Constraining the origin of recently deposited particles using natural radionuclides 7Be and 234Thex in deltaic sediments. (1st August 2018)

• Record Type: Journal Article
• Authors: Wu, Junwen; Rabouille, Christophe; Charmasson, Sabine; Reyss, Jean Louis; Cagnat, Xavier
• Is Part Of: Continental shelf research. Volume 165(2018)
• Page Start: 106
• Page End: 119
• Publication Date: 2018-08-01
• Subjects: 7Be -- 234Th -- Suspended particulate matter -- Rhône River -- 2008 flood -- Gulf of Lions; Continental shelf -- Periodicals; Submarine geology -- Periodicals; 551.41
• DOI: 10.1016/j.csr.2018.06.010
• Languages: English
• ISSNs: 0278-4343

Abstract

Abstract: 7 Be and 234 Thex 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 7 Be and 234 Thex 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 7 Be was more pronounced than that of 234 Thex, 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 137 Cs or 210 Pbex . Both 7 Be and 234 Thex inventories increased in 2008 compared to 2007, and are correlated to the cumulated SPM flux for normal and flood discharge. Moreover, the 7 Be/ 234 Thex 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 7 Be/ 234 Thex inventory ratio over 0.50 (surface area near river mouth ~ 7 km 2 ) is dominated by riverine influence; in contrast, Zone III at a distance superior to 8.5 km, with 7 Be/ 234 Thex inventory ratio less than 0.10 (surface area off river mouth beyond 150 km 2 ) is predominantly under a marine influence. In between, an intermediate area (Zone II at a distance between 3.0 and 8.5 km, with 7 Be/ 234 Thex 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. Highlights: Short-term deposition of particles near the Rhône mouth was traced by 7 Be and 234 Thex . Both 7 Be and 234 Thex inventories were correlated with the cumulated particle fluxes. Recent riverine particles are mainly deposited within 5 km off the Rhône river mouth. 7 Be/ 234 Thex inventory ratio was used to assess riverine and marine influence.