Varying organic content in fish otoliths: Effects on SIMS-based δ18O measurements and possible corrections

Abstract

Varying organic contents in otoliths have complex and sometimes counterintuitive effects on intra-otolith δ¹⁸O measurements. This is often addressed by roasting otoliths or applying fixed corrections, however, the underlying chemical and physiological mechanisms involved are poorly understood and have not been tested in a quantitative manner, potentially rendering such corrections unreliable. Using high-resolution secondary ion mass spectrometry (SIMS) measurements of δ¹⁸O values paired with OH/¹⁶O ion ratios as organic proxy, we derived quantitative relationships of these measurements over the entire life of a large sample of northern pike (Esox lucius) otoliths from freshwater and brackish habitats. We assessed OH/¹⁶O ion count ratio as an organic tracer, and estimated its relationship with δ¹⁸O determinations. We developed a pointwise correction approach that accounted for variations in otolith organic contents. OH/¹⁶O ion ratio profiles agreed with other organic proxies, confirming them as reliable tracer of organic content. We detected an inverse relationship between δ¹⁸O values and OH/¹⁶O ion ratio, with elevated OH/¹⁶O ion ratios near otolith cores. OH/¹⁶O ratios decreased with distance to the core. Pairwise corrections for the effect of OH/¹⁶O ratios on δ¹⁸O values resulted in a mean offset between uncorrected and corrected values of 0.52 ‰, suggesting an approximately 2°C bias towards warmer temperature if uncorrected data were to be used for δ¹⁸O thermometry. Simultaneous determination on organic- and inorganic-bound oxygen resulted in a negative offset of δ¹⁸O, which varies with the life history of individual fish. Varying offsets in δ¹⁸O values within individual life histories could be accounted for using our correction. We recommend future SIMS-based δ¹⁸O thermometry studies to estimate the local organic content to assess whether correction is warranted. We further offer more general recommendations on how future studies may assess whether corrections for organics are necessary.