U–Pb Zircon Age Constraining the Source and Provenance of Gem‐Bearing Late Cenozoic Detrital Deposits,Mamfe Basin,SW Cameroon

Zircons and other heavy minerals (corundum, rutile, ilmenite, magnetite, sillimanite) are identified in the Nsanaragati gem corundum placer deposit, in the western part of the Mamfe sedimentary basin, SW Cameroon. These alluvial minerals have different morphological characteristics and zircons, in particular, vary mostly in colour and shape. They are reddish, brownish, yellowish, pink or colourless. These minerals form rounded and sub‐rounded alluvial grains, prismatic, pyramidal or dipyramidal crystals. Reddish zircons retain their original crystallographic shape. Trace element and U–Pb isotopic geochemical analyses of these reddish zircons, using the LA‐ICP‐MS method give significant Hf (4576–6334 ppm), Th (46–1565 ppm) and U (66–687 ppm) contents, with Th/U ratio ranging from 0.6 to 3.0. The ²⁰⁶Pb/²³⁸U corrected mean age gave 12.39 ± 0.55 Ma, which characterizes an Upper Cenozoic (Serravallian) magmatic event. The zircons are probably sourced from a magmatic field in the South eastern boundary of the Cross River Formation. The Cameroon Volcanic Line of basaltic and alkaline lavas and intrusions which lie east of the Mamfe Basin mostly range in age from 37 Ma to <1 Ma. The zircons may also relate to the Mount Bambouto plateau lavas which lie northeast of the Mamfe sedimentary basin and have an eruptive age range of 21–14 Ma. The oldest Nsanaragati reddish zircon ages overlap within error with the end stages of the Bambouto eruptions. This eruptive or a related episode provides a potential source for megacrystic reddish zircons within the Nsanagarati placer deposit.     

Silver and lead in high-altitude lake sediments: Proxies for climate changes and human activities

High-altitude lake sediments are often used as archives for environmental changes and their chemical and isotopic compositions provide significant constraints on natural and anthropogenic long-term changes that have occurred in their catchment area. Here, trace-element concentrations and Pb isotopes are presented for two sedimentary cores from Lake Blanc Huez in the French Alps, to trace the impact of climate changes and human activities over the Holocene. Lead and Ag contents are very high and clearly dominated by input from a Pb–Ag vein located a few meters from the lakeshore, a vein that also buffers the Pb isotopes. Mining of this vein in medieval times is recorded in the corresponding lake sediments with high Ag content coupled with high Pb/U ratio. These chemical characteristics can be used to constrain the major Holocene climate changes. Significant advances of glaciers next to the lake produced sediments with Ag and Pb concentration peaks and high Pb/U ratios due to accelerated erosion of the Pb–Ag vein, similar to the effects of the medieval mining. In contrast, reduced glacier activity led to the formation of organic-rich sediments with high U and As contents and low Pb/U ratios. More generally, the observed combination of chemical changes could be used elsewhere to decipher environmental changes over long periods of time.     

Les vertébrés continentaux de l’Adrar Mgorn (Maroc,Paléocène); une dispersion de mammifères transtéthysienne aux environs de la limite mésozoïque/ cénozoïque ?

Résumé

— Le gisement paléocène sud-marocain de l’Adrar Mgorn recèle les plus anciens Mammifères placentaires africains, dont la plupart sont les premiers reconnus dans le domaine sud-téthysien. L’identification, parmi eux, d’« Insectivores » paléoryctidés, très proches des genres strictement nord-américains Cimolestes et Palaeoryctes (Crétacé supérieur et/ou Paléocène), suggère une liaison par voie continentale des domaines nord-américain et africain autour de la limite Crétacé supérieur/Tertiaire. Celle-ci suppose une traversée transtéthysienne dont nous discutons les possibilités à la lumière des données paléogéographiques.     

Assimilating altimetric data to control the tropical instability waves: an observing system simulation experiment study

Tropical instability waves (TIWs) are not easily simulated by ocean circulation models primarily because such waves are very sensitive to wind forcing. In this study, we investigate the impact of assimilating sea surface height (SSH) observations on the control of TIWs in an observing system simulation experiment (OSSE) context based on a regional model configuration of the tropical Atlantic. A Kalman filtering method with suitable adaptations is found to be successful when altimetric data are assimilated in conjunction with sea surface temperature and some in situ temperature/salinity profiles. In this rather realistic system, the TIW phase is roughly controlled with a single nadir observing satellite. However, a right correction of the TIW structure and amplitude requires at least two nadir observing satellites or a wide swath observing satellite. The significant impact of orbital parameters is also demonstrated: in particular, the Jason or GFO satellite orbits are found to be more suitable than the ENVISAT orbit. More generally, it is found that as soon as adequate sub-sampling exists (with periods of 5–10?days), the length of the repetitivity cycle of orbits does not have a significant impact.     

Unstructured-mesh modeling of the Congo river-to-sea continuum

With the second largest outflow in the world and one of the widest hydrological basins, the Congo River is of a major importance both locally and globally. However, relatively few studies have been conducted on its hydrology, as compared to other great rivers such as the Amazon, Nile, Yangtze, or Mississippi. The goal of this study is therefore to help fill this gap and provide the first high-resolution simulation of the Congo river-estuary-coastal sea continuum. To this end, we are using a discontinuous-Galerkin finite element marine model that solves the two-dimensional depth-averaged shallow water equations on an unstructured mesh. To ensure a smooth transition from river to coastal sea, we have considered a model that encompasses both hydrological and coastal ocean processes. An important difficulty in setting up this model was to find data to parameterize and validate it, as it is a rather remote and understudied area. Therefore, an important effort in this study has been to establish a methodology to take advantage of all the data sources available including nautical charts that had to be digitalized. The model surface elevation has then been validated with respect to an altimetric database. Model results suggest the existence of gyres in the vicinity of the river mouth that have never been documented before. The effect of those gyres on the Congo River dynamics has been further investigated by simulating the transport of Lagrangian particles and computing the water age.     

A morphometric analysis of gullies scoured by post-fire progressively bulked debris flows in southwest Montana, USA

In the fall of 2001, an intense thunderstorm in southwest Montana triggered many debris flows in the burned area of Sleeping Child Creek. In most instances, the debris flows cut deep gullies into previously unchannelized colluvial hollows and deposited large volumes of sediment onto the valley floor. The presence of rill networks above the gullies as well as the absence of landslide features indicate that the gullies were scoured by progressively bulked debris flows, a process in which dilute surface runoff becomes increasingly more laden with sediment until it transforms into a debris flow. In this contribution, we present a morphometric analysis of six of the gullies to better understand this relatively understudied process. We find that the locations of the rill heads and gully heads conform to slope-area thresholds that are characteristic of erosion by overland flow. Our data also suggest that the volumes of the debris flows increase exponentially with normalized drainage area, thus lending support to an assumption used in a recently proposed debris flow incision law. Finally, the debris flow fans have been relatively unaltered since deposition, suggesting that the valley may be currently aggrading while the hillslopes are being denuded.     

First Imaging of Coronal Mass Ejections in the Heliosphere Viewed from Outside the Sun – Earth Line

We show for the first time images of solar coronal mass ejections (CMEs) viewed using the Heliospheric Imager (HI) instrument aboard the NASA STEREO spacecraft. The HI instruments are wide-angle imaging systems designed to detect CMEs in the heliosphere, in particular, for the first time, observing the propagation of such events along the Sun – Earth line, that is, those directed towards Earth. At the time of writing the STEREO spacecraft are still close to the Earth and the full advantage of the HI dual-imaging has yet to be realised. However, even these early results show that despite severe technical challenges in their design and implementation, the HI instruments can successfully detect CMEs in the heliosphere, and this is an extremely important milestone for CME research. For the principal event being analysed here we demonstrate an ability to track a CME from the corona to over 40 degrees. The time – altitude history shows a constant speed of ascent over at least the first 50 solar radii and some evidence for deceleration at distances of over 20 degrees. Comparisons of associated coronagraph data and the HI images show that the basic structure of the CME remains clearly intact as it propagates from the corona into the heliosphere. Extracting the CME signal requires a consideration of the F-coronal intensity distribution, which can be identified from the HI data. Thus we present the preliminary results on this measured F-coronal intensity and compare these to the modelled F-corona of Koutchmy and Lamy (IAU Colloq. 85, 63, 1985). This analysis demonstrates that CME material some two orders of magnitude weaker than the F-corona can be detected; a specific example at 40 solar radii revealed CME intensities as low as 1.7×10⁻¹⁴ of the solar brightness. These observations herald a new era in CME research as we extend our capability for tracking, in particular, Earth-directed CMEs into the heliosphere.     

Effects of streamflow reductions on aquatic macroinvertebrates: linking groundwater withdrawals and assemblage response in southern New Jersey streams,USA

Abstract

A comprehensive hydro-ecological investigation was conducted to determine the ecological response of increased groundwater withdrawals from the Kirkwood-Cohansey aquifer system, an important source of water supply in southern New Jersey, USA. Collocated observations were made of aquatic-macroinvertebrate assemblages and stream hydrologic attributes to develop flow–ecology response relations. A sub-regional transient groundwater flow model (MODFLOW) was used to simulate three plausible high-stress groundwater-withdrawal scenarios which resulted in stream baseflow reductions of approximately 0.12, 0.20, and 0.26 m³ s^-1. These reduction scenarios were used to construct flow-alteration ecological response models to evaluate aquatic-macroinvertebrate response to streamflow reduction. For example, flow-alteration ecological response models indicate that if groundwater withdrawals diminish mean annual streamflow from 1.1 to 0.6 m³ s^-1, the abundance of intolerant taxa could be reduced by as much as 20%. These flow-alteration ecological response modelling results could be used by resource professionals to evaluate alternative water management strategies to determine maximum basin withdrawal rates that meet ongoing human water demand while protecting biological integrity.

Editor D. Koutsoyiannis; Guest editor M. Acreman

Citation Kennen, J.G., Riskin, M.L., and Charles, E.G., 2014. Effects of streamflow reductions on aquatic macroinvertebrates: linking groundwater withdrawals and assemblage response in southern New Jersey streams, USA. Hydrological Sciences Journal, 59 (3–4), 545–561.     

Sustainable groundwater extraction in coastal areas: a Belgian example

Water extractions in coastal areas have to deal with salt water intrusion and lowering of hydraulic heads in valuable ecosystems. Therefore, sustainable management of fresh water resources in these areas is crucial. This is illustrated here with two water extractions in the western Belgian coastal plain which extract groundwater from a phreatic dune aquifer. One water extraction faced problems with salt water intrusion, while lowering of hydraulic heads was an issue for both. To remedy the salt water intrusion, it was found that decreasing the extraction rate was the only solution. To offset this and to increase hydraulic heads around both extractions, it was decided to artificially recharge the aquifer of the second extraction with tertiary treated wastewater. By taking these interventions, the combined production capacity of the water extractions was increased with 56% whereas 27% less water was extracted from the dune aquifer itself. Extraction history and the effects of interventions are illustrated for both water extractions with water quality data and fresh water head observations. A more detailed insight in groundwater flow and fresh–salt water distribution in the aquifer is provided by simulating the evolution of the water extractions with a 3D density dependent groundwater flow model.