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In addition to nitrogen and oxygen, the Earth’s atmosphere contains a small amount of radioactive elements – radioisotopes that enter the atmosphere as a result of natural processes or human activity. Among them is lead, which is formed in the atmosphere during the decay of radon, a gas that does not react with other substances and is released from rocks, soil, mineral fertilisers and emissions from coal-fired power plants. Radioactive elements from the atmosphere find their way into the sea, and later into the bottom sediments. This is why one can use these elements, including stable lead (210Pb), to track the sources of sediments and the rate, at which they accumulate.
The scientists from the Vernadsky Institute of Geochemistry and Analytical Chemistry have measured the concentration of the radioactive isotope of lead in the bottom sediments of the Laptev Sea, which is located away from man-made radioactive substances released during human activity. The authors paid particular attention to areas that differ in the origin of bottom sediments and the rate of their accumulation. For instance, sediments accumulated faster in the eastern province of the sea due to a strong influence of the Lena River runoff, which has made it possible to recreate their history with the most accuracy. The influence of river runoff is lower in the western provinces of the sea, where sediments accumulate at a slower rate, which is why their history has been traced with less accuracy yet over a longer period.
The authors assessed the activity of lead decay based on the energy of gamma rays – particles released during the radioactive decay of lead – emitted by the samples. Based on these data, the researchers calculated the amount of the relevant isotope in the sediments of different ages: deep (older) ones and shallow (younger) ones. The scientists then developed a mathematical algorithm that makes it possible to determine the age of bottom sediments based on lead activity, and also created a program titled “Radio-chronological Model RUS2023”, which takes into account the size of particles in sediments and their ability to accumulate radioactive lead. These characteristics allowed them to improve the accuracy of dating and make the algorithm for calculating the age of marine sediments more reliable.
The researchers came to the conclusion that the decline in the activity of lead decay with the depth of bottom sediments occurs not gradually but in waves. The radio-chronological model made it possible to determine the age of these waves reflecting the highest rate of delivery of radioactive lead into marine sediments. These waves could have been caused by forest fires, which have become more frequent over the past 50 years.
“We believe that the increase in the radioactivity of marine sediments during certain periods can be attributed to forest fires in Siberia, Yakutia and Russia’s Far East. Mosses, lichens and peat are powerful accumulators of radioactive lead isotopes. During combustion, this element is released into the atmosphere of the Northern Hemisphere and later enters the sea, which causes fluctuations in the activity of lead in the bottom sediments,” Valery Rusakov, leader of the study and doctor of geological and mineralogical sciences, is quoted as saying by the Russian Science Foundation.
