[A-CG46] 北極域の科学

[ACG46-14] コリマ川の長期水質変動に対する温暖化および永久凍土融解の影響

*田代 悠人¹、檜山 哲哉¹、金森 大成¹、朴 昊澤²、Makarieva Olga³、Nesterova Nataliia³、鈴木 和良² (1.名古屋大学 宇宙地球環境研究所、2.海洋研究開発機構、3.Institute of Earth Sciences, St. Peterburg State University)

キーワード：永久凍土、生物地球化学、気候変動

The Arctic regions are now facing the environmental changes, including water and biogeochemical cycles, mainly due to permafrost degradation in response to the warming climate. To understand the impact of permafrost degradation on arctic river biogeochemistry, it will be beneficial to investigate long-term water chemistry data. In this study, we analyzed the river water chemistry data (Ca, Mg, SO₄, and Alkalinity) in the Kolyma River from 1980 to 2020, obtained from the Global Environment Monitoring System for Freshwater (GEMStat) and the Arctic Great Rivers Observatory (Arctic GRO). We also analyzed trends of annual/seasonal air temperature and precipitation in the Kolyma River basin from 1980 to 2020 using Global Historical Climatology Network (GHCN) monthly dataset version 4 on a 0.5 latitude/longitude and Multi-Source Weighted-Ensemble Precipitation (MSWEP) monthly dataset version 2.8 on a 0.1 latitude/longitude, respectively. Then we investigated relationships between interannual variations in air temperature, precipitation, and water chemistry from 1980 to 2020. Both long-term datasets (GEMStat and Arctic GRO) showed increases in the annual concentrations of Ca, Mg, and SO₄ from 1980 to 2020, while there was no clear trend of alkalinity in both datasets. Comparing the concentrations of Ca, Mg, and SO₄ in each month (April–October) in the 1980s and 2010s, the largest increases were found in late summer and autumn (August–October). Annual/seasonal air temperature (except winter) increased in the whole Kolyma River basin from 1980 to 2020, and this trend was stronger in the northern part of the basin. Annual and autumn precipitation also exhibited an increasing trend from 1980 to 2020, especially in the northern part of the basin. We also found that multi-decadal increases in Ca, Mg, and SO₄ were significantly correlated with annual air temperature anomaly during 1980–2020, and this correlation was stronger in the Kolyma lowland (northwest part of the basin) where ice-rich permafrost (Yedoma) is generally distributed. Based on these findings, it is reasonable to assume that increased concentrations of Ca, Mg, and SO₄ were resulted from promoted mineral weathering in soils due to higher soil temperature and increased exposure of minerals due to the degradation of Yedoma in recent decades. This interpretation is supported by relatively large increases in the concentrations of Ca, Mg, and SO₄ in late summer and autumn, when river water is predominantly fed by deep ground water (ice melt water). Weathering processes are known to produce alkaline components, but there was no clear trend of alkalinity. This inconsistent result may also be related to the universal distribution of ice-rich Yedoma in the Kolyma lowland. As alkalinity is a proxy of the residence time of the supra-permafrost groundwater flow, thawing of Yedoma likely supplies relatively new water to the main stream with less alkalinity.