Constraining the origin of recently deposited particles using natural radionuclides 7Be and 234Thex in deltaic sediments

7Be and 234Thex activities were determined in sediment cores off the Rhône River mouth (Gulf of Lions), in order to trace the initial transport and deposition of riverine suspended particulate matter (SPM) and evaluate the impact of flood events through 7 cruises carried out between 2007 and 2008. Consistently high 7Be and 234Thex inventories of 2000-3000 mBq cm−2 and 3000-5000 mBq cm−2, respectively, were observed within a ~5 km radius off the Rhône River mouth. Their spatial distributions showed a gradual decrease with increasing distance from the Rhône River mouth, and the decrease in 7Be was more pronounced than that of 234Thex, indicating that recent riverine SPM is rapidly deposited in the area located near the river mouth. This area is also characterized by high accumulation rates determined using 137Cs or 210Pbex. Both 7Be and 234Thex inventories increased in 2008 compared to 2007, and are correlated to the cumulated SPM flux for normal and flood discharge. Moreover, the 7Be/234Thex inventory ratio appears to be a potential tracer to identify the dominant influence of recently deposited particles between terrestrial and marine waters. This ratio provides an effective tool to assess river and marine influence: Zone I at a distance inferior to 3.0 km, with 7Be/234Thex inventory ratio over 0.50 (surface area near river mouth ~ 7 km2) is dominated by riverine influence; in contrast, Zone III at a distance superior to 8.5 km, with 7Be/234Thex inventory ratio less than 0.10 (surface area off river mouth beyond 150 km2) is predominantly under a marine influence. In between, an intermediate area (Zone II at a distance between 3.0 and 8.5 km, with 7Be/234Thex inventory ratios between 0.10 and 0.50) displays a mixed influence. This zoning could help in further understanding the spreading of particle-reactive contaminants and its initial sedimentary deposition in the Gulf of Lions.