Sources of colloidal and dissolved loads over the hydrological cycle in Siberian rivers

Abstract

High latitude permafrost-dominated areas such as Central Siberia present an atypical hydrological cycle punctuated by an important spring flood resulting from snow melting. However, chemical variations in river waters associated to these hydrological variations are rarely taken into account in environmental geochemistry studies. This study aims to work out how the highly contrasted hydrological cycle and the presence of permafrost influence the temporal pattern of chemical element migration. For this purpose, major and trace element concentrations as well as Sr and U isotopic ratios were analyzed in the dissolved load of two Siberian rivers regularly sampled over two hydrological cycles (2005-2007), Nizhniya Tunguska and Kochchumo rivers, following the classical approaches used in the lab (e.g. [1]). Our results highlight that DOC and traditionnaly insoluble elements such as Al and Fe, but also REE and Th, are mobilized as a major colloidal flux at the spring flood and to a lesser extent during the summer period. The data show the occurrence of two colloid sources, successively involved over time: during the spring flood, the main source of colloids is the uppermost organic soil horizon whereas later in summer colloids come mainly from deeper soil compartments. Similarly, the results point out that the dissolved load of these rivers have to be explained in terms of mixing between deep underground brines that dominate during winter and a summer suprapermafrost flow draining deep soils, with a minor contribution of shallow soil layers during spring flood. It is critical to consider these temporal variations in the intensity and in the nature of dissolved geochemical flux in order to establish reliable chemical budgets and to evaluate weathering rates in boreal regions