Inter-method differences (SIMS vs. IRMS) in oxygen isotope fractionation: Insights from Chinook salmon otoliths

Stable oxygen isotopes (δ18O) in biogenic carbonates serve as a valuable
proxy for reconstructing thermal history. Fish otoliths (ear stones) are
particularly useful, as they precipitate continuously throughout life,
creating a temporally resolved archive of water temperature. Here, we
calibrate the temperature-dependent oxygen isotope fractionation equation
for Chinook salmon (Oncorhynchus tshawytscha) using two analytical
methods—secondary ion mass spectrometry (SIMS) and isotope ratio mass
spectrometry (IRMS)—to evaluate method-dependent effects on fractionation
equations and their implications for temperature reconstruction. Juvenile
fish were reared for 15 weeks under controlled freshwater conditions
(salinity <0.1 ppt) with a stable ambient water δ18O of -5.54‰
(VSMOW) (±0.10, 1 SD) at three temperatures (11, 16, 20 °C). Otolith δ18O
values measured by SIMS showed a significant linear inverse relationship
with ambient water temperature: 1000lnα=11.51(±1.39,1SE)×103T(K)−1 −
10.94(±4.80,1SE) δ18Ootolith(VPDB) − δ18Owater(VSMOW) =
−0.14(±0.02,1SE)×T(°C)+0.64(±0.27,1SE) Applying this equation yielded
water temperature reconstructions with an accuracy of ± 1.97 °C and a
precision of ± 0.70 °C (1 SD). A paired comparison revealed SIMS δ18O
values were on average 1.97‰ lower than IRMS values, likely due to matrix
effects and organic content. This offset produced large differences in
equation intercepts, leading to reconstructed temperatures from IRMS-based
equations that deviated ~10 °C from observed temperatures when applied to
SIMS data. In contrast, the slopes (thermal sensitivity) of SIMS and IRMS
equations were highly consistent, indicating that relative temperature
changes can still be reliably inferred from SIMS δ¹⁸O values using
IRMS-based equations. Greater variability in SIMS δ¹⁸O values compared to
IRMS may partly reflect fine-scale isotopic heterogeneity within otoliths,
suggesting that SIMS-based temperature reconstructions may require larger
sample sizes or additional calibration tailored to specific contexts.
Across-species comparison of fractionation equations revealed that
inter-method differences exceeded inter-species differences, highlighting
the need for method-matched equations for accurate absolute temperature
reconstructions. Despite these challenges, once calibrated, SIMS-based
otolith thermometry provides a powerful tool for reconstructing fine-scale
fish thermal histories and assessing habitat refugia and resilience to
climate change.