Strontium in coral aragonite: 2. Sr coordination and the long-term stability of coral environmental records

Abstract

We have used X-ray Diffraction (XRD) and Sr K-edge Extended X-ray Absorption Fine Structure (EXAFS) to determine the structural state of Sr in a suite of coral aragonite samples. Our samples encompassed a selection of coral species ( Porites lobata, Porites lutea, Pocillopora eydouxi, Montastrea annularis, Pavona gigantea and Pavona clavus) including some commonly used for palaeoenvironmental reconstruction. Aragonite was the only carbonate observed by XRD. We refined the isolated EXAFS against structural models for Sr in aragonite and two-phase strontianite/aragonite mixes. Our data are indistinguishable from Sr ideally substituted in aragonite and strontianite was present below detection levels (estimated at <5% of Sr present). Comparisons of recent and ancient coral aragonite show no sign of exsolution, either by spinodal decomposition or by the direct nucleation of strontianite domains. Published diffusion rates of Sr in ionic solids support the view that exsolution would occur prohibitively slowly. Coral aragonites are metastable materials with slow diffusion kinetics that have the potential to encode environments over timescales of millions of years.