An Interior Pathway for Ventilation of the Abyssal South Pacific Ocean?

The distributions of the anthropogenic chlorofluorocarbons in the deep ocean have been used in numerous studies to identify the pathways and the spreading rates for ventilation of the abyssal ocean. In many of the ocean basins, this transport appears to be primarily occur via the equatorward-flowing deep western boundary currents (DWBC), its mixing with adjacent waters, and the recirculation in adjacent deep gyres. In the southwestern Pacific Ocean, there have multiple re-occupations of sections crossing the DWBC (i.e. P15S and P6) between 1990 and 2009 that have been used to estimate the timescales of transport and mixing of anthropogenic compounds into this basin along the western boundary. During the 2014 reoccupation of the P16S section along 150^oW and far from the western boundary, CFCs were measured in the bottom waters near 38^oS. The above blank level CFCs were observed in a feature that appears to be separate from the elevated CFC concentrations in the bottom waters to the south of 45^oS. The highest measured concentrations (CFC-12 >0.04 pmol kg^-1) are well above the detection limit, and the feature extends over several degrees of latitude in 2014. During the previous occupation of P16S in 2005, the CFC concentrations would have been near the detection limit, making this feature difficult to identify. The likely source of this bottom water feature is from the western boundary. A comparison of the pCFC values at 150^oW with those measured in the DWBC where crossed by P15S on the southern flank of the Chatham Rise (near 45^oS) are consistent with a timescale of less than 20 y from the western boundary to the P16S section. Repeated occupations with tracers and hydrography in the southwestern Pacific allow estimates of interior spreading rates for bottom waters.