Altervalent substitution of sodium for calcium in biogenic calcite and aragonite

Toshihiro Yoshimura, Yusuke Tamenori, Atsushi Suzuki, Hodaka Kawahata, Nozomu Iwasaki, Hiroshi Hasegawa, Luan T. Nguyen, Azumi Kuroyanagi, Toshitsugu Yamazaki, Junichiro Kuroda, Naohiko Ohkouchi

Research output: Contribution to journalArticlepeer-review

23 Citations (Scopus)

Abstract

Sodium concentrations in biogenic CaCO3 are several thousands of parts per million, and, on a molar basis, Na is among the most abundant constituent minor element in these carbonates. Nevertheless, the chemical form of Na in CaCO3 is not well constrained. We used synchrotron X-ray spectroscopy to identify the dominant molecular host sites for Na in biogenic calcite and aragonite precipitated by corals, bivalves, and foraminifera. We also used the K-edge X-ray absorption near-edge structure to investigate the chemical environment of Na in biogenic calcium carbonates and identify the altervalent substitution of Na into Ca sites in the lattice structures of calcite and aragonite. Minor cation and anion concentrations in biogenic CaCO3 suggest that the principal substitution mechanism involves charge compensation through the creation of CO32− vacancies. The mostly homogeneous Na concentrations in the skeletal microstructures of the various biota we examined indicate that environmental and biological controls, such as temperature, skeletal microstructure, and calcification rates, have only minor influences on skeletal Na concentrations. A decrease of Na:Ca ratios with increasing age of foraminiferal shells picked from a Quaternary sediment core, indicates progressive release of Na, which suggests that structurally-substituted Na in biogenic CaCO3 is readily leached during burial diagenesis. Whereas the sediment that undergo diagenesis release some Na back to the water column, sodium co-precipitation in biogenic CaCO3 serves as a potential sink of Na for the ocean.

Original languageEnglish
Pages (from-to)21-38
Number of pages18
JournalGeochimica et Cosmochimica Acta
Volume202
DOIs
Publication statusPublished - 2017 Apr 1

Keywords

  • Aragonite
  • Calcite
  • Sodium
  • Substitution
  • XANES

ASJC Scopus subject areas

  • Geochemistry and Petrology

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