Was the Arctic Ocean ice free during the latest Cretaceous? The role of CO2 and gateway configurations

The Arctic region is thought to play a key role in unraveling Mesozoic climate evolution. However, Late Cretaceous climate reconstructions in the high latitudes suffer from contradicting paleoclimatic interpretations. Toward the end of the Cretaceous hot-house, atmospheric CO2 concentration declined potentially enabling the formation of sea-ice in the Arctic Ocean. We use a coupled atmosphere-ocean climate model to investigate possible effects of different atmospheric CO2 levels and gateway configurations between the North proto-Atlantic Basin and the Arctic Ocean on the formation of Arctic sea-ice in the latest Cretaceous. Sensitivity tests were run with two atmospheric CO2 levels (840 and 1120 ppm, representing 3 x and 4 x pre-industrial concentrations, respectively) with six paleogeographic configurations. In the experiment with 840 ppm CO2, seasonal Arctic sea-ice is observed in each gateway configuration in December-June, while for 1120 ppm sea-ice in the central Arctic is either limited or absent, depending on gateway configuration. This suggests the existence of a CO2 threshold, estimated between 3 x and 4 x pre-industrial (PI) CO2 levels. For higher atmospheric CO2 levels seaice formation can only occur by the combined effect of cold winds blowing over the Arctic from continental North America during boreal winter and seawater freshening. The latter can be caused by either very limited or an absence of gateway connections between the Arctic and the open ocean. Such a configuration likely developed in the latest Cretaceous, i.e. close to the Cretaceous/Paleogene boundary interval.