Atmospheric chlorofluoromethanes in the deep equatorial Atlantic

Abstract

Waters that leave the surface of the ocean and enter the subsurface circulation contain concentrations of CCl₃F (fluorocarbon-11) and CCl₂F₂ (fluorocarbon-12), which reflects the temporal increases of these industrially produced compounds in the atmosphere. These chlorofluoromethanes (CFMs) are extremely stable in the troposphere and in natural waters, they have no known natural sources, and their histories of release to the atmosphere are fairly well known¹. The atmospheric distributions of CCl₃F and CCl₂F₂ are not strongly dependent on latitude^2,3, and their surface water concentrations can be expected to come into relatively rapid solubility equilibrium with the atmosphere⁴. Recent advances in analytical techniques^5,6 have made possible the routine use of these CFMs as oceanic tracers on a decadal timescale. The results we report here are from the first detailed surveys of CCl₃F and CCl₂F₂ distributions in the northern and tropical Atlantic Ocean. They show that CFM-bearing waters originating in the region of the Labrador Sea have reached the Equator in a well-defined western-boundary undercurrent located at a depth of about 1.6 km, in the Upper North Atlantic Deep Water. Using a simple dilution model, we calculate that this water has taken about 23 yr to reach the equatorial region, and has been diluted about five-fold by CFM-free waters.