Saghro Group in the Ougnat Massif (Morocco), an evidence for a continuous Cadomian basin along the northern West African Craton

The Saghro Group (SG) is a folded, low-grade volcano-sedimentary series up to 8 km thick that crops out within and to the north of the Pan-African suture zone in the central and eastern Anti-Atlas. Here we describe the SG of the Ougnat inliers that are exposed in the easternmost Anti-Atlas beneath the unconformable, Late Ediacaran Ouarzazate Group (OZG) volcanic rocks. The Ougnat SG mostly consists of volcaniclastic greywackes accumulated in a peritidal-to-shallow basin. The basin infilling was deformed by NNE-trending, mostly upright folds with axial-planar slaty cleavage and low-grade metamorphism. The deformed SG rocks were intruded by the ～550 Ma Mellab hypovolcanic granodiorite. The latter also crosscuts the lowest OZG rocks that are dated to 574–571 Ma in the western Saghro region. The SG rocks that form the Siroua and Saghro inliers have an oldest age of 620–610 Ma and were folded at ～610–580 Ma at the onset of the Cadomian orogenic events. We show that the SG rocks are similar to the “Série verte” (SV) rocks that are exposed in the Ougarta and western Hoggar east of the Pan-African suture. We infer that the SG and SV rocks accumulated in a same, continuous basin that was bounding the West African Craton to the north and the east. This strongly subsiding basin formed close to a volcanic arc and was folded during the last Pan-African synmetamorphic events. Fold orientation and age of folding differ however along the edge of the West African Craton. The orogenic greywackes that form the remnants of the SG-SV basin thus constitute a precious record of the diachronic Cadomian event s.l. along the West African Craton northern margin.     

Cambrian–early Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France: Continental break-up and rifting of the northern Gondwana margin

The Cambrian–lower Ordovician volcanic units of the South Armorican and Occitan domains are analysed in a tectonostratigraphic survey of the French Variscan Belt. The South Armorican lavas consist of continental tholeiites in middle Cambrian–Furongian sequences related to continental break-up. A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites. The Occitan lavas are distributed into five volcanic phases: (1) basal Cambrian rhyolites, (2) upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, (3) upper lower–middle Cambrian continental tholeiites, (4) Tremadocian rhyolites, and (5) upper lower Ordovician initial rift tholeiites. A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve. It renewed in the Tremadocian with abundant crustal melting due to underplating of mixed asthenospheric and lithospheric magmas. This main tectono-magmatic continental rift is termed the “Tremadocian Tectonic Belt” underlined by a chain of rhyolitic volcanoes from Occitan and South Armorican domains to Central Iberia. It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that forecasted the South Armorican–Medio-European Ocean as a part of the Palaeotethys Ocean.     

Paleomagnetic determination of emplacement temperatures of pyroclastic deposits: an under-utilized tool

Paleomagnetic data from lithic clasts collected from Mt. St. Helens, USA, Volcán Láscar, Chile, Volcán de Colima, Mexico and Vesuvius, Italy have been used to determine the emplacement temperature of pyroclastic deposits at these localities and to highlight the usefulness of the paleomagnetic method for determining emplacement temperatures. At Mt. St. Helens, the temperature of the deposits (T _dep ) at three sites from the June 12, 1980 eruption was found to be ≥532°C, ≥509°C, and 510–570°C, respectively. One site emplaced on July 22, 1980 was emplaced at ≥577°C. These new paleomagnetic temperatures are in good agreement with previously published direct temperature measurements and paleomagnetic estimates. Lithic clasts from pyroclastic deposits from the 1993 eruption of Láscar were fully remagnetized above the respective Curie temperatures, which yielded a minimum T _dep of 397°C. Samples were also collected from deposits thought to be pyroclastics from the 1913, 2004 and 2005 eruptions of Colima. At Colima, the sampled clasts were emplaced cold. This is consistent with the sampled clasts being from lahar deposits, which are common in the area, and illustrates the usefulness of the paleomagnetic method for distinguishing different types of deposit. T _dep of the lower section of the lithic rich pyroclastic flow (LRPF) from the 472 A.D. deposits of Vesuvius was ~280–340°C. This is in agreement with other, recently published paleomagnetic measurements. In contrast, the upper section of the LRPF was emplaced at higher temperatures, with T _dep ~520°C. This temperature difference is inferred to be the result of different sources of lithic clasts between the upper and lower sections, with the upper section containing a greater proportion of vent-derived material that was initially hot. Our studies of four historical pyroclastic deposits demonstrates the usefulness of paleomagnetism for emplacement temperature estimation.     

Modeling Surface Water-Groundwater Interaction with MODFLOW: Some Considerations

The accuracy with which MODFLOW simulates surface water-groundwater interaction is examined for connected and disconnected losing streams. We compare the effect of different vertical and horizontal discretization within MODFLOW and also compare MODFLOW simulations with those produced by HydroGeoSphere. HydroGeoSphere is able to simulate both saturated and unsaturated flow, as well as surface water, groundwater and the full coupling between them in a physical way, and so is used as a reference code to quantify the influence of some of the simplifying assumptions of MODFLOW. In particular, we show that (1) the inability to simulate negative pressures beneath disconnected streams in MODFLOW results in an underestimation of the infiltration flux; (2) a river in MODFLOW is either fully connected or fully disconnected, while in reality transitional stages between the two flow regimes exist; (3) limitations in the horizontal discretization of the river can cause a mismatch between river width and cell width, resulting in an error in the water table position under the river; and (4) because coarse vertical discretization of the aquifer is often used to avoid the drying out of cells, this may result in an error in simulating the height of the groundwater mound. Conditions under which these errors are significant are investigated.     

Modelling the Northern Hemisphere temperature for solar cycles 24 and 25

It is uncertain whether the solar cycle 24 will have a high or a low sunspot maximum number. In its last revision the Solar Cycle 24 Prediction Panel indicates that the low prediction is the most likely. Also, solar cycle 25 is considered to present an equal or lower activity than cycle 24. In order to assess the possible effect of the solar activity on temperature, in the present work we attempt to model the tendency of the Northern Hemisphere temperature for the years 2009–2029, corresponding to solar cycles 24 and 25, using a thermodynamic climate model. We include as forcings the atmospheric carbon dioxide (CO₂) and the solar activity by means of the total solar irradiance, considering that the latter has not only a direct effect on climate, but also an indirect one through the modulation of the low cloud cover. We use two IPCC-2007 CO₂ scenarios, one with a high fossil consumption and other with a low use of fossil sources. Also we consider higher and lower solar activity conditions. We found that in all the performed experiments the inclusion of the solar activity produces a noticeable reduction in warming respect to the IPCC-2007 CO₂ scenarios. Such reduction goes between ～14% and ～44%. In order to evaluate the efficiency of the TCM, we use the root mean square (RMS) between the observed and model temperatures for the period 1980–2003. We find that the RMS for the experiment using the CO₂ as the only forcing is 0.06 °C,while for the experiment that includes also the solar activity it is higher, 0.13 °C.     

Source of error and uncertainty in sheet erosion rates estimated from dendrogeomorphology

Dendrogeomorphology has been used since the 1960s to estimate sheet erosion rates. To date, most efforts have focused on accurately determining the first year of root exposure. However, an adequate methodological approach that takes into consideration the microtopography of the ground surface when estimating sheet erosion rates using dendrogeomorphology has not been proposed. In this study, terrestrial laser scanning (TLS) was used for the first time to examine how changes in microtopography determine the level of certainty in estimates. To this end, highly accurate TLS‐based digital elevation models representing exposed roots and their immediate vicinity were analysed using geographic information system tools. The results indicate that erosion rates calculated using the standard dendrogeomorphic method have been underestimated by up to 29% because the method does not take into account changes to the microtopography caused by the axial and radial pressure of the roots. Another source of uncertainty, which we estimate to be 50%, was also found and is the result of changes in the ground surface microtopography caused by variations in soil roughness. These findings do not invalidate the usefulness of dendrogeomorphology for assessing soil erosion, although they do show the need for correct characterization of the microtopography to guarantee reliability. Copyright © 2015 John Wiley & Sons, Ltd.     

A nested multivariate copula approach to hydrometeorological simulations of spring floods: the case of the Richelieu River (Québec,Canada) record flood

Floods have potentially devastating consequences on populations, industries and environmental systems. They often result from a combination of effects from meteorological, physiographic and anthropogenic natures. The analysis of flood hazards under a multivariate perspective is primordial to evaluate several of the combined factors. This study analyzes spring flood-causing mechanisms in terms of the occurrence, frequency, duration and intensity of precipitation as well as temperature events and their combinations previous to and during floods using frequency analysis as well as a proposed multivariate copula approach along with hydrometeorological indices. This research was initiated over the Richelieu River watershed (Quebec, Canada), with a particular emphasis on the 2011 spring flood, constituting one of the most damaging events over the last century for this region. Although some work has already been conducted to determine certain causes of this record flood, the use of multivariate statistical analysis of hydrologic and meteorological events has not yet been explored. This study proposes a multivariate flood risk model based on fully nested Archimedean Frank and Clayton copulas in a hydrometeorological context. Several combinations of the 2011 Richelieu River flood-causing meteorological factors are determined by estimating joint and conditional return periods with the application of the proposed model in a trivariate case. The effects of the frequency of daily frost/thaw episodes in winter, the cumulative total precipitation fallen between the months of November and March and the 90th percentile of rainfall in spring on peak flow and flood duration are quantified, as these combined factors represent relevant drivers of this 2011 Richelieu River record flood. Multiple plausible and physically founded flood-causing scenarios are also analyzed to quantify various risks of inundation.     

Efficiency of different mesh sizes for isolating fossil chironomids for stable isotope and radiocarbon analyses

We examined the effects of sieving with different mesh sizes on the efficiency of processing fossil chironomids from lake sediments for isotope analyses. Results obtained for three different sediments indicate that each of the studied sieve fractions (100–150, 150–200, 200–250, 250–300, >300 μm) contain a similar proportion of the overall mass of chironomid fossils in a sample. However, the sorting time needed to separate chironomids from other sieve residue is disproportionately large for smaller mesh sizes. Employing sieves with a 200-μm rather than the 100-μm mesh commonly used for standard palaeoecological analyses of fossil chironomids decreased processing time for a given mass of fossils by 30–58% in our study. For optimizing the efficiency of chironomid sample processing for stable isotope and radiocarbon analysis we therefore recommend a 200-μm mesh size sieve, although the sorting of all >100-μm fractions may be necessary in sediments with low chironomid abundances. Excluding certain small taxa from isotope analysis, may structurally bias isotope values of samples. Therefore, further studies on taxon-specific isotope analysis are required to quantify these effects.