2014/01/30 第109回 - 高田 裕行 博士

 第109回の懇談会は、釜山大学・海洋学科の 高田 裕行 博士の話題提供で行うことになりました。皆様お誘い合わせの上、ご参加下さいますようお願い申し上げます。

 


109回汽水域懇談会

 

島根県大橋川橋脚に産する有孔虫殻の酸素安定同位体比について

Isotope signatures of benthic foraminifera on hard substrates in the Ohashi River, southwest Japan

 

高田 裕行 博士(釜山大学・海洋学科)

Dr. Hiroyuki Takata (Marine Research Institute, Pusan National University)

 

日時:2014年1月30日(木曜日) 17:00〜18:00

場所:島根大学汽水域研究センター2階 セミナー室(201)

 


 

【発表の概要】

  本発表では,島根県大橋川の橋脚(松江大橋・中海大橋)に付着して生息する有孔虫殻の酸素安定同位体比についての概要を,報告する.貝類などと同様に,有 孔虫も炭酸カルシウムの殻を形成し,その炭酸カルシウム殻の酸素・炭素安定同位体比は,地質時代の古環境復元に,有用なツールとなっている.本研究では, 現生有孔虫に関して,殻の酸素安定同位体比を検討し,宍道湖−中海水系における(近)過去環境変動へ適用するあたって,それがどのような長所・短所を持ち うるかを論じる.(本研究は,David Dettman 博士,瀬戸浩二 先生,倉田健悟 先生,平塚純一 博士,Boo-Keun Khim 先生との共同で,実施したものです.)

 


 

Isotope signatures of benthic foraminifera on hard substrates in the Ohashi River, southwest Japan

 

Hiroyuki Takata 1, David L. Dettman 2, Koji Seto 3, Kengo Kurata 3, Jun’ichi Hiratsuka 4 and Boo-Keun Khim 1

1. Pusan National University, Korea; 2. University of Arizona, USA; 3. Shimane University, Japan, 4. Shimane Research Group of Wildlife, Japan

 

Stable oxygen and carbon isotope ratios of calcareous hard tissue of aquatic biota are an excellent tool for understanding modern and past aquatic environments. In the Lake Shinji–Nakaumi system, Sampei et al. (2005) investigated stable oxygen and carbon isotope ratios of the water and molluscan shells. They revealed the potential of stable oxygen and carbon isotope data in the reconstruction paleo-salinity using fossil molluscan shells. In this study, we focused on isotope signatures of Ammonia “beccarii” forma 1 on the hard substrates. Because we focus on calcareous hard tissues of a single species, we predict that the possible problem of inter-species difference will be reduced. Our approach may lead us to a better understanding of the detailed relationship between isotope signatures of calcareous hard tissues and the ambient waters. We investigated stable oxygen and carbon isotope ratios of A. “beccarii” forma 1 on the hard substrates in the Ohashi River, southwestern Japan, in order to learn the isotope signatures of carbonate shells of benthic foraminifera in the Lake Shinji–Nakaumi system.

 

The Ohashi River is located between Lake Shinji and Lake Nakaumi, southwestern Japan. The surface layer is typically oligohaline, whereas the bottom layer is characterized by mesohaline and often oxygen-depleted water. We studied benthic foraminifera at one of the two concrete piers supporting the Matsue and Nakaumi Bridges in the upper and lower reaches of the Ohashi River, respectively. Samples of benthic foraminifera from the bridge-piers were collected in October 2007, 2008, 2009 and 2010 by SCUBA diving: 100, 300 and 440 cm water depth at the Matsue Bridge and 100 and 200 cm water depth at the Nakaumi Bridge, respectively. Water samples were also collected at the same depths during the field survey. Stable oxygen and carbon isotope ratios of living (stained) “A. beccarii” individuals with 230–250 µm maximum test diameters were analyzed using an automated carbonate preparation device (KIEL-III) coupled to a gas-ratio mass spectrometer (Finnigan MAT 252) in the Environmental Isotope Laboratory, University of Arizona.

 

There was a generally positive relationship between stable oxygen isotope ratio of benthic foraminifera and salinity of the waters in the Ohashi River, similar to the δ18O–salinity regression lines of the waters or of molluscan shells (Sampei et al., 2005). However, the relationship does not have a linear trend in the low salinity regime (lower than a salinity of 15). This implies that the timing of test formation in bridge pier A. beccarii forma 1 may be biased toward times of higher salinity.

 

We estimated the oxygen isotope ratio of equilibrium calcite (δ18O eq. cal.) using both the average salinity and the average value of the highest 5% of salinity each day during the two months prior to the sampling time. The latter represents the highest daily salinity range at each depth. We compared these δ18O eq. cal. values to that of bridge pier A. “beccarii” forma 1 (δ18O foram). The expected δ18O eq. cal. was more consistent with measured δ18O foram if the average value of the highest 5% of salinity each day is used, rather than that of simple average salinity. Thus, it is highly probable that the oxygen isotope signature of bridge pier A. “beccarii” forma 1 shells in the Lake Shinji–Nakaumi system can be explained by the average value of the highest 5% of salinity each day, especially during times of lower salinity. Although the biological significance of the average value of the highest 5% of salinity each day needs to be investigated, bridge pier A. “beccarii” forma 1 probably biases production of calcareous hard tissues toward higher salinities, whereas it seemed to cease calcite precipitation under in low salinities. This suggests that the stable oxygen isotope signature of A. “beccarii” forma 1 may be useful for reconstructing paleo-salinity even under frequent salinity variations, when combined with the isotope signature of hard tissues of other organisms (e.g. molluscan shells).