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() Microchemistry and composition of fish otoliths.

Melancon, S.1, Fryer, B.1, Gagnon, J.1, 1 University of Windsor, Great Lakes Institute for Environmental Research (GLIER), Windsor, Ontario, Canada

ABSTRACT- Otoliths are the ear stones of fish and are mainly composed of calcium carbonate (CaCO3). Their continual growth is recognizable as concentric rings of alternating opaque and translucent zones. Otoliths are considered not subject to resorption, and as such, only ontogenic and environmental factors will result in changes to their composition. It is believed that once an element has entered the otolith it remains chemically and isotopically inert. All these factors make otoliths unique recorders of the environment and fish migrations. It is important to find elements that can serve as markers because it allows us to determine migration pathways and reconstruct the historical temperature and chemistry variations of the environment. Our project focuses on understanding mechanisms of metal uptake in otoliths and improving the laser ablation analytical technique for high spatial resolution (5-10 m) microchemical studies. We utilize the structural forms of calcium carbonate (aragonite and vaterite) and partition coefficients of heavy metals in each polymorph in lake trout otoliths. Raman spectroscopy is used to determine the structural forms and spatial distributions of CaCO3 and Laser Ablation combined with Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) to evaluate their impact on metal uptake. Initial results show that vaterite has lower levels of larger metal ions, like strontium (Sr) and barium (Ba) than aragonite. These results may be explained by the crystal structure of vaterite and its reduced ability to incorporate larger cations without creating instability because of serious crystal distortion (Casanova et al., 2004), compared to aragonite. We also noticed a similar trend for other metals as there is more zinc (Zn) and manganese (Mn) (smaller cations) than lead (Pb) (larger cation) included in vaterite. We also discovered that aragonite and vaterite grow simultaneously in separate growth rings that are divided by a boundary zone (20-30 m) composed of a mixture of aragonite and vaterite. To the best of our knowledge, this relationship has never been recognized before in the otoliths of fish. We plan to focus our laser ablation investigations on compositions of vaterite and aragonite chemistry growing at the same time (i.e. same growth ring) as it represents the same environment.

Key words: otolith, LA-ICP-MS, vaterite, trace-elements

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