The cover of Eos, 63(4), January 26, 1982, is a wonderful picture of varves with a strong 11 to 12 periodicity. Does it give evidence for a sunspot cycle in late Precambrian?
I want to suggest another interpretation. It may be the record of tides, in a cotidal flat deposition area, with an annual interruption, either by lack of sediments or by erosion. At present, we have some 25 tidal cycles per year. A close examination of the picture shows that the small cycles are much thinner on both sides of the dark lines, which may well correspond to several missing cycles.