Transit time distributions and oxygen utilization rates in the Northeast Pacific Ocean from chlorofluorocarbons and sulfur hexafluoride

Depth profiles of dissolved chlorofluorocarbon-11 (CFC-11) and sulfur hexafluoride (SF6) were measured during a September 2008 cruise in the Northeast Pacific Ocean. For each water sample, the two tracers were used in concert to estimate likely mean ages and widths of parameterized 1-D transit time distributions (TTDs). In shallow waters (<250 m), the TTDs’ mean ages were relatively loosely constrained due to the slow decrease of atmospheric CFC-11 since 1994. In the main thermocline (25.0–26.6 σθ, ∼300–550 m), the CFC-11/SF6 tracer pair constrained TTDs’ mean ages to within±10%. Deeper than 26.8 σθ (∼600 m), SF6 levels in 2008 were too low for the CFC-11/SF6 tracer pair to constrain the TTDs’ mean ages. Within the main thermocline of the subtropical North Pacific Ocean (20°–37°N along 152°W), the TTDs’ mean ages were used to estimate Oxygen Utilization Rates (OURs) of ∼11 μmol kg−1 yr−1 on 25.0–25.5 σθ (∼160 m), attenuating to very low rates (0.12 μmol kg−1 yr−1) by 26.8–27.0 σθ (∼600 m). Depth integration of the in-situ OURs implied an average carbon remineralization rate of 1.7±0.3 mol C m−2 yr−1 in this region and depth range, somewhat lower than other independent estimates. Along the 152°W section, depth integrating the apparent OURs implied carbon remineralization rates of 2.5–3.5 mol C m−2 yr−1 from 20°N to 30°N, 3.5–4.0 mol C m−2 yr−1 from 30°N to 40°N, and 2–2.7 mol C m−2 yr−1 north of 45°N.