4:15 PM - 4:30 PM
[MIS14-10] Reason for the high concentration of dissolved iron in the Amur River from 1995 to 1997 - possibility due to permafrost degradation -
Keywords:Biogeochemical cycles, Permafrost, Climate change
Amur River transport a large amount of dissolved iron (dFe) to the Sea of Okhotsk, which largely contributes to high primary production by phytoplankton. However, environmental factors and mechanisms that change dFe concentration in the Amur River are not well understood. From 1995 to 1997, a sharp increase in dFe concentration in the Amur River was observed, which could not be explained by the increase in groundwater discharge due to agricultural activities nor by enrichment of dFe due to low water discharge. Although the reason is still unclear, one possibility was pointed that permafrost degradation due to relatively high annual temperatures in 1990s increased bioavailability of Fe(Ⅲ)-(oxy)hydroxides and organic matter under anaerobic condition in deep soils, and consequently large amount of dFe was generated by Fe(Ⅲ)-reducing bacteria. If this hypothesis was true, dFe concentration in the Amur River could change significantly in the future due to ongoing climate change. The objectives of this study were (1) to investigate the long-term correlation between climate factors such as temperatures and precipitation and dFe concentration in the Amur River, and (2) to propose a concrete hypothesis including the mechanism for the relatively high concentration of dFe concentration in the Amur River from 1995 to 1997. This study includes not only biogeochemical contents but also hydroclimatological contents.
Amur River basin covers a large area (1,855,500 km2) in Eastern Eurasia, including the Russian Far East, North-Eastern China, and Eastern Mongolia. Permafrost is widely distributed in the northern part of the basin (Russian and Mongolian regions), although the distribution is quite scattered in these regions. In this study, the Amur River basin was divided into three regions: Russia, China, and Mongolia. For each region, annual mean air temperature (Ta) and late summer (July, August, September; JAS) mean net precipitation (precipitation minus evaporation ; P - E) were calculated from 1960 to 2000. The amount of P - E is the same as net recharge of water into the soil layer. Therefore, we investigated P - E to understand the long-term tendency of soil wetness and effect of groundwater discharge, which could be related to iron redox reaction and dFe movement. The data used for annual mean temperature and JAS mean P - E were Climate Research Unit (CRU gridded time series version 4.05) and Japanese Reanalysis Project (JRA-55), respectively.
Values of Ta in the three regions (Russia, China, and Mongolia) showed a similar trend from 1960 to 2000. It was remarkable that considerably high Ta was observed from 1988 to 1990. There was no correlation between the trend of Ta and annual dFe concentration in the Amur River. However, interestingly, cross-correlation between the two variables showed a significant positive correlation with a 7-year lag in the three regions (Russia R = 0.55 p<0.01, China R = 0.43 p<0.01, Mongolia R = 0.49 p<0.01). Considering the highest correlation in the Russian region where permafrost is widely distributed, this result suggests that intensive dFe generation occurred near the permafrost table in a hot year and dFe discharged into the Amur River with a 7-year time lag through the deeper part of the active layer. Moreover, JAS mean P - E showed continuously positive value (P - E > 0) from 1977 to 1997 in the three regions, especially in Russia and Mongolia. This fact implied that the active layer was wet in this period, leading to a huge amount of dFe generation in deep soils when permafrost degraded in the hot years from 1988 to 1990, and dFe discharged into the Amur River from 1995 to 1997. Although it is necessary to make this hypothesis more certain, these findings are expected to provide the relationship between permafrost degradation due to climate change and dFe concentration in the Amur River basin.
Amur River basin covers a large area (1,855,500 km2) in Eastern Eurasia, including the Russian Far East, North-Eastern China, and Eastern Mongolia. Permafrost is widely distributed in the northern part of the basin (Russian and Mongolian regions), although the distribution is quite scattered in these regions. In this study, the Amur River basin was divided into three regions: Russia, China, and Mongolia. For each region, annual mean air temperature (Ta) and late summer (July, August, September; JAS) mean net precipitation (precipitation minus evaporation ; P - E) were calculated from 1960 to 2000. The amount of P - E is the same as net recharge of water into the soil layer. Therefore, we investigated P - E to understand the long-term tendency of soil wetness and effect of groundwater discharge, which could be related to iron redox reaction and dFe movement. The data used for annual mean temperature and JAS mean P - E were Climate Research Unit (CRU gridded time series version 4.05) and Japanese Reanalysis Project (JRA-55), respectively.
Values of Ta in the three regions (Russia, China, and Mongolia) showed a similar trend from 1960 to 2000. It was remarkable that considerably high Ta was observed from 1988 to 1990. There was no correlation between the trend of Ta and annual dFe concentration in the Amur River. However, interestingly, cross-correlation between the two variables showed a significant positive correlation with a 7-year lag in the three regions (Russia R = 0.55 p<0.01, China R = 0.43 p<0.01, Mongolia R = 0.49 p<0.01). Considering the highest correlation in the Russian region where permafrost is widely distributed, this result suggests that intensive dFe generation occurred near the permafrost table in a hot year and dFe discharged into the Amur River with a 7-year time lag through the deeper part of the active layer. Moreover, JAS mean P - E showed continuously positive value (P - E > 0) from 1977 to 1997 in the three regions, especially in Russia and Mongolia. This fact implied that the active layer was wet in this period, leading to a huge amount of dFe generation in deep soils when permafrost degraded in the hot years from 1988 to 1990, and dFe discharged into the Amur River from 1995 to 1997. Although it is necessary to make this hypothesis more certain, these findings are expected to provide the relationship between permafrost degradation due to climate change and dFe concentration in the Amur River basin.