Abstract:

Continental lacustrine laminites deposited in low latitudes have been examined from the Devonian and Torridonian (Mesoproterozoic) strata of northern Scotland to investigate the consistency of sunspot periodicity through geological time. The thicknesses of 2448 annual laminae were measured from the Middle Devonian (c. 388 Ma) Achanarras Limestone Member, Lower Caithness Flagstone Group, and of 2821 laminae from the Mesoproterozoic (1.2 Ga) Poll a'Mhuilt Member of the Stoer Group. Time-series analysis resolved periodicities of 13.8 and 13.9 years in the complete Devonian and Mesoproterozoic datasets respectively. When sedimentological variance was considered and the data were examined as evolutive spectra those sections consisting of the most robust data consistently yielded periodicities between 8.6 and 13.7 years for the Devonian section and 9.2 and 10.6 years for the Mesoproterozoic section. These correspond well to Schwabe cycles. Evidence for the presence of the longer Hale cycles (20 years) was also recognized in the Devonian time series. The sunspot periodicities identified within the Devonian and Mesoproterozoic show little variance from published examples from the Quaternary and Eocene and therefore provide evidence for the consistency of solar processes and solar–atmospheric interactions for the past 1.2 Ga.

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