PMEL Publications Abstract

FY 2013

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

Sonnerup, R., S. Mecking, and J. Bullister

Deep-Sea Res. I, 72, 61–71 (2013)

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 σ_θ, ~ 300–550 m), the CFC-11/SF₆ tracer pair constrained TTDs’ mean ages to within ± 10%. Deeper than 26.8 σ_θ (~ 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 σ_θ (~160 m), attenuating to very low rates (0.12 μmol kg⁻¹ yr⁻¹) by 26.8–27.0 σ_θ (~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.

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