Organically bound tritium (OBT) and carbon-14 accumulation in the sediments off the mouth of the Rhône river

We have studied the recent history of organically bound tritium (OBT) and carbon-14 in suspended organic matter carried by the Rhône from the analysis of two cores collected in the sedimentary cone off the river mouth. Measured OBT and carbon-14 concentrations highlight characteristic anthropogenic traces in the suspended matter of the Rhône, with a peak of tritium and carbon-14 commencing at the start of the 1970s and extending over some 30 years. In spite of the influence of the atmospheric nuclear tests of the 1950s and 1960s, and of the discharges of carbon-14 by the nuclear industries located along the Rhône, carbon-14 levels are low due to the diluting effect of a refractory carbon component very low in carbon-14 present in the coastal sediments. OBT levels are well above the modern background created by the global imprint of bomb-tritium. Detailed study of the impact of the various contribution to the tritium peak indicates that these traces are mostly the result of the sediments in the Rhône being contaminated by past tritium releases by the watchmaking industry in the upper Rhône.