The Fish River canyon (Southern Namibia): A record of Cenozoic mantle dynamics?

Intracontinental elevated plateaus remain geomorphologic features which are poorly studied. Their genesis requires a coupling between climate and deformation. The Fish River canyon (southern Namibia), the second largest canyon of the Earth, carved the South-African plateau on 550 m along 65 km. This study reveals that the upper and the lower segments are shaped by NE–SW and north-south grabens, respectively. These deformations increased the meandering and the vertical incision mainly in the lower canyon. However the river main trend was not drastically modified attesting that the river was ancient and that the rate of the vertical displacement was slow compared to the erosion rate. The main incision occurred during a NW–SE stretching of Plio-Pleistocene. These stretching episodes belong to two deformation phases previously poorly described in the South-African plateau. These widespread stretching phases are interpreted as a result of deep mantle dynamics affecting the inner African continent.     

Alluvial architecture of mid-channel fluvial–tidal barforms: The mesotidal Lower Columbia River,Oregon/Washington,USA

Barforms of mesotidal to macrotidal fluvial–tidal transitions, regardless of fluvial-discharge, are currently thought to display a sedimentary architecture dominated by tidal signatures. Due to the scarcity of observations from modern mesotidal fluvial–tidal transitions, especially those of multi-channelled large-rivers (mean annual discharge ≥7000 m³ s⁻¹ and peak discharges ≥15 000 m³ s⁻¹) with mid-channel bars, this concept remains unproven. The present study analyses data produced by a combination of high-resolution ground penetrating radar and coupled shallow vibracores (<5 m depth), collected from modern fluvial–tidal mid-channel bars of the mesotidal multi-channelled Lower Columbia River, Washington/Oregon, USA, which can experience peak discharges ≥18 000 m³ s⁻¹. These data were used alongside time-sequenced aerial imagery to characterize the spatio-temporal sedimentological evolution of these barforms in singular flows or combined flows consisting of river, tidal and/or wind-wave oscillatory, current components operating in unique fluvial–tidal transition regimes. Results indicate that ca 75% of the Lower Columbia River fluvial–tidal transition produces braid-bars with basal to bar-top sedimentological architectures that are indistinguishable from fluvial-only braid-bars recorded in the literature. Barform stratal characteristics within the fluvial–tidal transitions of mesotidal large-rivers are therefore more likely to be dominated by downstream-oriented currents. Furthermore, a new style of low-angle (<5°) inclined heterolithic stratification found in bar-top accretion-sets within upper-mixed tidal–fluvial regime braid-bars is observed. This common stratification is created by combined-flows characterized by intrabasinal wind-wave oscillatory-currents and bidirectional tidal-currents. This inclined heterolithic stratification marks the initial downstream fluvial–tidal crossover point from Lower Columbia River up-dip fully-fluvial braid-bar architectures, to those possessing bar-top facies produced by the hydraulic-sedimentation response of combined intrabasinal wind-wave and tidal influence. When preserved, this form of mid-channel bar inclined heterolithic stratification provides a unique sedimentological signature of multi-channelled fluvial–tidal transitions that possess an open-water lower basin with intrabasinal wind-waves.     

Mercury in the Waters of the Jundiaí River, SP, Brazil: The Role of Dissolved Organic Matter

Many developing countries have regions of high demographic density, where untreated residuary waters from different sources are often discharged into rivers, streams and other water bodies. This paper discusses the reducing action of organic matter of anthropic origin on the mercury redox cycle in the Jundiaí River impacted by discharged wastes, and on the Piraí River, a non-impacted water body. The total mercury concentrations in these locations vary from 1.7 to 32?ng?L^?1 in the former and from 0.6 to 10.6?ng?L^?1 in the latter. Dissolved organic carbon concentrations of up to 68.3 and 6.5?mg?L^?1 were observed, confirming the higher impact on the Jundiaí River. It was found that an inverse correlation between the concentration of dissolved organic carbon and total mercury was stronger in the Jundiaí River, given that it receives higher organic loads, suggesting that organic matter exerts a reducing action on mercury, which is released as gas into the atmosphere. This correlation was not observed in the Piraí River, where the organic matter of natural origin is probably not sufficiently labile to act intensely upon the Hg redox cycle, favoring the metal transport.