Chlorofluoromethanes in the northeast Pacific Ocean: Measured vertical distributions and application as transient tracers of upper ocean mixing

The vertical distributions of the man‐made chlorofluoromethanes CCl 3 F (F‐11) and CCl 2 F 2 (F‐12) have been measured at two locations in the eastern North Pacific Ocean to depths greater than 500 m. At both sites (46°N, 125°W off the Washington‐Oregon coast and 50°N, 140°W in the Gulf of Alaska) the halomethane concentrations were found to fall off exponentially with increasing depth below the mixed layer. For F‐11 at 50°N, the surface concentration (3.2×10 −12 mol/l) was found to be in saturation equilibrium with the measured atmospheric concentration (190 pptv). The measured chlorofluoromethane profiles have been interpreted in terms of a one‐dimensional model for the vertical diffusion/advection of an exponentially driven, conservative tracer into a bottomless ocean. In the appropriate limit of ‘transient steady state’ the projected profiles are simple exponentials described by an advective‐diffusive scale depth H , which is a function of the vertical eddy diffusivity K z , the upwelling velocity w , and the characteristic time τ for the exponential growth of the tracer concentration at the boundary. At the two ocean locations studied the freon thermocline depth scales were in the range H ≃ 120–140 m, with F‐12 generally 10–20% deeper than F‐11. At 50°N the F‐11 and F‐12 vertical profiles gave consistent values of vertical diffusivity ( K z ∼ 1.2–1.3 cm 2 s −1 ) and upwelling velocity ( w ≃ 12–14 m yr −1 ). The model also allows a simple scaling from one exponentially driven, transient tracer (freon) to another (fossil fuel CO 2 ), leading to a predicted mean depth of penetration of fossil fuel CO 2 of approximately 300 m in the eastern North Pacific.