No abstract

The flux of bioavailable Fe from mineral dust to the surface ocean is controlled not only by the processes in the atmosphere but also by the nature and source of the dust. In this study, we investigated how the nature of Fe minerals in the dust affects its potential Fe solubility (Fepsol) employing traditional and modern geochemical, mineralogical, and microscopic techniques. The chemical and mineralogical compositions, particularly Fe mineralogy, in soil samples as dust precursors collected from North African dust source regions were determined. The Fepsol was measured after 3 days of contact with sulfuric acid at pH 2 to simulate acid processes in the atmosphere. Fepsol of the soil dust samples were compared with calculated predictions of Fepsol based on the amount of individual Fe‐bearing minerals present in the samples and Fe solubilities of corresponding standard minerals. The calculated Fepsol deviated significantly from the measured Fepsol of the soil dust samples. We attributed this to the variability in properties of Fe minerals (e.g., size of Fe oxides and heterogeneity of chemical compositions of clay minerals) in soil dusts in comparison to the standard minerals. There were, however, clear relationships between the degree of chemical weathering of North African soils and Fepsol. The Parker index and ratio of ascorbate plus dithionite Fe to total Fe ((FeA+FeD)/FeT) are positively and negatively correlated with Fepsol, respectively. In addition, the ratio of FeA/(FeA+FeD), which decreases with aging of the Fe oxides, was found to be positively correlated with Fepsol in the soil dusts. Overall, our results indicate that there is a significant regional variability in the chemical and Fe mineralogical compositions of dusts across North African sources, as a result of the differences in chemical weathering and aging of Fe oxides. Furthermore, the indices for these weathering processes can provide an estimate of the fraction of Fe which can be solubilized if acid processed in the atmosphere.

1] This paper combines archived remotely sensed data (airborne lidar and digital color air photographs) with nonsynchronous ground observations (including observations of topographic form and vegetation cover and growth) to test the hypothesis that colonization of exposed river sediments by riparian trees has an impact on channel form and to quantify any impact that is identified. This is achieved along a 21 km reach of the braided, gravel bed Tagliamento River, in northeast Italy, where the width of the braided corridor typically exceeds 800 m. Lidar data are analyzed to extract a 2 m resolution digital evolution model (DEM) and determine riparian vegetation extent, height, and structure within the active corridor. Aerial photographs are used to map the topography of the submerged parts of the corridor. These data are divided into 1 km length subreaches, which possess strong contrasts in vegetation height and extent. Joint analysis of vegetation and morphological properties of these subreaches reveals significant associations between vegetation properties and reach morphology. Residuals from a gamma function fitted to the topographic data for each subreach show a good fit with poorly vegetated reaches, but a weakening fit with increasing vegetation cover, largely as a result of the appearance of secondary peaks in the elevation frequency distribution associated with the heavily vegetated areas. Furthermore, the overall skewness and kurtosis of the elevation frequency distribution within each of the subreaches are both significantly correlated with vegetation extent, height, median elevation, and growth rate, indicating a clear topographic signature of vegetation development along this braided river that reflects sediment accumulation within and around the vegetated patches.Citation: Bertoldi, W., A. M. Gurnell, and N. A. Drake (2011), The topographic signature of vegetation development along a braided river: Results of a combined analysis of airborne lidar, color air photographs, and ground measurements, Water Resour. Res., 47, W06525,

An analysis of island and active corridor dynamics is presented for a 16 km island-braided reach of the gravel-bed Tagliamento River (Italy) based upon information extracted, geocorrected and registered to a common base from three map The active corridor width showed a general decline over the study period but with some recent widening. Adjustments in active corridor width were achieved through processes of floodplain avulsion, island attachment and progressive encroachment of the edge of the active corridor across gravel areas. These adjustments were accompanied by the preferential creation of dissection (floodplain avulsion) islands during periods of widening and the construction of mid islands within the corridor during periods of narrowing. Changes in island extent were achieved by rapid island turnover, which reached a maximum rate of over 50% per annum when corridor narrowing was most rapid between 1970 and 1991. Very few island surfaces were found to persist for more than 24 years.Despite this enormous dynamism and apparent cyclic behaviour, between 1944/6 and 2005 the ratio of island area to active corridor area remained relatively constant at around 0.08 and supported a consistently high bankfull shoreline to downstream length ratio of around 6 km Á km À1 . These intrinsic properties of the dynamics of the study reach and other island-braided channels need to be recognized and maintained by river managers because they represent a characteristic habitat dynamism that is crucial to the maintenance of ecological integrity.These areas are calculated as the area of changed cover between surveys divided by the number of years between surveys and then expressed as a percentage of the area that persists as island between the two survey dates.

From the deglacial period to the mid-Holocene, North Africa was characterized by much wetter conditions than today. The broad timing of this period, termed the African Humid Period, is well known. However, the rapidity of the onset and termination of the African Humid Period are contested, with strong evidence for both abrupt and gradual change. We use optically stimulated luminescence dating of dunes, shorelines, and fluviolacustrine deposits to reconstruct the fluctuations of Lake Mega-Chad, which was the largest pluvial lake in Africa. Humid conditions first occur at ∼15 ka, and by 11.5 ka, Lake Mega-Chad had reached a highstand, which persisted until 5.0 ka. Lake levels fell rapidly at ∼5 ka, indicating abrupt aridification across the entire Lake Mega-Chad Basin. This record provides strong terrestrial evidence that the African Humid Period ended abruptly, supporting the hypothesis that the African monsoon responds to insolation forcing in a markedly nonlinear manner. In addition, Lake Mega-Chad exerts strong control on global biogeochemical cycles because the northern (Bodélé) basin is currently the world’s greatest single dust source and possibly an important source of limiting nutrients for both the Amazon Basin and equatorial Atlantic. However, we demonstrate that the final desiccation of the Bodélé Basin occurred around 1 ka. Consequently, the present-day mode and scale of dust production from the Bodélé Basin cannot have occurred before 1 ka, suggesting that its role in fertilizing marine and terrestrial ecosystems is either overstated or geologically recent.