Improved estimates of ventilation rate changes and CO2 uptake in the Pacific Ocean using chlorofluorocarbons and sulfur hexafluoride

Depth profiles of dissolved sulfur hexafluoride (SF6) were measured at 10 stations during the 2006 Climate Variability and Predictability (CLIVAR) Repeat Hydrography reoccupation of the 1991 World Ocean Circulation Experiment P16 section along 152 degrees W between 14 degrees S and 52 degrees N. Ventilation ages calculated from the partial pressure of SF6 (pSF(6) ages) were generally 6-10 years younger than those calculated from concurrent CFC-12 partial pressures (pCFC-12 ages). A one-dimensional parameterization of transit time distributions is utilized to demonstrate the potential of SF6, as a complement to the CFCs, to constrain the impact of mixing on the tracer-derived ages. The two-tracer approach provides an estimate of the mixing-induced tendency of pCFC-12 ages to change with time by correcting their bias to younger ages in older waters and their bias to older ages in younger waters. Model scenarios that match both the 2006 pSF6 and pCFC-12 ages predict a mixing-induced increase in pCFC-12 ages of 6-10 years between the 1991 and 2006 occupations of P16, similar to the increase in pCFC-12 ages observed. Model scenarios tuned to the pCFC-12 and pSF6 ages from 2006 are used to calculate anthropogenic CO2 along the section and yield slightly smaller anthropogenic CO2 burdens than calculated solely from the pCFC-12 ages in waters with pCFC-12 ages > 36 years in 2006.