Nd-Sr isotopic compositions of dissolved and particulate material transported by the Parana and Uruguay rivers during high (december 1993) and low (september 1994) water periods

Our motivation is to better constrain the neodymium and strontium isotopic signatures of the closest continental riverine source (Parana and Uruguay rivers) to the Brazil/Malvinas Confluence zone in the Southwest Atlantic Ocean. We thus present new isotopic data on the Nd and Sr of the dissolved and suspended loads of the Parana and Uruguay rivers for two water periods forming the Rio de la Plata whose drainage basin is the second largest one in South America. The Parana dissolved material shows less radiogenic (εNd (0) ranging between -12.1 and -8.2) than the Uruguay one with a mean εNd (0) value of -6.3 ± 0.3. Suspended particulates display the same isotopic trend (mean εNd (0) value of -10.3 and -6.0 for the Parana and Uruguay rivers, respectively). Dissolved load 87Sr/86Sr in the Parana (0.7123) is found to be more radiogenic than the Uruguay one (0.7097); the suspended load follows the same trend with 87Sr/86Sr ratios of 0.7247 and 0.7115 in the Parana and Uruguay rivers, respectively. The relatively radiogenic Nd and non radiogenic Sr of the Uruguay River as compared to the Parana River could be attributed to a predominance of tholeiitic basalts in the drainage basin. A revisited estimation of the fluxes of Nd considering all South American rivers delivering into the western South and Tropical Atlantic Ocean yields a Nd particulate flux to estuarine water two orders of magnitude higher than the Nd dissolved flux. Considering the net dissolved and suspended fluxes of Nd reaching the Rio de la Plata, we have calculated a resulting εNd (0) equal to -10.2. The computation of εNd (0) of the Rio de la Plata outflow waters gives a value of -8.3, taking into account various removal processes within the estuary. (authors)