Greenhouse climate conditions that enveloped the Earth for long periods in the deep past – millions of years before humans added their current substantial contribution – were caused by the break-up of the supercontinent Pangaea.
Joint Australian-German research has found that steep increases in carbon dioxide levels during two periods – between 160- and 100-million years ago, and around 55-million years ago – coincide with the development of extensive rift systems within the giant land mass.
Pangaea formed approximately 335 million years ago, and existed primarily in the southern hemisphere. It started to break apart around 175 million years ago and eventually yielded the land masses that today make up Australia, Antarctica, India, Africa, North and South America, and Eurasia.
In a paper published in the journal Nature Geoscience, a team drawn from the University of Sydney and the Helmholtz Centre in Potsdam, Germany, model the effect of the gradual splitting apart of the supercontinent – an effect called rifting – and how much carbon dioxide the process released into the atmosphere.
To construct the basics for their modelling, scientists used previously published data on CO2 released at along the well-studied East African Rift.
The association between deep rift formation and the release of carbon dioxide into the atmosphere lies in the fact that much less than 1% of the carbon dioxide present on Earth is present in the air, soil or oceans. The overwhelming majority of it lies bound up in the deep Earth.
Some of this buried gas is released by means of volcanic eruptions, but over many millions of years much more has been released by rifting.
“Rift systems develop by tectonic stretching of the continental crust, which may lead to break-up of entire plates”, explains lead author Sascha Brune from the Helmholtz Centre.
“The East African Rift with a total length of 6,000 kilometres is the largest in the world, but it appears small in comparison to the rift systems which were formed 130 million years ago when the supercontinent Pangea broke apart, comprising a network with a total length of more than 40,000 kilometres.”
Brune and his colleagues combined the East Rift data with plate tectonic models covering the past 200 million years, and were thus able to reconstruct how the rift network developed over time.
They found two periods of heightened rifting activity as Pangaea slowly split apart. By simulating the effect of increased carbon dioxide release during those periods, they found that they matched other data showing greenhouse conditions at the same period.
Despite such a massive effect resulting from the tectonic break-up of the largest land mass that has ever existed on Earth, Brune cautions that the gas output was nowhere near the level it is today.
“The global carbon dioxide degassing rates at rift systems, however, are just a fraction of the anthropogenic carbon release today”, he says.