Isotopic Offsets of Riverine Supplies to the Ocean vs. Inland Precipitation - Case Studies of Boreal and Arctic Rivers

Stable isotopes incorporated within the water molecule (18O/16O and 2H/1H) are of particular interest to hydrological studies because they exhibit systematic spatial and temporal variations as a result of isotope fractionations that accompany water cycle phase changes and diffusion. Isotope fractionation produces a natural labelling effect within the global water cycle, which has been applied to study a wide range of hydrological and climatic processes at the local, regional, and global scales. Although the enormous potential for this tool is not challenged, only a few extensive datasets exist throughout the world for major river systems and the international community is only beginning to adapt water sampling networks to collect and analyse stable isotopes. The IAEA Global Network of Isotopes in Rivers (GNIR) is just currently being implemented whereas the PARTNERS network, sampling large rivers draining into the Arctic Ocean, has been active since 2003. Here we will focus on the relationship that exists between the isotopic composition of continental precipitation and large river discharge to the ocean in cold regions, notably the Atlantic and Arctic Oceans. Adequate spatial and temporal coverage is crucial to constrain both the hydrology and nutrient cycling within watersheds. Synoptic surveys have been carried out on the Mackenzie, Lena and St. Lawrence rivers. Furthermore, monthly to bi-monthly samples have been collected on these rivers as well as the outlet of the Yukon, Kolyma, Yenisey and Ob' rivers (PARTNERS dataset). From these surveys, it is clear that the major Russian rivers (Lena, Yenisey and Ob') experience very little open-water evaporation and that the isotopic composition of continental precipitation is more directly transferred to river waters at their outlet to the ocean. Conversely, for the Mackenzie and St. Lawrence rivers offset from the Global Meteoric Water Line (GMWL) in δ2H/δ18O space suggests significant open-water evaporation within tributary basins, mainly from lakes and wetlands. Intermediate responses are found for the Yukon and Kolyma Rivers.