Open AccessMagnesium Isotopic Composition of Tests of Large Benthic Foraminifers: Implications for Biomineralization
Author(s) -
Maeda Ayumi,
Yoshimura Toshihiro,
Araoka Daisuke,
Suzuki Atsushi,
Tamenori Yusuke,
Fujita Kazuhiko,
Toyofuku Takashi,
Ohkouchi Naohiko,
Kawahata Hodaka
Publication year - 2019
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2019gc008314
Subject(s) - calcite , benthic zone , geology , isotope fractionation , seawater , fractionation , environmental chemistry , isotope , stable isotope ratio , carbonate , biomineralization , isotopes of magnesium , mineralogy , chemistry , oceanography , paleontology , chromatography , physics , organic chemistry , quantum mechanics
The Mg isotopic composition in biogenic CaCO 3 is expected to reflect oceanic Mg cycles, yet its practical application for paleoenvironmental reconstruction is complicated by the influences of seawater temperature and biologically mediated isotope fractionation. In this study, we investigated the relationship between water temperature, growth rate, and the Mg isotope ratio (δ 26 Mg) of the biogenic high‐Mg calcite that forms the tests of large benthic foraminifers. We cultured asexually reproduced large benthic foraminifers ( Amphisorus kudakajimensis and Calcarina gaudichaudii ) at 21–30 °C. We measured Mg/Ca ratios, chemical Mg speciation, and δ 26 Mg for both species. Mg/Ca ratios showed significant temperature dependence for both species, while the δ 26 Mg values for different populations of C . gaudichaudii exhibited increasing discrimination of heavy isotope with increasing temperature. X‐ray absorption of near‐edge structure spectra of the foraminiferal tests showed Mg speciation to be consistent with that of inorganic calcite, indicating ideal substitution of Mg for Ca in the crystal lattice. The δ 26 Mg values for both species were close to those of inorganic calcite and other marine organisms that precipitate high‐Mg calcite. These results suggest that precipitation of high‐Mg calcite tests for both species is similar to that of inorganic calcite and that Mg isotope fractionation for both species is only slightly influenced by individual physiological processes related to growth temperature. The ∆ 26 Mg carb‐sol values of multiple field samples fell within the ∆ 26 Mg carb‐sol range for inorganic calcite, providing preliminary evidence that the effects of intraspecific differences in δ 26 Mg values can be averaged out by careful choice of samples for analysis.