δ13C tracing of dissolved inorganic carbon sources in Patagonian rivers (Argentina)

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 delta C-13 isotopic signature. The delta(13) C-DIC values vary between -12.8 and -1.8% 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, delta C-13(DIC) is mainly controlled by important exchanges between the river waters and atmospheric CO2, due to the presence of lakes and dams. For the second group, delta(13) C-DIC also appears to be controlled by the oxidation of organic carbon, showing a negative relationship between delta 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 delta(13) C-DIC alone to distinguish these contributions. The annual DIC flux exported by Patagonian Rivers to the South Atlantic Ocean averages 621 x 10(9) g. of C, i.e. a specific yield of 2-7 g m(-2) year(-1). The mean delta C-13(DIC) can be estimated to -4.9%, which is high compared with other rivers of the world. Copyright (c) 2005 John Wiley & Sons, Ltd.