Factors determining δ13C and δ18O fractionation in aragonitic otoliths of marine fish

Fish otoliths are aragonitic accretions located within the inner ear of teleost fish. The acellular nature of otoliths, along with taxon-specific shapes, chronological growth increments, and abundance in the fossil record suggest that the stable isotope chemistry of these structures may be unique recorders of environmental conditions experienced by fish in both modern and ancient water masses. To assess the factors determining δ13C and δ18O fractionation in fish otoliths, we reared Atlantic croaker (Micropogonias undulatus) larvae under controlled environmental conditions. Metabolic effects apparently generated large isotopic disequilibria in the δ13C values of M. undulatus otoliths. We found evidence of a negative regression between δ13C-carbonate-δ13Cwater (δ13C) and temperature: δ13C = −1.78 − 0.18 T°C However, this relationship was aliased to a degree by a positive correlation between δ13C and somatic growth and otolith precipitation rates. Oxygen isotopes were deposited close to equilibrium with the ambient water. The relationship between temperature and the 18O/16O fractionation factor (α) was determined empirically to be: 1000 ln α = 18.56(103T K−1) − 32.54 The fractionation factor was not affected by either otolith precipitation or fish growth rates. Reconstruction of water temperature histories should, therefore, be possible from the δ18O values of M. undulatus otoliths with a precision of 1°C, providing the δ18O of the ambient water can be estimated.