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Abstract:The analysis of physicochemical variables and selected dissolved elements was performed on the Apure River waters for 15 months. The variables pH, alkalinity, dissolved O 2 , conductivity and Na, Ca, Mg and Cd concentrations showed maximum values during low water, whereas K, Si, Fe, Al, Mn, Zn, Cu, Cr and dissolved organic carbon (DOC) showed maximum concentrations during rising and high water. Five important factors were found to control the amount and temporal variability of the dissolved elements: lithology, hydrology, vegetation-floodplain processes, redox conditions and organic complexation. Weathering of silicates, carbonates and evaporites in the Andes provides most of the proportion of Na, Ca, Mg and HCO 3 to waters. The temporal variability of these ions is controlled by a dilution process. Although Si can be taken up by the biomass, Si and K can be leached from the floodplain by weathering of clays. Microbial decay of the submerged plants in the floodplain during the inundation periods provides DOC and K to river waters and changes the redox conditions in water. The changing redox conditions control the solubility of Mn, Zn and Fe. Dissolved Mn is a function of pH-dependent redox process, whereas Zn solubility is controlled by scavenging of Zn during the oxidation of Mn 2C to MnO 2 . Positive relationships between Al, Fe, Cu, Cr and DOC suggest that these elements are complexed by organic colloids generated in the floodplain. Moreover, the binding capacity of Fe with DOC increases under reducing conditions. Although Cd seems to be provided by weathering in the Andes, several processes can affect the mobility of Cd during transport.

[1] This study describes the geochemistry of dissolved elements in the Caura River and gives information about weathering rates and associated CO 2 consumption in an Orinoco River subbasin. Physicochemical variables (pH, conductivity, HCO 3 − , dissolved O 2 , and temperature), dissolved elements (Na, K, Ca, Mg, Si, Al, Fe, Cu, Mn, Zn, and Cr), total suspended sediments (TSS), and dissolved organic carbon (DOC) concentrations were measured monthly from June 2007 to August 2008 in the Caura River. Major element concentrations (Na, Ca, Mg, and Si) showed an inverse correlation with discharge, whereas K showed high concentration values during the rising waters, probably due to biogenic sources. All these elements are provided mainly from weathering of plagioclases and K feldspars. The weathering rate (riverine flux of dissolved major cations and SiO 2 derived from weathering per unit area) and the CO 2 consumption rate in the Caura basin (15.4 tons km −2 yr −1 and 1.1 × 10 5 mol km −2 yr −1 , respectively) were higher than those reported for the Orinoco basin and other black water river basins. This fact can be due to several factors such as lithology (volcanic rocks), high runoff, and the presence of organic acids, which can enhance the chemical weathering. The variability of the trace elements showed a different behavior than major elements. Fe and Al concentrations were correlated with DOC. Dissolved Mn content was correlated with pH, whereas the low concentrations of Cu and Cr are possibly associated with the low content of small size organic colloids. The high values of Zn observed during the decreasing stage suggest biogenic input of Zn to river waters.

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Abstract:Monthly analyses of pH, conductance, alkalinity, total suspended solids, dissolved major cations (Na, K, Ca and Mg), and selected dissolved trace elements (Fe, Al, Mn, Cu, Zn and Cr) were conducted on waters of the lower Orinoco River from February 2004 to May 2006. The data show strong seasonal variability. Major cations had maximum concentrations at low water, and were correlated with pH, conductance and alkalinity because of the tributaries coming from the Andean zones, where the weathering of evaporites and carbonate shales controls water chemistry. K concentrations did not show any relation with evaporite weathering, probably because large amounts of K come from the Guayana Shield rivers as a consequence of feldspar weathering. The concentrations of dissolved Fe, Mn and Cu were highest during the high water stage. Concentrations of the elements K, Cr, Zn and Cu are correlated with each other but not with dissolved Fe and Al, which probably are complexed with humic and fulvic substances. Mn concentrations did not show relationships with other variables. Inter-annual variability of dissolved elements is explained by temporal changes in precipitation.

Comprehensive geochemical analyses (bulk parameters, major and trace elements, organic matter, mineralogy and particle analysis) were carried out on sediments from seven lakes on Fildes Peninsula, King George Island. The resulting compositional dataset was subjected to principal component analysis and cluster analysis. The results showed that four different lacustrine sedimentary environments can be distinguished: i) lakes 1 and 2 with relatively high values of secondary aluminous clay minerals, ii) lake 4 with anthropogenic enrichment of Co, Cu and Zn, iii) lake 7 with relatively high values of Ba and P and a high zeolite content, and iv) all the other studied lakes. Our results indicate moderate chemical weathering in all lacustrine sediments and that the distance to the glacial ice margin is one of the most important factors influencing the chemical weathering in the area. Furthermore, depositional processes have little influence on the geochemistry of the investigated elements and that local bedrock is the main source of sediments in the lakes on Fildes Peninsula.