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Fil: Recasens, Cristina. Columbia University; Estados Unidos

Mineral dust is a natural tracer of atmospheric composition and climate variability. Yet, there is still much to be known about the Southern Hemisphere dust cycle. Major efforts have attempted to solve the puzzle of the origin of the potential source areas contributing dust to the Southern Ocean and East Antarctica. Here we present a comprehensive geochemical characterization of a source area, whose role as a dust supplier to high latitude environments has significantly been underestimated. Sediments collected within the major dust-producing areas along the Namibian coast in Southern Africa (Kuiseb, Omaruru and Huab river catchments and the Namib Sand Sea region), were analyzed for radiogenic isotope ratios and rare earth element concentrations. We find that during warm periods, the Southern African dust signature can be found in archives of the Southern Hemisphere, especially in the Atlantic sector of the Southern Ocean and peripheral areas of the East Antarctic plateau.

The global chemical transport model GEOS‐Chem, implemented with a dust‐iron dissolution scheme, was used to analyze the magnitude and spatial distribution of mineral dust and soluble‐iron (sol‐Fe) deposition to the South Atlantic Ocean (SAO). The comparison of model results with remotely sensed data shows that GEOS‐Chem can capture dust source regions in Patagonia and characterize the temporal variability of dust outflow. For a year‐long model simulation, 22 Tg of mineral dust and 4 Gg of sol‐Fe were deposited to the surface waters of the entire SAO region, with roughly 30% of this dust and sol‐Fe predicted to be deposited to possible high nitrate low chlorophyll oceanic regions. Model‐predicted dissolved iron fraction of mineral dust over the SAO was small, on average only accounting for 0.57% of total iron. Simulations suggest that the primary reason for such a small fraction of sol‐Fe is the low ambient concentrations of acidic trace gases available for mixing with dust plumes. Overall, the amount of acid added to the deliquesced aerosol solution was not enough to overcome the alkalinity buffer of Patagonian dust and initiate considerable acid dissolution of mineral‐iron. Sensitivity studies show that the amount of sol‐Fe deposited to the SAO was largely controlled by the initial amount of sol‐Fe at the source region, with limited contribution from the spatial variability of Patagonian‐desert topsoil mineralogy and natural sources of acidic trace gases. Simulations suggest that Patagonian dust should have a minor effect on biological productivity in the SAO.

Fil: García, María Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina

The Chubut is a medium-size (42,000 km2) river basin that drains the arid-to-semiarid Patagonian seaboard and pours its waters into the southwestern Atlantic Ocean (ca. lat 43°20'S, long 65°04'W). The materials eroded from the continent and deposited in the sea are scarcely affected by chemical weathering (the chemical index of alteration of riverbed sediments is ∼55) and bear a typical chemical and mineralogical signature characteristic of volcanic arcs. Clearly, flowing toward a passive margin, the river carries the mineralogical and chemical signature of an active margin. Physically weathered andesites and basalts occupy only about 25% of the drainage area, and therefore most exported material must be supplied by outcropping sedimentary beds of variable age. The Chubut River headwaters are placed in a tectonically active region, soil formation is incipient (\"weathering-limited regime\"), and the rate of denudation (24.6 t km-2 yr-1) is much lower than the rates exhibited by similar rivers in other parts of the world. The depleted dissolved and particulate load is determined by scarce atmospheric precipitations (i.e., the drainage basin is in the Andean rain shadow) and by the protective effect of Cenozoic lava flows that often shield sedimentary formations from denudation. Although the index of chemical variability suggests that materials exported are products of the first denudational cycle, the geological history supports the view that most materials may have passed two or even three times through the exogenous cycle without acquiring a chemical or mineralogical signature indicative of repeated weathering. This is probably also true for other basins in temperate Andean climates.

Between June 1995 and November 1998, eight Patagonian rivers were sampled for the suspended and dissolved loads delivered to the SW Atlantic. The most important rivers (Negro and Santa Cruz) jointly deliver ;90% of the total Patagonian freshwater budget (;60 km3 y21). Of the total sediment load (;1.7 1012 g y21), 2.8% was accounted for by particulate organic carbon (POC), 0.9% by inorganic particulate carbon (PC), 0.7% by particulate nitrogen (PN), and 0.7% by particulate phosphorus (PP). The mean dissolved organic carbon (DOC) yield was ;0.50 g m22y21, and POC ;0.3 g m22y21. Nitrogen is the limiting nutrient in all rivers; the mean molecular C:N:P ratio is 37:1:1. POC:PN ratios (4.4–10) indicate an autochthonous origin for the organic matter in suspended particulate matter (TSS). Many factors, such as proglacial oligotrophic lakes, coal-bearing strata, wetlands, aridity, as well as various human impacts, suggest a complex typology. The analysis (Euclidean distance cluster analysis) of biogeochemical variables [SiO2, NO3 2, PO4 32, DOC, POC, PC, PP, PN, C:N, DOC:POC, PC:POC, POC (%)] indicates that runoff, superimposed on biogeochemical variables, plays an important role in Patagonian riverine typology: a) Low runoff rivers (,100 mm y21): the Chubut, Chico, Deseado are characterized by low yields and POC:PN ratios; the Coyle River, by high DOC:POC; b) Medium discharge rivers (100–300 mm y21): the Negro River has high dissolved yields and high POC(%TSS); the Colorado is distinguished by high PP and PC specific yields, and POC:PN ratio; c) High runoff rivers (.1000 mm y21): Santa Cruz and Gallegos rivers, are both characterized by above-average specific yields; the Gallegos has high POC(%TSS), POC:PN and DOC:POC.