Excess bottom 222Rn profiles and their implications in the northwestern Pacific Ocean

Vertical profiles of excess bottom 222Rn and potential temperature were measured at 23 stations in the northwestern Pacific Ocean. The Rn profiles were classified into the following three types: quasi-exponential (type E), benthic boundary layer (type B), and horizontally disturbed (type H). The ratio among types E, B and H was approximately 2 : 3 : 1. An apparent vertical eddy diffusivity ( K) was calculated by applying a one-dimensional diffusion model to the Rn profiles of types E and B. Type E had K values ranging from 15 to 180 cm 2 s −1 (average: 70 cm 2 s −1). As to type B, K values for the benthic boundary layer (4.5–260 cm 2 s −1, average: 120 cm 2 s −1) were always more than an order of magnitude larger than those for the upper layer (0.2–35 cm 2 s −1, average: 7 cm 2 s −1), indicating more active vertical mixing in the benthic boundary layer than in the upper layer. Rn profiles were measured in regions where the bottom topography is known. It was verified that the occurrence of type H related closely with local bottom topographic features accompanied by lateral transient supply of Rn-rich or Rn-poor water. A couple of Rn profiles at the same location, measured at time intervals of several years, were compared with each other for three locations. The general characteristics of Rn profiles were shown to remain unaltered with time, while the fine structure of Rn profiles may have short-term variations caused by local bottom topography and fluctuations of bottom current as indicated in type H.