The Arctic Ocean plays a pivotal role in global climate, yet its circulation under greenhouse conditions remains poorly constrained. Seismic, sedimentological, and drilling evidence from the Chukchi Shelf reveals large contourite drifts and bathyal carbonate mounds dated to the Campanian to Maastrichtian [~80 to 66 million years ago (Ma)], indicating persistent bottom currents and high productivity. These features point to the presence of a regionally sourced Boreal deep-water mass, likely driven by seasonal sea-ice formation and brine rejection. Bathyal carbonate mound development was sustained by tidal mixing, localized upwelling, and well-oxygenated conditions following Oceanic Anoxic Event 3. This vigorous circulation system collapsed at the Cretaceous-to-Paleogene boundary, coinciding with closure of the Western Interior Seaway and reorganization of global circulation. Our findings challenge the prevailing view of a sluggish, surface-dominated Late Cretaceous Arctic, demonstrating instead that it supported deep-water formation and localized carbonate factories, highlighting its key role in high-latitude climate dynamics under greenhouse conditions.
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