Uncovering variations in solar activity through tree-ring radiocarbon measurements

While the sun, as the primary source of energy on earth, is considered to have a constant energy output, small fluctuations can be observed over time. Historical records of solar activity (e.g., sunspot numbers) are however scarce, and only available over the last 400 years. Cosmogenic nuclides stored in tree rings (14C) or ice cores (10Be, 36Cl) can be used as proxies for solar activity and allow solar reconstructions reaching much further back in time1,2. However, only recently the presence of the eleven-year solar cycle could be revealed in an annually resolved 14C record from tree-rings covering the past 1000 years. The amplitude of this so called Schwabe cycles is found to correlate with the general level of the solar activity with high amplitudes during periods of strong solar activity and vice versa3. Here, we present the solar activity, and specifically the 11-year cycles, reconstructed from 14C in tree-rings covering two grand solar minima, one around 400 BCE and another one around 3400 BCE. The results will be compared with the two previously analysed grand solar minima (Spörer and Maunder) of the last millennia, coinciding with the Little Ice Age.   1 Bard, E. et al. (2000) Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus Series B-Chemical and Physical Meteorology 52, 985-992. 2 Muscheler, R. et al. (2007) Solar activity during the last 1000 yr inferred from radionuclide records. Quaternary Science Reviews 26, 82-97. 3 Brehm N. et al. (2021) Eleven-year solar cycles over the last millennium revealed by radiocarbon in tree rings. Nature Geoscience. 14(1), 10-15.