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

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.