Coastal water exchange rate studies at the southeastern Brazilian margin using Ra isotopes as tracers

摘要

The Sao Paulo Bight is the arc-shaped part of the southeastern Brazilian margin extending from 23°S to 28°S. To assess the cross-shelf Ra distributions in Sao Paulo Bight, four shore-perpendicular profiles were collected up to 100 km offshore from 23 to 26 February 2003 (summer). All samples studied here were taken in the selected area between latitudes 23°15′S and 25°50′S and longitudes 44°W and 46°W, in order to estimate coastal mixing rates and groundwater discharge fluxes. The activity concentrations of ~(223)Ra in surface seawater varied from 0.002 to 0.4 mBq L~(-1), ~(224)Ra in excess from 0.02 to 2.5 mBq L~(-1), ~(226)Ra from 1.2 to 1.8 mBq L~(-1) and ~(228)Ra from 0.4 to 4.4 mBq L~(-1). The ~(223)Ra/~(224)Ra and ~(228)Ra/~(226)Ra activity ratios observed in seawater samples ranged from 0,03 to 2.6 and from 0.3 to 2.4, respectively. These results seem to indicate that Ra isotopes from ~(232)Th series prevail in a major number of samples, when compared with Ra isotopes from ~(238)U and ~(235)U series. Considering the results obtained in the summer 2003, shore-perpendicular profiles of ~(223)Ra and ~(224)Ra in surface waters along the coast were modeled to yield eddy diffusion coefficients.