ontario, Canada. Two climate change scenarios were used. One was based on extrapolation of long‐term trends of monthly temperature and precipitation from a 129‐year data record, and another was based on a Canadian general circulation model (GCM) predictions. A monthly water balance model was calibrated using 26 years of hydrological and meteorological data, and the model was used to calculate hydrological impact under two climate change scenarios. The first scenario with a warmer and wetter climate predicted a smaller magnitude of change than the second scenario. The first scenario showed an increase in evaporation each month, an increase in catchment runoff in summer, fall and winter, but a decrease in spring, resulting in a slight increase in lake level. Annual runoff and lake level would increase because the precipitation change overrides evaporation change. The second scenario with a warmer, drier climate predicted a greater change, and indicated that evaporation would increase each month, runoff would increase in many months, but would decrease in spring, causing the lake level to decrease slightly. Annual runoff and lake level would decrease because evaporation change overrides precipitation change. In both scenarios, the water balance changes in winter and spring are pronounced. Copyright © 2009 John Wiley & Sons, Ltd.     

Dynamical evolution of NEAs: Close encounters, secular perturbations and resonances

We discuss the main mechanisms affecting the dynamical evolution of Near-Earth Asteroids (NEAs) by analyzing the results of three numerical integrations over 1 Myr of the NEA (4179) Toutatis. In the first integration the only perturbing planet is the Earth. So the evolution is dominated by close encounters and looks like a random walk in semimajor axis and a correlated random walk in eccentricity, keeping almost constant the perihelion distance and the Tisserand invariant. In the second integration Jupiter and Saturn are present instead of the Earth, and the 3/1 (mean motion) and v ₆ (secular) resonances substantially change the eccentricity but not the semimajor axis. The third, most realistic, integration including all the three planets together shows a complex interplay of effects, with close encounters switching the orbit between different resonant states and no approximate conservation of the Tisserand invariant. This shows that simplified 3-body or 4-body models cannot be used to predict the typical evolution patterns and time scales of NEAs, and in particular that resonances provide some “fast-track” dynamical routes from low-eccentricity to very eccentric, planet-crossing orbits.     

Behaviour of Li and its isotopes during metasomatism of French Massif Central lherzolites

Li behaviour and distribution in the mantle were investigated by ion microprobe in situ measurements on co-existing olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and amphibole (amp) in xenoliths from the French Massif Central. The fertile spinel lherzolites of this study record increasing degrees of mantle metasomatism, from unmetasomatised anhydrous samples through cryptically metasomatised samples to highly metasomatised amphibole-rich samples. In anhydrous lherzolites, Li is preferentially incorporated into olivine (1.1-1.4 ppm, average values) compared to pyroxenes (0.2-0.9 ppm). The hydrous samples clearly show enrichment of Li in ol (1.5-5.0 ppm), opx (1.1-2.4 ppm) and cpx (2.4-5.4 ppm), while amphibole incorporates less Li than the co-existing phases (0.8-1.3 ppm). Average δ⁷Li values range from +7.6 to +14.5‰ in ol, from 5.1 to +13.7‰ in opx and from 8.8 to +10.3‰ in cpx from the anhydrous lherzolites. A layered peridotite sample (Sdi) shows higher Li content in all phases, with lighter isotopic composition in opx and cpx (−0.6 and −2‰ average δ⁷Li values, respectively). In the hydrous lherzolites average δ⁷Li values both overlap and extend beyond these ranges in ol (up to 17.5 ‰) and in opx (up to 22.9‰), and vary widely in cpx (−2.7 to +9.7‰). Low δ⁷Li values are observed in some opx (−10.4‰) and cpx (−13‰) from sample Sdi, and in cpx from three hydrous samples (from −9.7 to −5.3‰). The different anhydrous phases from the hydrous samples show large intra-grain variations in Li isotopic ratios (e.g., up to 18‰) compared to the same phases from the anhydrous samples (mostly less than 6‰), excepting sample Sdi which has up to 20.4‰ variation in cpx. Similar to the anhydrous silicates, amphiboles show a wide variation of δ⁷Li values on the intra-grain scale (2-27‰). These variations are interpreted to result from fractionation processes during metasomatism by a silicate melt undergoing compositional changes as it percolates through and reacts with the peridotite phases. Thus Li abundances and isotopic in situ measurements are useful for tracing metasomatic processes but the heterogeneities observed in the samples preclude any identification of a specific mantle source by its Li signature.     

Oligotrophication from wetland epuration alters the riverine trophic network and carrying capacity for fish

Submerged aquatic vegetation (SAV) constitutes a major component of fish habitat, providing support for epiphytes and invertebrates as well as shelter from predators. The effects of wetland epuration from a mesotrophic to a nearly oligotrophic state were examined over a 15?km long reach of the St. Lawrence River under the direct influence of major farmland tributaries. We hypothesized that the nutrient-enriched zone would support a higher biomass of SAV, epiphytes, macroinvertebrates and fish than the nitrogen-deficient epurated zone located downstream of the wetland. Predictions included that the enriched habitat would support a richer fish assemblage, with higher biomass and growth of juvenile yellow perch than found in the epurated zone. Results supported these hypotheses, demonstrating the chain of effects of nutrient reduction on the biomass of SAV (fourfold drop), invertebrate prey (ninefold), small (threefold) and large (1.5-fold) fish between the two zones. In addition to the reduction in SAV biomass, the replacement of filamentous chlorophytes by benthic mats of filamentous cyanobacteria in the epurated zone resulted in a less complex 3-D habitat structure and a low invertebrate availability for fish. Oligotrophication by wetland epuration exerted negative effects on fish habitat quality, food quantity and availability, with an impairment of juvenile perch growth and recruitment. A generalized model of the changes in habitat carrying capacity occurring with epuration (oligotrophication) or eutrophication is presented, with examples of other aquatic systems in which strong linkages between trophic status, SAV, invertebrates and fish productivity were also demonstrated.     

Complex resistivity and radar investigation of building material: First results of field scale measurements

Moisture ingress is one of major damaging factors for masonry buildings. As the complex resistivity (CR) is sensitive to textural properties as well as to the pore fluid chemistry of wet porous media, its non-destructive application can provide helpful information for conservators. In a comprehensive laboratory study it has been shown that CR might even be able to distinguish between salt content and saturation degree in only one measurement. The combined use of electrical and electromagnetic measurement techniques in two field-scale flooding experiments has shown some unexpected differences. Possible reasons are discussed and it is shown that bringing together the information of both methods leads to a clearer picture.     

Are marine plastic particles transport vectors for organic pollutants to the Arctic?

Plastic litter accounts for 50-80% of waste items stranded on beaches, floating on the ocean surface and lodged in the seabed. Organic pollutants can be absorbed onto plastic particles from sea water, attached to their surfaces or included in the plastic matrix as additives. Such chemicals may be transported to remote regions by buoyant plastics and ocean currents. We have estimated mass fluxes of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and perfluorooctanoic acid (PFOA) to the Arctic via the main ocean currents and compared them to those in the dissolved state and in air. Substance fluxes with atmospheric or sea water currents account for several tons per year, whereas those mediated by plastics are four to six orders of magnitude smaller. However, the significance of various pollutant transport routes does not depend only on absolute mass fluxes but also on bioaccumulation in marine food chains.