doi:10.1016/j.gca.2005.03.052 Facilitation of barium uptake into fish otoliths: Influence of strontium concentration and salinity MELITA C. DE VRIES,BRONWYN M. GILLANDERS,* and TRAVIS S. ELSDON Southern Seas Ecology Laboratories, DP 418, School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA 5005 Australia (Received November 19, 2004; accepted in revised form March 22, 2005) Abstract—To reconstruct patterns of fish migration using otolith chemistry, it is essential to validate the relationship between elements in otoliths and the surrounding water, and in particular, how processes such as competition and facilitation among multiple elements influence otolith chemistry. Using a controlled labora- tory experiment, juvenile black bream (Acanthopagrus butcheri) were reared in both brackish and seawater spiked with different concentrations of Sr and Ba. The addition of Sr to the solution facilitated the uptake of Ba into otoliths of fish reared in brackish water, but not in seawater. Conversely, Ba did not facilitate nor compete with the uptake of Sr in either brackish or seawater. In brackish water, Sr incorporation into otoliths may create crystal defects within the CaCO 3 matrix, enabling greater incorporation of Ba. Ba:Ca partition coefficients (D Ba ) for brackish and seawater were 0.058 and 0.136, respectively, whereas Sr:Ca partition coefficients (D Sr ) for brackish and seawater were 0.463 and 0.287, respectively. The influence of Sr on Ba incorporation in fish otoliths is important to consider when reconstructing migration histories of fish, especially in brackish water environments. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION The concentration of foreign ions incorporated in calcified structures such as fish otoliths (ear stones) can be used to reconstruct migratory histories of fish (see Campana [1999] for a review of otolith chemistry). However, to determine migra- tions of fish using otoliths, we must first quantify the effects of the habitat variables of salinity, ambient elemental concentra- tion, and temperature on otolith chemistry (e.g., Elsdon and Gillanders, 2004). Although several experiments have related salinity and ambient elemental concentration to otolith chem- istry (see Elsdon and Gillanders, 2003a for a review), geo- chemical processes that control element incorporation have not been thoroughly investigated in the field of otolith chemistry. Two processes widely researched within the geochemical lit- erature of carbonate mineral precipitation are competition with or facilitation between elements, which subsequently influence elemental incorporation into the CaCO 3 matrix. The theory of elemental competition and facilitation in car- bonate crystals has been examined for several decades (Ichikuni, 1973; Mucci and Morse, 1983). Specifically, com- petition refers to the presence of one element preventing an- other element from being incorporated into the CaCO 3 matrix. Facilitation refers to the increased incorporation of one element into the CaCO 3 matrix as a direct result of the presence of a second element. Competition between elements occurs in cal- cite, an important crystal form in the investigation of divalent metal incorporation, as reactions at its surfaces can control divalent metal mobility and precipitation (Temmam et al., 2000). Mg has a competitive influence on Mn (Franklin and Morse, 1983), and Na on K (White, 1977). Conversely, it has been reported that Mg (Mucci and Morse, 1983) and Mn (Ichikuni, 1973) can facilitate the incorporation of Sr into calcite, and Sr can facilitate the incorporation of Ba into calcite (Pingitore, 1986). Competition or facilitation between elements such as Sr and Ba in calcified structures of living organisms, such as fish otoliths, has yet to be tested. Elements are thought to be incorporated into otoliths by substitution for Ca ions (Campana, 1999). As Sr and Ba are divalent cations and possess ionic radii larger than Ca, they are more likely candidates to substitute for Ca ions in aragonite (e.g., otoliths) than in calcite (see Eqn. 1). Calcite and aragonite are polymorphs of CaCO 3 (Campana, 1999), differing in their organisation and orientation of carbon- ate molecules and incorporation mechanisms (aragonite incor- porates larger cations than calcite; Falini et al., 1996). Substi- tution occurs within limits following Goldschmidt’s rules, which describe trace element partitioning between phases (Mason and Moore, 1982). Given that both Sr and Ba could substitute for Ca in otoliths, it seems likely that in seawater, Sr would outcompete binding of Ba because of its higher concen- tration (Sr concentration is up to 1000x greater than Ba; Elsdon and Gillanders, 2003b), thereby preventing Ba from being incorporated into the otolith. CaCO 3 + Sr 2+ → SrCO 3 + Ca 2+ (1) The use of partition coefficients (D Me ; see Eqn. 2) can aid in determining whether competition and facilitation of elemental incorporation occurs in otoliths, as they describe elemental discrimination and the relationship between ambient (i.e., rear- ing solution) concentrations and those in the calcified structures (Morse and Bender, 1990). Partition coefficients can be de- scribed by the following equation where Me represents a co- precipitated element; Me:Ca otolith = D Me Me:Ca water (2) Despite their potential to reveal elemental discrimination, par- tition coefficients are rarely reported in studies of elemental * Author to whom correspondence should be addressed (bronwyn.gillanders@adelaide.edu.au). Geochimica et Cosmochimica Acta, Vol. 69, No. 16, pp. 4061– 4072, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 4061