When cosmic radiation (primarily protons and alpha particles) from high-energy charged particles reaches the Earth, it produces cosmogenic isotopes through atmospheric nuclear reactions. These can be stable or radioactive isotopes. One such cosmogenic isotope is tritium, the radioactive isotope of hydrogen. If tritium is oxidized after its formation, it enters the terrestrial water cycle, i.e. it falls to the surface through precipitation. This means it is an excellent tracer for the terrestrial water cycle. Because the intensity of cosmic radiation depends on the magnitude of the Sun’s magnetic field, it follows that there should be a correlation between the 11-year solar cycle and tritium levels found in precipitation. By analyzing the time series of precipitation, derived from accurate tritium measurements, in the Hungarian city of Debrecen for more than 15 years, the researchers from the Institute for Nuclear Research (ATOMKI) were able to demonstrate that this connection exists for the first time in history. This had not been possible previously for several reasons. First, the measurements used to accurately determine the low tritium concentration in the precipitate were not sufficiently sensitive. In addition, atmospheric hydrogen bomb explosions since the 1950s have multiplied and increased the tritium content of the atmosphere more than a hundred-fold, thus rendering the tiny natural fluctuations caused by the solar cycle invisible. However, an article by Atomki researchers published in Scientific Reports provides clear evidence that the solar cycle influences precipitation tritium concentrations (Palcsu et al., 2018). This makes it easier understand various atmospheric physical processes, the terrestrial water cycle, and the potential for new hydrological applications.

To help confirm this relationship, the researchers intend to further investigate the tritium concentration of precipitation that definitely fell before the nuclear era. Such precipitation includes layers of ice deposited on top of each other from snow falling on the accumulation level of glaciers. In previous collaborations, the researchers had already participated in glacier research examining the age of ice layers by determining tritium concentrations (Shao et al., 2017; Shao et al., 2020), and launched their own expedition to the Swiss-Italian Alps in the summer of 2020 . Drilling was carred out at the top of the Colle Gnifetti Glacier, at 4,453 meters, in order to collect enough ice from a depth of 28 to 33 meters to represent snow that fell from 1930 to 1950. These layers still contain natural amounts of tritium, which are not contaminated with tritium from either hydrogen bombs or nuclear power plants. However, the half-life of tritium is just 12.32 years, and only less than 40% of the already low natural level is found in the ice due to radioactive decay, which means that an extremely sensitive yet accurate measurement technique is required. Measurements of ice cores providing data on individual years are expected to take eight months, as the required sensitivity does not detect radioactive decay of tritium, but uses sensitive noble gas mass spectrometry to determine how many 3He daughter products are formed from the decay of tritium in eight months. As a result, less than one thousandth of the natural level is detectable.

Researchers are also planning to drill a Tibetan glacier in the near future to show that the change seen in the tritium range is not only limited to the Alps, but is in fact global in nature, further reinforcing the relationship between the solar cycle and tritium.

 

References:

Palcsu L., Morgenstern U., Sültenfuss J., Koltai G., László E., Temovski M., Major Z., Nagy J.T., Papp L., Varlam C., Faurescu I., Túri M., Rinyu L., Czuppon G., Bottyán E., Jull A.J.T.: Modulation of Cosmogenic Tritium in Meteoric Precipitation by the 11-year Cycle of Solar Magnetic Field Activity, Scientific Reports 8 (2018) 12813.

Shao, L., Tian L, Cai Z, Cui J, Zhu D, Chen Y, Palcsu L, Driver of the interannual variations of isotope in ice core from the middle of Tibetan Plateau, Atmospheric Research 188 (2017) 48–54.

Shao, L., Tian, L., Wu, G., Naftz, D., Cai, Z., Wang, C., Li, Y., Palcsu, L.: Dating of an alpine ice core from the interior of the Tibetan Plateau. Quaternary International (2020) 544, 88-95.