[A-HW27_1AM1] Hydrological Cycle and Water Environment

[AHW27-05] Variation of ?carbon isotope signatures of particulate organic matter in the Kuzuryu River system in Japan

*Seiya NAGAO¹, Masaki KANAMORI², Takahumi ARAMAKI³, Shinya OCHIAI¹, Masayoshi YAMAMOTO¹ (1.LLRL, INET, Kanazawa University, 2.Grad. School of NST, Kanazawa University, 3.National Institute for Environmental Studies)

Keywords:POC, carbon isotopes, river waters, migration, water discharge

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 systems. This study discussed with the transport of particulate organic matter in the rivers from a river system with different watershed condition using carbon isotopic signatures. The Kuzuryu River system is located in Fukui Prefecture in the central part of Japan and consists of a main river, Kuzuryu River and a main tributary of the Hino River. River research was conducted at a fixed station in the Hino River (Fukatani site) and the Kuzuryu River (Nakatsuno site) during June-October in 2010. Suspended solid samples were collected from 130-140 L of river waters using continuous centrifugation. Prior to analysis, inorganic carbonates were removed by adding 0.1 M HCl solution, rinsing with Milli-Qwater, and drying again. Stable carbon isotopic ratio of a sample and the VPDB standard used for normalization were made by analyzing sub-samples of CO₂ gas generated during graphite production using a triple collector mass spectrometer with precision of ± 0.2 ‰ as the δ¹³C value. Radiocarbons were determined using accelerator mass spectrometry at the Japan Atomic Energy Agency. Radiocarbon values were reported as △¹⁴C corrected for sample δ¹³C with absolute error of less than 10 ‰. δ¹³C of organic matter in riverine suspended solids has -26.3 to -24.0‰ for the Kuzuryu River and -27.0 to -26.1‰ for the Hino River. The △¹⁴C vales of the Kuzuryu and Hino samples are -168 to -87‰ and -209 to -143‰, respectively. The △¹⁴C values of Hino River depleted in △¹⁴C rather than the Kuzuryu River. The averaged TOC/TN ratio is 9.1 ± 1.1 for the Kuzuryu and 8.2 ± 1.1 for the Hino. There is a positive correlation with the △¹⁴C vales and water discharge. These results indicate that the differences in POM character may be explained by the differences in watershed conditions and controlled by water discharge.