Evidence of subduction within cold filaments of the northern California coastal transition zone

Profiles of ²²²Rn, chlorophyll, nutrients, oxygen, temperature, and salinity taken within a cold filament observed in satellite images of the northern California coastal transition zone provide evidence that there is strong vertical transport out of the surface layer by subduction processes occuring within these features. Layers of chlorophyll found below the euphotic zone have associated with them other characteristics of water that at one time had been in contact with the sea surface, including deficiencies of ²²²Rn with respect to ²²⁶Ra, elevated oxygen cpncentrations, and warm water on plots of temperature and salinity. The data indicate that the subsurface maxima in chlorophyll are derived from subducted surface layers and are not produced by in situ phytoplankton growth nor derived by particle settling. The presence of a ²²²Rn deficiency at depth further suggests that the subducted water had been in contact with the surface within the past week and indicates that the rate of vertical transport is approximately 25 m/d. This has important implications for the fate of the filaments and for the high primary productivity within them.