If the first primitive animals crawling around on the seabed had somehow built telescopes, they would have seen something remarkable. The sun would have grown more and less active over an 11-year cycle, just as it does today. That鈥檚 because our star has maintained the same steady cycle for more than 700 million years.
The solar cycle is most visible as the number of relatively dark sunspots on the sun鈥檚 surface. During a solar maximum there are many sunspots, and hardly any during a minimum. Maxima and minima alternate over an 11-year cycle, which has been consistent over hundreds of years of observations, except for occasional 鈥済rand minima鈥� during which the cycle is temporarily suppressed.
Evidence from the geological record suggests that . The solar cycle during the dinosaur era was similar to that of today.
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Researchers led by and of the China University of Geosciences in Beijing have now reconstructed solar cycles from 810-715 million years ago. At this time, life on Earth was still relatively primitive and there probably weren鈥檛 any complex animals. The planet was about to almost entirely freeze over, for the second time in its history, becoming Snowball Earth.
Clues in the rocks
To find out what the solar cycle was like, the team studied sedimentary rocks called laminites, found in south China.
They formed on the bottom of a lake. In winter, silt was carried into the lake by a river and was laid down in a thin layer. Then in summer, calcite was precipitated and formed a second layer on top 鈥� followed by a layer of algae in the autumn. This pattern repeated every year, forming a repeating series of ultra-thin layers. Each set of three layers represents one year.
鈥淵ou can think of it like tree rings,鈥� says of the University of Edinburgh, UK, who was not involved in the study but .
The next step was to study the layers in detail and look for patterns. The team found that the thickness of the layers, and their chemical makeup, varied over a period of 11 years.
This probably occurs because the solar cycle can have subtle influences on the weather. When solar activity is low, more galactic cosmic rays enter Earth鈥檚 atmosphere and trigger extra cloud formation. This means more rainfall, and thus rivers carried more sediment into the lake at these times.
The finding is in line with other studies of prehistoric solar cycles. Even when Earth鈥檚 climate was changing dramatically, like during the Snowball Earth period, the sun kept doing its thing.
鈥淚t seems to be generally consistent even back to 1.6 billion years ago,鈥� says Hartley. and found 11-year solar cycles, but he says 鈥渢here is an argument that that鈥檚 not necessarily that reliable鈥�.
The current solar cycle began with a minimum in 2008, during which there were few sunspots on the sun鈥檚 surface. It reached a maximum in 2014 but this was relatively weak, with only a few dozen sunspots. The cycle is due to end this year. Some solar physicists think the sun is about to enter a grand solar minimum, in which the 11-year cycle is temporarily suppressed.
Precambrian Research
