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The causes of a sharp increase in the radiocarbon content in the Earth's atmosphere for periods with abnormally low solar modulation of galactic cosmic rays (GCRs) are considered. It is shown that such periods around 660 BC and 5480 BC can also be caused (in addition to the low activity of the Sun itself) by heliospheric compression as a result of the passage of the Solar System through a compact interstellar cloud with a density of nH ~ 100 cm–3 and characteristic dimensions of 10–100 AU. In this case, GCR modulation is suppressed, while solar cyclic processes are preserved. Wavelet analysis of radiocarbon data shows the presence of the 11-year solar-activity cycle in the period around 5480 BC. Based on the model of the interaction of cosmic rays with the Earth's atmosphere, the 14C production rates are calculated for various GCR model spectra typical for the local interstellar medium. [ABSTRACT FROM AUTHOR]

Abstract: Carbon isotope 14С is produced in the Earth's atmosphere by energetic cosmic-ray (CR) particles. The data on its atmospheric abundance are used to reconstruct the rate of its production and to analyze past levels of CR intensity and solar activity. These data are obtained via measurement of the 14С abundance in tree rings, with their real age determined dendrochronologically. The 14С abundance depends on the carbon dioxide concentration in the atmosphere and on climatic changes that affect the processes of carbon exchange between natural reservoirs. The present study reports the results of reconstruction of the production rate for cosmogenic 14С in the Earth's atmosphere in the interval from the onset of deglaciation (~17 000 BC) to the mid-Holocene. The climatic factors mentioned above were taken into account in this reconstruction. [ABSTRACT FROM AUTHOR]