Variations of oceanic and foraminiferal oxygen isotopes at the present day and the Last Glacial Maximum: Equilibrium simulations with an oceanic general circulation model

The stable water isotopes H218O and HDO are incorporated as passive tracers into the oceanic general circulation model MPI-OM, and simulations under present-day and last glacial maximum climate conditions are analyzed in detail. In present-day simulation, both δ18O and δD distributions at the ocean surface and deep ocean are generally consistent with available observations on the large scale. The modelled δD-δ18O relations in surface waters slightly deviates from the slope of the global meteoric water line in most basins, and a much steeper slope is detected in Arctic Oceans. The simulated deuterium excess of ocean surface waters shows small variations between 80°S and 55°N, and a strong decrease north of 55°N. The model is also able to capture the quasi-linear relationship between δ18O and salinity S, as well as δD and S, as seen in observational data. Both in the model results and observations, the surface δ-S relations show a steeper slope in extra-tropical regions than in tropical regions, which indicates relatively more addition of isotopically depleted water at high latitudes. Simulated oceanic isotope distributions at the last glacial maximum (21000 years ago) show features similar to the preindustrial in most basins but the northern North Atlantic. With the exception of the ice sheet impact, the oxygen-18 content variations in the last glacial maximum at sea surface are mainly controlled by the changes in boundary isotopic fluxes in most regions, while the changes from subsurface to bottom waters are mostly due to the differences in the water mass circulations. The changes in topography at the northern high latitudes have remarkable influence on the isotopic composition in the Arctic Ocean. Simulated LGM surface water δ-S relations are similar to present day in extra-tropical regions. The present-day and the last glacial maximum oxygen isotope compositions of calcite in the surface water and their difference are also calculated using the paleo-temperature equation. These results are compared with the observed values from different foraminifer species, and are in agreement with the observations in most regions. Additionally, the influences of seawater carbonate chemistry and life processes of foraminifera are also included in the simulations of the calcite oxygen isotope compositions. These results suggest that sea surface temperature and seawater oxygen isotope composition are the most important controls on the oxygen isotope partitioning in foraminifera. The variations in the seawater carbonate chemistry may contribute to around 10-20% of the oxygen isotope changes in foraminiferal shell. The understanding of the ‘vital effect’ impacts still needs inclusion of further knowledge about the life processes of foraminifera.