Improvement in analysis of dissolved organic radiocarbon in seawater: Evaluation method of accuracy
Dr Shigeyoshi Otosaka1, Mr Hyunduck Jeon1, Mr Yunxuan Hou1, Prof Yusuke Yokoyama1, Yosuke Miyairi1, Mr Takahiro Aze1, Dr Takahiro Watanabe2, Prof Hiroshi Ogawa1
1The University of Tokyo, Kashiwa, Japan, 2Japan Atomic Energy Agency, Toki, Japan
Accurate ¹⁴C analysis in oceanic dissolved organic carbon (DO¹⁴C) in seawater gives us useful information on the timescale of biogeochemical cycle of dissolved organic matter (DOM) in the ocean. However, due to the difficulty of analyzing DO¹⁴C, an easier-to-use and safer analysis system has been required. We have established a method for DO¹⁴C analysis combining decomposition of DOM with low pressure mercury lamp and accelerator mass spectrometry. This method greatly reduces the heat generation during the UV irradiation, and it enables us preprocess seawater samples more easily and safely than the previous method. In addition, with the sample volume ranging between 0.5 and 1 L, it is possible to decompose DOC with high efficiency (> 95%) without adjusting the sample volume. Since dilution is unnecessary, it also contributes to reduction of blanking due to dilution process. In order to evaluate the performance of such an analytical system, it is necessary to prepare an artificial DOM sample in which a certified reference material having a known ¹⁴C isotopic ratio is dissolved in pure water and analyze it in the same manner as the actual sample. We also prepared the artificial samples using IAEA-C7 and -C8 (oxalic acid) reference materials dissolved in pure water and confirmed that the 14C isotope ratio analyzed by this method agrees with the recommended value. However, when the experiment was carried out by dissolving the same reference materials in a solution containing chloride (at similar salinity as seawater), a slightly lower ¹⁴C isotopic ratio was obtained as compared with the experiment using pure water. We considered that plastic/rubber parts in the system were decomposed by chlorine generated by UV oxidation of chloride in sample, and contamination by a small amount of petroleum derived carbon reduced ¹⁴C isotopic ratio. Although it is needed to reduce the number of plastic parts in the system and to suppress the production of such “background carbon” by using a durable material, it is difficult to eliminate them completely. We thus propose to guarantee the accuracy of the ¹⁴C analysis data by conducting DO¹⁴C measurement of a standard sample aqueous solution containing salt, monitoring the status of the DO¹⁴C analysis system, and correcting the results as necessary. The method to evaluate the reliability of DO¹⁴C is feasible only with solid reference material which can be stored at room temperature and DOM free artificial seawater which is easily available. Therefore, this method is promising as a standard method for accelerating oceanic DO¹⁴C research.
Shigeyoshi Otosaka is an associate professor at the Atmosphere and Ocean Research Institute of the University of Tokyo and a chemical oceanographer. He received his Ph D degree from Hokkaido University, Japan, in 2004, and has been conducting research on the distribution and dynamics of radionuclides and trace elements in the ocean at the Japan Atomic Energy Agency and has been in his current position since 2018. He also studies the behavior of 14C and 129I in the marine environment based on analysis of these radionuclides using AMS.