Cosmogenic radionuclides in environmental archives – A paleo-perspective on space climate and a synchronizing tool for climate records

Sammanfattning: The Sun is the primary source for Earth’s climate system. Its fluctuations in irradiance are also known to have an impact on climate. In addition, changes in solar activity modulate the atmospheric production rates of cosmogenic radionuclides (e.g. 10Be, 14C, 36Cl) that all eventually deposit to different environmental archives. The signal of the changing solar activity through time can thus be retrieved and measured from these archives, such as ice cores, tree rings, or lake sediments. The Sun can also display a more chaotic behavior by erupting flashes of light, plasma and magnetic fields whereby energetic particles can be accelerated, and sometimes hit Earth. These all can damage our spacecraft technologies, harm astronauts, and also affect transformer, electric, and electronic infrastructures on the ground. In the case of extreme events, they may also pose a challenge to air-travel safety. At the same time, when solar energetic particles enter the atmosphere, they can enhance the production rate of cosmogenic radionuclides. The objectives of this thesis are twofold. First, the potential of using 10Be, 14C, and 36Cl as tracers of extreme solar storms is explored in depth. Second, the common production signal of 10Be and 14C caused by the longer term changes in solar activity is used to synchronize climate records from different environmental archives from different regions in order to assess the relative timing of a prominent climate oscillation, over 11,000 years before present. Two large signatures of cosmic-ray increase date to AD 774/5 and AD 993/4 are conclusively attributed to extreme solar energetic particle events that have hit Earth and left a clear imprint on the production rates of 14C as measured in tree rings all around the world, and of 10Be and 36Cl in ice cores from Greenland and Antarctica. The inferred energy spectrum and flux of particles of these events indicate that they were an order of magnitude stronger than any solar high-energy event observed during the space era. To infer the energy spectrum of ancient events, it is shown that the relative differences in the energy dependency of the production rates of 10Be and 36Cl by solar particles can be used. An additional, and similarly extreme, solar storm is also suggested to have hit Earth 2,610 years BP. The events from AD 774/5 and AD 993/4 are further explored to test, and thereafter reject, the hypothesis that nitrate enhancements in ice cores can be reliably linked to the occurrence of solar storms or to assess their magnitude. Two high resolution and continuous records of ice-core 36Cl concentration spanning the past several centuries are also presented. They show several increases in 36Cl possibly linked to solar energetic particle events including one that is coeval with the geomagnetic storm of September 1909 CE. These records also show that there is no enhancement in 36Cl production rate following the Carrington event of 1859 CE. A theoretical experiment also proposes that it is possible that major solar storms with a large flux of lower energy particles could lead to a significant increase in ice-core 36Cl concentrations but not in 10Be. Finally, it is shown that wiggles in cosmogenic radionuclides caused by longer-term changes in solar activity can be used to synchronize lake sediment records from Europe to Greenland ice cores. The investigation of both archives on the same time-scale suggests that the climate oscillations observed in Greenland, and subsequently in Western Europe could be attributed, in part, to solar forcing.