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Patagonia is considered to be the most important source of dust from South America that is deposited in surrounding areas, and we present here a systematic Sr and Nd isotopic study of sediment currently being exported. Eolian and suspended riverine sediments from Patagonia have a homogeneous chemical and isotopic composition that results from the mixing of by-products from explosive Andean volcanism, derived from the extensive Jurassic silicic Province of Chon Aike and pyroclastic materials from the basic to intermediate southern Andean Quaternary arc, which are easily denudated and dispersed. The main Andean uplift and the glaciations that began in the Late Tertiary account for the extensive distribution of these sediments in the extra-Andean region. The present geochemical signature of Patagonian sediments was produced during the Pleistocene, along with the onset of the southern Andean explosive arc volcanism. Previously published compositions of sediments from other southern South American source regions, assumed to be representative of Patagonia, are distinct from our data. Considering the alleged importance of Patagonia as a dust source for different depositional environments in southern latitudes, it is surprising to verify that the chemical and isotopic signatures of Patagonian-sourced sediments are different from those of sediments from the Southern Ocean, the Pampean Region or the Antarctic ice. Sediments from these areas have a crustal-like geochemical signature reflecting a mixed origin with sediment from other southern South American sources, whereas Patagonian sediments likely represent the basic to intermediate end-member composition.

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The Parana River was sampled and analyzed for carbon and mineral concentrations at the ParanaSanta Fe (Argentina) cross-section (-600 km above the mouth) between March 198 1 and November 1984. Within this period, the Parani was affected by an extraordinary flood caused by the 1982 El Niiio/ Southern Oscillation (ENSO) climatic anomaly. Under such conditions, with a flow 75% higher than the long-term mean, the river exported a total organic C (TOC) load of 8.43 Tg yr-l(3.0 g m-2 yr-I) of which 89% was accounted for by dissolved organic C (DOC) and the remainder by particulate organic C (POC). Under normal hydrological conditions, however, the ParanP has a TOC load that is roughly half of that determined during the ENSO-triggered flood (4.43 Tg yr-I, 1.6 g m-2 yr-I), but with a very different DOC : POC ratio (63 : 37).In July-August 1985 the Parani was sampled along a reach of over 1,300 km. At the time of sampling, the Paraguay River supplied -68% of the POC load, 59% of the labile particulate load (LPOC), and 7 1% of the DOC entering the middle reach of the Parani. Carbohydrates in LPOC appear to become more abundant than amino acids in the lower reaches, probably due a significant input from the floodplain of the ParanL.Understanding the temporal and spatial changes of the chemistry and the biology of major river systems -the biogeochemical dynamics-is a significant step toward comprehension of the circulation of matter and energy in natural systems. Many papers published ' To whom correspondence should be addressed.

Abstract:The main Patagonian rivers (Colorado, Negro, Chubut, Deseado, Coyle, Chico, Santa Cruz and Gallegos) were sampled between September 1995 and November 1998 to determine their chemical and isotopic compositions, the origins of the suspended and dissolved river loads and their inputs to the South Atlantic Ocean. This paper focuses on the dissolved inorganic carbon (DIC) transport and its υ 13 C isotopic signature. The υ 13 C DIC values vary between 12Ð8 and 1Ð8‰ and allow one to distinguish two river groups: (i) the Colorado, Negro, Chubut and Santa Cruz, which display the highest values and the lowest seasonal variations; (ii) the Deseado, Coyle, Chico and Gallegos, which show the lowest values and the highest seasonal variations. For the first group, υ 13 C DIC is mainly controlled by important exchanges between the river waters and atmospheric CO 2 , due to the presence of lakes and dams. For the second group, υ 13 C DIC also appears to be controlled by the oxidation of organic carbon, showing a negative relationship between υ 13 C DIC and the dissolved organic carbon. These biogeochemical processes interfere with the contribution of carbonate and silicate weathering to the riverine DIC and do not allow use of υ 13 C DIC alone to distinguish these contributions. The annual DIC flux exported by Patagonian Rivers to the South Atlantic Ocean averages 621 ð 10 9 g. of C, i.e. a specific yield of 2Ð7 g m 2 year 1 . The mean υ 13 C DIC can be estimated to 4Ð9‰, which is high compared with other rivers of the world.

Abstract:Most water in the Paraná River drainage basin is supplied by the tropical Upper Paraná (over 60% of the total annual water discharge, 550 km 3 . The total suspended solids (TSS) load (c. 80 ð 10 6 t year 1 , however, is essentially furnished (50-70%) by the mountainous, arid and mostly sediment-mantled upper Bermejo River drainage basin. This characteristic suggests that the Paraná River solid load (TSS, 600 km upstream from the mouth) is largely recycled sedimentary material, whose discharge-weighted mean chemical index of alteration is c. 71. The extended UCCnormalized multi-elemental diagrams are similar to those of other world rivers. Nevertheless, the detailed inspection of UCC-normalized rare earth element (REE) 'spidergrams' reveals a lithological source for the Paraná River TSS that might be compatible with either tholeiitic flood basalts (widespread in the upper drainage) or with young Andean intermediate volcanic rocks. In view of the Bermejo River's dominant role as a sediment contributor, we feel that the signature preserved in the Paraná's TSS is the latter. Conversely, the Uruguay River TSS REE signature is certainly determined by the extensive weathering products of Jurassic-Cretaceous tholeiitic basalts.