Thursday, 8 September 2005 - 1:20 PM

This presentation is part of: Biogeochemistry and Environmental Geochemistry II

Variations of Δ14C of humic substances in the Lake Biwa waters

Seiya Nagao, Faculty of Environmental Earth Science, Hokkaido University, Kita10, Nishi5, Kita-ku, Sapporo, 060-0810, Japan, Hiroki Kodama, Faculty of Agriculture, Kyoto Prefectural University, Shimogamo, Kyoto, 606-8522, Japan, Takafumi Aramaki, Environmental Chemistry Division, National Institute of Environmental Studies, Onogawa 16-2, Tsukuba, 305-8567, Japan, Nobuhide Fujitake, Faculty of Agriculture, Kobe University, 1-1 Rokkodai, Kobe, 657-8501, Japan, and Koyo Yonebayashi, Faculty of Agriculture, Ishikawa Prefectural University, 1-308 Suematu, Nonoki, 921-8836, Japan.

Humic substances are the largest fraction of dissolved organic matter (DOM) in river and lake waters, and play an important role in both pollutant chemistry and the biogeochemistry of natural waters and soils. Investigations on characteristics of aquatic humic substances (humic and fulvic acids) have yield insight into the production and turnover of soil organic materials, and their transport behavior within natural watersheds.

A radiocarbon (14C) is a useful tool for studying the dynamics of DOM in terrestrial environments. The 14C can provide unique information on the source, ages, and residence times of organic materials in aquatic environments. This study applied a combined Δ14C and δ13C approach to lake humic substances for better understanding the fate of aquatic humic substances in a lake and its watershed.

Surface waters were collected at the environmental standard sites in north and south part from Lake Biwa, and the sampling sites of Zao-Dam and Kawashiro-Dam. The humic substances were isolated and purified by the XAD-extraction method. 14C measurements were performed by accelerator mass spectrometry at the Marine Research Laboratory of Japan Atomic Energy Research Institute.

The Δ14C values of humic substances in surface waters from Lake Biwa and Zao-Dam were less than 0‰. The fulvic acids ranged from –316 to –55‰. The humic acids have the Δ14C values of –55‰ and –122‰. On the other hand, the Δ14C values of the Kawashiro-Dam are greater than 0‰. The International Humic Substances Society (IHSS) aquatic reference samples in 1986 are 149-209‰. The Δ14C values of the Biwa humic substances are ca. 400‰ lower than the IHSS reference samples. The surface water humic substances from Lake Biwa consist mainly of pre-bombed carbon and the IHSS samples must contain a substantial amount of bomb carbon. The amount of fast cycling and the humification for the watersheds of Lake Biwa may be lower and slower than in the Suwannee River and Nordic lake environments.

There is a negative correlation between the Δ14C and δ13C values (-25.6‰ ~ -27.8‰) of fulvic acids from the Biwa, the Zao-Dam, the Kawashiro-Dam and the IHSS samples. The Δ14C values of humic acids have no correlation with the δ13C values. The δ13C values of Biwa and Zao-Dam humic acids indicate a wide range of variation from –30.4 to –24.0‰. These results indicate that carbon isotopic ratios of the Biwa humic and fulvic acids depend on the watershed conditions.


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