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

Abstract

The vertical distributions of the man‐made chlorofluoromethanes CCl3F (F‐11) and CCl2F2(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 Kz, 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 (Kz ∼ 1.2–1.3 cm2 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 CO2), leading to a predicted mean depth of penetration of fossil fuel CO2 of approximately 300 m in the eastern North Pacific.