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 − 10.9..., , # Inter-method differences (SIMS vs. IRMS) in oxygen isotope fractionation: Insights from Chinook salmon otoliths

DOI: 10.5061/dryad.3n5tb2rvw

This README was generated on 2025-11-14 by K. Arai.

This README is in reference to the data sets and R codes (.Rmd) needed to recreate the figures and analyses in *“Inter-method differences (SIMS vs. IRMS) in oxygen isotope fractionation: insights from Chinook salmon otoliths”*, submitted by K. Arai.

There are five major analyses within the manuscript. The first script `01_tank_2025-09-10.Rmd` summarizes the water temperature and stable oxygen isotopes (water δ¹⁸O) measured in the temperature treatment experiment for juvenile Chinook salmon. The second script `02_sims_fractionation_2025-09-10.Rmd ` fits a linear regression to Secondary Ion Mass Spectrometry (SIMS)-measured otolith stable oxygen isotopes for juvenile Chinook salmon to calibrate a temperature-dependent fractionation equation. The third script `03_irms_2025-09-10.Rmd` compar...,