Seasonal change of geochemical sources and processes in the Yenisei River: A Sr, Mg and Li isotope study

Abstract

Abstract The Yenisei River has the highest annual discharge of all the rivers draining into the Arctic Ocean and therefore any change in the chemical flux composition could have a profound effect on the nutrient input to the ocean. The hydrological cycle of this region is predicted to change because of changing precipitation patterns and thawing permafrost. By studying the seasonal changes in the geochemistry of the river, information on which end-member sources are important under different hydrological conditions can be gained and therefore provide a basis to model future change. In this study we present a 7-month time-series of water chemistry samples collected at Igarka near the mouth of the Yenisei. We investigate seasonal variation in major cation chemistry, radiogenic Sr ( 87 Sr / 86 Sr ), stable Li ( δ 7 Li ) and stable Mg ( δ 26 Mg ) isotopes. Temporal variation in major cation chemistry was consistent with seasonal changes in the relative contribution of end-members previously identified from smaller rivers in this region: snow, shallow soil (organic layer and leaf litter) and deeper soil/groundwater. Temporal variation in 87 Sr / 86 Sr values, a source tracer, was also broadly consistent with seasonal changes in the contribution of the aforementioned end-members to river chemistry. However, an additional contribution from atmospheric dust, which is a minor component of major cation fluxes, is required to account for the observed increase in 87 Sr / 86 Sr values during the spring flood. Variations in Li isotopes are observed, with lower values during spring flood. Although there is a relationship between 87 Sr / 86 Sr and δ 7 Li , the variation in δ 7 Li values is unlikely to be due to changes in source inputs. Rather, they are consistent with decreased isotope fractionation during high discharge as a result of the short water residence time inhibiting processes known to fractionate Li isotopes such as adsorption and secondary mineral formation. The seasonal variation in δ 26 Mg values is negligible, but the inter-annual variation observed between 2012 and 2015 may be due to changes in permafrost extent.