Iodine 129/CFC 11 transit times for Denmark Strait Overflow Water in the Labrador and Irminger Seas

Abstract

Iodine 129 discharged from nuclear fuel reprocessing plants in France and the United Kingdom is transported into the Nordic Seas on timescales of 3–5 years. Tracer 129I is subsequently injected into intermediate waters that overflow the sills between Greenland, Iceland, and Scotland and ventilate the North Atlantic Deep Waters (NADW). During the early 1990s, discharges of 129I increased by 600%, resulting in a large, well‐resolved tracer “front” whose passage through the Nordic Seas is presently being observed by a time series of 129I measurements on the WOCE (AR7W) section in the Labrador Sea. The highest 129I levels were measured below depths of 3000 m in Denmark Strait Overflow Water (DSOW). These levels increased by about 300% between 1997 and 2001 to values >40 × 107 atoms/L, indicating that the leading edge of the tracer “front” from the early 1990s was being observed entering the NADW. Using a simple mixing/advection model, 129I and CFC 11 results were used to calculate transit times of 0.4–2.6 years for the flow of DSOW from formation regions in the Nordic Seas to the Labrador Sea. Measurements on samples collected in the Irminger Sea in 2001 gave transit times of 0.3 years for core DSOW immediately south of Denmark Strait and about 2 years for locations off the southern tip of Greenland. An increase in DSOW transit times observed in the Labrador Sea between 1999 and 2001 probably reflects a more general weakening in the subpolar gyre of the North Atlantic during the 1990s.