A study on evaluation of redox condition of Tokyo Bay using chemical states of sedimentary iron as an indicator by means
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A study on evaluation of redox condition of Tokyo Bay using chemical states of sedimentary iron as an indicator by means of Mössbauer spectroscopy Masashi Komori 1,2
& Katsumi Shozugawa
1
1
& Yun Guan & Motoyuki Matsuo
1
# Springer Nature Switzerland AG 2019
Abstract
We applied 57Fe Mössbauer spectroscopy to sediment cores and tried to build method of estimating historical redox condition of Tokyo Bay in Japan, by using chemical state of iron as an index. As a result of measurement, there are more iron (hydr) oxide and less pyrite in sediment core sampled at Yokohama (near center of Tokyo Bay) than sediment cores sampled at inner part of the bay. This is in good agreement with water quality data and indicates that the amount of dissolved oxygen at different points can be compared by the amount of iron (hydr)oxide or pyrite. We also observed pyrite in most samples and revealed that anoxic condition had widely occurred in Tokyo Bay. In addition, a good negative correlation was found between the pyrite abundance ratio in the sediment and dissolved oxygen in seawater in summer. It was suggested that the abundance ratio of pyrite can be a quantitative indicator for the occurrence of anoxic water masses. Keywords
57Fe Mössbauer spectroscopy . Hypoxia . Tokyo Bay . Sediment core . Pyrite
1 Introduction Hypoxia caused by a decrease in dissolved oxygen (DO) have exploded in the 1960s [1]. In Japan, hypoxia is a big problem particularly in inner bay such as Tokyo Bay. In the definition of hypoxia or oxygen-deficient water mass, it is proposed by Yanagi [2, 3] that the amount of dissolved oxygen in seawater is 2.5 ml L−1 (≒ 3.6 mg L−1) or less and is widely used. Under
Proceedings of the International Conference on Hyperfine Interactions and their Applications (HYPERFINE 2019), Goa, India, 10-15 February 2019 Edited by S. N. Mishra, P. L. Paulose and R. Palit
* Masashi Komori [email protected]
1
Graduate School of Arts and Sciences, The University of Tokyo, Tokyo, Japan
2
Yokohama Environmental Science Research Institute, Yokohama, Japan
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hypoxia, fatal effects are exerted on fish and benthos. Yanagi also proposed the definition of anoxia or anoxic water mass is that the amount of DO in seawater is 0.025 ml L−1 (≒ 0.036 mg L−1) or less. In such anoxic condition, reduction reactions such as nitrate reduction, manganese reduction, iron reduction, and sulfate reduction progress as the redox potential decreases. To understand and solve hypoxia / anoxia, it is important to know when or where those environmental condition occurred. However, water quality data of Tokyo Bay is absolutely lacking in terms of both frequency and number of locations. According to Ishii et al. [4], the hypoxic water mass in Tokyo Bay has appeared since 1956 when the observation started, but it is unclear before that. In addition, there are many places in Tokyo Bay where the depth is much deeper than the surroundings, such as ship routes or dredged trench, and complicated enviro
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