Tidal Stress Patterns on Europa's Crust

The ice crust of Europa probably floats over a deep liquid-water ocean, and has been continually resurfaced by tectonic and thermal processes driven by tides. Tidal working causes rotational torque, surface stress, internal heating, and orbital evolution. The stress patterns expected on such a crust due to reorientation of the tidal bulge by non-synchronous rotation and due to orbital eccentricity, which introduces periodic ('diurnal') variations in the tide, are shown as global maps. By taking into account the finite rate of crack propagation, global maps are generated of cycloidal features and other distinctive patterns, including the crack shapes characteristic of the wedges region and its antipode on the sub-Jovian hemisphere. Theoretical maps of tidal stress and cracking can be compared with observed tectonics, with the possibility of reconstructing the rotational history of the satellite.     

Exploring the hydrological effects of normal faults at the boundary of the Roer Valley Graben in Belgium using a catchment-scale groundwater flow model

Hydrogeology Journal - Faults may impact regional groundwater flow and transport, so it is important to include them during aquifer-system conceptualization and while constructing groundwater flow...     

Biomarkers in upper Holocene Eastern North Sea and Wadden Sea sediments

Total extracts of sediment cores from five different stations in the North Sea and Wadden Sea were analysed for their biomarker composition. Only sediments of the Skagerrak contained significant amounts of marine biomarkers (mainly alkenones), other sites contained predominantly terrestrial biomarkers. Bioturbation in the Skagerrak is, however, far too high to determine sea surface temperature (SST) changes within short time intervals. These results indicate that biomarkers contained in these sediments are not useful to reconstruct climate fluctuations during the upper Holocene. High amounts of α-, β- and ω-hydroxy fatty acids as well as small amounts of α, β-dihydroxy fatty acids were released from the insoluble organic material of the sediments from the Wadden Sea station, indicating a significant input of the eelgrass Zostera marina. This was confirmed by microscopic observations. This is the first time the α,β-dihydroxy fatty acids have been found in a sediment core and they have proven to be potential biomarkers for these seagrass species.     

<Emphasis Type="Italic">Phragmites australis</Emphasis> invasion and expansion in tidal wetlands: Interactions among salinity,sulfide, and hydrology

Through their physiological effects on ion, oxygen, and carbon balance, respectively, salinity, sulfide, and prolonged flooding combine to constrain the invasion and spread ofPhragmites in tidal wetlands. Initial sites of vigorous invasion by seed germination and growth from rhizome fragments appear limited to sections of marsh where salinity is <10‰, sulfide concentrations are less than 0.1 mM, and flooding frequency is less than 10%. In polyhaline tidal wetlands the invasion sites include the upland fringe and some high marsh creek banks. The zones of potential invasion tend to be larger in marshes occupying lower-salinity portions of estuaries and in marshes that have been altered hydrologically. Owing to clonal integration and a positive feedback loop of growth-induced modification of edaphic soil conditions, however, a greater total area of wetland is susceptible toPhragmites expansion away from sites of establishment. Mature clones have been reported growing in different marshes with salinity up to 45‰, sulfide concentration up to 1.75 mM, and flooding frequency up to 100%. ForPhragmites establishment and expansion in tidal marshes, windows of opportunity open with microtopographic enhancement of subsurface drainage patterns, marsh-wide depression of flooding and salinity regimes, and variation in sea level driven by global warming and lunar nodal cycles. To avoidPhragmites monocultures, tidal wetland creation, restoration, and management must be considered within the context of these different scales of plant-environment interaction.     

Carbon sequestration and environmental effects of afforestation with Pinus radiata D. Don in the Western Cape, South Africa

A three-step methodology to assess the carbon sequestration and the environmental impact of afforestation projects in the framework of the Flexible Mechanisms of the Kyoto Protocol (Joint Implementation and Clean Development Mechanism) was developed and tested using a dataset collected from the Jonkershoek forest plantation, Western Cape, South Africa, which was established with Pinus radiata in former native fynbos vegetation and indigenous forest. The impact of a change in land use was evaluated for a multifunctional, a production and a non-conversion scenario. First, the carbon balance was modelled with GORCAM and was expressed as (1) C sequestration in tC ha⁻¹ year⁻¹ in soil, litter, and living biomass according to the rules of the first commitment period of the Kyoto Protocol, and (2) CO₂ emission reductions in tC ha⁻¹ year⁻¹, which includes carbon sequestered in the above-mentioned pools and additionally in wood products, as well as emission reductions due to fossil fuel substitution. To estimate forest growth, three data sources were used: (1) inventory data, (2) growth simulation with a process-based model, and (3) yield tables. Second, the effects of land use change were assessed for different project scenarios using a method related to Life Cycle Assessment (LCA). The method uses 17 quantitative indicators to describe the impact of project activities on water, soil, vegetation cover and biodiversity. Indicator scores were calculated by comparing indicator values with reference values, estimated for the climax vegetation. The climax vegetation is the site-specific ecosystem phase with the highest exergy content and the highest exergy flow dissipation capacity. Third, the land use impact per functional unit of 1 tC sequestered was calculated by combining the results of step 1 and step 2. The average baselines to obtain carbon additionality are 476 tC ha⁻¹ for indigenous forest and 32 tC ha⁻¹ for fynbos. Results show that the influence of the growth assessment method on the magnitude of C sequestration and hence on the environmental impact per functional unit is large. When growth rate is assessed with the mechanistic model and with the yield table, it is overestimated in the early years and underestimated in the long term. The main conclusion of the scenario analysis is that the production forest scenario causes higher impacts per functional unit than the multifunctional scenario, but with the latter being less efficient in avoiding CO₂ emissions. The proposed method to assess impacts on diverse components of the ecosystem is able to estimate the general tendency of the adverse and positive effects of each scenario. However, some indicators, more specifically about biodiversity and water balance, could be improved or reinterpreted in light of specific local data about threat to biodiversity and water status.     

An inter-comparison of regional climate models for Europe: model performance in present-day climate

The analysis of possible regional climate changes over Europe as simulated by 10 regional climate models within the context of PRUDENCE requires a careful investigation of possible systematic biases in the models. The purpose of this paper is to identify how the main model systematic biases vary across the different models. Two fundamental aspects of model validation are addressed here: the ability to simulate (1) the long-term (30 or 40 years) mean climate and (2) the inter-annual variability. The analysis concentrates on near-surface air temperature and precipitation over land and focuses mainly on winter and summer. In general, there is a warm bias with respect to the CRU data set in these extreme seasons and a tendency to cold biases in the transition seasons. In winter the typical spread (standard deviation) between the models is 1 K. During summer there is generally a better agreement between observed and simulated values of inter-annual variability although there is a relatively clear signal that the modeled temperature variability is larger than suggested by observations, while precipitation variability is closer to observations. The areas with warm (cold) bias in winter generally exhibit wet (dry) biases, whereas the relationship is the reverse during summer (though much less clear, coupling warm (cold) biases with dry (wet) ones). When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation.     

Avulsions, channel evolution and floodplain sedimentation rates of the anastomosing upper Columbia River, British Columbia, Canada

Ages of channels of the anastomosing upper Columbia River, south‐eastern British Columbia, Canada, were investigated in a cross‐valley transect by ¹⁴C dating of subsurface floodplain organic material from beneath levees. The avulsion history within the transect was deduced from these data, and morphological stages in channel development were recognized. Additionally, floodplain sedimentation rates were established. The new data demonstrate that the upper Columbia River is a long‐lived, dynamic anastomosing system. Results show that anastomosis at the study location has persisted since at least 2700 cal. years BP, with avulsions occurring frequently. At least nine channels have formed in the studied cross‐valley transect within the past 3000 years. Channel lifetimes from formation to abandonment appear to be highly variable, ranging from approximately 800 to 3000 years. Log jams provoking avulsions and/or silting up of old channels are proposed as reasons for this variability. Long‐term average floodplain sedimentation rates appear to be significantly lower than previously proposed by Smith (1983, Int. Assoc. Sedimentol. Spec. Publ., 6, 155–168). A long‐term (4550 years) average of 1·75 mm year^?1 (after compaction) was based on ¹⁴C dates, while a short‐term sedimentation rate of 0·8 mm was determined for a single, relatively small, seasonal flood in 1994 using sediment traps. However, short‐term sedimentation rates vary considerably over the floodplain, with levees aggrading up to four times faster than floodbasins. Channels of the upper Columbia River anastomosed reach follow a consistent pattern in their development, with each stage being characterized by different morphology and processes. Channel evolution comprises the following succession: (1) avulsion stage, in which a crevasse splay channel deepens by scour and levee sedimentation; (2) widening and deepening stage, in which bank slumping and bed scouring dominates; (3) infilling stage, in which either channel narrowing (bank accretion) or channel shallowing (bed accretion) takes place; and (4) abandonment stage, in which the residual (remnant) channel is filled exclusively by silt, clay and organic material. Vertical stacking (super‐ imposition) of active channels on recent channel‐fill sand bodies is a notable feature of the upper Columbia River, which suggests that reoccupation of residual channels is a common process.     

Neoformation of Ni phyllosilicate upon Ni uptake on montmorillonite: A kinetics study by powder and polarized extended X-ray absorption fine structure spectroscopy

Wet chemistry kinetics and powder and polarized extended X-ray absorption fine structure (EXAFS and P-EXAFS) spectroscopy were combined to investigate the mechanism of Ni uptake on montmorillonite, at pH 8, high ionic strength (0.2 M Ca(NO₃)₂), initial Ni concentration of 660 μM, and solid concentration of 5.3 g/L. Approximately 20% of Ni sorbed within the first 24 h; thereafter, the Ni uptake rate slowed, and 12% of the initial Ni concentration remained in solution after 206 d of reaction time. Powder EXAFS spectra collected on wet pastes at 1, 14, 90, and 206 d showed the presence of Ni-Ni pairs at ∼3.08 Å in an amount that gradually increased with time. Results were interpreted by the nucleation of a Ni phase having either an α-Ni-hydroxide- or a Ni-phyllosilicate-like local structure. The latter possibility was confirmed by recording P-EXAFS spectra of a highly textured, self-supporting montmorillonite film prepared in the same conditions as the wet samples and equilibrated for 14 d. The orientation distribution of the c*-axes of individual clay particles off the film plane, as measured by quantitative texture analysis, was 32.8° full width at half maximum, and this value was used to correct from texture effect the effective numbers of Ni and Si nearest neighbors determined by P-EXAFS. Ni atoms were found to be surrounded by 2.6 ± 0.5 Ni atoms at 3.08 Å in the in-plane direction and by 4.2 ± 0.5 Si atoms at 3.26 Å in the out-of-plane direction. These structural parameters, but also the orientation and angular dependence of the Ni and Si shells, strongly support the formation of a Ni phyllosilicate having its layers parallel to the montmorillonite layers. The neoformation of a phyllosilicate on metal uptake on montmorillonite, documented herein for the first time, has important geochemical implications because this dioctahedral smectite is overwhelmingly present in the environment. The resulting sequestration of sorbed trace metals in sparingly soluble phyllosilicate structure may durably decrease their migration and bioavailability at the Earth’s surface and near surface.     

Bed load transport on the shoreface by currents and waves

Tide-driven bed load transport is an important portion of the net annual sediment transport rate in many shoreface and shelf environments. However, bed load transport under waves cannot be measured in the field and bed load transport by currents without waves is barely measurable, even in spring tidal conditions. There is, consequently, a strong lack of field data and validated models. The present field site was on the shoreface and inner shelf at 2 to 8.5 km offshore the central Dutch coast (far outside the surfzone), where tidal currents flow parallel to the coast. Bed load transports were carefully measured with a calibrated sampler in spring tidal conditions without waves at a water depth of 13–18 m with fine and medium sands. The near-bed flow was measured over nearly a year and used for integration to annual transport rates. An empirical bed load model was derived, which predicts bed load transports that are a factor of > 5 smaller than predicted by existing models. However, they agree with laboratory data of sand and gravel transport in currents near incipient motion. The damped transport rates may have been caused by cohesion of sediment or turbulence damping due to mud or biological activity. The annual bed load transport rate was calculated using a probability density function (pdf) derived from the near-bed current and orbital velocity data which represented the current and wave climate well when compared to 30 years of data from a nearby wave station. The effect of wave stirring was included in the transport calculations. The net bed load transport rate is a few m²/year. This is much less than predicted in an earlier model study, which is partly due to different bed load models but also due to the difference in velocity pdf. The annual transport rate is very sensitive to the probability of the largest current velocities.     

Globules and star formation

The principal properties of dark globules are reviewed. Optical and radio (CO) data suggest masses in the range 20 to 750M , radii 0.15 to 1.2 parsecs. There is no evidence for fragmentation. The dust density inside near-spherical globules varies inversely as the cube of the distance from the center. There is evidence to show that several globules are in a state of collapse.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.Much of the work was done while the author was a Visiting Scientist at Cerro Tololo Interamerican Observatory in Chile, supported by the National Science Foundation under Contract No. NSF-C866.