Stable Isotopes of Water in Permafrost Ecosystem

Abstract

Although the eastern Siberian climate is continental and severely dry, the permafrost hydrology system enables larch trees to survive. Stable isotopes of water have been widely used in studies of the water cycle because summer precipitation, winter snowfall, and ground ice can be isotopically discerned. In early summer, larch trees use soil water derived from winter snowfall; if the summer is wet, soil water from summer rainfall is subsequently used. However, in a dry summer, soil water originating from melting seasonal ground ice must be used. Soil moisture therefore varies quantitatively and isotopically from year to year depending on the balance of each source of water.

Transpiration by vegetation is also an important driver of the water cycle in dry-climate regions such as eastern Siberia. Typical diurnal variation in isotopes of atmospheric water vapor on a clear day exhibits an increase in the morning with increasing mixing ratio and a decrease in the afternoon. These increasing isotope ratios of water vapor in the morning correspond to increased transpiration, whereas the decrease in the afternoon shows the entrainment of free atmosphere, with lower isotope ratios. A clear correlation has been found between the isotope ratios of atmospheric water vapor and mixing ratio observed at midday, accompanied by high temperature and active transpiration. The isotopic compositions of transpired water vapor and background atmospheric vapor observed in this region lead to a high water vapor fraction from transpiration.