Determination of Plutonium and Uranium Radionuclides in Glacier Ice Samples by MC-ICP-MS

Díaz-Asencio

et al. 2022

Environ Sci Pollut Res

Here, we report on new data (75 analyses) of plutonium (Pu) isotopes to elucidate activity concentrations, inventories, sources, and their transport from the ocean surface to the seafloor from a collection of six deep-sea sediment cores (depths ranging from 257 to 3739 m) in the southern Gulf of Mexico. Sediment cores collected from the continental shelf and upper slope region of the Gulf of Mexico showed 240Pu/239Pu ratios of 0.15 to 0.26, and 239+240Pu-inventories ranging from 14.7 to 33.0 Bq m−2. Inventories and ratios are consistent with global fallout Pu for this tropical region. In contrast, sediment cores collected from the lower slope region and abyssal plain showed low 240Pu/239Pu ratios of 0.07 to 0.13 and much lower 239+240Pu inventories below 6.8 Bq m−2. This implies that only a small fraction of the expected global fallout Pu has reached the deep-sea sediments. The low 240Pu/239Pu isotope ratios indicate that fallout from the Nevada testing site was an important source of Pu in deep-sea sediments, and that this Pu was likely more efficiently scavenged from the water column than Pu from global fallout. We estimated that up to 44% of the total inventory of 239+240Pu in deep-sea sediments is due to the Nevada source. Low values and a progressive decrease of 240Pu/239Pu ratios and 239+240Pu inventories with increasing water depth have been previously reported for the Gulf of Mexico. Analysis of Pu isotopes in two sediment traps from the upper slope regions shows 240Pu/239Pu ratios comparable to those observed in global fallout. These results indicate that global fallout Pu is currently the main source of Pu in sinking particles in the water column. Therefore, a significant fraction of global fallout Pu must still be present, either in a dissolved phase, or as biologically recycled material in the water column, or scavenged on the shelf and shelf break. Our results bring to light important questions on the application of Pu isotopes to establish sediment chronologies in deep-sea sediments, since global fallout features such as the 1963 maximum are not available.

Section: Methodsmentioning

confidence: 99%

Distribution and source of plutonium in sediments from the southern Gulf of Mexico

Díaz-Asencio

et al. 2022

Environ Sci Pollut Res

“…The melt was dissolved in 175 ml 4.5 M HNO 3 and after silicates were precipitated, 1.4 ml aliquots were ltered and diluted for quantitative analysis of 238 U as reported elsewhere (Sahli et al, 2017). After ltration of the silicates, Pu and U radionuclides were separated and puri ed using TEVA and UTEVA extraction chromatography resins as described elsewhere (Röllin et al, 2020). This separation procedure was extended for 241 Am analysis, by coupling a DGA cartridge after the UTEVA column.…”

Section: Sampling and Sample Preparationmentioning

confidence: 99%

High resolution records of caesium, plutonium, americium and uranium isotopes in sediment cores from Swiss lakes

Sahli

et al. 2022

Preprint

Self Cite

The Aare river system in Switzerland, with two nuclear power plants on the banks of the river, and its intermediate lakes and reservoirs, provide a unique opportunity to analyze the contribution of different sources to the radioactive contamination. Sediment cores were collected from two lakes and a reservoir, all connected by the river Aare. In order to study the influence of the Chernobyl accident, one sediment core was collected from a lake in the southern part of Switzerland. The sediment cores were sliced and analysed with gamma ray spectrometry. Plutonium, americium and uranium were extracted radiochemically and their concentrations measured with a sector field ICP-MS. The uranium isotope ratios were further measured with a multi collector ICP-MS. The maximum 137Cs activity from the Chernobyl accident and the Pu and 137Cs activities associated to the 1963 global fallout maximum were well identified in sediments from all three lakes. High resolution records of plutonium isotopes in the zone of the sediments corresponding to the period of maximum fallout from the atmospheric nuclear weapon showed distinct fingerprints, depending on the different test activities. Pu isotope ratios could be used to detect non global fallout plutonium. The ratio 241Am/241Pu was used to determine the age of the plutonium. Despite of very low 241Pu and 241Am concentrations, the calculated plutonium production dates seemed to be reasonable for the sediment layers corresponding to the NWT tests. The calculated production date of the plutonium in the upper most 15 cm of the sediment core seemed to be younger. The reason for this could be additional non-global fallout plutonium or a higher mobility of plutonium compared to americium. For the lake sediments, natural ratios for 235U/238U and enriched or depleted ratios for 234U/238U were measured, depending on the lake. A small increase of the 236U/238U ratio could be recognized for the NWT zone in all three lakes and, for Lake Lugano, a further distinct increase in the Chernobyl layer.

“…Sediment samples were analyzed for Pu radioisotopes as described elsewhere (Röllin, Sahli et al 2009, Sahli, Röllin et al 2017, Röllin, Sahli et al 2020. Brie y, about 5 g aliquots were spiked with known amounts of 242 Pu (about 3 pg).…”

Section: Analysesmentioning

confidence: 99%

Records of plutonium isotopes in marine sediments from the southern Gulf of Mexico

Diaz-Asenciuo

et al. 2021

Preprint

Self Cite

Here we report on new data on plutonium (Pu) isotopes to elucidate activity concentrations, inventories, sources and their transport from the ocean surface to the sea floor from a collection of deep-sea sediment cores (depths ranging from 257 to 3739 m) in the Gulf of Mexico (GoM). Sediment cores collected from the continental shelf and upper slope region of the GoM consistently showed 240Pu/239Pu ratios of 0.15 to 0.26 and Pu-inventories ranging from 15 to 35 Bq m− 2. Inventories and ratios are consistent with global fallout Pu for this tropical region. In the continental shelf and upper slope regions, higher particle concentrations close to the margins favor significant scavenging and removal of Pu from the water column; in contrast with the deep-sea cores that show low 240Pu/239Pu ratios (0.07–0.13) and a much lower Pu inventory (< 7 Bq m-2) implying a small fraction of the expected global fallout inventory has reached into the lower slopes and abyssal plain of the GoM. Low values and a progressive decrease of 240Pu/239Pu ratios and Pu inventories with increasing water depth have been previously reported for the GoM. The low Pu ratios indicate that Nevada tests fallout was an important source of Pu to deep-sea sediments, and that this source was likely more efficiently removed from the water column than global fallout Pu. Analysis of Pu isotopes in two sediment traps from the upper slope regions show 240Pu/239Pu ratios comparable to the ones observed in the global fallout. These results indicate that global fallout Pu is currently the main source of Pu in water column particles. Therefore, a significant fraction of global fallout Pu must still be present; either in a dissolved phase, or as biologically recycled material in the water column, or scavenged on the shelf and shelf break. Our results bring to light important questions on the application of Pu isotopes to establish sediment chronologies, since these radionuclides are shown to be tracers of bioturbation rather than accumulation processes in deep-sea sediments of the GoM, similar to previously reported results from excess 210Pb.