Even In The Age Of The Dinosaurs, Orbital Patterns Shaped The Climate

These sediments in Dorset, UK, show the same Lower Jurassic cycles tracked in the Welsh mudstones that reveal orbital changes drove the carbon cycle that amplified climatic shifts then as now. Dr Micha Ruhl

Over the last 2.6 million years, the Earth has shifted between ice ages and warmer interglacials. It's known this pattern is driven by changes in the Earth's orbit that vary between orbital elongation and when the planet is closest to the Sun. One outstanding question has been how far back this pattern goes. New research reveals it was important during the Triassic Era.

The so-called Milankovitch cycles are a product of other planets' gravitational pull and have almost certainly been occurring since the Moon was formed, if not before. However, their direct impact on climate cycles is fairly small and would have been smaller still at a time when the continents were less concentrated in one hemisphere.

Prior to human interference, the modest changes in temperature induced by orbital cycles were enormously amplified by carbon dioxide. Dr Micha Ruhl of Trinity College, Dublin, has found something similar took place 200 million years ago.

At the end of each glacial period, an increase in sunlight hitting the Northern Hemisphere at sensitive times causes the release of carbon trapped in tundra. Combined with the melting of the outer edges of glaciers – making the planet a little darker and more absorbent to sunlight – this has a warming effect, which in turn leads to further carbon release and a feedback system that can melt rivers of ice miles high.

When dinosaurs ruled the Earth, continents were distributed differently and background atmospheric carbon dioxide was much higher. Yet Ruhl reports in Proceedings of the National Academy of Sciences, the carbon cycle was still a powerful influence.

Ruhl studied Welsh mudstone deposits laid down around the time of two of the most dramatic events of the dinosaur era: the Triassic-Jurassic extinction event 201 million years ago and the Toarcian Oceanic Anoxic Event 183 million years ago. Both have been heavily studied, but the periods before, after, and in between get less attention.

Deposited between the events, the mudstones reveal that "periodic changes in the shape of Earth's orbit around the sun impacted on the amount of energy received by Earth from the sun, which in turn impacted climatic and environmental processes, as well as the carbon-cycle, on local, regional and global scales," said Ruhl in a statement.

The shift between a circular and elongated orbit helped shape the climate of the Jurassic. Marisa Storm

Rainier periods caused increased weathering of rocks, pumping nutrients into the oceans that increased productivity and the drawing of carbon from the atmosphere. The 405,000-year cycle Ruhl found is different from those operating today, but the regularity marks the cause as astronomical, rather than volcanic.

The rocks do not record atmospheric carbon concentrations directly, but it can be calculated from ratios of carbon 13 to carbon 12 in organic material deposited within them.

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