Abstract
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 δ18Oc 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.
Highlights
Stable water isotopes (H126O, H128O and HD16O = HDO) are widely used tracers of the hydrological cycle (Craig and Gordon, 1965; Gat and Gonfiantini, 1981) and can be used to determine the origin and mixing pattern of different water masses (e.g., Jacobs et al, 1985; Khatiwala et al, 1999; Meredith et al, 1999)
Due to differences in their physical and chemical properties, stable water isotopes undergo fractionation processes at any phase transition within the hydrological cycle (Craig and Gordon, 1965). This leads to distinctive isotopic signatures for different freshwater fluxes, which are commonly expressed as δi (i with reference to the Vienna Standard Mean Ocean Water (VSMOW) standard and given as where R is the ratio of the abundance of the heavier water isotope H128O or HDO to the abundance of the lighter isotope H126O and RVSMOW = 2005.2 × 10−6 for δ18O (Baertschi, 1976) and 155.95 × 10−6 for δD
Stable water isotopes have been used as an important proxy in a wide range of climate archives, e.g., in polar ice cores which provide past temperature records reflecting climatic changes over the past glacial–interglacial cycles (e.g., Dansgaard et al, 1969; Epstein et al, 1970; Johnsen et al, 1972, 2001) as well as speleothems which reveal intensity changes and variations in the amount of monsoonal rainfall (e.g., Wang et al, 2001; Fleitmann et al, 2003)
Summary
Due to differences in their physical and chemical properties, stable water isotopes undergo fractionation processes at any phase transition within the hydrological cycle (Craig and Gordon, 1965). This leads to distinctive isotopic signatures for different freshwater fluxes, which are commonly expressed as δi (i = O or D). Δ18Oc records from sediment cores provide information on water mass changes
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