You probably picture the Earth's annual journey around the Sun as being more or less a circle, but that's only true some of the time. Models have long suggested that our home planet's orbit shifts from circular to elliptical and back again over hundreds of thousands of years, which plays a part in natural climate swings. Now, scientists have found the first physical evidence for the cycle, and traced it back to before the dinosaurs.

The Earth goes through regular, large-scale cycles of ice ages and warmer periods, driven by a whole range of factors. Some of the main contributors are what are known as Milankovitch Cycles, along with variations in the Earth's orbital path (cycling every 100,000 years), its axial tilt (on a 41,000-year cycle) and the "wobble" of its spin (on a 23,000-year cycle). These affect how much solar energy hits the Northern Hemisphere at different times of the year, and in turn affect the planet's long-term climate.

But scientists have long suspected another, much longer cycle sits over the top of them. Every 405,000 years or so, the shape of the Earth's orbit shifts from almost perfectly circular to slightly elliptical, thanks to the complex interactions between Earth and other planets, particularly our closest neighbor Venus and the huge gravitational influence of Jupiter. This has been traced back about 50 million years, but with so many moving parts it all gets a bit murky the further back you look.

"There are other, shorter, orbital cycles, but when you look into the past, it's very difficult to know which one you're dealing with at any one time, because they change over time," says Dennis Kent, lead author of the study. "The beauty of this one is that it stands alone. It doesn't change. All the other ones move over it."

Previously, this longer cycle has been hypothesized based on calculations of the movements of planets, but now researchers from Columbia University and Rutgers University have found the first physical evidence for it. The team drilled rock cores 1,500 ft (457 m) deep from the Petrified Forest National Park in Arizona, and compared them to similar deep cores from New York and New Jersey.

By analyzing the decay of isotopes in layers of volcanic ash dotted through the rock, the team was able to date the Arizona cores back between 209 and 215 million years ago. That places them in the late Triassic, around the beginning of the age of the dinosaurs. The researchers were also able to see clear signs of magnetic field reversals, a fairly regular cycle where Earth's magnetic poles flip every 200,000 to 300,000 years.

The researchers compared the Arizona cores to those from New York and New Jersey, and lined up the points of pole reversals. Combining both sets of data also showed that they formed at the same time and had similar characteristics that pointed to the influence of a longer-term cycle.

The cycle reared its head in the rocks in the form of alternating, extreme wet and dry periods. Darker layers indicate black shale, formed at the bottom of deep lakes as a result of wetter seasons, while lighter-colored rock indicates much drier conditions. These conditions cycled through every 405,000 years, with the darker/wetter times suggesting the Earth was in a highly-eccentric orbit, while the lighter/drier times represent a smoother, circular path.

The researchers say that this larger-scale cycle doesn't directly affect the Earth's climate – instead, it either strengthens or weakens the effects of the smaller Milankovitch Cycles.

"It's an astonishing result because this long cycle, which had been predicted from planetary motions through about 50 million years ago, has been confirmed through at least 215 million years ago," says Kent. "Scientists can now link changes in the climate, environment, dinosaurs, mammals and fossils around the world to this 405,000-year cycle in a very precise way."

The research was published in the journal Proceedings of the National Academy of Sciences.

Sources: Rutgers University, Columbia University

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