Beryllium-7 as a tracer of surface water subduction and mixed-layer history

Beryllium-7 is a cosmic-ray-produced radioactive nuclide (half-life 53.3 days) that is deposited upon the ocean surface by rainfall. Beryllium-7 profiles were collected and analyzed in May 1991 and March 1992 as part of the ONR SUBDUCTION program. The Be-7 activities were homogeneous in the mixed layer and ranged approximately from 100 to 200 dpm/1000 1. The activities dropped off rapidly in the upper thermocline. The Be-7 profiles were modeled using a simple seasonal progression of mixed-layer depth, indicating that seasonal changes in the mixed-layer depth control the main features of the profile. The observed temperature, salinity and Be-7 profiles from the May 1991 cruise show warm salinity maxima associated with a layer of high Be-7 penetrating into the upper seasonal thermocline. The high Be-7 concentration suggests that these waters were in relatively thin mixed layers within, approximately, the past month. The model results indicate that in late spring both salinity and Be-7 can respond to a shoaling mixed layer on a similar timescale leading to the observed anomalies in the subducted water layer. Copyright (C) 1996 Elsevier Science Ltd.