TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 46 (No. 2), pp. 77-87, 2012

Carbon- and oxygen-isotope compositions of a modern deep-water brachiopod Campagea japonica collected off Aguni-jima, Central Ryukyu Islands, southwestern Japan

Hideko Takayanagi,1 Ryuji Asami,2 Osamu Abe,3 Hiroyuki Kitagawa,3 Toshihiro Miyajima4 and Yasufumi Iryu1

1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Chikusa, Nagoya 464-8601, Japan
2Trans-disciplinary Research Organization, Subtropical Island Studies (TRO-SIS), University of the Ryukyus (c/o Department of Physics and Earth Sciences, University of the Ryukyus), 1 Senbaru, Nishihara, Okinawa 903-0213, Japan
3Hydrospheric-Atmospheric Science, Graduate School of Environmental Studies, Nagoya University, Chikusa, Nagoya 464-8601, Japan
4Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa 277-8564, Japan

(Received August 25, 2011; Accepted November 30, 2011)

Abstract: We present carbon- and oxygen-isotope compositions in a shell (δ13CS and δ18OS, respectively) of a modern brachiopod Campagea japonica collected at a water depth of 606 m off Aguni-jima, Central Ryukyu Islands, southwestern Japan. The δ13CS and δ18OS profiles of the outermost portions of the secondary shell layer along the axis of shell growth do not display significant seasonal variations. The δ13CS values display a gradual decrease with growth, whereas the δ18OS values are nearly constant. The δ13CS and δ18OS values along a particular growth line and from the inner shell surface are rather constant. The δ13CS values near the posterior shell edge of the ontogenetic series and the δ18OS values near the anterior shell edge of the inner series fall within the ranges of δ13C and δ18O values of calcite in equilibrium with ambient seawater (equilibrium calcite) (δ13CEC and δ18OEC, respectively). Therefore, they are the best portions to use as proxies of δ13C values of dissolved inorganic carbon (δ13CDIC) and temperature and/or δ18O values of seawater (δ18OSW), respectively. The δ13CS values of the ontogenetic series near the anterior shell edge and the entire isochronous and inner series display the smallest decrease (less than ~0.2‰) compared with the δ13CEC values. The δ18OS values of the entire ontogenetic series and an interior part of the inner series are slightly greater (less than ~0.3‰) than the δ18OEC values. Thus, these portions can be used as paleoenvironmental proxieif appropriate corrections are made. A significant positive correlation is recognized between the δ13CS and δ18OS values for the ontogenetic-series samples, which can be ascribed to a kinetic isotope effect. A metabolic isotope effect is not evident for the δ18OS values in these samples. Our study provides useful information for the appropriate selection of brachiopod taxa and shell portions that reflect isotopic composition of ambient seawater for paleoenvironmental studies.
Key words: brachiopoda, carbon-isotope composition, oxygen-isotope composition, Campagea japonica, paleoenvironmental proxy

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