An explanation of the ‘granulation boundary’ in the HR diagram

The granulation boundary is a line running in the HR diagram from about F0V stars to G1Ib. It divides the HR diagram in two regions in which the spectral line bisector behaves differently. To the right (cooler stars) the Fraunhofer lines in stellar spectra show the characteristics typical for convective motions. For hot stars it indicates the presence of large velocities involving a large part of the line-forming region. We give evidence that the opposite behaviour shown by spectra of hotter stars can be explained by gravity waves.     

Little Ice Age and neoglacial landforms at the Inland Ice margin, Isunguata Sermia, Kangerlussuaq, west Greenland

Neoglacial and Little Ice Age (LIA) limits occur within 2km of the Inland Ice margin in the Kangerlussuaq area on west Greenland. The LIA limit is clearly demarcated by ice-cored and non-ice-cored moraines, out-wash surfaces and trimlines. Rhizocarpon sp. thalli of ≥16mm on these landforms indicate a 1-2km retreat of the Inland Ice in the past c. 100 years, coincident with peripheral thinning of the ice. An older neoglacial moraine host of Rhizocarpon sp. thalli <40 mm indicates a minimum limiting age of <400 BP, whereas Optically Stimulated Luminescence (OSL) ages on aeolian silt capping the moraine yield close limiting ages of c. 2000 BP. Aeolian silt deposition beyond neoglacial limits yields OSL ages of c. 3000 BP, potentially coeval with advance of the Inland Ice. Aeolian sedimentation and the inferred age of the moraine are coincident with pronounced cooling inferred from palaeolimnological records from west and south Greenland. This neoglacial event at c. 2000 BP is probably of similar extent to the LIA maximum, because of the paucity of preserved moraine remnants.     

The effect of surface moisture on the entrainment of dune sand by wind: an evaluation of selected models

Abstract Reliable predictions of wind erosion depend on the accuracy of determining whether erosion occurs or not. Among the several factors that govern the initiation of soil movement by wind, surface moisture is one of the most significant. Some widely used models that predict the threshold shear velocity for particle detachment of wet soils by wind were critically reviewed and evaluated. Wind‐tunnel experiments were conducted on pre‐wetted dune sand with moisture contents ranging from 0·00 to 0·04 kg kg^?1. Sand samples were exposed to different wind speeds for 2 min. Moisture content was determined gravimetrically before and after each experiment, and the saltation of sand particles was recorded electronically with a saltiphone. Shear velocities were deduced from the wind speed profiles. For each moisture content, the experiments were repeated at different shear velocities, with the threshold shear velocity being determined by least‐squares analysis of the relationships between particle number rates and shear velocity. Within the 2‐min test runs, temporal changes in particle number rates and moisture contents were detected. A steep increase in the threshold shear velocity with moisture content was observed. When comparing the models, large differences between the predicted results became apparent. At a moisture content of 0·007 kg kg^?1, which is half the moisture content retained to the soil matrix at a water tension (or matric potential) of ?1·5 MPa, the increase in ‘wet’ threshold shear velocity predicted with the different models relative to the dry threshold shear velocity ranged from 117% to 171%. The highest care should therefore be taken when using current models to predict the threshold shear velocity of wet sediment. Nevertheless, the models of Chepil (1956; Proc. Soil Sci. Soc. Am., 20, 288–292) and Saleh & Fryrear (1995; Soil Sci., 160, 304–309) are the best alternatives available.     

The structure and evolution of the top layers of the water column across the marginal ice zone during spring 1988 in the Scotia-Weddell Sea sector of the Southern Ocean

The evolution of the wind-mixed layer and surface temperature in a period of ice retreat is studied with a one-dimensional turbulent erosion model. This model is established to provide values for a number of physical parameters to be used in ecophysiological modelling of plankton blooms in an area with a retreating ice cover. The wind-mixed layer (WML) model is driven by parameters obtained by regular on-board meteorological observations, visual ice cover estimates and global radiation measurements. The results of numerous conductivity-temperature-depth (CTD) casts performed during the EPOS leg-2 cruise provided the data for the initial conditions and validation.     

Long-term ferry-ADCP observations of tidal currents in the Marsdiep inlet

A unique, five-year long data set of ferry-mounted ADCP measurements in the Marsdiep inlet, the Netherlands, obtained between 1998 and 2003, is presented. A least-squares harmonic analysis was applied to the water transport, (depth-averaged) currents, and water level to study the contribution of the tides. With 144 tidal constituents, maximally 98% of the variance in the water transport and streamwise currents is explained by the tides, whereas for the stream-normal currents this is maximally 50%. The most important constituent is the semi-diurnal M₂ constituent, which is modulated by the second-largest S₂ constituent (about 27% of M₂). Compound and overtides, such as 2MS₂, 2MN₂, M₄, and M₆, are important in the inlet. Due to interaction of M₂ with its quarter-diurnal overtide M₄, the tidal asymmetry in the southern two thirds of the inlet is flood dominant. The amplitudes of all non-astronomic constituents are largest during spring tides, strongly distorting the water level and velocity curves. The M₂ water transport is 40° ahead in phase compared to the M₂ water level, reflecting the progressive character of the tidal wave in the inlet. The currents are strongly rectilinear and they are sheared vertically and horizontally, with the highest currents at the surface above the deepest part of the inlet. During spring tides, near-surface currents can be as large as 1.8 m s^− 1. Due to the relative importance of inertia compared to friction, the M₂ currents near the centre (surface) lag maximal 20° (3°) in phase with the currents near the sides (bottom). The tidal-mean currents are directed into the basin in the shallower channel to the south and out of the basin in the deeper channel to the north.     

Holocene flooding history of the Lower Tagus Valley (Portugal)

The present paper aims to reconstruct the Lower Tagus Valley flooding history for the last ca. 6500 a, to explore the suitability of pollen‐based local vegetation development in supporting the reconstruction of flooding history, and to explain fluvial activity changes in terms of allogenic (climate, human impact) and autogenic (system intrinsic) processes. The flooding history has been determined by cored sedimentary records located ～18 km apart in distal, low‐energy backswamps on both sides of the Tagus channel. In these low‐energy backswamps, fine‐grained sediment layers deposited from suspended load of overbank flood water reflect periods with multiple overbank floods. By means of a multi‐proxy approach (sedimentology, magnetic susceptibility, grain size, loss‐on‐ignition, carbonate content and pollen), sedimentary and environmental changes were identified. At both sites, synchronous lithological intervals accumulated, suggesting a common origin for the changes in fluvial activity since ca. 6500 cal. a BP. Based on lithological changes, three phases of high fluvial activity (6500–5500, 4900–3500 and 1000–0 cal. a BP) and two phases of low fluvial activity (5500–4900 and 3500–1000 cal. a BP) were identified. Two periods with dominant allogenic controls on fluvial activity in the Lower Tagus Valley were identified: relative sea level (6500–5500 cal. a BP) and human impact (1000–0 cal. a BP). During the intermediate period, changes in fluvial activity may have been caused by climate (5500–1000 cal. BP), but unambiguous correlations are difficult to make. This is due to the way allogenic controls are translated through the fluvial system, the geomorphological differences between upstream and downstream studies and autogenic processes. The comparison of local vegetation development and flooding phases as reconstructed using sedimentology shows a limited added value of using local palynology as a proxy for fluvial activity. Copyright © 2010 John Wiley & Sons, Ltd.     

Late Quaternary valley-fill succession of the Lower Tagus Valley, Portugal

The Lower Tagus Valley in Portugal contains a well-developed valley-fill succession covering the complete Late Pleistocene and Holocene periods. As large-scale stratigraphic and chronologic frameworks of the Lower Tagus Valley are not yet available, this paper describes facies, facies distribution, and sedimentary architecture of the late Quaternary valley fill. Twenty four radiocarbon ages provide a detailed chronological framework. Local factors affected the nature and architecture of the incised valley-fill succession. The valley is confined by pre-Holocene deposits and is connected with a narrow continental shelf. This configuration facilitated deep incision, which prevented large-scale marine flooding and erosion. Consequently a thick lowstand systems tract has been preserved. The unusually thick lowstand systems tract was probably formed in a previously (30,000–20,000 cal BP) incised narrow valley, when relative sea-level fall was maximal. The lowstand deposits were preserved due to subsequent rapid early Holocene relative sea-level rise and transgression, when tidal and marine environments migrated inland (transgressive systems tract). A constant sea level in the middle to late Holocene, and continuous fluvial sediment supply, caused rapid bayhead delta progradation (highstand systems tract). This study shows that the late Quaternary evolution of the Lower Tagus Valley is determined by a narrow continental shelf and deep glacial incision, rapid post-glacial relative sea-level rise, a wave-protected setting, and large fluvial sediment supply.     

Impact of groundwater flow on meandering; example from the Geul River,The Netherlands

The Geul River, located in the south‐eastern part of The Netherlands, is a meandering river with a planform shape characterized by large loops consisting of multiple bends. We evaluate the effect(s) of groundwater flow on the shapes of meanders as a possible explanation for the multi‐bend loops, using a combined meandering–groundwater computer model. In the model seeping groundwater enhances bank erodibility. Based on the simulation results, we present a conceptual, generalized model for groundwater–meandering interaction, based on wavelength selection and fixation effects. Wavelength selection occurs because of the positive feedback between growing meander bends and groundwater flow patterns and velocities. The promoted wavelengths have the same spatial scale as the groundwater flow system in the aquifer underlying the floodplain. In the case of the Geul River these wavelengths are of the order of 100 m. Since groundwater flow velocities are largest close to the recharging hill‐slopes, the seepage‐enhanced bank erodibilities are at a maximum near the floodplain limits. At these locations the difference in erodibility between banks facing the floodplain and those facing the hill slopes is large, so it is difficult for the river to migrate away from the floodplain limits. This causes long stretches of the river to be aligned along the floodplain limits, which we term a fixation effect. This mechanism best explains the multi‐bend loops of the Geul River. The general interaction between groundwater flow and meandering is site specific since it depends on climatic, fluvial and hydrogeological parameters. The Geul is characterized by a wide floodplain and steep hill‐slopes, and it is underlain by coarse‐grained deposits with good aquifer properties, favoring an important groundwater system. Since this kind of river frequently occurs, our results could apply to many other river systems. Copyright © 2008 John Wiley & Sons, Ltd.     

Coronal explosions as a signature of current loop coalescence in solar flares

The coronal explosions, discovered by De Jager and Boelee (1984), and interpreted by them as manifestations of plasma streaming out of the flare kernels, can also be interpreted as signatures of current loop coalescence in the flaring region.