North Atlantic Deep Water formation inhibits high Arctic contamination by continental perfluorooctane sulfonate discharges

Perfluorooctane sulfonate (PFOS) is an aliphatic fluorinated compound with eight carbon atoms that is extremely persistent in the environment and can adversely affect human and ecological health. The stability, low reactivity, and high water solubility of PFOS combined with the North American phaseout in production around the year 2000 make it a potentially useful new tracer for ocean circulation. Here we characterize processes affecting the lifetime and accumulation of PFOS in the North Atlantic Ocean and transport to sensitive Arctic regions by developing a 3-D simulation within the MITgcm. The model captures variability in measurements across biogeographical provinces (R-2 = 0.90, p = 0.01). In 2015, the North Atlantic PFOS reservoir was equivalent to 60% of cumulative inputs from the North American and European continents (1400 Mg). Cumulative inputs to the Arctic accounted for 30% of continental discharges, while the remaining 10% was transported to the tropical Atlantic and other regions. PFOS concentrations declined rapidly after 2002 in the surface mixed layer (half-life: 1-2 years) but are still increasing below 1000 m depth. During peak production years (1980-2000), plumes of PFOS-enriched seawater were transported to the sub-Arctic in energetic surface ocean currents. However, Atlantic Meridional Overturning Circulation (AMOC) and deep ocean transport returned a substantial fraction of this northward transport (20%, 530 Mg) to southern latitudes and reduced cumulative inputs to the Arctic (730 Mg) by 70%. Weakened AMOC due to climate change is thus likely to increase the magnitude of persistent bioaccumulative pollutants entering the Arctic Ocean. Plain Language Summary Perfluorooctane sulfonate (PFOS) is a persistent anthropogenic compound that has been released to the environment in large quantities and adversely affects human and ecological health. Its production was phased out in North America in 2000, but the ocean is the terminal PFOS sink. This study develops a new model for PFOS cycling in the North Atlantic Ocean and transport to sensitive Arctic regions based on best understanding of ocean physics and ecology. Results suggest that in 2015, 60% of cumulative PFOS discharges from North America and Europe were still present in the North Atlantic and 30% had entered the Arctic Ocean. Most of the remaining fraction was transported to the South Atlantic. The lifetime of PFOS in the surface ocean is less than 5 years due to rapid penetration into deeper waters where it persists for decades or longer before being transported to other basins. During peak production years, most continental PFOS pollution was transported to the sub-Arctic but large-scale overturning of the North Atlantic Ocean reduced the total burden entering the high Arctic by 70%. Climate-driven changes in ocean circulation that are weakening this process may thus increase the vulnerability of the Arctic to continental pollution.