The Cenozoic history (0 to 65 million years, Myr, ago) of the Arctic Ocean is largely unknown from direct evidence. Here we present the first Cenozoic palaeoceanographic record constructed from > 400 m of core from a recent drilling expedition to the Lomonosov Ridge. Our record shows a palaeoenvironmental transition from a warm greenhouse world, of the latest Palaeocene and early Eocene, to an icehouse world influenced by sea ice and icebergs from the middle Eocene to the present. We show: 1-2 cm per thousand years (ka) sedimentation rates occurred from the middle Miocene - Holocene (~18-0 Myr) in stark contrast to rates proposed in earlier studies; the first occurrence of ice-rafted debris (IRD) is in the middle Eocene (~45 Myr), ~35 Myr earlier than previously thought; prior to the IRD, fresh surface waters were present at ~49 Myr; and warm surface waters, ~10° warmer than previous estimates, occurred during the Palaeocene Eocene Thermal Maximum (PETM). The revised timing of Arctic cooling events coincides with those from Antarctica, supporting arguments for bi-polar symmetry in climate change.