Coastal water exchange rates at the southeastern Brazilian margin, using natural Ra isotopes as tracers

The knowledge of the four natural Ra isotopes in aqueous systems can be used to constrain important environmental processes occurring at the land-sea margin. The high variability of the 226Ra/228Ra activity ratio in waters, as has been reported by several authors, clearly demonstrates the suitability of Ra isotope relationships do determine fluxes and mixing rates of continental waters into oceans and estuaries and exchanges between ground water and surface water. Two main geochemical characteristics control the production and input of Ra isotopes in coastal areas: the existence of particle-reactive Th isotopes in sediments as its direct radiogenic parents and the vastly different environmental behavior of Ra in fresh water and salt water media. The short- lived Ra isotopes of the 222Th series, which are highly depleted in the ocean basins due to their rapid decay and the strong depletion in parent Th isotopes, have been used to track advection from the coasts. Because of the 5.7 year half- life of 228Ra, this isotope has been used effectively to estimate oceanic horizontal eddy diffusivities and coastal water residence times over timescales of less than 30 years. By using both 228Ra and 224Ra (t1/2= 3.7 days), timescales of less than 10 years can be investigated. In this paper, 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), on board the R/V Prof. Besnard. The Sao Paulo Bight is the arc-shaped part of the southeastern Brazilian margin extending from 23 deg. S to 28 deg. S. All samples studied here were taken in the selected area between latitudes 23 deg. 15'S and 25 deg. 50'S and longitudes 44 deg. W and 46 deg. W, in order to estimate coastal mixing rates and ground water discharge fluxes. This region is considered a tropical coastal area and the main geologic/geomorphologic feature is the presence of granites and migmatites of a mountain chain locally called Serra do Mar (altitudes up to 1,000 meters), which reaches the shore in almost all of the study area, and limits the extension of the drainage systems and of Quaternary coastal plains. Wave action is the most effective hydrodynamic phenomenon responsible for the bottom sedimentary processes in the coastal area as well as in the adjacent inner continental shelf. The terrestrial input of sediments is strongly dependent on the rainfall regime, leading to a higher contribution of sediments during summer season. During the summer, the advance of the South Atlantic Central Water (SACW) over the coast leads to the displacement of the Coastal Water (CW), rich in continental suspended materials, and to the transportation of these sediments to the outer portions of the continental shelf. During winter, the retreat of the SACW and the decreasing of the rainy levels restrict the input of sediments from the continental areas. The activity concentrations of 223Ra in surface sea water varied from 0.2 to 41 mBq 100 L-1, 224Ra in excess from 2.0 to 253 mBq 100 L-1, 226Ra from 122 to 181 mBq 100 L-1 and 228Ra from 39 to 442 mBq 100 L-1. The 223Ra/224Ra activity ratios observed in sea water samples ranged from 0.3x10-1 to 13, whereas 228Ra/226Ra AR varied in the interval from 0.3 to 2.4. These results seems to indicate that Ra isotopes from 222Th series prevail in a major number of samples, when compared with Ra isotopes from 238U and 235U series. Considering the results obtained in the summer 2003, shore-perpendicular profiles of 223Ra and 224Ra in surface waters along the coast were modeled to yield eddy diffusion coefficients. The results were also corrected by mixing, using the activities of 223Ra normalized to 228Ra