Impact of permafrost degradation on the extreme increase of dissolved iron concentration in the Amur River during 1995–1997

Abstract Dissolved iron (dFe) concentration in the Amur River increased considerably between 1995 and 1997, but the mechanisms of dFe discharge to the Amur River are poorly understood. We examined the long-term changes in climate variables in the Amur River basin between 1960 and 2000; net precipitation in late summer (July–September) has increased since 1977 throughout the basin; annual air temperature (Ta) was relatively high between 1988 and 1990. Most importantly, we found significant correlations between Ta and dFe concentration with a 7-year lag (r = 0.43–0.55, p < 0.01), which indicate a close relationship between high Ta in year Y and increased dFe concentration in year Y + 7. Correlation was the strongest in northeastern Amur basin where permafrost coverage is the highest. On the basis of our findings, we propose that (1) increased net precipitation since 1977 has increased soil moisture, which created favourable conditions for microbial iron reduction; (2) permafrost degradation during the warm years of 1988–1990 promoted iron bioavailability and led to the intensive generation of dFe in the deeper part of the active layer; and (3) dFe took approximately 7 years to reach the rivers and increased dFe concentration during 1995–1997.