[A-OS18] Coastal circulation and material cycle and freshwater discharge through rivers

[AOS18-03] Downward variation of δ¹³C and Δ¹⁴C in riverine particulate organic matter at Kumaki River, a small river in Noto Peninsula

*Seiya Nagao¹, Ryuunosuke Tahara², Akiko S. Goto³, Takashi Hasegawa³, Shinya Ochiai¹ (1.Institute of Nature and Environmental Technology, Kanazawa University, 2.Graduate School of Natural Science and Technology, Kanazawa University, 3.Institute of Science and Engineering, Kanazawa University)

Keywords:stable carbon isotope, radiocarbon, riverine suspended solids

Small rivers and streams have potential significance in the transport of sediments and organic matter, and its export flux is estimated as ~40% of riverine global organic carbon. Radioactive and stable isotopes of carbon (¹⁴C and ¹³C) can serve as powerful tools for identifying sources and estimating turnover times of organic matter in aquatic system. The present study aims to investigate downward variations in the characteristics of riverine particulate organic matter running through forest and paddy field. We selected the Kumaki River located at the central part of the Noto Peninsula in Ishikawa Prefecture, Japan. The river research was monthly conducted at fixed sites of the up-, mid- and down-stream during April 2016-December 2018.

δ¹³C and Δ¹⁴C values of organic matter in the riverine suspended solids of Kumaki River show the range of –27.2‰ to –25.1‰, and –130 to +3 ‰, respectively. The carbon isotope composition shows three groups corresponding the up-, mid- and down-stream sites at normal flow condition. After rain events, the carbon isotope values are shifted to lower value of δ¹³C and higher value of Δ¹⁴C at the three sites under higher flow condition. There is a positive correlation between particulate organic carbon concentrations and thier Δ¹⁴C values, and a negative correlation between the δ¹³C and Δ¹⁴C values at mid and down-stream sites. All data shows downward variation of carbon isotope composition from upstream to downstream. The Δ¹⁴C values decrease along the river line, but the δ¹³C increased downward in the Kumaki River. The results indicate that particulate organic matter at each sampling site is mainly supplied from each watershed area. The higher contribution from surface forest soil was observed at all sampling sites after rain events. Therefore, water discharge and land use type of the watershed are related to the transport of particulate organic matter in the small river system.