Glacial Lake Musselshell: Late Wisconsin slackwater on the Laurentide ice margin in central Montana, USA

Cosmogenic surface exposure ages of glacial boulders deposited in ice-marginal Lake Musselshell suggest that the lake existed between 20 and 11.5 ka during the Late Wisconsin glacial stage (MIS 2), rather than during the Late Illinoian stage (MIS 6) as traditionally thought. The altitude of the highest ice-rafted boulders and the lowest passes on the modern divide indicate that glacial lake water in the Musselshell River basin reached at least 920–930 m above sea level and generally remained below 940 m. Exposures of rhythmically bedded silt and fine sand indicate that Lake Musselshell is best described as a slackwater system, in which the ice-dammed Missouri and Musselshell Rivers rose and fell progressively throughout the existence of the lake rather than establishing a lake surface with a stable elevation. The absence of varves, deltas and shorelines also implies an unstable lake. The changing volume of the lake implies that the Laurentide ice sheet was not stable at its southernmost position in central Montana. A continuous sequence of alternating slackwater lake sediment and lacustrine sheetflood deposits indicates that at least three advances of the Laurentide ice sheet occurred in central Montana between 20 and 11.5 ka. Between each advance, it appears that Lake Musselshell drained to the north and formed two outlet channels that are now occupied by extremely underfit streams. A third outlet formed when the water in Lake Musselshell fully breached the Larb Hills, resulting in the final drainage of the lake. The channel through the Larb Hills is now occupied by the Missouri River, implying that the present Missouri River channel east of the Musselshell River confluence was not created until the Late Wisconsin, possibly as late as 11.5 ka.     

Quantifying successional land cover after clearing of tropical rainforest along forest frontiers in the Congo Basin

State-of-the-art impact-modeling studies in environmental and climatological sciences require detailed future deforestation scenarios that allow forest to be replaced by a mosaic of multiple successional land-cover types, rather than the simple conversion of forest to a single land-cover type, such as bare soil or cropland. Therefore, not only the amount and location of forest removal has to be known (as is typically provided by scenarios), but also knowledge about the successional land-cover types and their relative areal proportions is needed. The main objective of this study was to identify these successional land-cover types and quantify their areal proportions in regions deforested during the past 37 years around the city of Kisangani, D.R. Congo. The fallow vegetation continuum was categorized in different stages, adapted from existing classifications. Ground-truth points describing the present-day vegetation were obtained during a field campaign and used for supervised and validated land-cover classification of these categories, using the Landsat image of 2012. Areal proportions of successional land-cover types were then derived from the resulting land-cover map. The second objective of this study was to relate these areal proportions to time since deforestation, which is expected to influence fallow landscapes. Landsat images of 1975, 1990, and 2001 were analyzed. Present-day mature tree fallow is less abundant on areas deforested during 1975–1990. The relative areal proportions were used to refine a deforestation scenario and apply it to existing data-sets of LAI and canopy height (CH). Assuming a simple conversion of forest to cropland, the deforestation scenario projected a reduction of grid-cell-averaged CH from 25.5 to 7.5 m (within deforested cells), whereas the refined scenarios that we propose show more subtle changes, with a reduced CH of 13 m. This illustrates the importance of taking successional land cover correctly into account in environmental and climatological modeling studies.     

Morphosedimentary evolution of the macrotidal Seine estuary subjected to human impact

Many estuaries in the world have been subjected to significant human impact since the 19th century. Natural infilling and human activities, including building of embankments, dikes and jetties have modified both the morphology of these estuaries and the distribution of sedimentary facies. The Seine, a macrotidal estuary, provides a good opportunity to study such modifications, as the latest morphosedimentary observations could be compared to old sediment maps as well as to geotechnical drilling data. In the mouth of the Seine, originally a sand/gravel system sediment distribution, has now been transformed into a muddy system. The result is a regressive sequence several meters thick. It is typically fining-up and corresponds to a shift from a distal term (pebbles, gravel, and coarse sands) at its base to a proximal term (mud and fine sands) at its top. Civil engineering works have reduced the available amount of space within the estuary, leading to an increase in the natural downstream shift of the depocenter of mud brought by winter river floods. The deposition area of the mud is today in the open marine zone, where waves and tidal currents render the balance of this process precarious.     

Sixty-five years of solar radioastronomy: flares, coronal mass ejections and Sun–Earth connection

This paper will review the input of 65 years of radio observations to our understanding of solar and solar–terrestrial physics. It is focussed on the radio observations of phenomena linked to solar activity in the period going from the first discovery of the radio emissions to present days. We shall present first an overview of solar radio physics focussed on the active Sun and on the premices of solar–terrestrial relationships from the discovery to the 1980s. We shall then discuss the input of radioastronomy both at metric/decimetric wavelengths and at centimetric/millimetric and submillimetric wavelengths to our understanding of flares. We shall also review some of the radio, X-ray and white-light signatures bringing new evidence for reconnection and current sheets in eruptive events. The input of radio images (obtained with a high temporal cadence) to the understanding of the initiation and fast development in the low corona of coronal mass ejections (CMEs) as well as the radio observations of shocks in the corona and in the interplanetary medium will be reviewed. The input of radio observations to our knowledge of the interplanetary magnetic structures (ICMEs) will be summarized; we shall show how radio observations linked to the propagation of electron beams allow to identify small scale structures in the heliosphere and to trace the connection between the Sun and interplanetary structures as far as 4AU. We shall also describe how the radio observations bring useful information on the relationship and connections between the energetic electrons in the corona and the electrons measured in-situ. The input of radio observations on the forecasting of the arrival time of shocks at the Earth as well as on Space Weather studies will be described. In the last section, we shall summarize the key results that have contributed to transform our knowledge of solar activity and its link with the interplanetary medium. In conclusion, we shall indicate the instrumental radio developments at Earth and in space, which are from our point of view, necessary for the future of solar and interplanetary physics.     

Travel-time residuals of PP waves reflected under oceanic and continental platform regions

PP waves have been used to transform PP residuals with respect to Jeffreys-Bullen time into P-wave travel-time delays close to the reflection point. It is found that P-wave travel-time delays decrease with the age of the plate for oceanic structures. Average oceanic lithosphere (100 My) has a residual similar to that of a typical continental platform (Siberia). The absence of a P-wave low velocity zone under platform regions is sufficient to explain the small difference in residuals between old oceanic regions and the Siberian platform. Thus, from these observations, there is no evidence that would favour a substantial structural difference between continents and oceans at depths greater than 250 km.     

Local tomography and focal mechanisms in the south-western Alps: Comparison of methods and tectonic implications

We investigate how focal solutions and hypocenter locations may depend on the ray tracing algorithm and the strategy of velocity inversion. Using arrival times from a temporary seismological network in the south-western Alps, a local earthquake tomography has been performed by Paul et al. [Paul, A., Cattaneo, M., Thouvenot, F., Spallarossa, D., Béthoux, N., and Fréchet, J., 2001. A three-dimensional crustal velocity model of the south-western Alps from local earthquake tomography. J. Geophys. Res. 106, 19367–19390.] with the method developed by Thurber [Thurber, C.H., 1993. Local earthquake tomography: velocity and Vp/Vs-Theory, in Seismic Tomography: Theory and practice, Iyer, H.M., and Irahara eds., Chapman and Hall, New York, 563–583.]. Another inversion of the same data set is performed here using a different tomography code relying on a shooting paraxial method and cubic interpolation of velocities. The resulting images display the same main features, although Thurber's code appears to be more robust in regions with scarce ray coverage and strong velocity contrasts. Concerning hypocenter location in Piemont units, one major result is the concentration of hypocenters at the boundary between the mantle wedge of the Ivrea body and the European crust. Forty-six focal mechanisms are shown that were computed using both the take-off angles in the minimum 1-D model and in the 3-D velocity structures resulting from the two inversions. The sets of focal solutions are very similar, proving the reliability and the coherency of the focal solutions. The widespread extension in the core of the western Alps is confirmed whereas a few compressive solutions are found east of the Piemont units. These results constrain the sharp change of stress tensor and evidence a decoupling of strain beneath the east of Dora Maira massif up to beneath the north of Argentera massif. On a geodynamical point of view seismicity and focal mechanism distribution are compatible with the present day models published for the western Alps, where the major feature is the lithospheric thickening [Schmid, S.M., and Kissling, E., 2000. The arc of the western Alps in the light of geophysical data on deep crustal structure. Tectonics, 19, 62–85.], implying widespread extension in the core of the western Alps [Sue, C., Thouvenot, F., Fréchet, J., and Tricart, P., 1999. Widespread extension in the core of the western Alps revealed by earthquake analysis. J. Geophys. Res., 104, 25611–25622.]. However the existence of compressive events dealing at depth with the boundary of Ivrea body allows to postulate that this geological structure is still tectonically active. Even if field work has not shown this so far, the Insubric line appears to extend toward the south at depth, as a blind fault, and to play a key role in the dynamics of the south-western Alps.     

Simultaneous optical and ultraviolet observations of variability in HD 50896

In this paper, simultaneous optical and ultraviolet observations of spectral variability of the WN5 star HD 50896 are presented. Night-to-night and shorter timescale changes were found in the shape and intensity of most emission lines and P Cygni profiles in both wavelength ranges but there is no clear link between the ultraviolet and optical variability. There are some indications that the ultraviolet variations occur in phase with the well-known 3.766-day period often associated with this star.