Analysis of the seismicity in the central part of the Pyrenees (France), and tectonic implications

Two temporary seismological networks have been set up in 2000 and 2002 in the central part of the Pyrenees, in a region, which appears as a transition between two domains where both the seismic activity and the tectonic regime are different. Together with the permanent networks, they allowed us to obtain precise hypocenter locations for more than 400 events with local magnitudes ranging from 1.5 to 4.6, as well as 30 new focal mechanisms. The seismicity is distributed in several clusters, which are not located along the North Pyrenean Fault, considered as the major tectonic accident resulting from the suture of the Iberian and Eurasian convergent plates when the range formed. Several small fault segments dipping to the north are identified. The maximum focal depth varies from 10 to 20 km, with variations which are roughly parallel to those of the Moho, indicating a thickening of the seismogenic layer to the east of the studied area. The obtained focal solutions reveal a predominance of normal faulting to the West and reverse faulting to the East, with strike-slip motions in between. The largest fault segment to the East, with a length of about 20 km, could possibly be related to a large historical event which occurred in 1660, with intensity IX, close to cities which have become since then important touristic centres.     

Effect of aridity and rainfall seasonality on vegetation in the southern tropics of East Africa during the Pleistocene/Holocene transition

Fossil pollen analyses from northern Lake Malawi, southeast Africa, provide a high-resolution record of vegetation change during the Pleistocene/Holocene transition (~ 18–9 ka). Recent studies of local vegetation from lowland sites have reported contrasting rainfall signals during the Younger Dryas (YD). The Lake Malawi record tracks regional vegetation changes and allows comparison with other tropical African records identifying vegetation opening and local forest maintenance during the YD. Our record shows a gradual decline of afromontane vegetation at 18 ka. Around 14.5 ka, tropical seasonal forest and Zambezian miombo woodland became established. At ~ 13 ka, drier, more open formations gradually became prevalent. Although tropical seasonal forest taxa were still present in the watershed during the YD, this drought-intolerant forest type was likely restricted to areas of favorable edaphic conditions along permanent waterways. The establishment of drought-tolerant vegetation followed the reinforcement of southeasterly tradewinds resulting in a more pronounced dry winter season after ~ 11.8 ka. The onset of the driest, most open vegetation type was coincident with a lake low stand at the beginning of the Holocene. This study demonstrates the importance of global climate forcing and local geomorphological conditions in controlling vegetation distribution.     

Electrochemical measurements bearing on the oxidation state of the Skaergaard Layered Intrusion

The oxygen fugacities (fO₂'s) of magnetically-concentrated fractions (MCF) of three rock samples from the Skaergaard Layered Intrusion were measured between 800–1150° C using oxygen-specific, solid zirconia electrolytes at atmospheric pressure. Two of the bulk rock samples (an oxide cumulate and an oxide-bearing gabbro) are from the Middle Zone (MZ) and the other (an olivine plagioclase orthocumulate) is from the Lower Zone (LZ). All MCF define fO₂ versus T arrays that lie 1.5–0.5 log units above the fayalite-magnetite-quartz (FMQ) buffer. Experiments with different cell-imposed initial redox states (one from a reduced direction and one from an oxidized direction) were run on each sample in an attempt to achieve experimental reversibility. This was accomplished by imposing a known redox memory on the galvanic cell prior to loading each sample. Reversibility for each sample agreed to better than 0.2 of a log unit. Irreversible autoreduction of 0.2 of a log unit was observed on the two MZ samples at temperatures exceeding 1065° C. Scanning electron microscope and electron microprobe study of pre- and post-run products shows that reaction and textural re-equilibration occurred among the oxide phase assemblages under the experimental conditions employed. Careful characterization of pre- and post-run assemblages is clearly necessary before adequate interpretation of the experimental results can be made in these types of electrochemical studies. Different approaches to investigations of the fO₂ of the Skaergaard Intrusion, be it thermodynamic calculations or experimental methods, should yield concordant results or at least understandable discrepancies. Calculated fO₂'s using thermobarometry applied to the ilmenite-magnetite pairs in the post-experimental assemblages agree with the experimentally determined fO₂'s to within one log unit at a given temperature. These results are also consistent with previously calculated fO₂ values (Buddington and Lindsley 1964; Morse et al. 1980), but are considerably more oxidized than a previous electrolyte-based fO₂ study of a different sample suite from the Skaergaard (Sato and Valenza 1980) that include values close to the iron-wustite (IW) buffer from both MZ and LZ oxide separates. Differences between this electrochemical study and that of Sato and Valenza (1980) may be due to variations in the level of indigenous (or curatorially-introduced) carbon in the samples studied. Despite a number of experimental difficulties, electrochemical cells can provide an accurate and precise method of determining the oxygen fugacity of naturally occurring, complex oxide assemblages. Tight experimental reversals and reproducible values obtained in heating and cooling cycles are an indication of the precision and accuracy of the data recoverable with electrochemical cells.     

The impact of inter-flood duration on non-cohesive sediment bed stability

Limited field and flume data suggests that both uniform and graded beds appear to progressively stabilize when subjected to inter-flood flows as characterized by the absence of active bedload transport. Previous work has shown that the degree of bed stabilization scales with duration of inter-flood flow, however, the sensitivity of this response to bed surface grain size distribution has not been explored. This article presents the first detailed comparison of the dependence of graded bed stability on inter-flood flow duration. Sixty discrete experiments, including repetitions, were undertaken using three grain size distributions of identical D₅₀ (4.8 mm); near-uniform (σ_g = 1.13), unimodal (σ_g = 1.63) and bimodal (σ_g = 2.08). Each bed was conditioned for between 0 (benchmark) and 960 minutes by an antecedent shear stress below the entrainment threshold of the bed (τ*_c50). The degree of bed stabilization was determined by measuring changes to critical entrainment thresholds and bedload flux characteristics. Results show that (i) increasing inter-flood duration from 0 to 960 minutes increases the average threshold shear stress of the D₅₀ by up to 18%; (ii) bedload transport rates were reduced by up to 90% as inter-flood duration increased from 0 to 960 minutes; (iii) the rate of response to changes in inter-flood duration in both critical shear stress and bedload transport rate is non-linear and is inversely proportional to antecedent duration; (iv) there is a grade dependent response to changes in critical shear stress where the magnitude of response in uniform beds is up to twice that of the graded beds; and (v) there is a grade dependent response to changes in bedload transport rate where the bimodal bed is most responsive in terms of the magnitude of change. These advances underpin the development of more accurate predictions of both entrainment thresholds and bedload flux timing and magnitude, as well as having implications for the management of environmental flow design. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Tree ring analysis of hydro‐climatic thresholds that trigger ice jams on the Mistassini River,Quebec

This study documents the spatiotemporal variations in the frequency and magnitude of ice jams in the Mistassini River and applies that information to the identification of the hydro‐climatic threshold conditions associated with major events. Ice jams cause severe upheavals in water flow, which result in flooding upstream of the ice jam front, and therefore represent a significant geohazard to riparian populations. To analyze the spatiotemporal variations in the magnitude–frequency of ice jams, the Mistassini River was first divided into six different sites representing different geomorphological contexts. A 50‐year ice jam chronology was constructed from 85 damaged trees from all of the study sites. This chronology was then coupled with hydro‐climatic variables to construct classification trees, which helped identify the conditions and hydro‐climatic thresholds favourable to the triggering of ice jams in a predictive model. The results indicate complex interactions between the characteristics of flow, ice cover and river morphology that affect the frequency and magnitude of ice jam events on the Mistassini River. These factors affect the frequency and magnitude of ice jam events. The triggering of extreme events seems particularly influenced by exceptional ice conditions and sites with high sinuosity and islands. Copyright © 2015 John Wiley & Sons, Ltd.     

Galactic evolution and star counts in the milky way

A model of population synthesis for our Galaxy is used to estimate the constraints imposed by photometric and astrometric star counts. We construct a statistical method to compute the degree of compatibility of the model with the data in the form of a multidimensional functionf(V, B-V, U-B, ). We apply the method to the determination of the star formation history in the Galaxy from Schmidt plates towards the anticentre. Although the presently available data do not allow us to draw firm conclusions, we show that the same kind of data with a slightly better photometric accuracy and with proper motions would give new constraints on the past star formation rate of the Galaxy.     

Aquifer Discharge Drives Microbial Community Change in Karst Estuaries

Karst estuaries are unique systems governed by freshwater inputs that flow directly to the sea through karst conduits and/or matrices. Because they are found at the interface between terrestrial and marine environments, they can act as sentinels to climate change and as indicators of aquifer health. The physical and geochemical gradients formed in karst estuaries are a direct result of the interactions between inland hydrological conditions and sea level. To examine the influence these gradients have on microbial communities in the water column and sediment of a karst estuary, we studied the spring-fed Double Keyhole Karst Estuary in west central Florida for a 2-year period. Four sites were monitored within this system starting at the Double Keyhole Spring Conduit extending 2 km west through the estuary toward the Gulf of Mexico. Water column and sediment samples were collected quarterly at all sites from September 2011 through September 2013. Archaeal, bacterial, and microbial eukaryote communities were analyzed using quantitative PCR and length-heterogeneity PCR. The biological communities were analyzed in the context of hydrological, physical, and geochemical parameters in order to determine which factor(s) governed the observed changes of microbial abundance and richness. The major finding of this study was that microbial community change in this karst estuary was primarily driven by the volume of aquifer discharge and associated physical gradients, and less by the geochemical fluctuations within the system.     

North American Precambrian history recorded in a single sample: high-resolution UPb systematics of the Potsdam sandstone detrital zircons,New York State

Zircons separated from the Cambrian Potsdam sandstone of New York yield four distinct populations which can be defined by a number of analytical techniques. U-Pb isotopic analyses of small samples and monozircons of each population reveal a fine chronology not apparent in milligram-sized sample analysis, and define source area ages of 1180, 1320, 2100 and 2700 m.y. for the Cambrian detrital suite. These ages correspond to well-defined sources in the Superior and Grenville Provinces of the Canadian Shield (2700 and 2100 m.y.) and the well established Grenville age rocks of the Adirondack Mountains (1180 m.y.). The 1320-m.y. age appears to be derived from the Adirondacks, and suggests the existence of pre-Grenville basement in that massif. Our techniques allow the interpretation of the Precambrian history of a large portion of eastern North America from a single sample, and thus should be valuable in the definition of source areas in paleogeographic reconstruction, and in studies of continental crustal evolution.