Award: OCE-1132545

GEOTRACES is a multi-investigator, multi-institutional program with the objective of measuring and understanding the distribution of a suite of trace elements and isotopes that greatly impact the major biogeochemical cycles in the ocean. A component of the GEOTRACES program is the measurement of chlorofluorocarbons 11 and 12 (CFC-11 and CFC-12) and sulfur hexafluoride (SF6). These substances are man-made gases with well known atmospheric time histories (Fig. 1) that have only been in existence in significant quantities for the past 60 years for CFCs and 40 years for SF6. They enter the surface water of the ocean from the atmosphere and in regions where surface water sinks to deeper levels, they are transported with this water, providing information on how long it takes the subsurface water to reach a location away from its source region. This information is very important for understanding the distribution of trace elements, as well as many other substances, in the ocean. Results from the CFC and SF6 measurements are reported here. The distributions of trace elements and isotopes are reported separately by investigators who were funded to do these measurements. For the US North Atlantic GEOTRACES cruise, two legs were done, one from Portugal southward to about 17°N off the coast of Africa and from there to the Cape Verde Islands, and the other from Woods Hole southeastward intersecting the first cruise near the Cape Verde Islands (see map insets in Figures referred to below). The two-dimensional distributions of CFC-12 (CFC-11 is very similar to CFC-12 and not shown) and SF6 along the two legs are shown in Figures 2 and 3. In the western North Atlantic there are 3 vertical maxima in concentration. The upper maximum is from Subtropical Mode Water (STMW) that forms by convection in the northern subtropical region during winter, which is caused by the surface water cooling and becoming denser. This convection extends downward a few hundred meters from the surface and the water spreads throughout the subtropical North Atlantic ocean. A second maxima occurs at about 800 m adjacent to the continental slope of North America and deepens to about 1500 m in the interior. This is Labrador Sea Water (LSW) that forms by wintertime convection that extends to 1500 m and deeper due to severe cooling of the surface water in the Labrador Sea. Still deeper there is another maxima at about 3500 m depth. This is Denmark Strait Overflow Water (DSOW) that forms in the Norwegian and Greenland seas and flows into the North Atlantic through Denmark Strait, sinking to deep depths because of its relatively high density. The concentrations of CFC-12 and SF6 in LSW and DSOW are highest at the western boundary, which reflects the transport of these water masses in the Deep Western Boundary Current that is banked against the North American continental slope and flows equatorward. SF6 has not penetrated as far into the interior as CFC-12 because it has not been entering the ocean for as long as CFC-12 (Fig. 1). Concentrations in the deep water are lower in the eastern North Atlantic because the flow of deep water between the eastern and western basins is restricted by the mid-Atlantic Ridge. High CFC and SF6 concentrations reflect younger or more recently formed subsurface water. The year the water was at the surface can be estimated by dividing the CFC or SF6 concentration by the solubility of the gas at the temperature and salinity of the water to determine the concentration that would have been in the atmosphere when the water was at the surface, and comparing this with the atmospheric time history (Fig.1). The age, or time since the water left the surface, is the difference between this year and the year the water was sampled. The SF6 data provides the best estimate of the average age because its concentration in the atmosphere has increased nearly linearly for the past 3 decades. The increase of CFC-11 and CFC-12 slowed...