Short-term variability of the phytoplankton community in coastal ecosystem in response to physical and chemical conditions' changes

The short-term dynamics (time scale of a few days) of phytoplankton communities in coastal ecosystems, particularly those of toxic species, are often neglected. Such phenomena can be important, especially since these very species can endanger the sustainability of shellfish farming. In this study, we investigated the short-term changes in phytoplankton community structure (species succession) in two coastal zones in parallel with physical and chemical conditions. Mixing events with allochtonous waters could thus be distinguished from local processes associated with population growth when it was associated with a change in light or nutrient limitation. Mixing events and water advection influenced fluctuations in total phytoplankton biomass and concentration of dominant species, while local processes influenced delayed changes in community structure. The estuarine species Asterionellopsis glacialis increased in concentration when the water mass mixed with the nearest estuarine water masses. The biological response, measured as photosynthetic capacity, occurred after a time-lag of a few hours, while the changes in community structure occurred after a time-lag of a few days. Finally, the coastal water mass was constantly mixed with both the nearest estuarine and marine water masses, leading in turn to delayed changes in phytoplankton community structure. These changes in species composition and dominance were observed on a time scale of a few days, which means that some toxic species may be missed with a bi-weekly sampling strategy.     

Interannual relationships between Indian Summer Monsoon and Indo-Pacific coupled modes of variability during recent decades

Various SST indices in the Indo-Pacific region have been proposed in the literature in light of a long-range seasonal forecasting of the Indian Summer Monsoon (ISM). However, the dynamics associated with these different indices have never been compared in detail. To this end, the present work re-examines the variabilities of ISM rainfall, onset and withdrawal dates at interannual timescales and explores their relationships with El Ni?o-Southern Oscillation (ENSO) and various modes of coupled variability in the Indian Ocean. Based on recent findings in the literature, five SST indices are considered here: Ni?o3.4 SST index in December?CJanuary both preceding [Nino(?1)] and following the ISM [Nino(0)], South East Indian Ocean (SEIO) SST in February?CMarch, the Indian Ocean Basin (IOB) mode in April?CMay and, finally, the Indian Ocean Dipole (IOD) averaged from September to November, also, both preceding [IOD(?1)] and following the ISM [IOD(0)]. The respective merits and associated dynamics of the selected indices are compared through various correlation and regression analyses. Our first result is a deceptive one: the statistical relationships with the ISM rainfall at the continental and seasonal scales are modest and only barely significant, particularly for the IOD, IOB and Nino(?1) indices. However, a detailed analysis shows that statistical relationships with the ISM rainfall time series are statistically biased as the ISM rainfall seems to be shaped by much intraseasonal variability, linked in particular to the timing of the onset and withdrawal of the ISM. Surprisingly, analysis within the ISM season shows that Nino(?1), IOB and SEIO indices give rise to prospects of comparatively higher ISM previsibility for both the ISM onset and the amount of rainfall during the second half of the ISM season. The IOD seems to play only a secondary role. Moreover, our work shows that these indices are associated with distinct processes occurring within the Indian Ocean from late boreal winter or early spring onwards. The regression analyses also illustrate that these (local) mechanisms are dynamically and remotely linked to different phases of ENSO in the equatorial Pacific, a result which may have useful implications in terms of forecasting strategies since the choice of the better indices then hinges on the concurrent phasing of the ENSO cycle.     

The internal structure of spring mounds of Nefzaoua oasis in Tunisia: Formation of an original geomorphic structure

Spring mounds are specific geomorphological landforms in arid or semi-arid environments associated with playas and artesian springs. Spring mounds are found worldwide, especially in the great Artesian Basin of Australia and in North American and Egyptian deserts. They result from an exceptional succession of climatic, geomorphological and hydrogeological conditions, with processes that follow each other in a specific order. In Tunisia, in the arid zone, ca. 126 spring mounds have been identified in Nefzaoua province alone, especially in the oasis east of Chott el Jerid. They have a conical shape that ranges from 200 to more than 2000 m in diameter and 3–30 m tall, and their centre is hollowed out by an artesian spring of fresh water. Palm groves (Phoenix dactylifera) spread out at the foot of each mound. The springs have dried up because of the proliferation of borehole wells. Because of the low electric conductivity (EC) of the sediments (<1 mS cm⁻¹) at their base, spring mounds have been excavated and used as a soil amendment to expand new palm plantations. This excavation allows for analysis of their internal structure, which has never been observed well. In the present study, fine analysis of sediment layers in four sections of two representative mounds showed that vegetation had trapped sediment at their base. Fine strata of variable texture alternating with variable calcium carbonate, gypsum or organic carbon contents suggest a clear limnic origin. Many redoximorphic features, sometimes associated with the presence of old roots, suggest variation in the water level in the centre of the mound. The mounds are capped by a thick layer of indurated gypsum, which helps them resist hydric and aeolian erosion. The origin of the sediment components is discussed.     

New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group (Cameroon, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton

Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and ²⁰⁷Pb/²⁰⁶Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624?±?2?Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The ⁸⁷Rb/⁸⁶Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. ⁸⁷Rb/⁸⁶Sr isochron ages differ from sample to sample (599?±?3, 572?±?4, 554?±?5, 540?±?5?Ma) yielding the oldest Neoproterozoic age (~600?Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540?Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The ⁸⁷Rb/⁸⁶Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540?Ma) more closely to the border of the Congo Craton.     

A continuous velocity field for Norway

In Norway, as in the rest of Fennoscandia, the process of Glacial Isostatic Adjustment causes ongoing crustal deformation. The vertical and horizontal movements of the Earth can be measured to a high degree of precision using GNSS. The Norwegian GNSS network has gradually been established since the early 1990s and today contains approximately 140 stations. The stations are established both for navigation purposes and for studies of geophysical processes. Only a few of these stations have been analyzed previously. We present new velocity estimates for the Norwegian GNSS network using the processing package GAMIT. We examine the relation between time-series length and precision. With approximately 3.5 years of data, we are able to reproduce the secular vertical rate with a precision of 0.5 mm/year. To establish a continuous crustal velocity field in areas where we have no GNSS receivers or the observation period is too short to obtain reliable results, either interpolation or modeling is required. We experiment with both approaches in this analysis by using (i) a statistical interpolation method called Kriging and (ii) a GIA forward model. In addition, we examine how our vertical velocity field solution is affected by the inclusion of data from repeated leveling. Results from our geophysical model give better estimates on the edge of the network, but inside the network the statistical interpolation method performs better. In general, we find that if we have less than 3.5 years of data for a GNSS station, the interpolated value is better than the velocity estimate based on a single time-series.     

Morphology and geology of the ILD in Capri/Eos Chasma (Mars) from visible and infrared data

Layered deposits have been observed in different locations at the surface of Mars, as crater floors and canyons systems. Their high interest relies in the fact they imply dynamical conditions in their deposition medium. Indeed, in opposition to most of the rocks of the martian surface, which have a volcanic origin, bright layered deposits seems to be sedimentary outcrops.Capri Chasma, a canyon located at the outlet of Valles Marineris, exhibits such deposits called Interior Layered Deposits (ILD). A large array of visible and infrared spacecraft data were used to build a Geographic Information System (GIS). We added HiRiSE images, from the recent MRO mission, which offer a spatial resolution of 25 cm per pixel. It allowed the mapping and the analysis of morphologies in the canyon. We highlighted that the ILD are several kilometers thick and flat-top stratified deposits. They overlap the chaotic floor. They are surrounded and cut by several flow features that imply that liquid water was still acting after the formation of these stratified deposits. The density of crater on the floor of Capri Chasma was quantified. The current topography was aged to 3 Gyr. All these morphological information allow us to suggest a plausible geological history for Capri Chasma. We propose that the Interior Layered Deposits have formed during the Hesperian, during or after the opening of the canyon. Some observations argue that water discharges have happened at several times before and just after the formation of the ILD. Liquid water must have played a major role in the formation of these deposits after 3.5 Gyr, implying that it was present in surface at least locally and temporarily. If this can be applied to ILD in others canyons of Valles Marineris, it would imply that liquid water was stable in surface or sub-surface during the Hesperian. Or in the actual conditions, with a cold and dry martian surface, long-term standing water bodies are not possible. Thus we suggest that either the climate at the Hesperian was cold, but wetter, or as warm as the Noachian climate, what is less likely. Nevertheless, the global climate change which has occurred at the beginning of Mars history may have been later than announced.     

Distribution and chronology of Pleistocene permafrost features in France: Database and first results

Numerous periglacial features (polygons, nets, soil stripes, ice‐wedge pseudomorphs and sand‐wedge casts, involutions) have been recorded in France by examining bibliographical sources and aerial photographs. These data show that a large part of France was affected by permafrost during the Pleistocene and only the southern Aquitaine Basin and Languedoc seem to have been beyond its maximum extent. The first OSL ages obtained from the aeolian infill of wedge structures indicate that at least two phases of thermal contraction cracking occurred in southwestern France between ～25 and 36 ka. Chronostratigraphical data from loess in northern France indicate that these episodes correspond to the formation of ice‐wedge networks associated with tundra gleys. In the latter region, two additional permafrost episodes probably occurred during the Last Glacial, the older one corresponding to the end of Marine Isotope Stage (MIS) 4 around 60 ka and the more recent one to MIS 2 around 19–16 ka. Although stratigraphical data indicate that these episodes were relatively short (about one millennium), relict permafrost may have existed for longer periods in northern France.     

Multi-technique comparison of troposphere zenith delays and gradients during CONT08

CONT08 was a 15 days campaign of continuous Very Long Baseline Interferometry (VLBI) sessions during the second half of August 2008 carried out by the International VLBI Service for Geodesy and Astrometry (IVS). In this study, VLBI estimates of troposphere zenith total delays (ZTD) and gradients during CONT08 were compared with those derived from observations with the Global Positioning System (GPS), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and water vapor radiometers (WVR) co-located with the VLBI radio telescopes. Similar geophysical models were used for the analysis of the space geodetic data, whereas the parameterization for the least-squares adjustment of the space geodetic techniques was optimized for each technique. In addition to space geodetic techniques and WVR, ZTD and gradients from numerical weather models (NWM) were used from the European Centre for Medium-Range Weather Forecasts (ECMWF) (all sites), the Japan Meteorological Agency (JMA) and Cloud Resolving Storm Simulator (CReSS) (Tsukuba), and the High Resolution Limited Area Model (HIRLAM) (European sites). Biases, standard deviations, and correlation coefficients were computed between the troposphere estimates of the various techniques for all eleven CONT08 co-located sites. ZTD from space geodetic techniques generally agree at the sub-centimetre level during CONT08, and??as expected??the best agreement is found for intra-technique comparisons: between the Vienna VLBI Software and the combined IVS solutions as well as between the Center for Orbit Determination (CODE) solution and an IGS PPP time series; both intra-technique comparisons are with standard deviations of about 3?C6?mm. The best inter space geodetic technique agreement of ZTD during CONT08 is found between the combined IVS and the IGS solutions with a mean standard deviation of about 6?mm over all sites, whereas the agreement with numerical weather models is between 6 and 20?mm. The standard deviations are generally larger at low latitude sites because of higher humidity, and the latter is also the reason why the standard deviations are larger at northern hemisphere stations during CONT08 in comparison to CONT02 which was observed in October 2002. The assessment of the troposphere gradients from the different techniques is not as clear because of different time intervals, different estimation properties, or different observables. However, the best inter-technique agreement is found between the IVS combined gradients and the GPS solutions with standard deviations between 0.2 and 0.7?mm.