Cold sea, hot sea? Barberton rocks reveal 3,5 billion-year-old story
For the past three decades, scientists have maintained that the sea was hot when life first began on Earth some 3,5 billion years ago – somewhere between a scorching 30 and 80?C. But at the same time, research has shown that the sun was much less intense, generating about 25% less heat than it does today.
For Nelson Mandela Metropolitan University’s Prof Maarten de Wit, the two theories just did not add up. “The planet should have been frozen unless the atmosphere was dense with greenhouse gases.”
So, together with co-researcher Harald Furnes from the University of Bergen in Norway, he sought proof in the rocks of the Barberton greenstone belt, exposed in the Makhonjwa Mountains in Mpumalanga, which are among the very oldest on earth, and arguably contain the earliest fossils. The two have just released a paper in the journal Science Advances, which disproves the earlier “hot earth” theories.
Working on the Barberton rock records (which would once have lain at the bottom of a deep ocean) over the past six years, the pair found two particular pieces of evidence against the hot ocean theory:
The first was the discovery of hydrothermal pipes that formed in a deep ocean environment – which suggest the hot water in question did not come from the ocean, but rather from hydrothermal vents that sprung from the earth’s volcanic crust two to four kilometres below the ocean surface, and that were active at that time, at about 200?C, “just as we find in modern oceans”, said De Wit. They also found hydrothermal fields of slightly younger age, which would have operated as hot springs near and above sea level, between about 30 and 270?C .
In addition, and quite separately, they also discovered evidence of glacial deposits, at and above sea level; and sulfate minerals (e.g. Gypsum) in the ocean sediments deposited two to four kilometres below sea level, which today only grow in deep sea environments where there is cold water.
These key findings disprove the earlier theory and present a new one: that the sea was cold – and that life was already thriving and may even have originated in this cold sea.
“What is unique in our findings is that there is evidence from both deep oceans and shallow oceans that ocean water was cold,” said De Wit. “Since some of the shallow water deposits were subaerial [above sea level], it follows that the atmosphere was also cold.”
“The shallow water hydrothermal springs can be compared to the hot springs at Yellowstone in the United States, where hot water spouts up at regular intervals. Those temperatures are very high and exactly the temperatures we find in the Barberton rock signatures. They are not the temperatures of the ocean, but rather the volcanic recycling of ocean water … The earlier interpretations never considered hydrothermal fields being the driver for the local high temperatures.”
Using paleomagnetic signatures in the ancient rocks, the scientists were able to determine that they would have formed somewhere near the equator.
“We don’t have glaciers at surface level at the equator. This shows there was a very special cold period in the planet as a whole – the whole world was cold.” There are younger periods in the geological past that also record global freezing-over of earth, referred to as “snowball earth”.
The new discoveries and geochemical work suggest that life might have started in a cold environment.
However, the hydrothermal fields De Wit describe may have provided a nurturing warm environment for bacteria. He and Furnes, in an earlier Science article, were able to show that early life existed in the same rocks of this study.
“These are arguably the oldest (trace) fossils described from Earth, that formed like bacteria in modern pillow lavas in the deep, cold ocean waters, and we now suspect also that the hydrothermal fields we describe may have provided a nurturing warm ‘home’ for bacteria too – as they do in modern oceans around its deep ocean hydrothermal vents, and in shallow waters such as Yellowstone, United States and Taupo, New Zealand and other volcanic regions like Japan.”
Image: Scientists have discovered ancient hydrothermal vents in the Barberton greenstone belt, exposed in the Makhonjwa Mountains in Mpumalanga. In this photograph, the vents are situated at the bottom of the small hill in the centre. (The scientists’ camp site is just visible at the site.)
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