Anomalous chlorofluorocarbons at the bottom of the eastern subtropical North Pacific Ocean
Friday, 19 December 2014
Chlorofluorocarbons (CFCs) are man-made compounds which have been widely used as decadal-scale transient tracers for ocean circulation and ventilation processes. These compounds have well-known atmospheric histories and solubilities. Information derived from CFC concentration and ratio measurements in the global ocean allows us to infer the past history of deep water formation and the rates of spreading and mixing processes. These tracers have also been extensively used to compare and calibrate large-scale ocean circulation models and to estimate the oceanic uptake of anthropogenic CO2. During the P02 CLIVAR/CO2 Repeat Hydrography section along 30°N in 2004, low but detectable levels (>0.005 picomole kg-1) of CFCs (esp. CFC-12) were unexpectedly observed in deep waters (>3000-4000m) along an extensive area between Hawaii and California in the subtropical North Pacific Ocean. Near-bottom CFC concentrations tended to be higher toward the California outer continental shelf region (i.e. Southern California Borderland Basins or SCBB). Potential analytical errors or sampling contamination possibilities were carefully checked during the expedition. These anomalous bottom CFC features were observed again in the same region during the P02 revisit cruise in 2013 by an independent group. The presence of CFCs at the levels measured is inconsistent with presently understood rates and pathways of deep ventilation processes in this region. Potential mechanisms for generating these anomalous features will be discussed, including: a) previously unknown deep ventilation processes in this region; b) release of CFCs from sunken ships or other objects; c) spreading of high-CFC content deep waters from the SCBB along the continental slope; d) vertical transport of CFCs by adsorption/uptake by sinking particulates (including particulate organic matter and/or plastic debris) originating in the surface ocean and re-release in the bottom waters. Studies of sediment trap, sediment cores, uptake and release of chemical compounds by plastics, and uptake and transport by organisms in various sizes (from plankton to baleen whales) would shed light in this mysterious feature and potential knowledge in material transport from the surface ocean to the ocean floor.