| 21 Jan 2026
Including the triple isotopic composition of dissolved oxygen in the ocean into the iLOVECLIM model (version 1.1.7): development and evaluation
Abstract. Contributing around half of the oxygen produced on Earth, marine photosynthetic production is one of the main mechanisms for carbon fixation, with a central role in the oxygen cycle. The triple isotopic composition of atmospheric oxygen (17Δ), measured in ice cores, provides a global integrator of past biospheric oxygen fluxes, and by extension carbon fluxes. However, deconvolving the signal of 17Δ requires to isolate the oceanic biosphere productivity (17Δocean). Here, we present the first implementation of 17Δocean in the intermediate-complexity climate model iLOVECLIM. The three main processes controlling 17Δocean, i.e. photosynthesis, respiration, and air-sea gas exchange, are explicitly represented and evaluated under preindustrial conditions. Model results show overall good agreement with available measurements, particularly in the Pacific Ocean. In contrast, systematic overestimation is found in the Southern Ocean. At fixed stations, seasonality is reproduced but with underestimated amplitude. These discrepancies mainly reflect challenges in representing remineralization and oxygen minimum zones, and highlight opportunities to refine the representation of primary productivity and vertical mixing. Overall, this new implementation provides the first coupled model framework for simulating 17Δocean, both as a diagnostic of biogeochemical processes and as a tool for reconstructing past changes in marine productivity. Extending the implementation to the terrestrial biosphere will further allow reconstruction of the past global biosphere and direct comparison with 17Δ records from ice cores.
Competing interests: At least one of the (co-)authors (DMR) serves as topic editor for the special issue to which this paper belongs.
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The 17ΔO2 preserved in ice cores is a key proxy for reconstructing past global biosphere productivity, yet interpreting this signal requires a robust understanding of both the oceanic and terrestrial end-members. Previous studies have incorporated oxygen isotopes into the ocean biogeochemical component of CESM, but a fully coupled Earth system modeling framework has remained unavailable. This study addresses this gap for the oceanic component by implementing the triple oxygen isotope composition of dissolved oxygen (17Δocean) into the intermediate-complexity Earth System Model iLOVECLIM.
The authors incorporate the three principal processes controlling the isotopic composition of dissolved O2—photosynthesis, respiration, and air–sea gas exchange—and evaluate model performance against a compilation of 2482 observational measurements under pre-industrial boundary conditions. The manuscript is generally well structured, the methods are described in sufficient detail, and the model–data comparison is comprehensive. I believe this work represents a valuable contribution and is suitable for publication in Geoscientific Model Development after the authors address the comments below.
Major comments
Minor comments
1, In Eq. (7), should the asterisks on Osurf and Oeq be removed? Please verify the notation.
2, Line 184: “the the model” contains a duplicated word.
3, Table 1 uses inconsistent formats for reporting parameter uncertainty. Some entries are presented as ranges (e.g., [0.982–0.990]), whereas others are reported as mean ± uncertainty (e.g., 0.980 ± 0.003). Consider adopting a consistent format throughout the table.
4, In Table 2, the simulated global GPPO2 value (9.68 × 1015 mol O2 yr−1) is compared with estimates from Huang et al. (2021). However, the Huang et al. estimates represent the modern ocean, whereas the model is run under pre-industrial boundary conditions. Although the difference in global productivity between these two states may be modest, this mismatch should be acknowledged. The same caveat applies to the comparison of isotope tracers with modern observational datasets.
References
1, Yeung, L. Y., Young, E. D., & Schauble, E. A. (2012). Measurements of 18O18O and 17O18O in the atmosphere and the role of isotope-exchange reactions. Journal of Geophysical Research, 117(D18).
2, Li, B., Yeung, L. Y., Hu, H., & Ash, J. L. (2019). Kinetic and equilibrium fractionation of O2 isotopologues during air–water gas transfer and implications for tracing oxygen cycling in the ocean. Marine Chemistry, 210, 61–71.