Solar Cycle Detected in Natural Tritium of Ice Layers Before the Nuclear Era

The 11-year cycle of the solar activity affects the production rate of cosmogenic isotopes. For tritium in precipitation, it has been just recently proven that this link exists. Here we show, for the first time, a tritium ice core time series which extends back to 1923, covering a time period that avoids the presence of artificial tritium from the thermonuclear weapon tests of the early 1950s. Accurate analyses of low-level tritium enables us to estimate the natural level of tritium in the study site of Colle Gnifetti, Swiss-Italian Alps, as well as its variation. Statistical analyses using sunspot number and the count rate of cosmic ray secondary neutrons have confirmed that the modulation of the solar activity does affect the tritium concentration of ice layers accumulated earlier than the first hydrogen bomb tests. The tritium level of the ice, as well as in precipitation, is still slightly decreasing during the last three decades. The natural level of tritium obtained in this work fits very well to early tritium analyses of European wine samples, as well as model calculations with the isotope-enabled atmospheric general circulation model MIROC5-iso. Further sensitive tritium analyses of ice cores around the world will provide the opportunity to validate these models.