Dating young Holocene coastal sediments in tropical regions: Use of fallout ~(239,240)Pu as alternative chronostratigraphic marker

摘要

We present a study on the use of ~(239,240)Pu as a chronostratigraphic marker to constrain ~(210)Pb ages in young Holocene sediments from four coastal locations of Cuba (Caribbean Sea). The selected sites typify a variety of deposition environments such as a bay, a gulf, a coastal lagoon and a river estuary. The vertical distributions of ~(210)Pb (and ~(226)Ra), ~(239,240)Pu and ~(137)Cs in the sediment cores were determined in order to quantify the sedimentation processes. Mean activity ratios of 0.04 ± 0.01 for ~(238)Pu/~(239,240)Pu and 0.3 ± 0.2 for ~(241)Am/~(239,240)Pu indicated that atmospheric nuclear weapon tests (NWT) fallout was the main source of the anthropogenic radionuclides. Marine sites (Havana Bay and Gulf of Batabano) were characterized by low or negligible signals of ~(137)Cs, which impeded its use as a chronostratigraphic marker. In the Sagua River Estuary and the Guanaroca Lagoon, where mixing of fresh and saline waters take place, depth profiles of ~(137)Cs and ~(239,240)Pu were in good agreement. A simple model that assumes a well-mixed reservoir in surface ocean waters and constant sediment accumulation was used to interpret the~(239,240)Pu profiles. The model corroborated the ~(210)Pb ages, except in the case of the highly mixed sediment cores from the Gulf of Batabano. The apparent mean residence time of ~(239,240)Pu in the well-mixed reservoir ranges from 5 to 50 years, with longer timescales for marine sites. ~(239,240)Pu showed to be a useful time tracer in coastal sediments where the ~(137)Cs signal is very low.