Oxygen stable isotopes during the Last Glacial Maximum climate: perspectives from data–model (iLOVECLIM) comparison

Abstract. We use the fully coupled atmosphere–ocean three-dimensional model of intermediate complexity iLOVECLIM to simulate the climate and oxygen stable isotopic signal during the Last Glacial Maximum (LGM, 21 000 years). By using a model that is able to explicitly simulate the sensor (δ¹⁸O), results can be directly compared with data from climatic archives in the different realms.

Our results indicate that iLOVECLIM reproduces well the main feature of the LGM climate in the atmospheric and oceanic components. The annual mean δ¹⁸O in precipitation shows more depleted values in the northern and southern high latitudes during the LGM. The model reproduces very well the spatial gradient observed in ice core records over the Greenland ice sheet. We observe a general pattern toward more enriched values for continental calcite δ¹⁸O in the model at the LGM, in agreement with speleothem data. This can be explained by both a general atmospheric cooling in the tropical and subtropical regions and a reduction in precipitation as confirmed by reconstruction derived from pollens and plant macrofossils.

Data–model comparison for sea surface temperature indicates that iLOVECLIM is capable to satisfyingly simulate the change in oceanic surface conditions between the LGM and present. Our data–model comparison for calcite δ¹⁸O allows investigating the large discrepancies with respect to glacial temperatures recorded by different microfossil proxies in the North Atlantic region. The results argue for a strong mean annual cooling in the area south of Iceland and Greenland between the LGM and present (> 6 °C), supporting the foraminifera transfer function reconstruction but in disagreement with alkenones and dinocyst reconstructions. The data–model comparison also reveals that large positive calcite δ¹⁸O anomaly in the Southern Ocean may be explained by an important cooling, although the driver of this pattern is unclear. We deduce a large positive δ¹⁸Osw anomaly for the north Indian Ocean that contrasts with a large negative δ¹⁸Osw anomaly in the China Sea between the LGM and the present. This pattern may be linked to changes in the hydrological cycle over these regions.

Our simulation of the deep ocean suggests that changes in δ¹⁸Osw between the LGM and the present are not spatially homogeneous. This is supported by reconstructions derived from pore fluids in deep-sea sediments. The model underestimates the deep ocean cooling thus biasing the comparison with benthic calcite δ¹⁸O data. Nonetheless, our data–model comparison supports a heterogeneous cooling of a few degrees (2–4 °C) in the LGM Ocean.