Studies of vertical mixing in the Southern California Bight with cosmogenic radionuclides 32P and 7Be

We measured concentrations of the two cosmogenic nuclides 32P and 7Be and radiogenic 234Th in the Southern California Bight during June and October 1989 in water depths up to 80 m. The depth of the mixed layer was 18 and 28 m, respectively, and the thermocline was well developed. The depth profiles of 32P are similar to an earlier profile in September 1987 but the column inventories are ∼5 times lower. 32P activity is fairly uniform in the mixed layer and decreases monotonically with depth below it. On the other hand, 7Be concentrations are nearly uniform to depths of 60–80 m. The integrated inventories of both dissolved 32P and 7Be are appreciably lower, by factors of 5–10, than their expected secular equilibrium concentrations for their mean oceanic injection rates. We show that the low inventories of 32P and 7Be are not due primarily to their decreased injection rates but rather to vertical mixing. The vertical eddy diffusivity and the consequent sustained new production based on upward NO3‒ fluxes have been estimated as >7 cm s and ∼1 g C m‒2 d‒1 from the observed profiles of 7Be, 32P, and NO3‒.