Abstract. We present the first results of the implementation of stable water isotopes in the Massachusetts Institute of Technology general circulation model (MITgcm). The model is forced with the isotopic content of precipitation and water vapor from an atmospheric general circulation model (NCAR IsoCAM), while the fractionation during evaporation is treated explicitly in the MITgcm. Results of the equilibrium simulation under pre-industrial conditions are compared to observational data and measurements of plankton tow records (the oxygen isotopic composition of planktic foraminiferal calcite). The broad patterns and magnitude of the stable water isotopes in annual mean seawater are well captured in the model, both at the sea surface as well as in the deep ocean. However, the surface water in the Arctic Ocean is not depleted enough, due to the absence of highly depleted precipitation and snowfall. A model-data mismatch is also recognizable in the isotopic composition of the seawater-salinity relationship in midlatitudes that is mainly caused by the coarse grid resolution. Deep-ocean characteristics of the vertical water mass distribution in the Atlantic Ocean closely resemble observational data. The reconstructed δ 18 O c at the sea surface shows a good agreement with measurements. However, the model-data fit is weaker when individual species are considered and deviations are most likely attributable to the habitat depth of the foraminifera. Overall, the newly developed stable water isotope package opens wide prospects for long-term simulations in a paleoclimatic context.

Depth transects of benthic foraminiferal oxygen isotopes from the Atlantic Ocean show that glacial‐interglacial changes are larger at deep (> ~2,000 m) than at intermediate water levels. Our model results suggest that the smaller changes in the upper 1,000 m of the water column are a result of the glacial sea level lowering of about 120 m, leading to warmer temperatures of around 1 °C and hence a smaller glacial‐interglacial stable oxygen isotope difference. In contrast, a shoaling of the water mass boundary to ~2,000‐m water depth between the northern and southern source waters is caused by the expansion of a cold (close to the freezing point) southern source water in the abyssal ocean, increasing the oxygen isotope values of benthic foraminifera from the Last Glacial Maximum in the deep Atlantic. These two effects explain the different amplitudes of glacial‐interglacial stable oxygen isotope differences in the upper and deeper water column of the Atlantic Ocean.

Abstract. We present the first results of the implementation of stable water isotopes in the ocean general circulation model MITgcm. The model is forced with the isotopic content of precipitation and water vapor from an atmospheric general circulation model (NCAR IsoCAM), while the fractionation during evaporation is treated explicitly in the MITgcm. Results of the equilibrium simulation under pre-industrial conditions are compared to observational data and paleoclimate records (the oxygen isotopic composition of planktic foraminiferal calcite). The broad patterns and magnitude of the stable water isotopes in annual mean seawater are well captured in the model, both at the sea surface as well as in the deep ocean. However, the surface water in the Arctic Ocean is not depleted enough, due to the absence of highly depleted precipitation and snow fall and slightly enriched river runoff. This shortcoming is also recognizable in the isotopic composition of the seawater-salinity relationship in mid-latitudes. Deep ocean characteristics of the vertical water mass distribution in the Atlantic Ocean closely resemble observational data. Apart from the systematic offset of the modeled oxygen isotopic composition of planktic foraminiferal calcite towards lower values, the comparison with proxy data shows a good agreement. We summarize that the offset is mainly caused by gametogenic calcification and a matter of choice of the applied paleotemperature equation. Overall, the newly developed stable water isotope package opens wide prospects for long-term simulations in a paleoclimatic context.