The history of the Arctic Ocean remained poorly known until the 2004 IODP coring of Lomonosov Ridge sediments. Early studies of the recovered sequence demonstrated the existence of an Eocene ‘lake-stage’ prior to the transition to marine conditions. The marine stage onset was inferred to be ∼17.5 million years -Ma- ago, thus implying a nearly 26 Ma gap between the lacustrine and marine episodes, and an unusual tectonic history for Lomonosov Ridge, in order to explain this gap. More recently, Rhenium-Osmium (Re-Os) isotope measurements of the transition from the lacustrine to marine sediments suggested a much earlier inception of marine conditions and the absence of any significant gap between both episodes. Here, an improved Osmium isotope stratigraphy and Re-Os data concur to assign a Late Eocene age (∼36 Ma) to the marine invasion, consistent with a relative change in sea level on top of Lomonosov ridge, either from tectonic origin or from another cause.