A basin-wide Arctic ice shelf reverses deep ocean circulation

Geological evidence suggests that a kilometre-thick ice shelf may have covered the Arctic Mediterranean (AM)—comprising the Arctic Ocean and Nordic Seas—during one or more glacial periods. However, the broader climatic and oceanographic implications of such a scenario remain poorly understood. Here, for the first time, we use a comprehensive Earth system model that explicitly resolves sub-ice-shelf circulation to explore the consequences of a fully glaciated AM. Our simulations reveal a fundamentally altered glacial ocean state: the expansive ice shelf reverses the overturning circulation within the AM—from dense-water sinking to deep-water upwelling—and suppresses North Atlantic Deep Water formation through sustained meltwater discharge from the AM. The persistent connection between the AM and the North Atlantic is maintained by a continuous Atlantic inflow through the Faroe–Shetland Channel, which prevents the AM from freshening. Hence, our results do not support the previously proposed hypothesis that freshwater could have accumulated beneath such a gigantic ice shelf, underscoring the critical role of ice shelf–ocean interactions in shaping glacial deep-water circulation.