NON-STEADY-STATE PB-210 FLUX AND THE USE OF RA-228 RA-226 AS A GEOCHRONOMETER ON THE AMAZON CONTINENTAL-SHELF/

The recovery of quasi-cyclic Pb-210 profiles from the foreset region o f the Amazon subaqueous delta has raised questions concerning the non- steady-state flux of Pb-210 to this region of the shelf. Radioisotopic profiles (Pb-210, Th-234, Ra-228/Ra-226, Cs-137) from seven kasten co res, along with the analysis of grain size, organic carbon, and clay-s ized mineralogy are used to investigate the time scale and the control ling mechanisms of the Pb-210 cyclicity. A common assumption used in P b-210 geochronology is that the initial specific activity is constant with time; an assumption that is invalid in this region. Variations of particle affinity for Pb-210 have been shown to affect the initial sp ecific activity reaching the seabed in various other studies. However, investigations of grain size, organic carbon, and mineralogy have rev ealed no relationship with the fluctuating Pb-210 activity in this are a. A new geochronometer, Ra-228/Ra-226, is used to determine sediment accumulation rates over a 30-year time scale, which range from 10 to 6 0 cm/yr in the study area. Spectral analysis of the Pb-210 cyclicity h as revealed that the major component of Pb-210 cyclicity has a frequen cy of approximately 2-3 years. In addition, components of the Pb-21O c yclicity occur on annual and semi-annual frequencies, The controlling mechanism for the Pb-210 cyclicity on these time scales appears to be the seaward transport and accumulation of low-activity fluid muds from the topset region.