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Mg structural state in coral aragonite and implications for the paleoenvironmental proxy

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Abstract

Thermodynamic calculations and inorganic precipitation experiments indicate a relationship between aragonite Mg/Ca and water temperature. This offers a route to reconstruct seawater temperatures from fossil corals. Fundamental to this is the assumption that Mg2+ exchanges for Ca2+ within carbonate. We present X-ray Absorption Fine Structure (XAFS) data to indicate the structural state of Mg in modern Porites coral skeletons. We show Mg is not in aragonite, but hosted by a disordered Mg-bearing material. Mg may be predominantly hosted in organic materials or as a highly disordered inorganic phase, e. g., a nanoparticulate form of Mg carbonate or hydroxide. Reported correlations between seawater temperature and coral Mg/Ca are unlikely to be consistent between corals and hence analysis of Mg/Ca in fossils is unlikely to produce accurate climate reconstructions. We anticipate XAFS will be applied widely to environmental proxies and become an important tool in identifying those that reconstruct accurate climates.

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Original languageEnglish
Article numberL08704
Number of pages5
JournalGeophysical Research Letters
Volume35
Issue number8
DOIs
StatePublished - 19 Apr 2008

    Research areas

  • Amorphous Calcium-carbonate, X-ray, Organic matrix, Skeletons, Porites, Biomineralization, Temperature, Diagenesis, Records, Growth

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