Caesium-137 dating applied to slackwater flood deposits of the Llobregat River, NE Spain

During a palaeoflood study of the Llobregat River (NE Spain), analysis of 137Cs was carried out on slackwater flood deposits to obtain an improved flood chronology for the most recent flood sediments. At the Monistrol study reach, 137Cs was measured in flood deposits preserved in valley side rock alcoves, with a range of 137Cs concentrations of 2.80-10.06 mBq/g measured. The 137Cs was associated with sediment mobilised from the upstream catchment by erosion and transported to the alcoves during floods. The onset of measured caesium activity in the sedimentary profiles permitted the deposits to be divided between those from floods that occurred before the mid-1950s and those from events after this period. Combining the information derived from the slackwater flood stratigraphy, the minimum discharge estimates associated with the flood deposits, 137Cs analysis and the instrumental peak discharge series, it was possible to identify the particular flood events responsible for the majority of the slackwater flood units. At Vilomara, however, 137Cs activity was measured in palaeoflood deposits dated to the Late Bronze Age and located at an elevation not reached by the floodwaters of the largest modern events. The data from this site indicated that the 137Cs was not associated with sediments transported by floods but was rainfall-derived, the rainwater reaching the deposits through fissures in the rock. To ensure the successful application of caesium dating to slackwater flood deposits preserved within rock alcoves, or caves, some general guidelines are proposed. The technique is best applied: (1) when there is a sedimentary profile of greater than 1 m depth; (2) when the profile is sampled away from zones affected by direct percolation or springwater entering the alcove; and (3) where there are various levels of contrasting textures within the profile.