Strontium speciation in archaeological otoliths

Abstract

Fish otoliths (“ear stones”) are major environmental indicators used in ecology and fisheries sciences. Otoliths consist of a biomineral material containing an organically-templated mineral calcium carbonate, normally aragonite, in which strontium is incorporated at trace to minor levels depending on water chemistry and individual physiology. Sr content and fluctuations inform on the life histories of ancient specimens and provide data for palaeoenvironmental reconstructions. Identifying the impact of post-mortem alteration is a critical question to assure the reliability of such work. A central parameter for the reliability of Sr content as a palaeoenvironmental proxy is whether the mode of incorporation can be considered as stable and homogenous at the microscale in otoliths over thousands of years. In addition, it is important to know whether a different kind of speciation of Sr is observed, especially at the outer surface of the sample in contact with the soil and local environment. Here, a novel combination of synchrotron microscale point analyses and raster-scanning X-ray absorption spectroscopy is implemented and used for the first time to study otoliths at different length scales, spanning from millimetres down to micrometres. Strontium is found in substitution for calcium in aragonite in all our analyses of five Holocene otoliths and their three modern counterparts; the first set of samples from the Peruvian coast, up to 11 000 years old, are studied for their potential as palaeoenvironmental proxies. The chemical environment of strontium in otoliths is independent of content of this element, location in the otolith, species, and archaeological age. This is shown with a high lateral resolution (about 10 μm) over wide fields of view, as a way to consolidate macro-scale approaches. To our best knowledge, this work is the first report of the chemical environment of strontium in ancient otoliths. Our work opens the way to new approaches to validate palaeoenvironmental studies of biocarbonate paleoproxies.