Meromictic lakes in the Arctic Archipelago were analyzed to determine whether the isotope content of the water molecule might confirm their oceanic origin. The 25 m bottom layer of Lake Garrow, on Little Cornwallis Island, is filled with a homogeneous sodium chloride solution with 2.6 times the chloride concentration of seawater (52 g L−1). Its δ18O value is around −10‰ with respect to V-SMOW, and its δ2H is eight times the oxygen value (~ −85‰). In Lake Sophia on Cornwallis Island, the deep stratum (25 m thick) is less concentrated (35 g L−1 Cl−1) and shows slightly higher δ18O and δ2H values (−9 and −75‰, respectively).Chemocline and surficial waters result from mixing between bottom waters and meteoric waters. The chemical and isotopic signals are interpreted as evidence of diagenesis through subpermafrost or intrapermafrost freezing of aquifers once filled with seawater, at some time during postglacial emergence.