Early Earth less hellish than previously thought

Calvin Miller is shown at the Kerlingarfjoll volcano in central Iceland. Some geologists have proposed that the early Earth may have resembled regions like this. -  Tamara Carley
Calvin Miller is shown at the Kerlingarfjoll volcano in central Iceland. Some geologists have proposed that the early Earth may have resembled regions like this. – Tamara Carley

Conditions on Earth for the first 500 million years after it formed may have been surprisingly similar to the present day, complete with oceans, continents and active crustal plates.

This alternate view of Earth’s first geologic eon, called the Hadean, has gained substantial new support from the first detailed comparison of zircon crystals that formed more than 4 billion years ago with those formed contemporaneously in Iceland, which has been proposed as a possible geological analog for early Earth.

The study was conducted by a team of geologists directed by Calvin Miller, the William R. Kenan Jr. Professor of Earth and Environmental Sciences at Vanderbilt University, and published online this weekend by the journal Earth and Planetary Science Letters in a paper titled, “Iceland is not a magmatic analog for the Hadean: Evidence from the zircon record.”

From the early 20th century up through the 1980’s, geologists generally agreed that conditions during the Hadean period were utterly hostile to life. Inability to find rock formations from the period led them to conclude that early Earth was hellishly hot, either entirely molten or subject to such intense asteroid bombardment that any rocks that formed were rapidly remelted. As a result, they pictured the surface of the Earth as covered by a giant “magma ocean.”

This perception began to change about 30 years ago when geologists discovered zircon crystals (a mineral typically associated with granite) with ages exceeding 4 billion years old preserved in younger sandstones. These ancient zircons opened the door for exploration of the Earth’s earliest crust. In addition to the radiometric dating techniques that revealed the ages of these ancient zircons, geologists used other analytical techniques to extract information about the environment in which the crystals formed, including the temperature and whether water was present.

Since then zircon studies have revealed that the Hadean Earth was not the uniformly hellish place previously imagined, but during some periods possessed an established crust cool enough so that surface water could form – possibly on the scale of oceans.

Accepting that the early Earth had a solid crust and liquid water (at least at times), scientists have continued to debate the nature of that crust and the processes that were active at that time: How similar was the Hadean Earth to what we see today?

Two schools of thought have emerged: One argues that Hadean Earth was surprisingly similar to the present day. The other maintains that, although it was less hostile than formerly believed, early Earth was nonetheless a foreign-seeming and formidable place, similar to the hottest, most extreme, geologic environments of today. A popular analog is Iceland, where substantial amounts of crust are forming from basaltic magma that is much hotter than the magmas that built most of Earth’s current continental crust.

“We reasoned that the only concrete evidence for what the Hadean was like came from the only known survivors: zircon crystals – and yet no one had investigated Icelandic zircon to compare their telltale compositions to those that are more than 4 billion years old, or with zircon from other modern environments,” said Miller.

In 2009, Vanderbilt doctoral student Tamara Carley, who has just accepted the position of assistant professor at Layfayette College, began collecting samples from volcanoes and sands derived from erosion of Icelandic volcanoes. She separated thousands of zircon crystals from the samples, which cover the island’s regional diversity and represent its 18 million year history.

Working with Miller and doctoral student Abraham Padilla at Vanderbilt, Joe Wooden at Stanford University, Axel Schmitt and Rita Economos from UCLA, Ilya Bindeman at the University of Oregon and Brennan Jordan at the University of South Dakota, Carley analyzed about 1,000 zircon crystals for their age and elemental and isotopic compositions. She then searched the literature for all comparable analyses of Hadean zircon and for representative analyses of zircon from other modern environments.

“We discovered that Icelandic zircons are quite distinctive from crystals formed in other locations on modern Earth. We also found that they formed in magmas that are remarkably different from those in which the Hadean zircons grew,” said Carley.

Most importantly, their analysis found that Icelandic zircons grew from much hotter magmas than Hadean zircons. Although surface water played an important role in the generation of both Icelandic and Hadean crystals, in the Icelandic case the water was extremely hot when it interacted with the source rocks while the Hadean water-rock interactions were at significantly lower temperatures.

“Our conclusion is counterintuitive,” said Miller. “Hadean zircons grew from magmas rather similar to those formed in modern subduction zones, but apparently even ‘cooler’ and ‘wetter’ than those being produced today.”

Searching for ‘Martians’ in ancient rocks

'The rocks kind of resemble Swiss cheese,' explains Nicola McLoughlin.
‘The rocks kind of resemble Swiss cheese,’ explains Nicola McLoughlin.

They are looking for traces of micro-organisms which literally eat rock. Not just any kind of rock, however: volcanic glass is necessary in order for these tiny organisms to survive.

Such volcanic glass is often found between pillow lava, which are formed when magma comes into contact with water. Pillow lava does not have crystal structures, which means that the microorganisms can manage to “eat” their way through them.

As the microbes eat their way through the glass, they leave behind small cavities shaped like tiny bubbles or pipes.

Searching on the ancient seabed

“The rocks kind of resemble Swiss cheese,” explains Nicola McLoughlin, a post-doc at the University of Bergen. She explains that such microbes are commonly found on the modern seabed around the Mid-Atlantic Ridge. Ms McLoughlin is hunting for ones that may have lived 3.5 billion years ago. In order to study them she needs a seabed which is at least that old.

And that is not easy to find. As a result of the plate tectonics, new seabed is continuously being formed in areas such as the Mid-Atlantic Ridge. But this in turn leads to the older seabed being pushed outwards and destroyed. As a result, the age of the seabed usually does not exceed 170 million years.

“But sometimes processes occur which lead to a part of the seabed being stripped off, and transported up to the continents, so-called ophiolites,” explains Ms McLoughlin.

Controversial finds

In South Africa and Australia, signs of such “rock eating” microorganisms have been found in seabed which is approximately 3.5 billion years old.

“These finds are controversial since it is practically speaking often impossible to find traces of organic material in rocks that are so old. However, we have observed that many of the same tubular or pipe structures that we find after modern rock eating microorganisms are present,” explains Ms McLoughlin.

Even though the rock has been periodically subject to high pressures and temperatures for billions of years, the cavities are preserved intact because they have been filled up with different materials after they arose. When geologists date the age of the material which has filled the pipe-shaped cavities, and the surrounding volcanic glass, they find a discrepancy.

Radiometric dating

For example, in one of the places she has been searching for such ancient micro-biotic life, North Pilbara, the former seabed is in fact 3.5 billion years old. However, the materials which filled the cavities are “only” 2.9 billion years old.

“The fact that the cavities are filled with materials is what makes it possible for us to find these traces such a long time afterwards,” says Ms McLoughlin of the Centre for geo-biosphere research.

The results of the research conducted by Ms McLoughlin and her colleagues are of interest to those who are searching for life in completely different places. Places like Mars for example.

Need to perfect our methods here on the Earth first

The reason for this is that there is a great deal of volcanic rock on Mars. If there was once water on Mars, and if it is possible to find pillow lava there similar to that which exists here on Earth, then conditions may be right for discovering traces of microbial life.

“But first we must perfect our own methods and thoroughly test the criteria that form the basis for our claim that we have found such structures on the terrestrial seabed. Until we can trust our results from here on the Earth, we cannot use our methods on another planet,” emphasises Ms McLoughlin. In the exploration of this phenomenon, she has teamed up with Ingunn Thorseth, Harald Furnes and Neil Banerjee.