Quantifying seawater exchange rates in the Eocene Arctic Basin using osmium isotopes

The closure of seaways that connected the Arctic Ocean to the global ocean during the early Paleogene led to severe hydrographic restriction. We present new osmium isotope datafrom organic-rich sediments deposited in the central ArcticOcean during the Early – Middle Eocene. The new data show that the long term isotopic composition of osmium in Arctic seawater began to diverge from that of the global ocean at ∼ 54 Ma, after the Eocene Thermal Maximum 2 hyperthermal event. This divergence was prob-ablycausedbythegradualclosure of seaways connectingtheArcticOceantothe global ocean. The Os data are used to calculate water exchange rates between the Arctic and surrounding oceans and to calculate Arctic Ocean salinity during the Early Eocene. The results show that the development of severe, long term Arctic Basin restriction after ∼ 54 Ma occurred as open ocean seawater input decreased below ∼ 0.01 Sv, resulting in a mean basin salinity between 8 – 16 PSU, depending on model assumptions.