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

Varying organic contents in otoliths have complex and sometimes counterintuitive effects on intra-otolith δ 18 O 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 δ 18 O values paired with OH/ 16 O 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/ 16 O ion count ratio as an organic tracer, and estimated its relationship with δ 18 O determinations. We developed a pointwise correction approach that accounted for variations in otolith organic contents. OH/ 16 O ion ratio profiles agreed with other organic proxies, confirming them as reliable tracer of organic content. We detected an inverse relationship between δ 18 O values and OH/ 16 O ion ratio, with elevated OH/ 16 O ion ratios near otolith cores. OH/ 16 O ratios decreased with distance to the core. Pairwise corrections for the effect of OH/ 16 O ratios on δ 18 O 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 δ 18 O thermometry. Simultaneous determination on organic- and inorganic-bound oxygen resulted in a negative offset of δ 18 O, which varies with the life history of individual fish. Varying offsets in δ 18 O values within individual life histories could be accounted for using our correction. We recommend future SIMS-based δ 18 O 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. • SIMS-measured OH/ 16 O correlates closely with organic-associated elements. • Intraotolith organic content biases SIMS δ 18 O measurements towards lower values. • Organics in otoliths may bias temperature reconstruction in northern pike by 2 °C. • Pairwise correction with OH/ 16 O offers a biologically realistic correction of otolith δ 18 O in SIMS approaches.