A 2-D SECTION OF RA-228 AND RA-226 IN THE NORTHEAST PACIFIC

Seawater samples collected in the northeast Pacific from 112 degrees 5 0'W to 126 degrees 36'W along a latitudinal band (21-25 degrees N) hav e been analysed for Ra-228 and Ra-226. Both nuclides exhibit their cha racteristic distributions. In the surface water, the exponential-like decrease of Ra-228 away from Baja California can be interpreted by hor izontal water mixing with eddy diffusion coefficients (K-x) of 1 x 10( 6) cm(2) s(-1) and 5 x 10(7) cm(2) s(-1) for scale lengths of 200 km a nd 1000 km, respectively. In the bottom waters, the decrease of Ra-228 away from bottom sediments can be modeled by vertical eddy diffusivit ies (K-z) of 15-30 cm(2) s(-1) except at one station (24 degrees 16.9' N, 115 degrees 8.9' W) where a value of 120 cm(2) s(-1) is obtained. The Ra-228-derived diffusivities were used to compute the mass balance of 226Ra using a two-box model. The model results show a mean mixing coefficient of 3.8 cm(2) s(-1) for the thermocline and a mean upwellin g velocity of 7.7 m y(-1) in the study area, both are about two or thr ee times higher than those generally quoted for the Pacific. (C) Elsev ier, Paris.